Sizing a propane direct vent wall furnace

[Arky217]

I'm trying to figure what size of a propane direct vent wall furnace to buy
for backup when the electricity is off for an extended time, like during an ice storm.

I'm looking specifically at the Williams brand because it does not require any electricity.
They manufacture 3 sizes, 14000 btu, 22000 btu, and 30000 btu.

The location is western Arkansas; the house is one story, 1152 sq.ft.
It is very well insulated, R20 walls, R40 ceiling, R27 floor, with double pane windows.
It is currently heated with a 2 ton, 13 seer heat pump that never sees the emergency heat strips come on.
With no electricity, I could shut bedroom doors
which would leave about 600 sq. ft. that would need to be heated.

I would like a unit large enough to keep the open area (about 600 sq. ft.)
at 70 degrees when it's about 30 degrees outside.

Which of the above 3 sizes would be necessary ?

Thanks,
Arky

[hpinson]

I've got a 25,000 BTU direct vent freestanding heater (Hearthstone Tuscon) that does an adequate job of heating 320 sq. ft. bedroom on 0-10F degree days. 2 bedroom walls are adobe and two are un-insulated stick framed. Ceiling/ roof is well insulated, probably R30 or so.  At 32 degrees, it barely needs to run.

The real key to getting good performance with it is a fan or blower to distribute heat.  Also placing the thermostat well away from the heater.

[MushCreek]

Our house in upstate SC is about 1400 square feet, and at 21 degrees outside temperature, our heating load is only 12K BTU. Our walls are R-24 (ICF) and the ceiling is R-50. The house is very tight with very good windows. If you want to do it scientifically, you can get an online Manual J program and plug in all of the numbers. I used HVACCalc which is about $50 for a single-use residential edition that expires at 45 days, I think. I think there might be free programs as well.

[Ridgeracing]

I have a 600 Sq. Ft. cabin with open ceilings and use a Williams vented wall furnace 50K btu and it works great. However it does require electricity for fan blower. I dont no if it operates without electricity, never tried it. I would imagine it would not be near as efficient without fan either. I use it to keep camp 40deg. when I am not there for weeks on end in between snowmobile trips in NY and to get camp up to temp then pellet stove takes over.