Heating a greenhouse / indoor pond

hen11(z7 Eng UK)February 5, 2007

Hi there

I'm just finishing off building my new greenhouse. It's slightly bigger than my last one so I was thinking of some cheaper/more reliable ways of heating the place (i've had some bad experience with heater failures recently). I'm thinking of putting in a pond to grow some tropical lillies which will have to be heated, but I was wondering whether I could just use the water to heat the whole greenhouse?

I've read the other thread about heating a greenhouse with a hot-tub, I've also heard from another Victoria grower that he used his pond to heat his large stove-house all year (at a temperature of > 20C). The only problem I'm having is thinking of a suitable, cheap and safe way of actually heating the pond. I've heard of people using domestic immersion heaters with an exterior tank which water is pumped through to heat. This sounds quite good but i'd probably need an electrician to come and install it. I've also been looking at stock-tank heaters but there is very little info about these. Any ideas would be much appreciated.

Also, i'm wondering how large the surface area of the pond would have to be in relation to the greenhouse? The rough floor area is 20' x 10', I was thinking of having a pond area of 6' x 10', would this be enough?

Does anyone else heat their ponds? If so, how?



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garyfla_gw(10 Florida)

WATER stayed at 60 but air even directly above the large pool dropped to 50. Parts of my grow area I want to fall into the 40 's but the central part I'm hoping I can maintain above 50 no matter what Ma Nature dishes out.
I just guessed at the 1500 Watts, have no idea if this will be enough. I opted for 3 heat sources in case one fails
These are just standard submersible,aquarium heaters
Probaly not much help for your situation

    Bookmark   February 5, 2007 at 5:23PM
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watergal(z6/7 Westminster, MD)

How warm do you want the air and water to be? Stock tank heaters are designed to keep the water liquid, but not much warmer than that - I think they have a thermostat to keep water in the 35 to 45 degree F range. Probably not enough to heat the air very much.

Do you want to keep the tropical lilies alive or get them to bloom? They like to be pretty warm to keep from going dormant, above 70 degrees F is the goal.

I would look into heaters for koi ponds. However, they are quite expensive. Maybe a good aquarium store can help you find something appropriate?

    Bookmark   February 6, 2007 at 7:34AM
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Hi Hen
I think you may have problems with condensation.
It would definitely be a plan to use a pond cover at night when the greenhouse air temperature drops, especially in the colder months.

Heatloss (and evaporation) from the pond is related to it`s surface area. Any sort of airflow will speed things up through the evaporative cooling effect.
How economical it will be to maintain the pond temperature will mostly depend on how well insulated the greenhouse and pond is.

It`s great to see some innovative thinking, so the very best of luck with your greenhouse and pond :)

    Bookmark   February 6, 2007 at 10:09PM
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garyfla_gw(10 Florida)

I'll add a couple of comments to bump this thread up.
Maybe I misunderstood but isn't your purpose in heating the water to keep thee air warm?? Covering the pool would obviously defeat that. Obviously condensation is terrific
but I don't see the problem if your talking tropical.
Heating water especially with electricity is very easy and effiecent compared to air. What's wrong with submersible heaters??? gary

    Bookmark   February 7, 2007 at 5:13AM
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Hi Gary,
If we had some of your sunshine it wouldn`t be a problem :)
With typical uk temperatures the pond will have a hard time keeping itself warm let alone heating the greenhouse as well.

The heat coming from the pond will be significantly less than the amount the greenhouse is losing to the outside air.
To act as a heater the pond has to heat the internal air and overcome the heatloss from the building.
Judging from the rough figures Hen gave.. the greenhouse may lose over 5x as much heat as the pond can supply.
Condensation isn`t too bad if the structure is rotproof, but moulds, fungi and a whole host of bugs also love warm moist environments.

    Bookmark   February 7, 2007 at 8:05AM
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nathanhurst(VIC Aust)

As discussed in the thread http://forums2.gardenweb.com/forums/load/strucs/msg0120023511440.html?32
if you're evaporating water you going to be wasting a lot of heat energy. It's probably best to heat the greenhouse with a separate loop of hydronic heating and let your pond find a natural balance temperature with the air. So put an insulated storage tank under a central aisle or similar, heat it with solar and backup heat and run that water under the plants using the special tubing mentioned by cuesa.

    Bookmark   February 7, 2007 at 11:15AM
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garyfla_gw(10 Florida)

Why are you wasting heat energy?? In my case the main purpose is to keep the water warm the air is a by product.
Sure a lot easier to heat water than air. gary

    Bookmark   February 7, 2007 at 5:42PM
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hen11(z7 Eng UK)

Hi There,

thanks for all the excellent replies! I like hearing the different opinions and ideas about this. A few more details:

The greenhouse is a three-quarter span, homemade (wood) greenhouse up against a 6' brick wall. One end has a waist-height brick wall and the side has a 3 1/2' stone wall. I'm planning on two/three layers of large-size bubble wrap on the areas of glass only. I'm trying to maintain at least 70% humidity, with a rise at night of up to 95% so condesation shouldn't really be a problem.

