How big a furnace or heat pump does your home need? The answer starts with how much heat the house loses on a design cold day. Enter the areas below and the calculator adds them up the way our technicians do on site, using the Alberta Indoor Comfort heat loss worksheet built on the CSA F280 method.
Loading the Home Heat Loss Calculator.
How the calculation works
Each part of the house is multiplied by a factor in BTU per hour for every square foot, set by how well it is insulated. The results are added in three groups and then adjusted for the heating system.
- Subtotal A, above grade: windows, doors, walls, exposed floors and the roof.
- Subtotal B, below grade, air and ventilation: basement walls, slab, air leakage based on how tight the home is, and mechanical ventilation.
- Subtotal C, duct work: 10% of Subtotal B for insulated ducts in unheated space, 25% for uninsulated ducts, nothing for ducts inside the heated house.
- Furnace loss: 5% of A + B + C for a 95% furnace, 25% for an 80% furnace.
The total is the heat input the equipment has to supply, in BTU per hour.
The factors used
| Part of the house | Construction | BTU/h | Subtotal |
|---|---|---|---|
| Windows | Single | 97 per ft² | A |
| Windows | Double | 47.1 per ft² | A |
| Windows | Triple | 31.7 per ft² | A |
| Doors | Wood door (no storm) | 36.6 per ft² | A |
| Doors | Wood door (with storm) | 26.2 per ft² | A |
| Doors | Insulated metal door | 13.7 per ft² | A |
| Walls | No insulation | 34.3 per ft² | A |
| Walls | R13 (4″ wall) | 7.2 per ft² | A |
| Walls | R19 (6″ wall) | 4.9 per ft² | A |
| Walls | R25 (sprayfoam / 8″) | 3.7 per ft² | A |
| Walls | Masonry (no insulation) | 35.8 per ft² | A |
| Walls | Masonry (1″ insulation) | 16.7 per ft² | A |
| Exposed floors | No insulation | 34.3 per ft² | A |
| Exposed floors | R13 (4″) | 7.2 per ft² | A |
| Exposed floors | R19 (6″) | 4.9 per ft² | A |
| Exposed floors | R25 (sprayfoam / 8″) | 3.7 per ft² | A |
| Roof | Uninsulated | 34.3 per ft² | A |
| Roof | R13 (3.5″ insulation) | 7.2 per ft² | A |
| Roof | R19 (5.5″ insulation) | 4.9 per ft² | A |
| Roof | R28 (8″ insulation) | 3.3 per ft² | A |
| Roof | R40 (11″ insulation) | 2.3 per ft² | A |
| Roof | R50 (14″ insulation) | 1.9 per ft² | A |
| Roof | R60 (16″ insulation) | 1.6 per ft² | A |
| Foundation (basement walls) | No insulation | 22 per ft² | B |
| Foundation (basement walls) | R12 on full wall (4″ wall) | 5.7 per ft² | B |
| Foundation (basement walls) | R19 on full wall (6″ wall) | 4.7 per ft² | B |
| Foundation (basement walls) | R25 on full wall (sprayfoam / 8″) | 4.2 per ft² | B |
| Foundation (basement walls) | R12 on less than full wall | 8.4 per ft² | B |
| Slab on grade | Slab on grade | 1.8 per ft² | B |
| Infiltration | Tight home (home floor area) | 2 per ft² | B |
| Infiltration | Standard home (home floor area) | 5.9 per ft² | B |
| Infiltration | Drafty home (home floor area) | 8.1 per ft² | B |
| Infiltration | Uninsulated drafty home (home floor area) | 15.8 per ft² | B |
| Ventilation | Mechanical ventilation | 56 per CFM | B |
For walls, measure the wall perimeter times the wall height, then subtract the window and door area. The calculator can check that number for you.
What to do with the result
Use the total to compare with the output rating of your current furnace or a quote you are considering. A furnace much larger than the heat loss short-cycles, wastes fuel and leaves rooms uneven; one much smaller cannot keep up on the coldest nights. This is an estimate for planning. Final sizing should come from a room-by-room calculation of your actual home.
Planning a replacement? See furnace installation in Calgary or heat pumps, or call 587-792-1456.
Frequently asked questions
What does a heat loss calculation tell me?
It estimates how much heat your home loses through its windows, doors, walls, roof, basement, air leaks and ventilation on a design cold day, in BTU per hour. That number is the heat your furnace or heat pump has to replace, so it is the starting point for sizing equipment.
Why not size a furnace by square footage?
Two homes of the same size can lose very different amounts of heat. Window type, insulation levels, basement insulation and how drafty the house is all change the load. Sizing by square footage alone often produces an oversized furnace that short-cycles, wastes fuel and leaves rooms uneven.
What is CSA F280?
CSA F280 is the Canadian standard for calculating the heating and cooling capacity a house needs. This calculator follows the worksheet our technicians use in the field, which applies F280-style multipliers to each part of the building.
Why does the result include duct and furnace losses?
Heat lost from ducts running through unheated spaces, and the heat a furnace cannot deliver because of its efficiency, both add to what the equipment has to produce. A 95% furnace adds 5% on top of the building load; an 80% furnace adds 25%.
Is this calculator enough to choose new equipment?
It is a good estimate for planning. Final equipment sizing should come from a room-by-room calculation of your actual home, so ask for one when you get a furnace or heat pump installation quote.