It is the first genuinely cold night of the year, the house is 61 degrees and dropping, and the furnace is doing nothing. Before you call anybody — including us — there are five things worth checking. They take about ten minutes total, and in our experience a good share of no-heat calls end at one of them.
None of this involves opening the furnace cabinet or touching gas or high-voltage wiring. If a step here asks you to do that, stop and call. Everything below is checkable from the outside.
1. The switch that looks like a light switch
There is almost always an ordinary-looking toggle switch on or near the furnace, frequently at the top of the basement stairs. It controls power to the unit. It also gets flipped by people carrying laundry, by kids, and by contractors who were down there for something unrelated three weeks ago.
Flip it on. Wait a full minute — most modern furnaces run a self-check before they will fire. If nothing happens, flip it off and back on once more, then move on.
2. The thermostat, and specifically its batteries
Confirm the thermostat is set to Heat and not Auto-changeover or Off, and that the setpoint is meaningfully above the current room temperature — bump it up five degrees, not one.
Then check the batteries. A dying battery in a digital thermostat produces a display that still looks alive but cannot close the circuit that calls for heat. If yours takes AAs, swap them. It costs two dollars and rules out a surprisingly common failure.
3. The filter
A filter that has not been changed since spring can restrict airflow badly enough that the furnace overheats and shuts itself down on a safety limit. It will often run for a few minutes, quit, and try again — a cycle that looks like a control problem and is really just a clogged filter.
Pull it out and hold it up to a light. If you cannot see light through it, replace it. A standard disposable filter should be changed monthly during the heating season.
4. The breaker — both of them
Check the electrical panel for a tripped breaker labeled for the furnace or air handler. A tripped breaker is not always obviously in the middle; sometimes it looks nearly on. Push it firmly to Off, then back to On.
If the breaker trips again immediately, stop. That is an electrical fault, not a nuisance trip, and repeatedly resetting it is how a bad situation becomes a fire.
5. The condensate drain
High-efficiency furnaces — anything with PVC venting out a sidewall — produce condensate, and that water has to drain. When the drain line or the float switch that watches it clogs, the furnace locks out on purpose to avoid flooding your basement.
Look for standing water in or around the drain pan or a small inline switch on the drain tubing. If you see water where water should not be, that is very likely your answer, and it is a legitimate service call rather than a DIY fix.
When to stop and call
- You smell gas — leave the house first, then call the gas utility from outside.
- The breaker trips again as soon as you reset it.
- The furnace ignites and then shuts down repeatedly (short-cycling) after the filter has been replaced.
- You see soot, scorching, or rust around the burner compartment.
- The blower runs continuously but the air is never warm.
Any of those is a real diagnostic problem. Call us — dispatch at Lanz is live 24 hours a day, seven days a week, and there is no additional charge for nights, weekends, or holidays.
The version of this that never happens
Every item on this list except the thermostat battery is something a fall precision maintenance visit would have caught. The filter gets changed, the drain gets cleared, the safeties get tested, and the burner gets cleaned while it is 70 degrees outside and nobody is stressed.
That is the whole argument for scheduled maintenance: not that it prevents every failure, but that it moves the failures from January to October.