If my maths is right, a 6' x 10' x 1' pond is only around 200 gallons. I was thinking of having a chamber where water was pumped through, heated, then let flow out again. I was hoping that this could be made into a waterfall, mainly for aesthetics but also to help the water lose heat into the air. How much did your 800watt heaters cost gary? How many do you think I might need?

A lot of people are saying that evaporating water loses energy, that sounds fair enough but can anyone explain why that happens? I don't have a vast knowledge of physics but I would have thought that through random collisions, the particles which gain enough energy to evaporate would then let the enrgy back out again in the form of heat. I suppose that this might be being bit naive though? Could anyone explain this a bit further for me?

I don't see why mildew or fungus would become a problem, all these plants come from areas of constant high humidity. As long as the air is always moving inside and any rotting/dead plant material is removed daily then there shouldn't be a chance for and mould to set in, should there? Has anyone encountered these kind of problems already?

I was also thinking of maybe adding a series of tubes (as mentioned by ceusa) underneath a centre path and pumping water from the pond through these. Maybe even having the tubes leading to a small rock-pool to act as a secondary radiator at the other end of the house. The Pond is probably going to have to be heated anyway, I've got some victoria seedlings on the go now which i'm planning to put into the pond this spring. Really its just about trying to find a way to heat the greenhouse without having to have a separate heater for the pond and the air.

Thanks for all the replies so far! Any views on the new ideas?

    Bookmark   February 7, 2007 at 7:46PM
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garyfla_gw(10 Florida)

I'm afraid I'm not much help as our climates are soooo different. My main purpose for heating the pools is for the fish rather than the plants. The heating system has only been in place since Sept and 41 for 3 hours has been the low. So not really much of a test. The aquarium heaters kept the water at 60 as set uncovered as well as covered
..The air above the pool fell to 50's but then I have no coverings on the roof ,just lattice.
In the US, aquarium heaters, are made to 300 watts but you can order at least to 1000 watts that I'm aware of .I went with 3 at 500 watts each in 3 locations of the system.
They cost 28 bucks each and they have digital temp controls but I would certainly recommend metal rather than glass tubes. Have no idea if this will be enough wattage
I just guessed and this is what I found available. If we get a severe cold spell I'll shut down the system and just maintain the aquarium. Actually I want parts of the structure to fall into the forties for a cool rest period for orchids but would like to maintain a central area above 50. I'm still experimenting lol In the past I've always pumped heat from the house with various types of coverings
all of which have serious drawbacks.
Sorry I can't be of more help. I apologise too old to get the metric system through my head lol gary

    Bookmark   February 8, 2007 at 8:00AM
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stressbaby(z6 MO)

"Why are you wasting heat energy?"

You know how water, when it freezes, releases a lot of heat? This is the latent heat, more specifically the latent heat of fusion, and this is why people put barrels of water in the greenhouse. The heat released as the water freezes can keep the GH right at the freezing mark and provides some protection in the GH.

Water also has what is called the latent heat of vaporization. Just as moving from solid to liquid phase requires heat, moving from liquid to gas phase requires heat as well. Think evaporative cooling...that's the same principle.

Nathan can correct me if I'm wrong, but I think that the latent heat of vaporization for water is actually much higher than the latent heat of fusion for water.

    Bookmark   February 8, 2007 at 9:49AM
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Hi Hen
Having a high humidity will reduce the temperature difference needed to drop the air to dewpoint which will cause the water vapour to condense.
If the humid air finds a surface cool enough, you will get condensation, raising the RH actually makes it easier ;)
For example, at 70%RH you need a 10F drop for the air to get to dewpoint. At 95% RH it just needs a tiny 1.5F drop.

I think the layers of bubblewrap will reduce the incoming light levels somewhat but this may not be an issue depending on the plants light requirements.

Regarding your question about heatloss and evaporation, a quick google search for "sensible and latent heatloss" should tell you all you need to know :)

Judging by the amount of water in your pond (60 cubic feet), a heater around 1.09kw, should be able to raise the pond water temperature 1F/hr assuming no losses.

    Bookmark   February 8, 2007 at 9:52AM
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Hi Stressbaby,
i guess we posted at the same time :)
To raise a pound of water 1F it takes 1btu, to vapourise a pound of water takes over 1000btu.
When the vapour condenses it releases most of this energy in the process. In a greenhouse situation that usually means the cold exterior glazing, where the energy is instantly lost to the cold exterior.

A very effective system for transporting large quantities heat from the pond to the street :)

    Bookmark   February 8, 2007 at 10:05AM
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nathanhurst(VIC Aust)

yes, it's been well explained by stress and hex. I'll just add that most people like to ventilate their greenhouses, if you've made that air humid you're blowing money out the window.

Physicswise water gas molecules have a lot more energy than water liquid ones, so once those molecules launch into the air they've absorbed lots of heat (1000btu/lb or 2.2MJ/kg) which they only give up when they turn back into liquid.

The solutions are a) don't ventilate or b) don't heat open water. On the plus side, your pond will never get very hot in summer for exactly the same reason.

