Hi,
I've built a 12 x 16 greenhouse with 8 foot sidewalls .. 10 foot in the center. It's covered with 6 mil greenhouse vinyl. The BTU calculaters that I've found on the web say I need a 24k BTU heater to maintain 55 F on my coldest expected nights of the winter. My question is do I need a "vented" or "non-vented" propane heater. The non-vented appear to be cheaper and easier to install ... but I've also read that they can damage some greenhouse plants. Which works best to keep my growing citrus population happy during the winter ??? Also, I've got four black 55 gallon drums of water (one in each corner). It only appears to keep my inside temperature about 5 degrees warmer than the outside temp at night ... should it be more ?? Thanks in advance

I have a 72 X 32 foot greenhouse, I have 102 fifty five gallons drums filled with water, I use the drums both as a heat source and as greenhouse benches. The drums are painted black on the south side, and white on the north side. The black side faces the sun and absorbs the sun during the day thus heating the water. The white side reflect light around the greenhouse. I have two vented overhead Modine style heaters. One is 250,000 BTU and the other is 200,000 BTU. I also have two infrared heaters that runoff of a pilot light, which do not require electricity. The infrared heaters are a back up in case of a power failure. You want vented heaters, because non-vented heaters release ethylene gas into the greenhouse, which kills citrus, and many other plants. A 55-gallon drum of water contains 440 pounds of water. Take the temperature of the water at sunset, and then again in the morning. For each 1 degree drop during the night the drum releases 440 BTU of heat into the greenhouse. The more drums, the more heat is released into the greenhouse. One word of caution, about maintaining a 55 degree greenhouse night time temperature. A citrus tree in a greenhouse will receive a lot of direct sunlight on the leaf surface. During mid day and for several hours in the afternoon, the leaf surface can reach temperatures at, near, or in excess of 100F. If the root zone temperature is not at a minimum of 65F your tree will experience winter leaf drop (WLD). Therefore, you will need to be sure to maintain a soil temperature of at least 65F, or shade the tree from the direct sun. You WILL NEED a soil thermometer so that you can manage the soil temperature. Because of evaporation from the surface of the container, the soil will always be 5 - 15 degrees lower than the average air temperature.

Thanks ! The 24k heater was going to be alittle overkill for my small setup .... so maintaining 65 F shouldn't be a problem. So far about the best deal I've seen is for a Modine HD30 for $432 plus shipping. If anyone knows of a better deal, please let me know where to find. It's small, can go overhead, and runs on 110 volt. I'd sure hate to lose all the little lemons and limes that have formed this summer

I would not look for the best deal, when it come to a greenhouse heater. Modine is an excellent brand of a greenhouse heater, and will give you a long lasting and faithful service.

My friend has commercial sized greenhouse like that of Millet's. The winter heating bills, from November to February is $1,500 per month per greenhouse, Northern California conditions, maintained at 70 deg F.

He has a big propane tank that provides the heating for 4 of his commercial greenhouses. So the heating bills are $6,000 a month.

He recently is experimenting on hoophouses, the cheap version of a commercial greenhouse of the same volume and floor area, and using double plastic sheets as cover. The heating bill on those are just half, and could pay itself for the labor and cost of the plastic sheet installation in one season. The plastic cover could last 7 years if the grape growers don't spray in the area, and only 3 years if they are spraying. The sprays used by grape growers contains sulfur which can make the covers brittle. He plans to make more hoophouses nonetheless, it makes more economic sense to him now that the propane prices are sky rocketing.

What's the typical monthly average heating during the winter in your area?

I have a 90'x22' double poly hoop house. It is heated by a 165K Reznor heater, which is more than adequate. I try to keep it between 55 and 60 degrees. Holding it at 70 greatly increases heating costs. Of course this is necessary for many crops, but not for citrus.

The big problem with the type greenhouse I have is keeping the temperture down in spring and summer. I have found that it is best to just take the poly off, and use shade cloth alone. My poly clips on and I can change in 1 1/2 hour.

Ned

Ned is certainly correct. My Colorado greenhouse has a heating bill of $500.00 per month. My greenhouse is covered with a double air inflated poly covering. Double poly covering air inflated to separate the two sheets by two inches, is currently the most cost effective method for heating. The greenhouse runs north and south. The north wall is a solid insulated wall with the inside painted white to reflect the sunlight back into the greenhouse. North walls reduce the amount of light in a greenhouse, because the light just passes through to the outside, the greenhouse is much better off with a reflective north wall, both for light and heat. The southern end and both sides are double wall polycarbonate. A 1,000 gallon propane tank fuels the heaters. Generally, the small hobby greenhouse sold are among the most expensive greenhouse structures to heat on a square foot basis, and a nightmare to cool during the summer. Therefore, give your plans a great deal of thought before you build.

