The furnace will not start. What to check first.

Check four things before you call anyone: the thermostat batteries, the breaker for the furnace, the switch on or beside the unit that looks like an ordinary light switch, and the filter. Between them they account for a large share of no heat service calls, and all four take ten minutes and no tools.

The four, in order

1. The thermostat. Replace the batteries even if the display looks fine, set it to heat, and set the target several degrees above the current room temperature so you know it is calling. If it is a smart thermostat, check it has not lost its wiring power or its schedule.

2. The breaker. Gas furnaces still need electricity for the controls, the igniter, and the blower. Find the breaker labeled for the furnace or the air handler, push it fully off, then on.

3. The switch that looks like a light switch. There is usually a service switch on or next to the unit, and often at the top of the basement stairs. It is the single most embarrassing cause of no heat and it gets switched off by decorators, cleaners, and children constantly.

4. The filter. Pull it out and hold it up to a light. If you cannot see light through it, that is your answer. A blocked filter starves the system of airflow, which makes it overheat and shut down on a safety limit, and it will do it again ten minutes after you reset it.

Then the next few

The condensate line, on a high efficiency furnace. These produce condensate, and a blocked trap or a frozen external pipe trips a float switch that stops the unit dead. A frozen condensate pipe in a cold snap is extremely common, and warm cloths on the external section sometimes fix it in minutes.

The door panel switch. Most furnaces will not run with the front panel off, and a panel that is not seated properly holds that switch open.

The gas supply. If other gas appliances have nothing either, the problem is upstream. If the meter has a valve that has been turned off, do not turn it back on yourself if you do not know why it was closed.

The vents and returns. Too many closed vents, or blocked returns behind furniture, will trip a system on airflow.

The flue or intake, on a unit that vents through a wall. Snow, ice, or a nest blocking either will stop it, and clearing it from outside is reasonable.

The error code. Most modern units have an LED that flashes a pattern, and the legend is usually printed inside the panel door. Write down the pattern before you reset anything, because a reset clears it.

What resetting does and does not do

There is often a reset button, and a system will sometimes restart and run. Understand what has happened: a safety device operated for a reason, and resetting clears the symptom rather than the cause. If it trips again, stop resetting it and call.

Repeatedly resetting a furnace that keeps locking out on a safety limit is the same mistake as repeatedly resetting a breaker that keeps tripping, and on a combustion appliance the stakes are higher.

When to stop and call

• Any smell of gas, at any point.
• Any carbon monoxide alarm activity.
• Soot or scorching around the unit.
• A cracked or rusted flue, or a flue that has come apart.
• Water pooling around the base that is not condensate you can explain.
• It starts and then shuts down repeatedly.
• The same fault after one reset.
• You do not know what you are looking at.

What to have ready when you call

• Make, model, and rough age of the unit.
• The error code or flashing pattern.
• What you have already checked.
• Whether it is gas, oil, electric, or a heat pump.
• Whether anyone in the house is vulnerable, and whether the house is at risk of freezing.

That last point is not a small thing: it moves you up most priority lists, and it is true.

While you wait

Protect the pipes and keep the people safe, and if you use a portable heater, use it properly.

The short version

Thermostat batteries, breaker, the service switch, the filter. Then the condensate line, the door panel, and the flue. Write down the error code before you reset. Stop at any smell of gas or any carbon monoxide alarm, and do not take a gas appliance apart.