Probably best to put the heating tubes directly under the plants as it is often the roots that need the heat rather than the leaves. The leaves will heat up when they get sun on them, but the roots take too long in cold weather.

    Bookmark   February 8, 2007 at 5:44PM
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I use solar energy and water to heat my greenhouses. I have two 10x30 greenhouses, and I pump water to roof-mounted solar heaters that circulate the heated water through large tanks in the greenhouses between 10 am and 4 pm each day. I get about a 20 degree temperature increase in the water on a clear but cold day. Today, the panels heated the water to 78 degrees. The night temperatures outside are in the 30s and the water keeps the greenhouses warm. I have maybe 500 gal. in each greenhouse and use 4x10 foot solar panels, one per greenhouse.

    Bookmark   February 8, 2007 at 9:46PM
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garyfla_gw(10 Florida)

Far too technical for me lol. I have an aquarium that heats a room. Have never checked the air temps as I'm only concerned with the water but it's immediately obvious when you walk in the room. lol
If he wants to raise Vics he will definitely have to keep the water above 65.
I'm using about the same method as he describes.5x10x3,2x6x3 pools and a 150 gallon aquarium, All 3 have heaters. Water is pumped from large pool to wall where it flows down into aquarium from the it goes to small pond and retured to large pond. The water is kept at my ideal and I actually get above outside temps in the air.
I want to maintain he normal humidity at 70 percent or above so evaporation is a bonus for me. I seriously doubt
it will keep air more than 10 degrees above outside but I can guarantee that frost will not settle on the pools. Outside that I don't know.
So far the system has kept water at ideal while air is actually above what I want. except when I used the insulation. About the only difference I can see is that the heaters worked harder but then heating the air was not necessary.
But for me the system provides many other advantages surplus heat is just a bonus.
How far I can push this I have no idea. I think it's the ideal solution in my climate gary

    Bookmark   February 9, 2007 at 10:32AM
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hen11(z7 Eng UK)

Hi there,

Still loving the great suggestions!

It seems as if the general consensus is that if I use this method of heating, the way I look after the greenhouse would have to change. The problem with running hot water under the plants is that my plants are mostly epiphytes mounted on branches/hung from the cieling. Any terrestrial plants I have tend to be planted in beds which are dug out, lined with membrane and filled with a loam-based compost. I suppose that pipes could be run under the beds but they may be too deep to really make a difference.

The plants that I grow need a hot and humid atmosphere, simply finding the cheapest way of making the air warm in the greenhouse is not my aim. It is now my understanding that my original thought of heating a pond to do both those jobs, is possible, but would require far too much energy to make it practical (at least I hope I've taken the right meaning from StressBaby and Hex2006's very detailed explanations) If this is the case, what if I used a koi-pond inline heater to heat a network of plastic pipes which ran inside the pond and under the path/ around the side walls. This way the air would be heated as well as the pond, meaning a reduced percentage of the heated water being vaporised (an amount of water still being vaporised to raise humidity). Then also incorporate some kind of shut-off valve for summer when only the water would need to be heated.

I'm quite interested in these solar-heaters. Are they just heat exchangers using the suns heat or is there some power included in there somewhere? I've been thinking of making a few wind turbines, but thats just a pipe-dream really. How reliable do you find this method is? For example, today and yesterday we've had the biggest covering of snow for years (If we're lucky, we normally have one day a year where there is just enough snow for a snowball fight before 11am), how would the heating fare on a day like this when it is most needed? Do you use a backup heater?

Any views on this idea or is it really best to just use two separate systems?

Thanks for all your help and patience!

    Bookmark   February 9, 2007 at 11:06AM
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hen11(z7 Eng UK)

Sorry gary, didn't see your post.

Do you have any pictures of your set-up? Is your shade-house open mesh or does it have some kind of covering? When I first added bubble wrap to my greenhouse (one layer of small-bubble) it raised the temperature 10C. What proportion of the floor is used by uncovered water?

    Bookmark   February 9, 2007 at 11:15AM
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garyfla_gw(10 Florida)

The shade house was originally aa attached screenroom that has been modified over the years to hold the orchids and some of my birds. Have tried many types of coverings over the years and I'm in the process of doing it again.
Decided to go with the 3 inch diagonal lattice with no screen and use vines as the main shade source. Heat is my main problem and everything I've tried in the past made it worse lol.
The larger pond 5x10x3 is outside about 25 feet from the shadehouse while the 2x6 is in the SE corner inside. I plan on expanding the shadehouse so that the large pool will also be inside. The waterfall is located on the north wall above the aquarium 6x18x23 inches deep on an 18 inch stand. A submersible pump is located in the large pool and is fed to the waterfall underground.It is around 4 feet above the tank, falls down the "rocK' face into a marsh area then into the tank. Via a standpipe it overflows into the small pool and this overflows back into the large pool via PVC.pipe. The heaters are just placed on the bottom of the 3 locations they are set at 60 degrees.
I have pix of the setup before this system was installed
but of course nothing shows and the area is too large to get into one pic.. I have more recent pix of the waterfall as I'm experimenting with expanding foam painted with acrylic. Sort of a semi hydroponic setup. More of a seep wall than waterfall.
The heaters are not permanently installed untill I see how they work.. Probably doesn't help you a bit I'm sure.
Never freezes here and frosts are in hours and it usually gets above 60 the day after. Normally the night time lows are in the 60's during winter. Have had no frost since 2001 but recently had days in the mid fifties..also days in the mid eighties lol.
If your trying to grow tropical water plants I'd definitely heat the water as it's so easy and cheap but I suspect you'd have to heat the air also.

