Load calculations explained: CSA F280 in plain words

What size furnace do you need?

You need a furnace whose heat output covers your home's heat loss at the local design temperature, plus a modest margin. That heat loss, in BTU per hour (BTU/h), comes from a calculation, not a chart. In Canada the standard method is CSA F280: it adds up how fast each room loses heat through walls, windows, ceiling, floor and air leaks on a design-cold day.

Watch input versus output. Furnaces are sold by input, the gas they burn, but output heats your home. A 60,000 BTU/h input furnace at 96% AFUE puts out about 57,600 BTU/h; an old 80% furnace with the same input put out about 48,000. Each furnace line comes in a few sizes (40,000, 60,000, 80,000 BTU/h input and so on), so the designer picks the smallest one that covers the load. When you line up furnace quotes for your home, check whether each size is input or output.

What a load calculation measures

It answers one question: how much heat leaves each room per hour at 22°C inside and design-cold outside? Heat escapes six ways, and each is measured.

1. 1

   Ceiling and attic

   Ceiling area and attic insulation depth.

2. 2

   Walls

   Wall area, framing and insulation.

3. 3

   Windows and doors

   Size, count and glass type. Glass loses far more than a wall.

4. 4

   Air leakage

   Drafts around windows, doors and the attic hatch.

5. 5

   Basement

   Depth below grade and wall insulation.

6. 6

   Ventilation

   Fresh air from an HRV or ERV, which must be heated too.

The outside number is not the coldest night on record. It is the design temperature in the Ontario Building Code's climate data (Supplementary Standard SB-1): the January 2.5% value, which January hours drop below only 2.5% of the time. Caledon's, for example, is -21°C. A good report names the design temperature it used, so you can check it matches your municipality.

In summer the survey runs in reverse: heat gain counts sun through each window, people, appliances and humidity, and sizes the air conditioner.

Why it is done room by room

The whole-house total picks the furnace. The room numbers do the rest: each room's load sets how much air it needs, and that airflow sets its duct and register size. That is how a designer finds the cold bedroom over the garage, or a duct too small for a new system.

Heat loss and gain summary

CSA F280-12, room by room

Sample

Designer ID here

House

Two-storey, 1,900 sq ft, built 1994

Design temperatures

-20°C winter, 31°C summer, 22°C indoors

Room	Heat loss BTU/h	Heat gain BTU/h	Airflow CFM
Living room	6,400	3,900	135
Dining room	3,700	2,000	80
Kitchen	4,100	3,200	90
Family room	5,600	3,400	120
Primary bedroom	4,900	2,900	105
Bedroom 2	3,300	1,900	70
Bedroom 3	3,100	1,800	65
Bathrooms (2)	2,200	900	45
Halls, stairs and laundry	2,400	1,000	50
Basement	9,100	1,000	195
Whole house	44,800	22,000	955

Furnace

60,000 BTU/h input, two-stage, 96% AFUE: about 57,600 BTU/h output, 129% of the heat loss

Air conditioner

2 ton (24,000 BTU/h): 109% of the heat gain

Ducts

Bedroom 2 branch upsized

The furnace's output is 129% of the heat loss and the air conditioner 109% of the heat gain: a sensible margin, not a big one. For cooling, industry design training allows roughly 80% to 125% of the calculated load. For heating, an older edition of F280 capped output at 140% of the heat loss. The 2012 edition dropped the cap, but it is still a useful line between margin and oversizing.

Rules of thumb: a rough first check only

Square-foot rules are good for one thing: spotting a quote that is wildly off. The usual Ontario range is 25 to 45 BTU/h of heating per square foot, depending on age and insulation, and about one ton of air conditioning per 500 to 700 square feet.

The small 1950s bungalow can need nearly as much heat as a new house twice its size. The sample 1990s house above measured 44,800 BTU/h, 33% below its rule-of-thumb figure of 66,500. For a ballpark price rather than a size, try the furnace price estimator.

Why "same as the old one" usually oversizes

• It was probably sized generously. Many older furnaces were picked by rule of thumb plus a safety margin.
• The house has changed. New windows, attic insulation and air sealing all cut the heat loss.
• Same input, more output. A new 96% furnace with the old 80% unit's input rating delivers about 20% more heat.

In the sample house, the old 80,000 BTU/h input, 80% furnace put out about 64,000 BTU/h, already 143% of the load. A 96% furnace with the same input would deliver 76,800, or 171%. Natural Resources Canada's sizing guide agrees: there is no precise way to work out a home's load from the equipment already in it.

Oversizing brings short cycling (quick on and off bursts that wear the igniter, inducer and controls), comfort swings between rooms, noise from more air than old ducts were built to carry, and clammy summers, because an oversized air conditioner shuts off before it removes much humidity. Undersizing is a risk too; the calculation protects you both ways.

Why it matters even more for a heat pump

A furnace puts out the same heat at -20°C as at 5°C. A heat pump does not: its output falls as the temperature drops, just as your heat loss rises.

• Check the output in the cold. Capacity is often quoted at 8.3°C (47°F). Ask for the output at -8.3°C (17°F), at -15°C and at your design temperature. Cold-climate models publish these, for example on the NEEP list.
• Know the balance point. Below the temperature where heat loss exceeds the heat pump's output, backup heat makes up the gap: electric strips, or your gas furnace in a dual fuel system.
• Mind the ducts and summer. Heat pumps often need more airflow than the old AC, and one oversized for winter can leave the house humid in July.

Our heat pump quote page covers the options, with typical prices of $9,000 to $18,000 for a ducted cold-climate system and $11,000 to $20,000 for dual fuel, before HST.

What the contractor measures at the visit

A real calculation takes a tape measure and every floor. Expect the person doing it to:

How to tell if one was really done

The proof is on paper: a printout or PDF with the design temperature, room-by-room losses and gains, the totals and the equipment selected. The quote should name equipment that matches it.

If a quote states a size with no report, ask for it before you sign. It is one of the questions to ask before signing, and the sizing belongs on the quote itself, as our guide to what a furnace quote should include shows.

The building code and permits

Canada's building codes, Ontario's included, point to CSA F280 for sizing heating equipment in new houses. For a replacement, requirements depend on your municipality. Toronto, for example, lists a flat-fee furnace replacement permit for houses, and its HVAC permit guide asks for heat loss and gain and duct design calculations. Ask the contractor whether your job needs a permit and whether the sizing report goes with it. More guides are in the Quote Guide.