Work from the wall to the furnace
A furnace needs three things before it will do anything: power, a call for heat from the thermostat, and every safety switch closed. Silence means one of those is missing.
- The thermostat. A blank screen is usually batteries. Check the mode is Heat and the set point is above the room temperature. A thermostat that has lost its settings after an outage may be back on a default schedule.
- The furnace switch. An ordinary-looking light switch on or beside the furnace. In a garage or a crawl space it is easy to knock off.
- The breaker. Labeled furnace or air handler. Reset a tripped breaker once. If it trips again, leave it off.
- The blower door. Most furnaces have a safety switch that cuts all power when the lower panel is off or not seated. If someone has had the panel off to change the filter, make sure it is pushed fully home.
The safety that stops a high-efficiency furnace
A high-efficiency furnace makes water from its own exhaust, and that water drains away through a trap and a hose, sometimes into a small pump. When the drain blocks, a safety switch stops the furnace. If there is water on the floor by the furnace and it will not start, that is very likely the reason. The high-efficiency furnace page explains.
When it is none of those
If the thermostat is calling, the breaker holds, the switch is on and the door is seated, the fault is inside: the transformer that powers the controls, a blown low-voltage fuse on the control board (itself usually the sign of a short somewhere), a broken thermostat wire, or the control board itself. Those are tested with a meter, with the power isolated.
What to tell us when you call
- Whether anything at all happens when you raise the set point
- Whether the thermostat screen is lit
- Whether a light is blinking in the furnace's inspection window, and how
- Whether there is water on the floor near the furnace
- Whether the furnace is gas or electric, if you know
If you can hear the furnace start but no heat arrives, the not heating page is the better place to start.
No heat? Tell us what the furnace is doing.
Gas, electric and high-efficiency furnaces across Caldwell, Nampa, Middleton and Parma. Start with the symptom and your ZIP code.
You'll be asked for your ZIP code first, so your call reaches a technician who covers your area.