A homeowner in Mahogany called us to do a second-opinion visit before a central AC installation. Another contractor had already quoted the job and was ready to book it. We came out, looked at the system, and found the furnace fan motor was too small to move adequate airflow for cooling.
The contractor who quoted the job hadn’t looked at the fan. He’d matched the condenser size to the house square footage and priced accordingly. The install would have technically worked. It would also have cooled poorly, cycled constantly, and worn out the compressor prematurely.
Installing central AC on an existing furnace means checking four things before any equipment gets ordered.
The Four Things That Have to Line Up
Fan capacity. Evaporator coil sizing. Refrigerant line routing. Electrical capacity. All four have to work together. One weak link undermines the whole system.
Most Calgary homes that already have forced-air heating have a furnace and existing ductwork. The furnace fan, the evaporator coil, and the outdoor condenser have to work as a matched system even though the furnace and condenser are separate pieces of equipment from separate manufacturers.
Evaporator Coil Sizing
The evaporator coil sits inside the furnace plenum, above the heat exchanger. Refrigerant passes through it, absorbs heat from the indoor air, and carries that heat to the outdoor condenser.
Coil sizing has to match the condenser capacity. A 2-ton condenser needs a properly sized 2-ton coil. Mismatches reduce efficiency and can cause icing or liquid refrigerant flooding back to the compressor.
The coil also has to physically fit the existing plenum. Plenums on older Calgary furnaces run various dimensions. Some accommodate standard coil casing sizes without modification. Others require a custom transition or a plenum replacement. Worth measuring before ordering anything.
The Furnace Fan: Where Things Go Wrong
This is the one that caught the Mahogany homeowner’s job.
Heating airflow and cooling airflow have different requirements. A furnace fan sized for heating a Calgary home in January may or may not have the capacity to move adequate airflow for cooling. The industry standard for cooling is roughly 400 to 500 CFM per ton of cooling capacity. A 2-ton AC needs 800 to 1,000 CFM. Some older furnace fans don’t reach that.
Undersized fan airflow produces high static pressure across the coil, which causes icing in humid conditions and reduces effective cooling capacity. The system runs but doesn’t perform.
Variable-speed furnace fans are the most flexible. They can be programmed for a higher airflow setting for cooling than for heating. Single-speed and two-speed furnace fans are less adaptable.
Fan capacity shows on the furnace spec sheet or the data plate on the unit. Matching it against the cooling system requirements is a quick check. Skipping it is how contractors get into trouble.
Electrical: What Usually Needs Upgrading
Central AC requires a dedicated 240-volt circuit for the outdoor condenser. The condenser draws somewhere between 15 and 45 amps depending on size. A dedicated breaker and properly sized wire from the panel to the disconnect box at the outdoor unit is required.
Homes in Calgary built before the mid-1990s often have 100-amp electrical service. A 100-amp panel can support a central AC, though it may be tight depending on what else is on the service. An electrician assessment makes sense before the HVAC installation is booked, not after.
100-amp to 200-amp panel upgrades run $1,500 to $3,000 in Calgary depending on the complexity of the run and the current panel location. That cost belongs in the total project budget.
Refrigerant Lines
Line sets run from the indoor unit (the coil) to the outdoor condenser. Pre-existing line sets from a previous AC installation may be reusable depending on their condition, diameter, and whether the refrigerant type has changed.
R-22 refrigerant was phased out in 2020. New equipment uses R-410A or R-454B. Line sets that ran R-22 refrigerant are not automatically compatible with new equipment. The internal diameter requirements differ between refrigerant types, and old line sets may have contaminants from the previous refrigerant. A technician can assess whether existing lines are reusable or need replacement.
New line sets run from the indoor unit through the wall to the outdoor condenser. Routing matters for cosmetics. Line sets that have to run across a finished exterior wall, through an attic, or around obstacles add installation time and cost.
The Mahogany job ended with a furnace fan upgrade, a properly matched coil, new line sets, and a panel assessment confirming the existing 200-amp service could support the load. Calgary Air Heating and Cooling Ltd checks all four elements before quoting any add-on AC installation. The goal is a system that performs as expected, not one that technically runs.
Calgary Air Heating and Cooling Ltd Contact Information
Address
95 Beaconsfield Rise NW, Calgary, AB T3K 1X3
Phone
+1 (403) 720-0003
Hours of operation
7 a.m.–11 p.m. (including weekends)
Website
https://calgaryair.ca/air-conditioning-repair-calgary/