What size air conditioner does your Surrey home actually need?

Ask three contractors what size air conditioner your house needs and you might get three different answers. One squints at the square footage, one matches whatever the furnace label says, and one actually measures the house. Only the third one is doing it right, and the difference shows up every hot afternoon for the next 15 years.

Why bigger is not better

It feels intuitive that a bigger unit cools better. It doesn't. An oversized air conditioner blasts the thermostat down to temperature quickly, then shuts off. That's called short-cycling, and it causes three problems. The equipment wears out faster from constant starting and stopping. The house never runs long enough to pull humidity out of the air, so it feels clammy at 23 degrees. And you paid more for the bigger unit in the first place.

An undersized unit has the opposite problem: it runs flat out on the hottest days and never quite catches up. The sweet spot is a system that runs long, steady cycles and holds the house exactly where you set it.

What a real load calculation looks at

Proper sizing is a heat-load calculation, and it accounts for the specific house, not a rule of thumb. The inputs that matter most:

• Floor area and ceiling heights, room by room.
• Window area and which way it faces. A wall of west glass in South Surrey can add more load than an entire extra bedroom.
• Insulation levels in the attic and walls, which vary hugely between a 1975 Newton two-storey and a 2018 Clayton build.
• Air leakage, doors, and how the house is shaded.
• The ductwork. If the ducts can't move enough air, the biggest condenser in the world won't help.

The Surrey wrinkle: our homes weren't built for heat

Most Lower Mainland housing was designed to hold heat in, not keep it out. Big south and west windows, dark roofs, minimal shading. That's great in January and brutal in a July heat wave, especially in the inland parts of Surrey that run hotter than the coast. It's also why two houses with the same floor plan can need different systems depending on orientation and shading.

What about the ducts?

Cooling needs more airflow than heating, and a duct system that was fine for a furnace is often too small for AC. Part of honest sizing is checking whether the returns and supply runs can actually deliver the air. Sometimes the right answer is a slightly smaller unit plus a duct fix, and it outperforms the bigger unit every time.