google thermocube they plug into your outlet and turn on/off at specific temps. Some people here have had good luck with them. Keeping it at the low 30's wont really "grow" anything. It should keep leafy greens alive but they will grow very slow. A small 1500 watt heater will do just fine but it will run quite a bit on really cold nights.

I'm a bit confused. If it only gets into the low 20s F, doesn't that make you a zone 9?

austin-
An aquarium thermostat will not have the capacity to handle a space heater: it will fail and may even start an electrical fire. If you are going to use a space heater, get an oil-filled one with a built-in thermostat...about $50 for a decent one. Also bear in mind that even a small heater of 1500 watts will use almost the full capacity of a 15 amp circuit, so if anything else is running on the circuit you plug it into, it may trip a breaker. Lastly, if you are going to be using an extension cord with this, you need a heavy duty one: at least 12 ga.

are these suitable?

http://www.homedepot.com/h_d1/N-5yc1v/R-100669058/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

http://www.homedepot.com/h_d1/N-5yc1v/R-100669063/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

charles-
Since you don't say how big a space you're heating, or what temperature you expect to maintain, or how cold it gets where you are, or what your greenhouse glazing is, the question can't really be answered. However, if you are looking for something that delivers up to 1500 W, and that's adequate for your heating needs, I would still choose a somewhat different heater.

Both of the ones you linked to appear to heat by electric resistance coils with a fan that blows out hot air, and having hot air blown on your plants is not ideal. In addition, neither one has an anti-freeze setting. Finally, the oscillating model means that there are moving parts which will break down sooner or later.

For about the same amount of money you can get one like the one I've linked to. It does not have a fan; instead it is oil-filled so it heats by radiating instead of blowing. It also has an anti-freeze setting. This is the kind I have used in my greenhouse for the last 4 years. I don't try to keep my greenhouse very warm because it's so expensive. Instead, I put my oil-filled heater on the low setting, and the anti-freeze feature makes sure that the temperature never drops below about 38F.

One other thing I have is heated propagation mats, which don't use a lot of electricity, but can easily keep the soil in small pots around 70F, even in a cool greenhouse.
Between the heater on low and the mats on a thermostat, I'm able to propagate, grow, and protect a lot of plants, even if I can't grow tomatoes in January.

Here is a link that might be useful: Heater

FWIW: I have in my 6 by 8 HFGH a space heater. (China made, Ace hardware) It is 1500 watt, but so far I have it set at the lower setting ot 750 watts. Thermostat is set so that it maintains at about 45 degrees minimum. It is placed so that the fan blows down the alley and not at any plant. So far it has worked well this spring and now here in Colorado. How much longer into the winter I can go with it is yet to be determined. The latest tomatoes planted are begining to flower nicely, with several marble sized fruits. I have just added supplementary lights and try to learn as I go.

fwiw

Forgot to say that I use a good external thermostat. The one built into the space heater is useless--it heats when the heater comes on and cycles endlessly.

Make sure that you have a mechanical on/off switch, not digital. Digital will turn off and not come back on if you have a power glitch.

Kudzu

whats the drawback to using an forced air heater and having it hit the plants. I use one but I make sure the plants are about 2-3 feet away. My greenhouse is 8x6 and I put it in the corner so it oscillated the whole area. I used it last winter and didnt notice any problems. What could happen so I know what to look out for. I have thought about an oil filled but all the ones I have used seem to just radiate the heat straight up on not out much.

mike

mksmth-
Hot air blowing against plant foliage will increase transpiration and possibly cause leaves to dry out. If you had no problems, then maybe it's not an issue for what you're growing. It was just a general observation. Radiant heaters will radiate in all directions, but, since hot air rises, you will get some heat going up, just as you will with any other heater. In addition, blown air will cause more heat transfer to occur as it circulates around and past the cold structure and glazing of your greenhouse, so it may cost somewhat more to heat as your electricity use will be a bit higher. My goal is to keep the space inside the greenhouse just warm enough so that I don't lose sensitive plants, and radiating heat without hot air being blown around works best for me. If your setup works for you, then that's fine, too.

thanks I that does make sense. I also have a radiant heater that can mount on a wall or ceiling. I may try it just to see how differently it works.