    Bookmark   February 10, 2007 at 6:58AM
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Ok. No more stinkin' words. Time for some pictures.

Here's the basic water tube in my greenhouse:

Here are the water tubes (which are linked with a 1" pipe) with a tilapia fish tank in between them:

Here is a picture of the water tube that gets the returned warm water from the solar panel (note the trickle of water from the pipe in the upper left):

Here's a solar panel before installation:

Here's the solar panel mounted to the roof of the adjacent shed, facing south and collecting solar energy:

Here are the happy little tomatoes and fish that thrive in the heat from that collector:

Get the general idea?

    Bookmark   February 10, 2007 at 1:09PM
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nathanhurst(VIC Aust)

You're going to lose a lot of heat with that trickle of water. I use much the same design to actively cool my greenhouse.

Where did you get that lovely clear tank though?

It's a very nice looking greenhouse.

    Bookmark   February 10, 2007 at 6:43PM
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Hi stoneunhenged
A very nice aquaponic greenhouse system.

I would think a stainless steel heat exchange coil for the panel would be better, but if it ain`t broke why fix it?
Water cooling that light is a potential source of heated water in the winter months too ;)
It`s great to see tomatoes grown with aquaponics, most people think it`s only good for lettuce and leafy stuff.
My modest freshwater aquarium grew chillies aquaponically from seedling to harvest, but the tomatoes just keeled over and died lol.
Guess i just need to add several hundred extra fish :)

Nice work.

    Bookmark   February 10, 2007 at 9:34PM
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The 'trickle' is 700 gph. The scale of the picture is deceptive.

The fiberglass cylindrical tank is available from Aquatic Ecosystems in Florida.

I supplement the fish water for the tomatoes with organic nutrients from Earth Juice.

I'm glad you guys like the system. It's fun to watch it do its thing.

    Bookmark   February 10, 2007 at 10:31PM
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garyfla_gw(10 Florida)

HOLY COW!! You spent a lot of bucks ?? lol My entire budget for my project is 3 grand and that must include a lot of the new stock.. I'm hoping to reproduce some of the
stuff and maybe break even?? Right that's going to happen lol
Interesting looking system but some questions. For the water reservoir. The acrylic cylinder. Wouldn't a standard aquarium been more useful and far cheaper?? Acrylic I can understand because of less conductivity. but round?? I use a 150 glass aquarium mainly because it was free lol but would have liked acrylic.
As to the fish .Why tilapia?? I opted for more commercial species hoping to get a dime or two back lol
Have many other questions but last. How many square feet of grow area does the systen require ??
So far I've lost nothing to equipment though some space has been lost to pure "looks" lol
Anyway, thanks for the pix gary

    Bookmark   February 11, 2007 at 7:29AM
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I read that you expect your 10 ft by 6 ft 12 in deep pond to be only 200 gal but that is way off. It would actually be 550 gal. nice reading otherwise

Here is a link that might be useful: build a tank

    Bookmark   February 11, 2007 at 9:01AM
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A ferrocement tank would be real cheap to make and could be any shape/size you like.
Tilapia are fast growing vegetarians. They eat algae and vegetable matter..maybe cheaper to feed perhaps? They do like the water pretty warm though.

Don`t forget that the actual shape of the greenhouse will have a huge impact on the heating/cooling costs. The best shape to maximise the floor area and minimise the external surface area is to use a geodesic.
Inexpensive too if you make it yourself :)

    Bookmark   February 11, 2007 at 9:22AM
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You can certainly buy cheaper water tanks than those fiberglass cylinders I have. For my set up, they're dual purpose: to hold water for thermal mass and to grow algae to feed the tilapia. So, I needed large clear tanks with a small footprint.

I grow tilapia because I eat them.

If you're looking for a large, inexpensive tank, I highly recommend buying a Rubbermaid stock tank. They have a threaded port for plumbing and are black and absorb solar energy.

    Bookmark   February 11, 2007 at 12:11PM
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hen11(z7 Eng UK)


I'm really getting interested in this solar idea! Stoneunhenged, do you use any backup heaters or supplement the heat in any way? Do you grow all year round? Is the Cichlid tank heated by the solar aswell?

