If you're adding extra insulation, that will change the btu calculation.

The problem with those giant heaters is that they are meant for big spaces and blast a lot of very hot air that your plants won't like to be around in such a small space. You would be better off to wire a sub-panel into the greenhouse with 15 or 20 watt circuits and use several smaller heaters. It would be much cheaper as well. A 1500 watt heater costs about $20 and puts out about 5000 BTUs.

But wouldn't I have to run 3 12 gauge wires to the greenhouse to power three small space heaters? That would end up being more expensive than just buying one wire that can take a larger load. And I would also have to get three thermostats for each outlet to control the heaters, which would also be somewhat costly.... I just feel that in my situation, 3 outlets would end up being more expensive than one. I found a dayton g73 online that is in good condition for about 130 dollars, which I think is a pretty good deal for a heater like that. The wattage can also be adjusted on it, so I can lower down the heat if it's too strong for my plants. I'm not sure why, but the description made by dayton says not recommended for greenhouse use although I have heard of people using them in greenhouses.

to run 3 120v heaters yes you would need 3 20amp circuits

are you planning to install a 30 amp circuit with at least #10 wire for this heater? Thats the only way youll get full use of it.

Mike

I was planning to use an 8 gauge since it's going over a relatively far distance. Is it worth the money and will it work well as a greenhouse heater?

Is it worth the money and will it work well as a greenhouse heater

id say if you can score one for a good price, yes. its probably bigger than you would ever need for that size GH but thats not a bad thing.

mike

But wouldn't I have to run 3 12 gauge wires to the greenhouse to power three small space heaters?

That problem is overcome by having a sub-panel. You still run one big wire to the greenhouse sub-panel, then the panel splits it into lower-amp circuits.

Those big heaters you are looking at are meant for pole barns and job sites, which tend to be a lot bigger than greenhouses. When you put them in a small space, it's hard to heat evenly. It can be 150 degrees in front of the heater and 50 behind it. That cooks poly and kills plants, which is why they tell you up front that it is not recommended for a greenhouse.

I am in zone 6 in Pa and next year will be trying to keep my 6X8 ft Harbor Fr. Gh at above freezing for some plants. As of now my greenhouse is still in the box. I have already purchased 2 doghouse heaters. They are electric. They are still in their box. Has anyone ever tried them? I liked the idea that the units probably provide constant warmth inste ad of intense heat. My greenhouse will be right up against my house, but I still need to make a hole to feed the cord through the basement window. The house is brick and I would have to drill though brick and the cement block of the basement if did't go through the basement window, the plug is right under the window in the basement. The doghouse heaters are under the name Hound Heater tm. It has a bracket to mount it on. It says plug into a GFI outlet. It has a theremostat that has Low, Med and High. It has a 150 watt heating element. It says when you set the thermostat the unit will run at full power until the desired temperature is reached, but says Turning it to the maximum amount will not make it run any hotter. It mounts on to a side wall. I have yet to try it, so I can't recommend it yet, but it is worth checking out.

I put a link below that will help you calculate your heating needs.

Forgive me for being critical, but I don't think you will get much of a temperature gain from only 300 watts of heat. It's frustrating to heat a greenhouse, because they don't hold in heat well at all.

Here is a link that might be useful: Heat Loss Calculator

I like the greenhouse heat calculator below.

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

I was thinking about electric heater but a friend of minesuggested apropane heater. I bought a small one from HD I am very happy with it. Ithink the smaller the heater the better theresults because greenhouses are very small areas or atleast mine is . All I have is 10x20.

I will need to add some heat storing things to next years greenhouse. As for now i am waiting for spring.

The calculator says 4125 BTUH for my 6X8, if it is 8 ft tall. And if it's ground to ground, which I think means a square or oblong box set on a no frills wood foundation. If I just want it above freezing when it is 13 F above on some nights, most nights will be in the 20's. Maybe the dog house heaters will be better for early sowing of some plants in spring, long after the worst of winter. Just enough to stop the frost from getting some tomato and pepper plants, maybe start some annuals.

Remember also that you needn't heat the entire space, which is likely going to be wasteful. A more do-able idea is to create a small space that you keep warm within the GH. For example you can use soil heating cables or heat mats, then cover the space with hoops and poly and/or frost blankets. This will take a fraction of the energy and money.

See a thread by Hudson_WY for a perfect application of this idea. The thread is called something like "alternative to Greenhouse heating."

Somebody else recently posted an idea where they surrounded the bed with gallon water jugs, then put a cover over that. I thought that was a clever way to create a warm spot.

Soil Heat Cables did work well for us this winter. We harvested our lettuce and spinach several times during the winter without heating the GH. We placed the heat cables 4"s under the soil in a 30" x 8' area - then covered the area with polycarbonate sides and lid - creating a heated grow box. Much less expensive than heating the entire GH! I have moved most of our Geranium starts to the GH grow box now to make room for our tomato starts under the indoor grow lights and the Geraniums are loving it. Here is a photo taken in December when inside GH temperatures reached as low as -2 degrees:

{{gwi:304315}}

The soil heat cable is on a thermostat set @ 55 degrees. Even though we will continue to use the GH grow box in years to come during the cold winter months - we are installing a GH heater this year to maintain the temperature in the GH to approximately 50 degrees starting in the late March - April time frame to get an early start in our GH. The daytime temperatures on sunny days is already up to 100 degrees when not vented. The cost to heat the entire GH to 50 degrees at this time of year will be very inexpensive - especially when compared to the winter months! We are installing a propane GH heater with blowers as soon as the weather permits and I can get the concrete walks and heater platform poured.

This post was edited by Hudson...WY on Tue, Mar 5, 13 at 10:05

I've never heard of soil heating cables, don't worry I'll look it up. I have seen the coils you put in water to keep it liquid vs. frozen for outdoor animals. I would imagine it's similar. The idea of adding protection within the green house is good, I wouldn't want to pay for the heat it would take to keep heat in to grow all that I would like to. Propane is probably expensive too, and hard to set up for a 6X8 HFGH.