Greenhouse heat advice, please

I recently bought 4 GH's from Lowes, and each measures 6'x8'x7'tall. I combined 3 and placed a Delonghi oil filled radiator space heater in the center, but it's only raising the overnight temp by about 6 degrees. On my back porch, which measures 16'x18', and the ceiling slopes from 12' to 8', one of these heaters raises the temp 15-20 degrees. I won't have a 240v connection for the GH's until next year, and I'm wondering if there's a 120v heater that would do a better job than the Delonghi. Please advise.

Never mind. Just found out there's a greenhouse forum. Yay!

Donna I use a box fan with a milk house heater along with 2ltr soda bottles painted black. The 2ltr soda bottles are great for sitting plants on that don't like cold feet. They collect heat in the day and release it at night .. passive solar heating. My greenhouse is about half the size of yours but how I heat mine would work with your size.

I use a box fan at one end of my greenhouse and a milk house heater at the other end along with a portable thermostat .. definitely a must and not that expensive.

http://www.kkontrols.com/poth8ftco.html

The box fan is on a timer and goes on at 7pm and turns off at 9am. Helps keep algae down and distributes the heat when it comes on. I keep the thermostat behind the heater and I have my heater set to come on at 40 degrees .. All it does is keep things from freezing and I really haven't noticed a large spike in my electric bill over the last 4 years since I've been using it.

Algae on top of the soil and spidermites will be your worst problems so keep an eagle eye out for the spidermites and try to catch them before they get out of hand .. the algae doesn't really hurt anything but looks awful and can sometimes fool you into thinking a plant doesn't need to be watered.

Thanks for the info, MmeX. I have a box fan circulating air, but I don't have a thermostat yet. Today I added a second Delonghi heater, and I'm anxious to see what the temp is tomorrow morning. There's always at least one bitter cold night here when temps dip below 15f, so I need a heat source that will keep temps above freezing on those coldest nights.Putting passive heat bottles below plants is a great idea - will have to put word out that I need plastic soda bottles. I taped the GH's together, but the tape isn't adhering very well. I'm thinking about covering the GH's with a layer of plastic to keep the warm air in during the coldest upcoming winter months. Will also keep an eye out for spider mites. I'm hoping to have a large greenhouse built by next fall, and align 2 of the Lowes GH's on either side. Should also have a 240v connection by then, too. I am prepared to move more plants into my backporch if all else fails this winter. Thanks again!