New greenhouse too hot

penfold2(4b, MN)August 25, 2010

Ok, so I built my first greenhouse and it was 10 times more work than I expected, but it's roughly finished. The problem I'm having is that it's a lot hotter than I expected. I've read that exhausting the entire air volume 1-2 times per minute should keep a greenhouse within 4-8° of outdoor temperature. I've estimated my greenhouse to be about 1300ft³ and I'm using a 16" fan rated at 1900cfm, which means I'm changing the air about 1.5 times per minute. This should put me within 6° of outdoor temperature. Yet with the exhaust fan running this afternoon the greenhouse was at 89° while the outdoor temperature was 70°, which is a much bigger differential than 6°. Also, if I turn off the exhaust fan and open the door, the temperature stays about the same. Shouldn't the exhaust fan be having a greater effect than an open door? I know there are other methods of lowering temperature, but this makes me wonder if my exhaust fan is even working properly. It seems to be moving a lot of air, but I just don't know.

Can anyone tell me if this is to be expected, or if there is something else I should be looking at?

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mksmth zone 6b Tulsa Oklahoma(6b)

first off I love your greenhouse!

Your fan may be rated for enough cfm, but does your vent allow the same cfm to enter the house? If not it could be choking your airflow.

I dont know how accurate this online calculator is, but its something I stumbled upon once.

Here is a link that might be useful: greenhouse cfm calculator

    Bookmark   August 25, 2010 at 3:40PM
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Even with the hot air being exhausted you are getting radiant heat from the surfaces which themselves become hot relative to the sun's intensity. A little bit of shade cloth should really help.

    Bookmark   August 25, 2010 at 3:57PM
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I noticed that the intake vent is located beside the door and the exhaust fan is at the top. In theory that should pull cool air in at the bottom and vent out all the hot air at the top. But what I think is happening is you have hot air staying in the top above the intake vent.

My fan and vent are both located near the peak, at opposite ends, where all the hot air pools. So when the fan comes on the vent opens and the air coming in pushes the hot air out at the other end. I also have two other fans in the GH just to keep all the air moving in there. If the air isn't moving in all areas of the GH, you'll have all the other issues that come with it, mildew, mold, bugs and diseases. It has to be cool and moist and breezy all over the place all the time.

Other than that dead air pocket, I think your GH is dynamite!! Cheryl

    Bookmark   August 25, 2010 at 4:07PM
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penfold2(4b, MN)

Thanks for the compliments and suggestions guys! I do plan on adding some circulation fans soon, and probably an evaporative cooler over the intake shutter for next summer. I just thought the exhaust fan should be doing more than it is so far. I have a 24" intake for the 16" fan, so that shouldn't be a problem.

I'll keep at it. Any other suggestions are welcome.

    Bookmark   August 25, 2010 at 4:26PM
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An evaporative cooler doesn't work in humid weather. If your "dew point" is up in the 50s and 60s you get very little cooling. Check for dew points.

You would get better results from shade cloth.

    Bookmark   August 26, 2010 at 8:50AM
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Thats a nice looking GH. I can't help you except to say that I ran into the same thing. I was worried about keeping it warm and found a small electric hearer took care of that. I think the one for one, exhaust fan may be way under estimated. Here with the sun at 5000 foot Colorado, the amount of heat was unbelievable. I did my 500 cu foot GH with a 3000 cu ft swamp cooler. But then I am in a dry climate.
Good luck, Kenny

    Bookmark   August 26, 2010 at 9:28AM
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imstillatwork(8-9 Oregon Coast / Ca Border)

Most fans are rated for how much air they can move in an open environment, not how much air they can evacuate, so it's a good idea to buy a much bigger fan than you think you need. Once installed in a greenhouse it's creating vacuum and the cfm it's moving changes (lower) drastically.

The lower vent is good. I would add a Lasko 16" oscillating fan (cheap, good air movement, walmart) attached to the ceiling to blow hot air towards the exhaust fan. This should pull more cool air up and even the temps out. You should have one or two 10" or so fans doing the plant level circulation.

In the summer keep the hot air high and exhausted, in the winter tilt the osculating fan downward to take the hot air and warm up the plant level. seems to work for me, I have a 15* difference top to bottom in the winter.

    Bookmark   August 26, 2010 at 1:10PM
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penfold2(4b, MN)

I'm thinking my exhaust fan is just far too small to keep up with the massive solar input. I went by the standard advice of exchanging air 1-1.5 times per minute, but maybe that only applies to greenhouses with significant shading. I'm growing quite a few high light plants, so I could only shade the back half of the greenhouse.

So now I'm considering either upgrading my exhaust fan or adding a second exhaust fan. The downside is I will have to enlarge my intake shutter as well, which means framing a larger hole in the wall.

    Bookmark   August 26, 2010 at 4:28PM
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What do you consider high light plants? What are their light requirements and will you be using the g'house in the dead of summer? I put shade cloths on about May first in my latitude, and they come off by September first. It's a good idea to find out exactly how much light is getting to your benches by metering it. Then compare that to how much light your 'high light' plants actually want. You may find out you are still getting better light transmission in summer than you actually need.

    Bookmark   August 27, 2010 at 3:02PM
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lily51(OH 5)

My greenhouse had problems with heat build up this year, too. Tried a shade cloth on the inside on the back 1/3, which is south. It helped some..will put it on the outside next year.

There are two intake vents in the "front" (one on the east side, one on the west) which is north and my fan is on the opposite wall (SW ) There was a heat pocket in the back SE corner, which another separate fan helped. Am going to try a fan/mist system such is used for cooling livestock.

We had some extrmemly sunny, warm weather all of April,and then again later in the spring,so that made the situation different than the year before when excess heat was not a problem.

The horticulture teacher at a nearby school said he would spray paint the surface of their greenhouses white (special paint?) on the outside, and as the summer went on, it washed/peeled off. I would guess the material your greenhouse walls are made of would also determine if this is a solution for you.

If nothing else, we know what is meant by "greenhouse effect".

    Bookmark   August 27, 2010 at 4:52PM
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stressbaby(z6 MO)

I bet if you put some shade cloth on there and fill it with plants you will be fine.

Nice looking house.

    Bookmark   August 27, 2010 at 10:56PM
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penfold2(4b, MN)

Yes, I think I'll try some shade cloth on much of the roof, leaving just the front raised bed unshaded for my succulents. Then I'll see how much further I need to go.

Now I need to decide what percentage shade cloth to use for my aroids, orchids, and other epiphytes.

Thanks everyone.

    Bookmark   August 27, 2010 at 11:33PM
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