"I read that you expect your 10 ft by 6 ft 12 in deep pond to be only 200 gal but that is way off. It would actually be 550 gal. nice reading otherwise - Jim"

oops! sorry about that. I got 450 US gal and 370 UK gal from here: [URL]http://www.fishlore.com/ConversionCalculator.htm[/URL] (120" x 72" x 12")

I'm thinking of trying to use a solar system with a supplemented heater to heat both the pond and the greenhouse. Here's my (crappy) diagram of what I had in mind:


I you can understand the poorly drawn paint diagram, the water is pumped from the solar panel on the roof to a heavily insulated underground heat storage tank. The water is then pumped into a heat exchanger for the pond (filled with pond water directly), a separate pump circulates water from the pond through the tank (in the form of a waterfall maybe? Or will this lose too much heat?). I'll either have to use non-copper tubing or cover the copper with silicon to avoid it contacting the pond water. After this, the water in the pipes continues around under the floor or walls and then back to the solar panel. In summer, I'll have some sort of short circuit pipe with a shut-off valve to stop the water going around the house and heating it unecessarily (with the possibility of pumping the pipes around the house to a cooling tank somewhere outside the greenhouse).
To regulate temperature, a thermostat will be attatched to the heater to switch off at say below 65C, and one to the pump to switch off above 75C. So when the water being circulated is too cold, the heater kicks in, and when it's too hot, the pump cuts out - stopping water from the hot solar panel running to the pond.

Does this sound like something that could work? Would I need a very large panel?


    Bookmark   February 11, 2007 at 12:12PM
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nathanhurst(VIC Aust)

I'll either have to use non-copper tubing or cover the copper with silicon to avoid it contacting the pond water.

I think people's fear of copper in ponds is misplaced - put a few plants in there and you'll need to be adding copper to get growth.

Does this sound like something that could work? Would I need a very large panel?

Work backwards - you know your climate, you know how big your greenhouse is, work out the heat losses for your climate, work out the length of time you want to store heat when it's cloudy, you won't be able to get much hotter than 50C so work out how much energy your tank will need to hold, work out how long it will need to be hot for, work out how large the solar collector will need to be in your climate.

I would use an evacuated tube collector as they have much better cloudy day performance and are really cheap now.

Put valves in so that you can isolate sections.

    Bookmark   February 11, 2007 at 4:41PM
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Hen 11:

The small tilapia tank in the photo is not directly heated by the solar panel. It has a 500w submersible heater that keeps it warm, if necessary. During the day, it is rarely on. When the temperature drops into the 20s (F) at night, I use a single burner propane heater to supplement the heat.

In a separate greenhouse I have a 450 gal. tilapia tank that is directly connected to a solar panel.

I use copper tubing and it has no effect on the fish.

    Bookmark   February 11, 2007 at 7:13PM
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I guess we should try to help Hen 11 out in a practical way that vaguely fits the situation.
The worst case scenario would be to keep the air temp of the greenhouse at least the same as the desired pond water temperature.. say 60F as a minimum? (i know it`s aiming high but still)
Assume the absolute minimum outside temp won`t go below 15F (-7C) and the average sunshine hours is 1750/year at the most ;)
To see what that entails we need more information about the building to calculate the heatloss.
Assuming Hen`s lean-to greenhouse is 20ft long x 10ft deep (6ft high at the front and 8ft high at the back) the total exposed surface area will be around 460 sq ft (worst case scenario)
If the glazing has R-1.5 with the 60F target internal temp and external temp of 15F, the difference is 45F!
The structure will lose 460x45/1.5 = 13,800btu/hr so the worst case is you may need upto 4kw of heat per hour to maintain 60F :)
1750 hrs of sun per year in the UK with a mean average of 300 btu/hr/sqft = 525,000 btu/yr/sqft (154kw/yr/sqft)
Averaged over a year this equates to about 60 btu per sqft of panel per hour, assuming 100% efficiency :)

227 sqft of panels would be marvellous but as the 4kw won`t be needed 24/7 for a year.. a 60 sqft panel (25%) might be a reasonable size to start with.

Simply a worst case scenario estimate, there are lots of variables not taken into account. Don`t take these figures as set in stone ..more as a rough idea of where to begin ;)

    Bookmark   February 12, 2007 at 2:06PM
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hen11(z7 Eng UK)

WOW, Thanks! I've beenlooking for some formulas like that but had no luck. Impressively not far off the actual dimensions! It's a 3/4 span (meaning it has a small bit of roof slanting back into the wall) and is thick wood framed. It's sunken about 18" but the ground drops away before the front face so its all exposed. There is a 1ft thick stone wall on the smaller side (wall around 3ft, glass 2ft, whole side is 5ft tall), same on back face. The peak is about 8 ft and the tall side (which is a brick wall with a thicker skirt along the bottom) is about 6 1/2ft tall. The long slanted roof side is 6ft and faces SE. The whole house is 19 1/2ft long. It has odd dimensions but a quick, rough surface area (counting only exposed glass) is ~320sqft.

60F sounds about right, Some of the Anthuriums can't go much lower than that. I don't know what R - 1.5 means but I'll assume its some kind of conductance rating? I'm using standard greenhouse glass with a few layers of bubble wrap, does this make a difference? So, 320 x 45 / 1.5 = 9600 Btu/hr. Maybe more like 3kw (without any bubble wrap)?

Just to make sure, this chunk of working is generic to the whole of the UK?

