Every winter we drive out to no-heat calls that turn out to be a fan setting or a tripped switch. We do not mind — it is a legitimate call and we would rather you rang — but you would probably rather have the ten minutes and the money.
Work through these first. If none of them fixes it, the problem is genuinely ours and you will have given us useful information for the phone call.
1. The thermostat fan setting
If the fan is set to ON rather than AUTO, the blower runs continuously — including between heating cycles, when it is pushing unheated air around. That air feels cold because it is room temperature and moving.
Set it to AUTO. If the air is now hot when the furnace runs, nothing was ever wrong.
This is comfortably the most common one, and it happens most often after someone has been in the thermostat menu for another reason.
2. Thermostat batteries and mode
Sounds insulting; is not. A thermostat with weak batteries can display normally and still fail to call for heat reliably.
Check the mode is on HEAT, not COOL or OFF. Check the setpoint is actually above the current room temperature — a programmable schedule that ran out or reset can leave it lower than you assume. If it takes batteries, replace them.
3. The filter
A clogged filter chokes airflow. On many furnaces that trips a high-limit safety switch: the burner fires, overheats because heat is not being carried away, shuts the burner down, and leaves the blower running. The result is exactly what you would describe as “blowing cold air”.
Pull the filter out and hold it up to a light. If you cannot see light through it, that is your answer. Replace it, give the system twenty minutes, and see whether normal operation returns.
Filters are also the single cheapest thing you can do for the life of the equipment. Our maintenance plans include four a year for exactly this reason.
4. The furnace switch and the breaker
There is usually a switch that looks like an ordinary light switch on or near the furnace, sometimes at the top of the basement stairs. It gets turned off by mistake — during cleaning, by children, by anyone assuming it is a light.
Check it. Then check the breaker panel for a tripped breaker on the furnace circuit.
5. The gas supply
If you have other gas appliances, check whether they work. If the stove lights, gas is coming into the house and the problem is at the furnace. If nothing gas-fired works, the issue is upstream and it is a FortisBC matter, not an HVAC one.
Also check the gas valve at the furnace has not been closed.
6. The condensate drain
High-efficiency furnaces produce condensate and drain it away. If the drain or its trap blocks, a float switch shuts the furnace down to prevent water damage — again leaving you with a blower and no heat.
Look for standing water around the furnace base or a full condensate pan. In coastal conditions these block more often than people expect, which we cover in what coastal damp does to HVAC equipment.
When to stop and call
Stop immediately and call — us or anyone — if:
- You smell gas. Leave the house first, then call FortisBC’s emergency line from outside. Do not troubleshoot.
- There is a burning or electrical smell. Turn the furnace off at the switch.
- The unit is short cycling, firing and shutting off repeatedly. That is a fault, not a setting.
- Your carbon monoxide alarm has sounded. Out of the house, then emergency services. This is not negotiable and it is not a topic for a checklist.
If you do not have a working CO alarm near the sleeping areas, get one this week. It is the cheapest safety equipment in the house by a wide margin.
What to tell us if you do call
You will get a faster diagnosis if you can say:
- What the thermostat is set to, and whether the display is on
- Whether the burner is firing at all — many furnaces have a small sight glass
- Any flashing error code on the control board (count the flashes; the legend is usually printed inside the access panel)
- How old the furnace is, if you know
- Whether it is intermittent or completely dead
That is often enough for us to arrive with the right part on the van.
The uncomfortable one
If your furnace is past twenty years old and this is the third fault this season, the checklist is not really the question. We go through the arithmetic we actually use — repair cost against remaining life against what a replacement changes — in repair or replace.
Otherwise, we do furnace repair across the Lower Mainland, and during heating season we prioritise no-heat calls. Phone early in the day for the best chance of same-day attendance. The rest of what we do on heating is there if you want to read around it first.