Furnace Blowing Cold Air in Calgary? Check These 6 Things First

Cold air from the vents is actually good news compared with silence. It means the blower has power and the control board is alive. What has usually happened is that the burner is either never firing, or it fires and then shuts down while the fan keeps running.

Work through these in order. Stop as soon as one of them explains what you are seeing.

1. Is the thermostat actually calling for heat?

Set the mode to Heat, not Auto and not Cool, and set the fan to Auto rather than On. A fan left on On runs continuously and pushes room-temperature air through the ducts between burner cycles, which feels exactly like a broken furnace and is not one.

Then push the setpoint five degrees above the room temperature and wait. If the screen is blank, replace the batteries before you do anything else. A dead thermostat is the single most common no-heat call that turns out to be nothing.

2. Has the switch beside the furnace been turned off?

There is a switch on or near the furnace that looks exactly like a light switch, usually at the top of the basement stairs or on a joist beside the unit. It gets flipped off by accident constantly, most often by someone carrying something down the stairs or cleaning around the unit.

Turn it on. Give the furnace a few minutes. Many furnaces run a purge cycle before they will light again.

3. Has the breaker tripped?

Find the breaker labelled furnace in your panel. If it has tripped, push it fully off and then back on. Reset it once.

4. When did you last change the filter?

Hold the filter up to a light. If you cannot see light through it, it is choking the airflow. A furnace starved of return air overheats, the high limit switch cuts the burner to protect the heat exchanger, and the blower keeps running to cool things down. The result is exactly what you are feeling: a fan blowing cold.

In Calgary a one inch pleated filter is realistically a three month part, and closer to one month if you are in a new build with construction dust still settling or you have pets. Change it and run the furnace again.

5. Are the vent pipes outside blocked with snow?

A high-efficiency furnace vents through two white plastic pipes that come out the side of the house, usually a metre or so off the ground. If either one is buried by a drift, blocked by ice, or packed with snow from a plough or a roof slide, the pressure switch will not let the burner light.

Clear a wide space around both pipes with your hands or a broom rather than a metal shovel, and check that neither one has an ice collar formed inside the opening. This is the most Calgary-specific item on this list and it accounts for a real share of winter no-heat calls.

There is more on this in our piece on frozen furnace vents.

6. Does the burner light and then go out after a few seconds?

If you can watch the furnace and see it light and then shut down within about five to ten seconds, repeatedly, that pattern points at the flame sensor. It is a thin metal rod that confirms to the control board that a flame actually exists. When it films over with oxide the board reads no flame, shuts the gas off for safety, and tries again.

It is a genuinely small job and a common one. It is also not a homeowner fix, because it means opening the burner compartment on a live gas appliance.

When should you stop and call?

• The breaker trips a second time
• You smell gas at any point, in which case leave the building first and call 911 or ATCO Gas at 403-245-7222 from outside
• The furnace lights and drops out repeatedly
• You hear a repeated clicking with no ignition
• Everything on this list checks out and the air is still cold
• The house is below about 15C and falling

In a Calgary winter an unheated house can reach freezing pipe territory in well under a day, and a burst supply line costs far more than the repair that would have prevented it. If you are working through this list at night with the temperature dropping, make the call while you work.