[i]1750 hrs of sun per year in the UK with a mean average of 300 btu/hr/sqft = 525,000 btu/yr/sqft (154kw/yr/sqft)
Averaged over a year this equates to about 60 btu per sqft of panel per hour, assuming 100% efficiency :)[/i]

So on the coldest night of the year (we probably only get a handful of nights like that), I might need up to 3Kw of power and 160sqft of solar panel?

but seeing as the house would only need heat 7 or 8 months of the year, 2/3 x 160 = 107sqft assuming no supplemented heat, 100% efficiency and a constant temp of 15F for 5 months of the year :)

But say I want it to be solar efficient down to 40F, that makes only a 20F rise. 320 x 20 / 1.5 = 4270btu/hr which is now down around 2kw, bringing the solar panel to 70sqft if I want it alone to keep the greenhouse heated down to 40F (assuming 100% efficiency). But I think that the bubblewrap will greatly reduce this figure.

All in all, that original estimate of 60sqft seems pretty accurate!

Thankyou everyone for all you help (especially Hex, stone unhenged and Gary!). I really feel like I might be able to get this to work. Does anyone know of a website with all of these energy/efficiency formulas on it? They're really useful!

    Bookmark   February 12, 2007 at 8:37PM
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Hi Hen
It`s not an exact science..far too many variables in the mix.
Heat can be lost in so many different ways, so vague ballpark is as good as it gets ;)
The R-value is Resistance to heat flow.
(R and btu/sqft are "old money" values.. W/m2/K and U-values are the "new money" terms)

Single layer glass is around R 1.0, a couple of inches of extruded rigid polystyrene is around R8.0 - R10.0
Glass plus the bubblewrap is unlikely to give R1.5 imho, maybe R1.2 at best.

All materials have an R-value which you can use to help calculate the overall amount of heat flow from the building (Area x Temperature Difference/ R-value)
The greater the temperature difference the greater the heatflow.
To put it in some context, your loft may have R50 or more, so R1.5 glazing isn`t really slowing heat down much at all.

The main use for R-values is for conductive heatloss, so R-value won`t help a great deal for convective or radiant losses. There`s also windchill, humidity and latent heat issues...many variables :)

If you gear everything to the lowest likely outdoor temperature and highest desired indoor temp it should account for most scenario`s but it`ll still only be a very rough guess.
The weatherman predicts a heatwave this year though, so you should be ok.

    Bookmark   February 12, 2007 at 9:23PM
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Hi Hen
Just to add,
The figures do relate to the uk ,as close as conflicting sources will allow anyway.

I don`t think downsizing of the panel to reflect lower temperature targets would be a good plan.
The hours of sun were taken over the entire year to get the average, but unfortunately, the sun doesn`t work quite like that with the bulk hours being gained in the height of summer.

Ideally the panel could to be sized for the worst possible month for solar gain.(worst case scenario)

You might find it`s cheaper to move to the caribbean though ;)

    Bookmark   February 13, 2007 at 12:17PM
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If you're looking to minimize costs, my guess is that a homebuilt hot air collector would be the cheapest way to provide free heat to your greenhouse. You can build them for very little money; it's essentially a glassed-in black box with a small fan to move warm air. If you want to move up a rung on the solar tech ladder, a solar hot water collector is probably a good bet. I don't know your personal circumstance, but if wood smoke would not irritate your neighbors, there is also the possibility of a wood-fired boiler that feeds hot water through tubes to radiate heat. I'm currently planning on using one of these for a larger plant-growing project, and if I can make it work, will report on it in this forum.

Or, you could move to Florida. Low taxes, and with global warming, eventually everyone gets their turn to own beachfront property!

    Bookmark   February 13, 2007 at 7:56PM
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hen11(z7 Eng UK)

HI there,

I think i'm going to try using a 5ft x 6ft solar tube panel to begin with, along with a supplemented heater strong enough to heat the greenhouse by itself. Mostly because that's the largest i have room for/can afford at the moment (The house is in a protected area so can't touch the roof). If the panel provides a good amount of heat, after a year or so I might try investing in another and joining it on.

If it's a heatwave coming i'll have to start thinking about cooling the greenhouse. Looking outside now, it's strange to think the greenhouse will ever need to get colder!

I would move to Florida but it's difficult to find someone willing to sell a house to a 16-year-old :) I did consider a wood burning stove at one point, but I wont always be around to keep it going.

one quick question which I can't seem to find the answer to on any website: When the water flows out into the solar panel at night, is there any heatloss from the panel? Or is there some kind of shut-off?

Also, do you use a slow-flow system or have you found that the panel is just as effective at a high flow-rate?



    Bookmark   February 13, 2007 at 8:15PM
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You're 16? Sounds like you're in for an interesting life.

I put the solar panel pump on a timer: on at 10:00 am, off at 4:00 pm during the winter.

The fast flow rate doesn't seem to matter much.

    Bookmark   February 15, 2007 at 9:58PM
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Great thread!

""I did consider a wood burning stove at one point, but I wont always be around to keep it going""

That is the advantage of using water as a heat storage and transport system. You can use any form of energy you can think of to heat the water but nothing inside your greenhouse (home) needs to be changed.

