Best Method to choose furnace size

[dchance]

Comparison shopping for indoor wood add ons is somewhat confusing. What is the best way to know if a model will be sufficient? I have found that the rule of thumb in my area is 40-45 BTUs per square foot. Which means my 3400 sq ft home needs at least roughly 150,000 BTU furnace correct?

Then looking at the Drolet lineup, the Tundra clams a 310,000 BTU but only that is serves up to 2500 sq ft. Other brands claim up heat up to 2500 sq ft and are around the 140,000 BTU. Why so inconsistent???

 

[3fordasho]

So that 310,000 btu is probably over the duration of a burn cycle, not per hour.
Some rough calcuations:
Tundra has a 3.6 cuft firebox
Average red oak or ash has a dried weight of about 40lbs/ cuft
All wood types at 20% moisture content give up about 6700 btu/pound
Assume Tundra efficiency of 60%

3.6cu ft firebox that we need to leave air space because we cant get a solid chunk of wood in the fire box
so I am going to say 60% of firebox is solid wood on a full load.

3.6 cuft x .6 x 40lbs/cuft x 6700 btu per lb x .6 (60% appliance efficiency) = 347,000 btu delivered to the house over the duration of a 8 hr burn.

The furnace output will ramp up fairly quicky at the start, peak and then slowly ramp down until there are only coals and very little output. Then it's time for another load and burn cycle.

So the real factors determining overall output per cycle is the lbs of wood you can stuff in there and how efficient the wood furnace is.

As far as why the inconsistent ratings, the furnace manufacturers have used btu output ratings as a marketing thing, and we all know how much we can trust marketing departments ;-)

 

[dchance]

So that 310,000 btu is probably over the duration of a burn cycle, not per hour.
Some rough calcuations:
Tundra has a 3.6 cuft firebox
Average red oak or ash has a dried weight of about 40lbs/ cuft
All wood types at 20% moisture content give up about 6700 btu/pound
Assume Tundra efficiency of 60%

3.6cu ft firebox that we need to leave air space because we cant get a solid chunk of wood in the fire box
so I am going to say 60% of firebox is solid wood on a full load.

3.6 cuft x .6 x 40lbs/cuft x 6700 btu per lb x .6 (60% appliance efficiency) = 347,000 btu delivered to the house over the duration of a 8 hr burn.

The furnace output will ramp up fairly quicky at the start, peak and then slowly ramp down until there are only coals and very little output. Then it's time for another load and burn cycle.

So the real factors determining overall output per cycle is the lbs of wood you can stuff in there and how efficient the wood furnace is.

As far as why the inconsistent ratings, the furnace manufacturers have used btu output ratings as a marketing thing, and we all know how much we can trust marketing departments ;-)

OK, So what would I look at to determine if it would be sufficient for my 3400 sqft home? Is the product description the best way?

 

[3fordasho]

OK, So what would I look at to determine if it would be sufficient for my 3400 sqft home? Is the product description the best way?

Previous utility bills can give you an idea of your homes heat load. Gallons of propane, fuel oil, therms of NG, kilowatt hour needs over the heating season or even just the month of January or Febuary can give you a pretty good idea of the heat load during a high demand month.
I can get the total propane usage for my place per heating season, it's even better if it's metered like natural gas or electric. If you've always supplemented with wood it's not as easy.

 

[3fordasho]

OK, So what would I look at to determine if it would be sufficient for my 3400 sqft home? Is the product description the best way?

Per the other thread, if there is only a few hundred difference in price between HeatPro and one of the smaller versions and considering your 3400 sqft home size- just go with the big one, the HeatPro.