Heating Greenhouse w/ Hot Tub Heat??

birdwing(z4 MN)March 20, 2006

I referred to this on another thread w/ the photos of my GH in progress but I wondered if anyone else had ever done this or heard of it being done. My husband has a plan about how to do this & he has some pretty unorthodox methods as a rule but I'd like to know what you all think..

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nathanhurst(VIC Aust)

The biggest problem I can see is the bromine anti-algal, anti-bacterial stuff may harm the plants. Otherwise, it seems like a very good idea. You could use some evacuated tube solar collectors to keep it hot too, saving lots of money on the heater.

You should probably still use a cover to keep the tub warm - most of the heat loss in a pool is due to evaporation.

    Bookmark   March 20, 2006 at 5:44PM
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jimmydo2(z9 Calif)

I am thinking that it would not use the heated steamy air from the Hot Tub, because if you are in a cold climate, you are not going to want to draw air away from above the hot tub.

You could run some air pipes over/through the water and run the heated air back to the greenhouse, but again you are just drawing heat away from the Hot tub.

What would make more sense would be to pipe heated air gathered from above the Hottub heater, (this would just be waste heat anyhow), into the greenhouse.

I will be doing something similar with the exhaust air from our clothes drier, diverting the drier exhaust to the greenhouse during the colder months. We usualy do laundry at night anyhow.

    Bookmark   March 20, 2006 at 9:46PM
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nathanhurst(VIC Aust)

Simpler, and more fun might be to put the hot tub in the greenhouse. Having warmer, more humid air around the hot tub in use might mean you can run it at a lower temperature.

We talked a while back about using the dryer air for greenhouse heating, search the archives.

    Bookmark   March 20, 2006 at 10:59PM
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jimmydo2(z9 Calif)

With the HotTub in the greenhouse it would be like bathing in a tropical forest ;)

    Bookmark   March 21, 2006 at 3:40AM
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mraroid(USDA Zone 8b)

Jimmydo, you are just trying to find a way to talk your wife into putting a Hot Tub in that HFGH. :-)


    Bookmark   March 21, 2006 at 8:19PM
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jimmydo2(z9 Calif)

I do not see how that would be such a bad idea, I already have the HotTub, now I know where to build the Greenhouse, but it would not leave much room for plants, I think the hot tub is 8 foot by 12 foot

    Bookmark   March 22, 2006 at 12:00AM
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nathanhurst(VIC Aust)

You'll just need a bigger greenhouse. What. A. Pity. :)

    Bookmark   March 22, 2006 at 3:51AM
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The plants might not like the chlorine vapors coming off the hot tub. I'm assuming that you will use some chlorination in the hot tub to control thermophilic bacteria.

Pumping the hot water from the tub to an exchanger in the greenhouse would be a better idea provided your cost of heating the hot tub water is much less than simply heating the greenhouse by some other means.

    Bookmark   March 23, 2006 at 10:32AM
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nathanhurst(VIC Aust)

bmoser: I did mention the bromide treatment, and the hot tub isn't really hot enough for thermophilic bacteria, more just your common legionella type things I think.

A heat exchanger sounds like a very bad idea. Using the hottub cogeneration (heat loss from tub equals heat gain in GH) makes a lot of sense, but if you are pulling heat out of the tub you are adding to the cooling losses, not decreasing them!

And heat exchangers are either expensive or don't last long.

    Bookmark   March 23, 2006 at 4:38PM
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birdwing(z4 MN)

Thanks for all your comments..I think I forgot to link this to my e-mail so I thought no one was responding (I assumed you must think it was just plain crazy!) Here's what I've been able to get out of him about how he plans to do it. He says he would run water lines in the floor and put a pump in it so it's a radiant floor. The water would circulate from the tub to the waterlines. I may not be describing it quite right. He is too busy thinking up crazy schemes to explain further. I expect he also doesn't have the bugs quite worked out..

In this way the chemicals wouldn't be an issue. This plan is kind of like what bmoser above was suggesting. If that's what you mean by a heat exchanger..

    Bookmark   March 24, 2006 at 4:39PM
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nathanhurst(VIC Aust)

That would be very wasteful of energy.

