Hi Cheryl,
We used a single head outdoor propane heater as backup for the past 5 years. Hubby modified the valve setting to super low because they heat FAST and hot in a small area (8x11x9). We averaged 60 hours per 20lb tank at a temp of 55F.
Humidity will be an issue because these dry out everything. But they work well and are fairly cost efficient. $18 for refill, $50 for the single head, $59 for the tank deposit.