As an example, you could use electricity to heat your water (If I recall you can get cheap off peak electric in the UK. Used to be the "white meter" thing. They turn on your water heaters and storage heaters at night when they have spare electric) But say on Sat and Sun when you are around you can run a wood stove to top up your water temp.

As to flow rates. Our needs are different to the normal usage of solar water heating panels. The "normal" use of a solar panel is to make hot water for showers, laundry etc.

You need a relativly slow flow through them so that the water that comes out of them is hot. However the hot water in the panel radiates heat back out so the hotter you run them the less efficient they are. (OK the glass enclosed evacuated ones do a lot better)

Our needs are different. We need to capture as much energy as we can, so the cooler you can run your solar panels the better. Less captured heat loss. We don't need 140F water.

For us a faster flow rate, a cooler panel, is better.

    Bookmark   February 15, 2007 at 11:37PM
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garyfla_gw(10 Florida)

I doubt they would care if you were a toddler as long as you had money lol. Only problem is bring LOTS of money.

    Bookmark   February 16, 2007 at 7:53AM
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hen11(z7 Eng UK)

Hi again,

sorry to bring up an old thread but could anyone tell me what they think of this solar panel?


To me it looks a bit dodgy and slightly tacky but maybe thats just me. They're selling at a much better price than any of the others on ebay at the moment. Also, does anyone know how much more effective the tubular style panels are than the flat plate ones? only because the flat plate kinds are far cheaper on ebay as people are selling them second hand.



    Bookmark   March 4, 2007 at 11:33AM
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hen11(z7 Eng UK)


sorry, just found a cheaper one:


Looks far better, bigger, more reputable (the other company's site didn't even include specifications (the seller linked to some specifications of what appears to be a different product))

at the time of posting it's going for £106

what do people think of this one aswell?

    Bookmark   March 4, 2007 at 11:42AM
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nathanhurst(VIC Aust)

Buy em both - you'll be thankful for the extra capacity.

    Bookmark   March 4, 2007 at 4:50PM
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Hen, You sure got a lot of information that has helped me as well. I am planning on building an earth contact GH in the midwest. We normally have 90 heating days (or more) per year where the outside air temps run from mid 20's to low 40's (F) for the daytime highs and lows. So from your estimation I am going to need about double your solar collector capacity to heat the same amount of area. I also have enough ground I am planning on running a geothermal ground loop heat pump, along with a propane fired boiler for supplemental heat. (I live in a rural area and do not have access to natural gas.)

Well, thanks for the information you have harvested, and above all for your drive at such a young age. Most 16 year olds here would rather concentrate upon playing video games, hanging out at the mall, and doing other things than work or study.

    Bookmark   April 1, 2007 at 1:44AM
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I think that I posted a link to this once before.
I still think that it's an interesting way to heat, if you have the room. I don't see why the efficiency can't be vastly
I can attest to the amount of heat generated by wet hay. A section of my feederbarn had a leak, last year, and
I spent some time pulling out hot steaming bales before they could start a barn fire.
The wet hay was added to my compost pile anyway. With this, things are more controled.

I'd like to try it with a large water storage tank in the middle of the pile and see what you get.
Maybe, you could just run far more tubes through it instead and have the water stored under the greenhouse.

Either way, I will try it. I really wish that I could tap into an underground geothermal source.
The water 30 miles from me comes out hot. They're a lot
closer to the dormant/extinct volcanic area.


Here is a link that might be useful: Composting Greenhouse

    Bookmark   April 2, 2007 at 2:20PM
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Scott, a geothermal heat pump system relys upon the natural temperature of the soil below a certain level. In my area I go down 5 to 6 feet and the soil stays at a constant 50 degrees! All I need to do is raise it to a level where I get the desired heat from it. I do agree with you though, if I lived in the rockies I would love it if I was close enough to tap into either a natural hot spring or volcanic vent. I saw where a family in Utah were doing just that. In fact they get enough high pressure to steam to operate an electric generation plant, and to heat thier greenhouse operation.

    Bookmark   April 17, 2007 at 12:32PM
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rubbleshop(UK zone 9)

Wow I would love a volcanic vent!

    Bookmark   April 17, 2007 at 4:36PM
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Hello do you use your indoor pond water to water your plants with? and do you have fish in it.

    Bookmark   October 12, 2008 at 12:06PM
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buyorsell888(Zone 8 Portland OR)

I have a rubber lined wood framed pond in my greenhouse approx 65 gallons for tropical water lilies. I just bought an Ebo Jager fully submersible aquarium heater to keep the water warm enough for them. Heating the greenhouse did not keep the water warm enough last year and they all rotted. Papyrus, taro and cyperus were fine though. I don't know if the heated water will help warm the greenhouse or not but will find out this year.

    Bookmark   October 16, 2008 at 12:45PM
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ladylotus(Z3/4 ND)


I have a pond in my greenhouse and I have been using the water to water my plants and intend to continue with using the pond water. If you have fish in your pond it makes for healthy plants as they are getting fertilizer too. Using the pond water will allow me to continually perform 30% weekly water changes to keep my fish healthy.

