Greenhouse temperatures

Hi everyone,

Does anyone know if natural gas can be used to heat a greenhouse? We have natural gas for our inside heater and also for our water heater and stove. So since we already have it for those things, it would be easy to run a line out to the greenhouse. We have a guy that we hire at times to run the lines for us. He did it for our stove. So getting someone to do it is not a problem. I was just wondering if they make natural gas heaters for greenhouses and, if so, do they have to be vented?

Oh, one other question. What do you all keep your temperature set at in your greenhouses? We have this huge attic fan that we sort of took out of the attic because that was all we had at the time. We had just installed both of them in the attic, one to pull air in, and the other to blow it out. So we took one of the fans and put it in the greenhouse. It has a thermostat on it that ranges from 60° to 140°. What should I have it set on? I think it is actually too big for the greenhouse but that is all I have at this time. It says it cools attics up to 2400 sq ft and our greenhouse is only 144 sq ft. Should I continue to use this or should I get a smaller one? Here is a photo of the front and side of the box it came in. I hope it is ok but it if isn't, please tell me so I can get the proper thing. I know one thing for sure, it is never hot in my greenhouse.

Thanks,
Jesse

You can certainly use natural gas to heat the greenhouse. I do.

Check Stressbaby's posts on heating costs for good information.
I suppose greenhouse heaters don't HAVE to be vented, but I feel better when they are. The vent will add some amount of cold air infiltration but adds a margin of safety (carbon monoxide). The type of heater also affects the efficiency of energy use and the cost.

The temperature you set your thermostat depends on several factors. First - how much can you afford? The higher you set the temperature, obviously the higher your fuel cost will be. Second - What are you growing? Many tropicals prefer minimum temperatures of 60 degrees but may be undamaged at 50. Some plants will just become dormant while others may be damaged. I grow mostly Bromeliads and my thermostat is set at 53.

That said, you will find that different areas of the greenhouse will have different temperatures depending on where the heater is, how you are distributing the heat, where is the door, where are the fan inlets, etc. Microclimates abound even indoors.

As far as ventilation, you almost can't have too much. The power used by the fan depends on the volume of air the fan is moving and the differential pressure the fan is exposed to. The larger the opening opposite the fan the less energy you will use. For example, a fan running in open air blows a great deal of air but uses very little energy while a fan pulling air through a very long greenhouse will use more energy. The 1540 cfm rating of the fan assumes a certain installation and as they say, "your mileage will vary." My gut feel is that you're fine as is.

I notice that your fan has a built-in thermostat. The setting you use will depend on where the fan is installed. Generally you want the fan at the top of the greenhouse where the air is the hottest, but this isn't always possible. Once again, the setting depends on the type of plants you are growing. In the Summer, your microclimates inside the greenhouse will be different. The temperatures near the inlet will generally be cooler than near the fan. And the temperatures at ground level will always be cooler. I set my thermostat so that the fan comes on when the temperature hits 92 degrees at about four feet off the ground. Every thermostat will have a different "deadband", that is the temperature when the fan will turn off. This may or may not be adjustable.

I hope my fellow Missourian Stressbaby will chime in on this topic.
Dave.

Do you have a shade cloth on your greenhouse? I have one that is 75% shade that I have to put on in May and I remove it in Oct. otherwise it will get up to 130 in the greenhouse. I would not let it go over 90 degrees. Over that temp your plants will not want flower. I have my vents set to open right now at 80 degrees. Otherwise they automatically shut below that temp.
I found my orchids getting burned, but since I got the shadecloth, all is well.
I have two fans that open at the front lower part of the greenhouse and suck air in, then have fans placed throughout aiming towards the back. The two fans the suck the air out in the back are up high. So you are bringing in the air low and taking out high in the greenhouse. helps keep it cooler. Maybe you should put another fan in the front part too? Just some thoughts.....good luck!!

I agree with Dave, nat gas is fine for the GH. There are people who have no concerns or problems with unvented heaters. I feel more comfortable with vented heaters and I use direct-vented Empire models with propane.

That fan should be fine as long as you don't have uncomfortably strong air flow across the GH.

Dave has accurately outlined the various factors that will determine your temps. My minimum temp last winter was 54F, but I hope to keep it a little warmer this year with some improvements in insulation. I grow tropicals and subtropicals. My summer temp settings are: intake vents and ridge vents: 83F, exhaust fan 85-87F, overhead mist 90F, fogger 95F. I'm still working out the best temps for the mist and fogger. I disable the ridge vent in the middle of the summer for better flow of air across the GH.

If your thermostat is built in, make sure it is not in the sun. The best thermostat setup I have seen are thermostats remote from the fan or vent, about waist or chest high, in the middle of the GH, enclosed in white boxes for shade with holes for air circulation.

SB

My greenhouse faces North West, so the right front corner faces North. Half of that is blocked by a structure added on to make a laundry room. So the North only shines on half of my greenhouse. The back, or South East is blocked by the house itself. So the sun never shines directly on that part. The South West backs up to the back of the house so it never gets any direct sun. And the left side of the greenhouse faces West. So the only direct sun comes from above and on the West and North West walls. I hope this makes sense. I tried to draw it using PSP but I'm not good enough at that yet. I hope this makes sense.

What I am concerned about is the fan being too stong for the size of the greenhouse. I don't want it too cool in there. What do you all think? Also, he is a photo of the fan in the greenhouse. It is cool today so we don't really need it.

Jesse

Good points by foreverdeen and SB

You will need shadecloth. I use 40% coming on about March 15, and replace it with 55% around May 30. Then back to 40% around Sept 15 and off around Oct 15. The amount of shade you need also depends on the lie of the greenhouse and trees (especially deciduous trees on the west side which provide more shade in the Summer when you really need it). It obviously also depends on what you're growing. Specialists have it little easier, but most of us grow high and low-light plants. Vertical stacking helps here with light levels and termperatures.
My overhead misting is on timers but I'm usually out there anyway and can manually turn them on earlier - or off on overcast days.
All this sounds complicated, but you ARE going to have fun! There's nothing like walking into the greenhouse on a cold, sunny, winter day to bolster the spirits!
Don't worry about it being too cool in the Summer. The thermostat will turn the fan off.
Dave.

Hi Dave,

Could you tell me more about your overhead misting system? Most of my plants could use that except for my African Violets. I keep them all together so I could just keep them separated from the other plants.

Thanks,
Jesse

Plantcrazii, am I correct in understanding that your GH doesn't get any sun from the southwest or southeast? If so, you need to be careful in the winter when the sun is at a low angle in the sky. Light will be scarce in the southern half of the greenhouse. A foil reflective coating on the north walls may increase the light in the southern sections. I'm not sure how that would work with the house on a diagonal, but it is something to consider.

Many types of misting systems are available. Mine is the kind used for propagation. It uses 1/2" PVC and saddle nozzles and makes a fairly fine mist. You can use the misting systems available at HD or Lowe's, they will work. Foggers have the smallest droplet size and are quite effective. Dave, I would also like to hear about your setup.

One other point about your fan. Oversizing can potentially be a problem in terms of plant shock. That is, a fan that is oversized for the greenhouse can, on a cool day, draw in enough cold air so fast (before it shuts off) that it shocks the plants.

SB

I'm starting a new thread on misting - hope I don't get too long winded.
Dave.