No heat at all: what to check before you call.

The three that cost nothing

Do these in order. They resolve a surprising share of no-heat calls.

• Thermostat — is it set to HEAT, and is the target temperature above the room temperature? Replace the batteries if it takes them.
• The furnace switch — the light-switch-looking one on or near the unit. If someone has been in the basement, it gets knocked.
• The breaker — check the panel. A tripped breaker needs to be pushed fully OFF before it will go back ON.

Then check the filter and the vents

A badly clogged filter can shut a furnace down on its safety limit. Replace it if light will not pass through.

Walk the house and make sure supply vents are open and not blocked by furniture. Starving the return air does the same thing a dirty filter does.

If you have a high-efficiency furnace

High-efficiency units produce condensation and drain it away. When that drain clogs, a float switch cuts the furnace off deliberately to stop water backing up into the unit. Water around the base of the furnace is the tell.

The furnace also draws air and vents exhaust through PVC pipes that usually exit the side of the house. After snow or heavy leaves, those can block. Clearing the outside end is something you can safely do yourself.

Protect the pipes while you wait

If the house is getting genuinely cold and the repair is hours away, open the cabinet doors under sinks on exterior walls and let faucets drip. A burst pipe turns a repair bill into a renovation.

What to tell whoever you call

Tell them what you already checked, whether the blower runs at all, whether the burners light, and whether there is water near the unit. It is the difference between a diagnostic visit and a technician arriving with the right part on the truck.