Proper Furnace Sizing

The size of an outdoor furnace must be matched to the requirements. While over-sizing is acceptable, under-sizing is not. All manufacturers use either a BTU rating or square footage capabilities. Since BTU’s in wood vary, the only true way is to state maximum heating capabilities in square feet. The key word here is that the stated capability is the maximum under ideal circumstances. In most parts of Canada you will not find these ideal conditions and you will be forced to estimate the capabilities. Here are some guidelines to follow:

Base Calculations

In Canada, start your calculations by using 50 percent of the stated maximum square footage. This is usually a safe place to start under most conditions. When you exceed this amount, be very careful in your calculations. Upgrade to the next size of furnace long before you reach the maximum heating capability.

Ceiling Height

Calculations are based on having 8 ft. ceilings. If for example the ceiling height is 12 ft., then add 50% to the square footage. Also allow for any vaulted or cathedral ceilings.

Basement, Upstairs

Add the square footage of the basement and the upstairs and any other area to be heated.

Crawl Spaces

These are killers on heating demand even if you don’t heat them. Allow lots of square footage for these.


The more windows, the higher the heat loss, so be sure to take this into consideration. Check for these features and adjust accordingly: gas filled, single pane, double glaze, triple glaze, coated glass, roof line windows, location of windows, air leaks.

Building Age

The older the building the less likely it is to have adequate insulation. New R2000 buildings have lower heat demands on the furnace.


Find out the type and R value of the insulation. Any building under R2000 rating will require more heating.

Geographic Location

Buildings in Wisconsin are easier to heat than those in Manitoba. Take the location into consideration.

Lot Location

Is it on top of a cliff, out in the middle of a field, in the trees, on the shore of a lake, facing north? All of these will have a bearing.

Heating System

Floor heating will reduce heating demand, but only if properly insulated between pad and ground. Forced air systems call for more heat.

Type of Building

Garages, barns and shops usually take more to heat than homes. Add 25% or more on to the demand. And add another 25% more per garage door. Are the other doors insulated or single – it all adds up.

Number of Buildings

It takes approximately 25% more to heat two 1,000 ft. buildings than it does to heat one 2,000 ft. building due to perimeter areas and underground heat loss.

Distance From Furnace to Buildings

The longer the distance, the greater the heat loss and the larger the furnace size required.


The use of proper and adequate underground insulation will have a bearing on the size of the furnace.

Easy Rule of Thumb Solution

Once you pass the halfway mark on square footage, consider moving up to the next size of outdoor furnace. Over-kill never hurts; it will allow more time between fills. You do not want to regret buying a small unit that is maximized.