Bigger Isn’t Better: Why HVAC Sizing Matters
One of the most common things we hear when quoting a new furnace or AC in Vaughan is “go one size bigger, just to be safe.” It sounds sensible. In practice, it usually makes the house less comfortable.
What happens when a system is too big
- Short cycling: it heats or cools the house fast, the thermostat is satisfied, it shuts off, and a few minutes later it starts again. Every start is hard on parts and wastes some energy.
- Uneven temperatures: short run times don’t give air time to reach the far rooms.
- Damp air in summer: an oversized AC doesn’t run long enough to pull moisture out, so the house feels cool and clammy.
- Noise: more air than the ducts were designed for means whistling registers and a loud furnace.
- Higher cost up front, for a bigger unit you don’t need.
And too small?
An undersized system runs nearly non-stop on the coldest and hottest days and may not hold the setpoint. It’s less common, but it happens, usually after an addition or basement finish that the old system never accounted for.
How sizing should be done
A heat-loss calculation (for heating) and heat-gain calculation (for cooling) looks at the home itself: wall and attic insulation, windows and which way they face, air leakage, the number of floors, and the local design temperature. For new homes, the Ontario Building Code requires this calculation (CSA F280). For replacements it isn’t always required, but it’s the only way to know the right size.
The old unit’s size isn’t a reliable guide. Many existing furnaces were oversized to begin with, and most homes have had insulation, window or door upgrades since.
Where two-stage and variable equipment help
Two-stage and modulating furnaces, and variable-speed ACs and heat pumps, run at low output most of the time and only ramp up when needed. They’re much more forgiving of sizing and keep the house more even.
When we quote a replacement, we size it for your house, not your old equipment. Estimates for new installations are free. See our heating and cooling services, or contact us at 647-657-2977. We’re based in Vaughan.
