Only 1/2 joking - whichever type you can best afford to run. ;)

Seriously, the cost of heating is the biggest expense to greenhouse gardening - a bit less for you than for me maybe - but still high.

Plus what power sources do you have available? Gas or just electricity? Propane or natural gas? What is the price of kerosene in your neck of the woods? Kerosene is quite beneficial heat for a GH because of the CO2 but the cost of kerosene (what we used to use) is prohibitive here so we have converted to wood heat and water sumps for passive solar. Thermostatic control of the heat can be a middle of the night time saver AND a crop saver.

For 30 feet in length then yes, unless you are going to partition/enclose various parts, I think you would need 2 sources with the GH divided into thirds and fan forced circulation of the heat is almost mandatory.

Height of the heat source is also important - heat rises - so what is table height if using tables or ground level temps relevant to the heat source?

Lastly , how much heat will be required all depends on what you plan to grow. For just seed starting a small enclosed space can be heated easily. But for established plants widely scattered throughout the GH, heating becomes more of a problem as cold pockets develop (lots of thermometers are needed).

Be sure to also ask over on the GH forum here for even more tips and recommendations.

Dave

A gas heater made for greenhouses is usually the best option. Second best - and the one I use - is a wood heater with an electric fan to stir the heat around. Why is a wood heater second best? Because there is a LOT of work involved in cutting and splitting wood and getting up at 1:00 a.m. to put more wood in the heater. There are wood burning furnaces designed to heat greenhouses. They are a better option in many ways than plain wood heaters.

Regardless of heat source, you MUST have some type of fan in most greenhouses, otherwise the heat will rise to the top and the cold air will fall down around the plants. I've lost plants from failing to understand this basic requirement. Air circulation does two things. It keeps warm air down low where it can do some good and moving air does not as easily form frost on leaves.

DarJones

at 20 degrees night temp, you're going to need a pretty big unit to heat a 12x30 if you're heating the whole thing. Remember, you need a heater that will handle your lowest possible temperature at the minumum you set. Even if you only get that temp once a year, that's the size heater you need.
In our 12x26, we needed a 50,000 btu lp modine unit.

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

But yes, the best heater is one that won't bankrupt you running it! Its all dependant on surface area, tempature rise and r-value. Try this link for the greenhouse heater calculator.

www.littlegreenhouse.com/heat-calc.shtml

I use a propane Mr Heater in my green house only for the cold nights below 25 degrees,otherwise I just cover my plants with 2oz row cover freeze protection blanket.

I 'manage' a greenhouse at UH with the same dimensions as yours. Fire issues restrict me from using anything more than a ceramic heater (can be had for $70). For those few nights when it really is necessary, it works great at keeping the temperature well above freezing, like 55 F. Like everyone above says, it is imperative to keep one or two fans on to spread the heat around. And no, putting a few lightbulbs inside won't work. The one time I tried it, the snow on the glass above right above the lamps melted, and no temperature difference was observed.