As a first followup to my article that looked in to the economics of heating my garage (The Garage Heater), I ordered myself a TEMPer USB temperature sensor to do some fact checking. Note - I use the term fact pretty loosely... more like "numbers I made up using formulas I found on the internet."
The software I'm using to record the data is called ThermoHID (http://www.thermohid.co.uk) - a freeware app which is far superior to the terrible software that comes with the sensor. For this test, I went out on a 5 degree day, bought a bunch of propane, and did my best to keep the garage 60 and 85 degrees for three hours or so:
The first thing I would like to point out in the data is the pause in the initial heat up. It's darn cold out - the garage started around 38, which it seems to be able to maintain with just heat leaking in from the house (there's one finished wall w/ a house on the other side), so when we tried to heat it up to 85 the first time, the propane tank froze up and we had to switch to a warm one. That's just what happens when you use a 106,000 BTU propane heater with a 20lb tank. It simply can't support that rate (something about vaporization rates).
And finally - it's interesting that as time goes by, it takes less time to heat the garage back up and it takes more time to lose the heat. I'm pretty sure this is due to all the "stuff" in the garage(cars, garage slab, some engines, refrigerators, tools, workbenches, etc). Heating up air is one thing, but heating up stuff - that takes more energy and it has its own rate of heat loss. But when your "stuff" loses heat because the air around it is cooler, the heat goes in to the air - the air we're trying to keep hot. Just an interesting observation. I'm also fairly sure that 106K BTU heater is putting out nowhere near that hooked up to a 20lb tank, it simply couldn't have been based on how fast that much energy should be able to heat the garage. Now to move on to the inevitable - the Natural Gas heater.