    Bookmark   March 24, 2006 at 5:13PM
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birdwing(z4 MN)

That's the kind of input i'm looking for. does anyone think there would be a way to do it that wouldn't be wasteful or that wouldn't cause the chemicals to be introduced into the greenhouse?

i told him about the dryer air thing and his ears perked up..

    Bookmark   March 24, 2006 at 7:11PM
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nathanhurst(VIC Aust)

Put the hot tub in the greenhouse. (build bigger greenhouse if necessary)

That way the heat that leaks out of your hot tub heats your greenhouse, and reduces the losses from the hottub. Keeping the water vapour around is very effective for reducing heating costs too. If you are worried about the bromide or chlorine antimicrobial, put the hot tub in a separate partition in your greenhouse, perhaps as a tent with tent, or sharing a common wall. I'm not convinced there will be a problem, particularly if people here splash bleach around their greenhouses regularly without trouble. If you can't smell the chlorine with the cover on, it probably isn't an issue.

There are also non-volatile anti-microbials such as salt, ppm copper and chloramine. If you change the water often enough, you don't need anything. To do that would require a heat exchanger between incoming water and outgoing water. If you don't have a treatment you can use the waste water on your garden, it may require as much as a 50% replacement a day though (ask a real expert at say a university or hospital, not someone who sells treatment products - of course they will tell you you need their product ;).

Heat exchanging outgoing and incoming water can have a serious impact on your total energy consumption.

Another possibility is to run the dump water through some black poly pipe in your greenhouse to cool it off before sending down the sewer. 100m of 13mm(half inch) would probably be enough (and would cost about $20 here).

    Bookmark   March 24, 2006 at 7:28PM
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garyfla_gw(10 Florida)

I maintain a heated aquarium in my shadehouse and was rather surprised at how much it heated the immediate area.
I'm expanding the idea to include the 1000 gallon lily pool.have no idea how effective this will be as an air heat source but would suspect it will frostproof the shadehouse.
bear in mind that I'm in zone 10 so the problems are much different.

    Bookmark   March 25, 2006 at 4:10AM
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jimmydo2(z9 Calif)

I still think the only way this would work (aside from having the hot tub in the green house, would be to capture lost heat.

Now my hot tub is very well insulated, especialy above the tub. My hot tub is a self contained tub, the heater is contained in the tub enclosure. But I think this method would work even better with an inground hot tub.

Idealy, if the hot tub is not installed yet, you could build an inground hot tub (the heater is seperate with an inground hot tub). Install the just the heater inside the greenhouse.

Otherwise, the place I have found the most Waste heat with my Tub, is above the heater. So hputting some sort of heat exchanger above the heater to capture the heat lost from the heater would work very well.

    Bookmark   March 26, 2006 at 4:37AM
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nathanhurst(VIC Aust)

If the heater is directly fossil powered (nat gas, propane or whatever) you get the additional benefit of increasing the CO2 levels in the greenhouse, which growing plants appreciate.

    Bookmark   March 26, 2006 at 4:53AM
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birdwing(z4 MN)

Our tub is the kind with the heater inside and it is above ground. Our greenhouse is 12x9 our tub is 8x8. So putting the tub in the greenhouse really isn't an option. The greenhouse is nearly finished (see my photos in the thread bluehouse photos) I'm not sure how it will be heated. Since we're in
zone 4 though and it gets below 0(not below freezing but below 0) fairly routinely here in the winter I"m starting to think heating it is going to be wasteful no matter what method I use. We'll see I guess..

    Bookmark   March 26, 2006 at 9:59AM
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Sorry to pull up such an old thread, but i am planning on putting my hot tub in my greenhouse (in progress), and hoped to get some opinions. Do you think that if the GH is well insulated, it is possible to keep the greenhouse at the minimums for growing in winter? say tomatoes :-)
our winters cold side of average around 30 F, and rarely get down into the 20s
GH 10x20
hot tub 7'3" x 7'3"

    Bookmark   March 24, 2009 at 7:47PM
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I was thinking of adding those plug in water thawing contraptions. At the farm and garden places they have those heat cables for farm animals water troughs. In my zone 6, using them in a small water trough should keep some plants from freeze until it is safe to put outdoors.

    Bookmark   May 25, 2013 at 5:14AM
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