recommended greenhouse heater
We have a fiberglass enclosed 8x12 greenhouse in south Reno. We need to get a heater/fan that would keep the greenhouse at an acceptable temperature to grow all year long. Do you have a recommendation on which type would work? Thank you, Cathie Graham
Washoe County Nevada
What a neat greenhouse. I've never seen one quite like that. Thanks for including the picture. I'm curious what you are growing.
Sizing and selecting a heater for your greenhouse depends on a couple of things.
1. Desired inside temperature (generally 65 F for starting, perhaps higher for certain crops and during production months).
2. Minimum outside temperature (Reno, NV looks like it can get to -5 F in January, but I suspect you're not growing at that time.)
3. The envelope (outside material). Looks like a single layer of translucent fiberglass which my reference  indicates has a U-value of 1.2 BTU/hr-F-ft2. This is a measure of how much heat is lost per unit of surface area, per unit of temperature different (inside to outside) per hour.
4. Preferred / available fuel. Most of the greenhouse growers in my area use propane since the use is so seasonal and since we pay $0.15 / kWhr for electricity. But you can use any heater / fuel.
With the above info (1-3) you can estimate the maximum heat loss (coldest night). There are a number of ways to do this. I summarize three of them here: http://blog.uvm.edu/cwcallah/2015/04/01/calculating-greenhouse-and-high-tunnel-heat-loss/
I used the online calculator (simple) and entered 7 ft width, 10 ft length and 0 ft side wall height, then indicated it was a "ground to ground" style. With 65 F inside temperature and 30 F outside temperature, the calculator indicates 5,555 BTU/hr of heat loss. We can convert that to electric wattage by knowing that there are 3,412 BTU per kW. So a 1.6 kW (or 1600 watt) space heater can keep your greenhouse at 65 F on a 30 F night. Consider putting it on a thermostat to prevent overheating and to only have it come on when needed. Also take care to follow the heater instructions related to fire safety.
 Bartok, J., & Aldrich, R. (1994). Greenhouse Engineering, NRAES - 33. Natural Resource, Agriculture and Engineering Service (NRAES). Retrieved from http://host31.spidergraphics.com/nra/doc/Fair%20Use%20Web%20PDFs/NRAES-33_Web.pdf see p.66
Cathy, I just re-read your question and see that you would like to grow year-round. So that changes things a bit. But not too much. To "grow" year round, I suspect you will grow late into the fall and "hold" in the depth of winter. Daylight likely starts to become the limiting factor for actual growth. Anyway, it depends a great deal on what you're trying to grow. If it is winter greens, many in our area do this without heat addition by using row cover. But some use small heaters sized to keep things at 45 F or so.
I also now realize you included dimensions in your original question which I failed to see (long week).
So these things combined...
8'x12' - heated to 65 F on a 30 F night takes 7,574 BTU/hr (2200 Watt heater)
the same house heated to 45 F on a -5 night takes 10,820 BTU/hr (3200 Watt heater)