Why Your Contractor's Heat Pump Size Recommendation Might Be Wrong

And what to ask before you sign anything

A homeowner in Ottawa recently shared something in a Facebook group that stopped me cold. His heat pump calculator results came back at 6 to 9 tons. His HVAC contractor told him a 3-ton unit might already be too big for summer humidity control.

Both numbers came from people who know what they're doing. So how can they be that far apart?

Here's what's actually happening.

Heating Load and Cooling Load Are Not the Same Number

Most contractors in Canada size heat pumps for the heating load — how much BTU you need to keep the house warm on the coldest design day. In Ottawa, that's a serious number. Old house, original windows, uninsulated walls — you could be looking at 60,000 BTU or more.

But your cooling load — what you need to keep the house comfortable on the hottest summer day — might be a fraction of that. 15,000 to 20,000 BTU in a 1,600 square foot bungalow is not unusual.

If your contractor sizes for the heating load and ignores the cooling load, you end up with a unit that's two or three times too large for summer. That unit short-cycles — it hits temperature and shuts off before it's had time to pull humidity out of the air. Your house feels clammy. You run it more. You wonder why your hydro bill is high.

The Duct System Makes It Worse

Gas furnace duct systems were designed around gas furnace airflow. Heat pumps move more air at lower temperatures. Stuff a large heat pump into a duct system built for a smaller furnace and you've got pressure problems, noise, and uneven distribution on top of the oversizing issue.

A qualified contractor sizes the heat pump to what the duct system can actually handle — not just what the heating load demands.

What to Ask Before You Sign

• Did you run a CSA F280 load calculation, or is this based on square footage?
• What is the cooling load separately from the heating load?
• Which load is constraining the equipment size?
• Can my existing duct system handle this unit's airflow requirements?
• If the heating load is larger than the cooling load allows, are we looking at a hybrid system?

If your contractor can't answer those questions clearly, get a second opinion.