Propane Heater/Tank Question

I have a 15000-30000 blue flame, ventless propane heater I need to install. I am wondering what size propane tank I should be getting. Our winter nights here are typically in the 30s, with temps dipping into the 20s about 2-3 weeks out of the year. Will try to keep the greenhouse in the 40s. Any thoughts on propane tank size???
Thanks!
Glenda

better prices, and less headaches with larger tanks.

A couple of things are going to make this nearly impossible to figure out. What size is the greenhouse, How many BTUs is the furnace? How much or how often does it run, What is the greenhouse made of? Are you trying to mantain a 73 degree temp or just keep it above freezing? I would see if you could find someone who has one of the same construction, same model, that keeps it at the same temp in the same zone... Or I would buy one that is at lease like 400 gallons. You can also lease those from the propane company.

Drew

Thanks to you both for your input!
Glenda