How to choose a gas furnace in BC: staging, AFUE, blowers and sizing explained

Canada's Energy Efficiency Regulations require residential gas furnaces manufactured on or after July 3, 2019 to have an AFUE of at least 95% (with narrow exceptions for through-the-wall units with integrated cooling and furnaces for manufactured homes). In practice every furnace you will be offered condenses flue gas, needs plastic venting and a drain, and sits between 95% and 98.5% AFUE.

The difference between 95% and 97% is about two dollars of gas in every hundred. It is real but it will not by itself repay a large price premium. Where the higher-tier furnaces earn their money is in staging and blower control, not in the last two points of AFUE.

AFUE is also position dependent on many furnaces. Carrier's Performance 96, for example, is certified at 95.0% to 96.7% upflow depending on size, and 95.0% in every size when installed downflow or horizontal. Ask which figure applies to your installation.

Single-stage furnaces run at full fire every cycle. They are the least expensive and, correctly sized, perfectly adequate in a small or well-insulated home. Their weakness is that on a mild day full fire satisfies the thermostat quickly and shuts off, so temperatures swing and far rooms lag.

Two-stage furnaces have a low fire (typically around 65% of full input) and a high fire. The control runs low fire most of the time and steps up only when the house is losing heat faster than low fire can replace it. Cycles are longer, quieter and steadier. Carrier's 59TP6C, for instance, runs 39,000 BTU/h on low and 60,000 BTU/h on high in the 060 size.

Modulating furnaces vary the gas rate continuously, often down to 40% or less of full input, and match output to the house almost exactly. They are the most comfortable and the most expensive, and they depend on a compatible thermostat and careful set-up.

The blower moves air through the ducts and is chosen independently of the burner. A PSC motor runs at fixed speeds and uses the most electricity. A constant-torque ECM (Carrier's "variable 25-speed" motor on the Performance 96 is one) is more efficient and quieter with several selectable speeds. A constant-airflow ECM measures and holds a target airflow as filters load or dampers move, and is standard on top-tier furnaces.

A variable-speed blower does not make a furnace modulating. Sales copy often blurs the two. Check the staging and the blower type separately on the manufacturer's product data.

The right furnace output is the house's design heat loss on the coldest expected day, calculated room by room from insulation, windows, air leakage and orientation. Square footage rules of thumb overshoot badly in newer or upgraded homes. Carrier's product data for the Performance 96 warns that excessive oversizing may cause the furnace or vent to fail prematurely and can affect warranty coverage, and that none of its sizes should be used where the load is 20,000 BTU/h or lower.

Ask the contractor how the size was determined. "Same as the old one" is not an answer; the old furnace was probably oversized too.

Because every new furnace condenses, the quote must cover plastic venting to an approved termination, combustion-air intake, a condensate drain or pump, and any chimney work for a water heater that used to share a chimney with the old furnace. In Burnaby and several other Lower Mainland municipalities the gas permit comes from the city; elsewhere in BC from Technical Safety BC. The contractor pulls it.

Ducts limit what the furnace can do. A furnace that needs 1,400 CFM on high fire will trip its limit on ducts that can only pass 900 CFM, however good the furnace is. An assessment should include static-pressure measurement, not just a look at the plenum.

A gas furnace can share its blower and ducts with a heat pump in a dual-fuel arrangement: the heat pump heats in mild weather and the furnace takes over when it is cold. Whether this makes sense depends on electricity and gas prices, the house and the electrical service. Rebates for heat pumps do not apply to gas furnaces; this site only lists rebate or financing programs once they are verified for the specific equipment.