Rob,

Oil starts jelling at around -20. When temps start getting into the -teens
I start adding an anti jelling agent to the oil. Tanks in a heated area
would get rid of this headache and cost. At a minimum, the tank should be
plumbed with dual lines as the return line helps warm the oil in the tank.
If your greenhouse is big enough and you do a little fast talking, I bet
your oil supply company to be will have a real deal for the tank. Tank size
is an issue here, if they are too big they come under EPA regs. Smaller
tanks are your friend and the more frequent fillups are not your headache.

My main source for heat is oil hot air. Sunderman makes a fine unit in two
sizes. It is basically a Beckett burner, heat exchange and blower covered
with sheet metal. They looks cheap but have proven to be workhorses that
locals can work on. I do not think there is anything special about a
greenhouse heater except for size. For smaller greenhouses I expect a house
unit would work just fine. In fact I use a house boiler for under bench
heating in my seedling area.

If your pockets are deep, two smaller units are much safer than a single
unit. If you only have one main heater some sort of backup heat is a must.
When it is zero outside and the heater fails you just do not have much time,
and if you can get a repairman out at 3 am on a snowy night, he will not
have the right parts on the truck.

Talking about boilers, they can bite you. To operate a boiler in a
commercial setting I must have it state inspected every year and carry a
special insurance (that would be boiler insurance). It is worth doing some
checking before going the boiler route. I know a guy who heated his
greenhouse with a hot water heater and a circulating pump. The hot water
heater was nothing more than a low cost boiler.

I was in a greenhouse the other day that was heated using radiant heat. I
do not know much about it, but I really liked the feel of the heat. It
seemed to heat the plants on the benches and not all the air in the
greenhouse. It has to be cheaper operate. Might be worth looking into.

Pat