I have an outside screened porch with plastic storms on the inside for the winter and I use 2 heaters - 1500 watt electric radiant heater that works great until the temps drop below 30f. Then I use a
Mr Heater (Models MH12T & MH12TS - Single Tank-top Propane Heater w/Piezo Ignitor ) that puts out around 8000 to 14,000 btu using a single propane tank. This propane heater goes for $50 to $70 at Gander Mountain and other sporting goods stores. This tank-top heater does get hot in front, so you have to be careful if you have little ones around (I don't have that problem anymore here).
Using any propane heater requires some ventilation so you don't get carbon monoxide poisoning. My porch has an exhaust fan so there are no venting issues here.
Good luck with which ever one you choose.
