Why Sizing Matters More Than Most People Realize
A heat pump that's too large for the space it's heating will short-cycle — turn on, reach temperature quickly, shut off, repeat. Short-cycling means the system never runs long enough to dehumidify effectively in summer, wears out the compressor faster, and delivers less consistent comfort than a properly sized unit.
An undersized heat pump runs continuously on cold days, never quite hits the setpoint, and in the coldest weather may need to rely heavily on backup electric resistance heat — which is expensive to operate.
Getting the size right matters for comfort, longevity, and operating cost.
What "Sizing" Actually Means
Heat pump capacity is measured in BTUs per hour (BTU/h) or tons (1 ton = 12,000 BTU/h). The capacity required depends on a Manual J load calculation — an engineering calculation that accounts for:
- Total conditioned floor area
- Ceiling height and volume
- Insulation levels (walls, attic, crawl space or slab)
- Window area, orientation, and glazing type
- Local climate data (Burnaby's design temperatures)
- Air infiltration rate (how drafty the building is)
- Internal heat gains from occupants and equipment
Rules of thumb (like "400 sq ft per ton") are often used for rough estimates, but they're not reliable for sizing — a well-insulated modern home needs significantly less capacity per square foot than a drafty 1960s bungalow.
Burnaby's Climate Context
Burnaby's heating design temperature (the outdoor temperature used to calculate peak heating load) is around -7°C to -9°C depending on the specific location and design standard used. Most cold-climate heat pumps are rated to deliver full capacity at temperatures well below this, which means a properly sized system handles the worst Burnaby winters without issue.
Cooling loads in Burnaby are generally moderate — summers are milder than eastern Canada or the US Pacific coast interior. This means the cooling season rarely drives sizing decisions here; heating load is usually the dominant factor.
Multi-Zone Sizing
For multi-zone mini-split systems, each indoor head is sized for its zone, and the outdoor unit is sized to handle the combined load with some diversity factoring. The outdoor unit doesn't need to equal the sum of all indoor head capacities — it needs to handle the realistic simultaneous demand, which is typically lower.
Your installer should calculate each zone separately and select the outdoor unit accordingly. Outdoor units that are oversized relative to the indoor heads they serve can cause similar short-cycling problems.
Questions to Ask Your Installer
Before accepting a quote, ask:
- Was a load calculation performed, or is this sized by rule of thumb?
- What outdoor design temperature was used?
- Is the outdoor unit sized to the calculated load, or is it the next size up?
- If multi-zone, how was the outdoor unit capacity determined relative to the indoor heads?
A contractor who can answer these questions with specifics is doing the work properly. One who can't usually isn't.
The Bottom Line
Proper sizing is one of the most important parts of a heat pump installation — and one of the least visible. The right system is one that matches the specific load of your specific property, not the largest unit that fits on the side of the house.
We calculate heating and cooling loads for every installation before specifying equipment. It's not optional.