need help calculating propane usage.

[Off Grid Rving]

Hello guys, I know that its possible to get a rough calculation going of how much propane I will use with my system.

I have 48sq feet at 6x8 floor space in here by 6ft 2in tall at the highest point. so we will use those numbers for figuring the air space.

=297

the heater is a pro com 10,000 btu heater with thermostat. hooked with a two stage regulator to a 20lb propane bottle.

the insulation value is r 7-8 walls.

what is a rough est that the propane bottle will last on a single refill?
other variables, keeping temp at 69ish

[aggie79]

It's been awhile - 35+ years - since I've done any thermal load calcs; however, I think you need to supply one more piece of information to get an estimate of usage.

What are the outside temps you expect to encounter?

[Off Grid Rving]

I live in Delavan wi,
lately its dropping down to low 30's through the night. max i've seen it drop down in dead of winter is -17

I only need to keep this heater on its lowest setting to keep it toasty in here.

[jimqpublic]

A pound of propane is worth a bit over 20,000 BTU's. 20 pounds of propane (a "5 gallon" tank) will run your 10,000 BTU heater on high for 40 hours.

I could do energy calc's on your trailer, but honestly the intangibles are the most important. How much venting do you have? How much glass window area? How good are the door and window seals? How often does the door get opened? Wind speed? Solar gain? Temperature profile?

Air volume isn't that important (except when considering air exchanges and energy needed to heat the new air). Most important is surface area and R-value (actually the inverse: U-Factor) of each surface type.

So instead I'll just give an example. We took my much bigger, leaky, less insulated Chalet trailer camping in the snow last winter. Daytime highs were mid twenties, overnight lows were teens. During our stay we got over 2 feet of snow. Between the 19,000 BTU furnace, 12,000 BTU water heater, and stove we used less than 20 pounds of propane for 4 nights & 5 days.

[jimqpublic]

More blather:

From my experience with our Chalet, a new weatherstrip around the door and some pipe insulation jammed into the seams between walls and roof seemed to cut furnace run time by about half. Cold air blowing through gaps pushes lots of heat out:

Energy Calcs:
Glass = R 0.9
Wall = R 7.0
Hole = R 0

At 60 degrees inside and 20 outside, there's a 40 degree differential.

1 square foot of wall will pass 40/7.0 = 5.7 BTU's per hour. (not much- going to thicker insulation doesn't do much)
1 square foot of window will pass 40/0.9= 44.4 BTU's per hour. (lots more. Curtains cut this at least in half)

Then there is the air going through gaps. If you have a 1/16" gap all around a 24"x60" door and a breeze is blowing:
5 mph breeze (7.33 feet/sec) through 1/16" x 168" gap square inch (0.0729 square feet) x 3600 sec/hr = 1924 CF/hr. Density about 0.08, Specific heat 0.24 BTU/lb- degree F.
1924 CF/hr * 0.08 lb/CF * 0.24 BTU/lb-DegF * 40 Degree F differential = 1478 BTU/hr

So obviously if you have air gaps (greater than what's needed for safe/comfortable ventilation), you will lose far more heat through leaks than through the walls. Especially if you bump the wind speed up.

[Off Grid Rving]

its 51 outside right now roughly since noon. at noon I fired up the heater to 78 degree's inside and have had it shut off for the last 5 hours and the temp is 73.8 inside

both temps taken at stomach height in the room.

[bobhenry]

I went around the barn the redneck way.......

Most of us heat our trailers with a 750 / 1500 watt electric heaters during our winter shivaree.

Mine is approx 275 cubic feet interior ( 230 on the sleepy side w/o galley)and I turn the heater down or off after warm up to remain comfortable.

[ I have woke up to 110 degree inside temps ]

This said , and a bit of less fancy math

a watt = 3.41 btu's

A 20 pound cylinder of propane contains 430270 btu's

I found a site that calculated heating load , and for a poorly insulated 6x8 room 8' tall the heat load was 3571 btu.

During our winter shivaree's we have proven that a teardrop can be comfortable in "teen" degree weather with between 750 watts (2600 btu) and 1500 watts ( 5115 btu)
If we average these 2 numbers we arrive at 3857 btu. This is real close to the 3571 btu from the calculator making me feel good about this number.

So lets say 4000 btu are needed as a round number. 430270 divided by 4000 = 107.56 hours of run time.

Jim's numbers were spot on and the more subtle nuances were definatly factors to concider ! One simple fact remains you just can't use 10,000 btu batches of heat in this small a trailer and remain comfortable.
So by reduceing the 10,000 btu to 4,000 btu you have stretched your run time from 40+ hours to 107 +/-

One quick hint the heat will stratify at the roof. add a ceiling fan and push the air upward and wash the heat back down the walls to where you sleep for greater comfort.

OK OK not a paddle fan just a decent sized small air circulation fan. Here is the one I use.



Yep it's a see thru roof not much " R" value there

[Off Grid Rving]

so are you saying that a 10,000 btu heater with thermostat operation will not last as long on a 20lb propane tank than a 4000 btu heater will on the same propane tank?

[bobhenry]

Is the 4,000 btu also thermostatically controlled ?

[Off Grid Rving]

the 4000 btu heater is hypothetical right now. but if I did find a thermostatic controlled 4k btu heater. logically thinking ( in my head) would they not use the same amount of propane if they were the same efficiency. as BTU is the deciding factor here and a btu is a btu?

[bobhenry]

Sorry didn't mean to leave ya hanging I was having trouble getting my head around this question.

The 20 lb cylinder contains 430,000+ btus of heat. No matter what you burn it in it will still contain 430,000+ btus. If you run the larger unit at its lowest setting it should have a comperable run time to the 4000 on a higher setting. I have gone to sleep in the teardrop with the electric heater on high (1500 watts or 5115 btus) and woke up hot and uncomfortable and turned it down to low (750 watts or 2557 btus) Some nights this is still too much and I wake up and shut it off and on as needed or open a window to further adjust the low to "somewhat lower"

If you want to sleep neked under nuttin' but a silk sheet in a record low blizzard best that you have a 10,000 btu heater. I have been warm an comfortable under my blankets in my old comfey sweatpants and a long sleve cotton tee shirt in single didgit weather.