home furnace sizing tips

Most installed furnaces are oversized without any advantage. Oversizing a furnace can diminish its efficiency and be a cause of higher energy bills and a smaller lifespan...

Furnace oversizing is particularly critical in homes that have benefited from insulation and air sealing... The size of furnaces in these homes is likely to be grossly oversized.

Furnace capacity
The heating capacity of a furnace is measured in thousands of BTU (British Thermal Units), but the heat it produces isn’t the same as its input BTU. The heat output depends also on the AFUE of the furnace, that is, on its efficiency: a furnace with an input BTU of 100,000 BTU and a 80% efficiency/AFUE will produce 80,000 BTU of heat (the BTU output), while a furnace with the same 100,000 input BTU and a 95% AFUE will produce a higher heat output: 95,000 BTU. Manufacturers have several sizes for each furnace model. The same model can involve furnaces with very different very capacities, usually from 40,000 to 150,000 or more BTUs.

Why not to oversize

A well-sized furnace will run continually and smoothly during the colder winter days.

Undersizing will cause a deficit of heated air and comfort, while an oversized furnace will make the furnace to turn on and off too shortly (something technically called “short cycling"), without ever coming up to the best rhythm and efficiency and causing higher energy bills and temperature swings and discomfort.

Note: New multi-stage furnaces are not subject to the problems of oversizing, but it doesn't bring them any advantage either.

The capacity of the furnace

Remember: the heat output of a furnace is rated in BTU (British Thermal Units) and depends on two elements: the input BTU and the energy-efficiency of the furnace, measured by its AFUE (Annual Fuel Utilization Efficiency). If a furnace has a 50,000 BTU input and an efficiency/AFUE of 90%, it's heat output will be 90% x 50,000 BTU = 45,000 BTU (see box at right).

Remember this first tip: pay attention to the type of BTU, when assessing a furnace capacity. There is an input BTU and an output BTU. The true capacity of a furnace is measure by its output BTU.

Good and bad Sizing methods

There are several methods or tips to size a furnace, but most of them are inaccurate and untrustworthy.

Sizing Tip 1
The “method” of replacing an old unit with one with the same capacity is obviously wrong, unless your home hasn't benefited from any improvement in what concerns insulation, air sealing, windows... and the old unit is undoubtedly well sized. To know the size you currently have you should look for it in the name plate of the furnace, often located somewhere inside it.

Sizing Tip 2
Simple rule of thumbs, based on the home’s size, its window area and other indicators can provide first estimates, but just that.

Rules of thumb often begin with a general sizing rule like 30 or 35 output BTUs per home square foot in cold climates (not very harsh). But this is a vary abstract value; it can be a useful tip to get a first estimate of the size of the furnace, but never a final estimate. The right value depends largely on the exact climate and the on the home's insulation levels.

How to size a furnace

Sizing a furnace is a job for certified heating contractors (some utilities also provide that service), and should consider more than just the practical tips quoted above:

1) the local climate,
2) the sun-orientation of the building;
3) its shape and size;
4) its insulation and sealing levels (walls, attic, ceiling) and the
5) the quality and type of windows and glass and enter or glass doors...
6) home appliances...

Typically, contractors use some basic computer software to get the right furnace size, and not just general tips. In North America it's common to use the Manual J, "Residential Load Calculation" from the ACCA (Air Conditioning Contractors of America).

A qualified contractor can provide a written list with the results of his assessment.