I was not going to respond to all the additional posts as this is a rather old post. However, I have noticed a few things with my pond and thought they may be useful for anyone just beginning the planning stages for a greenhouse and perhaps an indoor pond.

1. I had only 15 to 30% humidity in my greenhouse prior to adding the water in my pond. Please note where I live. It is very dry here and we very seldom have much humdity. It has increased my humidity to a very high rate 60's and 70% humidity sometimes even higher requiring me to turn on my exhaust fans.

2. If you have a wooden structure (as I do) think twice about a pond OR make sure you are using a very good primer and outdoor high gloss paint. I am flabbergasted at the amount of moisture that just drips off the polycarbonate panels I've framed into the ceiling, not to mention the moisture which drips from the walls.

3. I had initially thought the water in my gh would heat up drastically from the sun during the day and produce an abundance of heat during the evening. Not so, perhaps it's the depth of water I have. My pond is 4' deep to house my Koi for the winter months. The water has never really heated up and I can't see an enormous amount of heat that the pond is giving off into my greenhouse.

4. I tried building my pond so that I would not waste a lot of space in my gh. I built it so that if I run out of room I could build a table or plant shelves over the top of my pond allowing me to use all the space I'm using in other parts of my gh except the floor directly under the table/plant racks over my pond.

Just a little food for thought when considering a pond in your greenhouse. Honestly, I love mine and hope to keep it. However, if it causes damage to my structure, I will fill it in, or just not fill it with water for one year to see if that makes a difference.

Happy Greenhousing Everyone!

    Bookmark   October 16, 2008 at 1:19PM
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I am planning to build a 11 x 20 lean-to GH against a brick west facing wall. It will be about 12 feet high at the inside wall. I am thinking about using cypress or perhaps pressure treated wood because I have around 40 salvaged double paned sliding glass door panels. My idea is to remove the aluminum frames and set them onto wood panels. I plan to pour a concrete foundation and then come up with about 2 layers of concrete blocks. I also have 2 4x8 solar collection panels that I want to use to heat the GH. I am thinking about pouring a concrete floor that is well insulated from the ground and embedding pex line into it and heating it with the solar panels. Does this sound like a valid idea or would it be better to heat water in say 8 55 gal barrels and use them as foundations for the shelves. Any ideas will be appreciated.

    Bookmark   October 30, 2008 at 1:55AM
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I'm new here, so forgive me if I'm repeating someone else. Has anyone tried using a solar pump to pump the water from the pond or tank to the roof where it would flow through black hoses facing the sun and then back down. A kind of manifold? I think drip lines between two hoses? It only works when the sun hits the pump and thats when you want it too. Let me know if that would work?

    Bookmark   October 30, 2008 at 5:43PM
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We have an A frame style solar greenhouse that we put 3/4' x 100' PE pipe and circulated to a stock tank in our aquaponics system via a pond pump. It helps bring our water temp up. It is run off thermostat that also runs a small axial fan pulling heat down from the peak. A 1/2 hour sun break on a cloudy day can give us a couple of degrees of water temp. Check the link below later in the week. Good luck with your gh!

Here is a link that might be useful: See it here

    Bookmark   November 4, 2008 at 2:01AM
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I am researching this same issue. I plan to raise tilapia, and am looking at maintaining an outdoor pool to extend my grow out ability. I will have 2 300-gallon tanks in a heated attached garage no problem, but fish capacity is an issue. I was thinking about building a greenhouse, but not a traditional one. I am thinking about more of an insulated shed. I would use double pained windows insulated doors etc. Heating it though has me a bit stumped. If you don't care about the cost of heating there are tons of options. Solar is not very practical to try and keep the greenhouse at 70 plus degrees. You would need a massive solar array, and a bunch of batteries to maintain it. I could install a gas wall heater, and run it for 15 years before I would recoup the cost difference. Now I am building some solar panels to run thru a grid tie inverter that will offset the cost of running my air pumps, water pumps, fans, and misc electronics. That is a far cry from the 3k watts of electric consumption of tank heaters, or straight electric heat. I believe for fish, the biggest concern is the heat loss of the structure itself. I have looked at bio-fueled rocket heaters that are very interesting, but I would hate to lose 1500 fish because I forgot to add wood one night. One idea might be a HPTAC unit like the ones motels use. The nice thing about those is they have come down dramatically in price. The use a heat pump, and provide the ability to control humidity too. Of course insulating the pool, tanks, windows will be needed as well.

    Bookmark   January 7, 2011 at 8:39PM
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I heard that simply having a large container of water is a great natural way of colling and heating a greenhouse simple but effective.

    Bookmark   October 31, 2011 at 9:11AM
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Could you not increase the amount of water containers you have if the greenhouse is larger than normal. I have an 8 x 6 and a greenhouse heater works perfectly. I would recommend greenhouses and greenhouse heater from Waltons which is where I got mine.

Here is a link that might be useful: Greenhouses

    Bookmark   October 31, 2011 at 9:26AM
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Have any of you tried to use the heater above?

Here is a link that might be useful: Pond Heater

    Bookmark   December 23, 2013 at 10:50PM
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