Ya'll are starting to scare me My friend, who has a greenhouse similar to the one I built, says he filled up his 250 gallon propane tank 2 years ago and has yet had to refill it. He raises alot of tropicals. My first experience with a greenhouse was with one of those Harbor Freight 10 x 12 polycarbonite kits. After 3 weekends of aggravation trying to get the instructions interpreted, I had it put together. The first typical spring thunderstorm that rolled thru imploded the dang thing !! The aluminum framing crumpled and the wind sent panels into the trees on the other side of the neighbor's 5 acre field. Luckily, it as within the 30 day "no questions asked" warranty ... what came in 3 boxes was taken back in 2 boxes ... and I got my money back. After I got over the frustration, I decided to build my own out of treated lumber.

If you only want to keep your plants from freezing, your fuel bill should be low. They will rise sharply as you try to maintain a higher temperature.

Many modern greenhouse designs allow for the roof to the opened when the temperature increases beyond a set point. This is a good way to vent excessive heat. I am sure such a feature could be incorporated into a home greenhouse design (I believe I have seen a few), but ventilation requirements would have to be calculated for it to work properly. Also there are considerations such as; do you want your venting system to be automatic, or will you always be available to manually open the vents?

Ned

So you guys can calculate just how much a piece of banana will cost due to the heating bill expenses?
I can buy a 10 piece bananas at Costco for $1.30 (lol).

Ned,

I've got my fan & vents set up to automatically open based on a thermostat. Even with a 60% shade cloth over the top, it got up in the 90s when we had that major 100+ heat wave this summer.

Thanks,
George

Ned
Would the "poly clips" that you mentioned be wiggle wire and channels? I am going to face the same overheating problem as you since I live in Zone 9.
Karl

To maintain a summer temperature that provides good growing conditions, the greenhouse cooling system must include evaporative cooling pads on the opposite end of the greenhouse from the exhaust fans. My exhaust fans are capable of completely exchanging all of the inside air every one minute. One last note: the propane truck came today to fill our five propane tanks, (one 1000 gallon, two 500 gallon, and three 250 gallon tanks.) The cost of propane this year is $1.85 per gallon, up .75 cents from last year, and up $1.95 from 2005. At these prices, I will not be able to maintain a 60F temperature throughout a Colorado winter.

I will offer a couple of thoughts.

First, in regard to cooling, turning the air over more often can reduce summer temps. The standard advice is to turn the air over once a minute. However, a friend of mine who is an international GH consultant showed me a study once which described the average GH temps compared to outside temps based on the rate of air exchange. At one exchange per minute the GH averages 8F over outside temp. Increasing that to one exchange every thirty seconds and the GH temps drops to an average of 4F over outside temp. (At one exchange every 2 min the figure is 15F > ouside temp.)

Second, there are ways of taking advantage of the latent heat of vaporization of water besides evaporative cooling pads. That is to say, evaporative cooling and a favorable growing environment can be obtained with foggers and a mist system as well.

Third, to come back to the issue of vented or unvented heaters...there is no such thing as an unvented combustion heater. Even an "unvented" heater will require some source of fresh air. There are many GH growers who swear by their unvented heaters and have had no problems. I suspect these users are careful with respect to ventilating their unvented heaters. There are also cases of disasters with unvented heaters, and this is the reason that most GH guides published by university faculty or extension departments will recommend against unvented heaters.

SB

Karpes, Here is one supplier of the poly clips:

http://www.jaderloon.com/merchant2/merchant.mvc?Screen=CTGY&Store_Code=J&Category_Code=C-FFS-KKE

I use the clips on a tubular steel frame; I am not sure how they work on wood frame houses. I am not sure if you have to be a professional or wholesale grower, or not, to order here, but I don't believe you do. They have a place in SC and I think, in Texas. Someone at the supplier can tell you what you need to know.

Rottford, if you only had a 10 degree increase on the inside of your greenhouse, you were doing better than most hoop houses I have seen. Opening the roof seems to me to be the most cost effective and efficient way to cool the house. Evaporative coolers work best where the humidity is low. Here, where I am, the humidity is virtually always high, especially in summer; the drop in temperature across a cooler is small in comparison with dryer climates. I suspect Millet's humidity is low by comparison, so a cooler would be much more effective there.