Thanks, Kudzu, for the thoughts on a heater for the greenhouse. I ordered one on Amazon and it arrived just in time for the first freeze of the season. I want to grow tomatoes and basically a veggie garden all winter, so needed to keep it warmer. I'm loving it - just a little concerned about the electric bill. At this point, I'm turning the thermastat down in the house, to help compensate because I'm really happy with my winter-summer garden.

kudzu9, i plan to get a smal 6x6 portable greenhouse. I live in zone 9, coldest it get will be in the low 30's/high 20's and i want to keep the greenhouse at bout low 40's.

the link you sent me is an electric heater at home depot correct?

charles-
Since I saw you were shopping at Home Depot, I posted a link for a heater there that I thought would be a better choice. I'm sure you can also get a similar or identical one at places like Amazon.com, but, if you have a Home Depot or Lowe's near you, maybe you can get it at the store and avoid shipping charges. Just look for one that has the features I mentioned.

Kudzu,

Thanks for the info! I saw your recommendation here and bought a similar model at Lowe's today. Up until now I have been trying to avoid using the added energy of a heater, but I am stuck with a dilemma of a greenhouse brimming with produce, and a forecast in the teens tonight. I opted to go with the heater and use it only in situations like these.

The heater is currently running in the GH as a test before nightfall and I am headed in there now to do another big harvest and make some pickles!

-Karin

cost is easy to figure. Look at your home bill and divide the TOTAL cost by the kilowatt hours used. Mine is about 10 cent per KW hour. Real good, some are up to 20 cents.
My heater is set at the low setting of 750 watts, so roughly about 10 cent an hour to run. For the last couple of weeks it has kicked in for about 2 hours each morning, so about 20 cents a day. But today we have 10 inches of snow and the high is expected to be 32 degrees so---$2.40 per day for today and tomorrow. I hope the 750 watt setting will be enough. The heater does have a 1250 watt setting. My numbers are conservative and approximate and only meant to show you a guess.
KennyP

Karin...how did the heater work out for your produce?

Bought a heater similar last week, but wont use until mid March when this winters hosta seedlings will go into the greenhouse.

Trudy, the heater seemed to do just fine and it bought me the time I needed to harvest a massive haul of veggies. I think I only used it for a week or so before it got too cold to fight it anymore, but I will use it to bridge gaps in otherwise good growing weather.

I set the thermostat on 45 and the power on Med and it did fine. The thermometer inside the GH read a little colder than the thermostat temp on the heater, ie if the GH thermometer read 43 and the heater was set to 41, it did not turn on. So I upped the thermostat setting to be a bit warmer to keep the outside edges of the greenhouse above freezing. The heater did exactly what it should have and everything was great.

Also, the reviews of that heater say that it has a bad chemical smell at first, but the manufacturer must have fixed that because it was not bad at all (and I am sensitive to stuff like that).

Thanks Karin for the update on the heater.

We are looking for a heater that will also bridge the gap on the colder nights, not heating the greenhouse during the winter months.

Oh, I just re-read my post in the light of day and realized I swapped the numbers. I meant to say that if it was 41 in the GH, and the heater was set to 43, it did not turn on. In any case, the main point remains the same, which is that I set it a bit warmer than I actually needed to make up for that.

Serves me right for posting after bedtime! :)

I use a small heater like the one sold at HomeDepot. I set the temp (trail and error) to come on at 50. I also have to two heat lamps. (The heat lamps turns off at 7am)
I chose one that oscillates to keep the air moving around some. I have a smaller greenhouse (10X24), so not that much to heat. Also have large water containers (hydro use) that heat up during the day under the sun. I am home most of the day, so I can monitor the heat in the gh to open the doors if it gets too warm from the sun (need an auto door opener, temp control).
I am considering getting water heaters for my two water solution tanks (35 and 40 gal), to warm up the water to around 60. Has anybody tried that?

In my new 8 by 14 lean-to GH, I have two open windows into the house with a small window fan circulating air continually. This allows me to not have to vent and waste hot air to the outside as well as help moderate temps. I added one of the small oil filled electric radiators listed above, trying to keep the temps above 55 at night.

With the radiator, continual fan and 15 hour a day lights, I was worried how crazy the electric bills would be - could end up with some expensive tomatos. But after the the expense of building the GH and loving growing the plants, I would not have changed a thing. I did turn the thermastat down in my home to help compensate for the added GH.

I just got the first electic bill since adding the heater. I used 3KWhours per day less this month than the same month last year. Maybe I should add more lights. :)
Carol

Here is a link that might be useful: pictures of lean-to greenhouse

I would also recommend the garden heaters from Haslet Products

https://www.hasletproducts.com/shop/p/gmh