These checks are safe for anyone. None involve opening the burner compartment or touching gas or refrigerant. If you smell gas at any point, stop, leave the house, and call your gas utility from outside.
1. The thermostat
- Is it set to HEAT (or COOL), not OFF or FAN?
- Is the set temperature at least 3 degrees above the room temperature for heat, or below for cooling?
- Is the screen blank or dim? Replace the batteries, even if it seems to have power. Many thermostats lose their connection to the system before the screen fades.
- Is a schedule or “away” mode holding the temperature?
2. The breakers
Your furnace or air handler has its own breaker, and a central air conditioner has a second, larger one for the outdoor unit. A tripped breaker sits in the middle position. Push it fully off, then on. If it trips again, leave it off and call.
3. The furnace service switch
Near the furnace, usually on a wall or the side of the unit, is a switch that looks like a light switch. It is easy to flip by accident while reaching for something on a shelf. Make sure it is on.
4. The filter
A badly clogged filter can make a furnace overheat and shut down on its safety limit, or make an air conditioner freeze up. Pull it and look. See our filter guide for sizes and ratings.
5. The condensate drain
High-efficiency furnaces and air conditioners produce water that drains through a small tube or pipe. If the line clogs, a float switch shuts the system off to prevent a flood. Look for a full drain pan, water around the unit, or a small box on the drain line. Clearing the line with a wet-dry vacuum from the outside end often fixes it.
6. The vents
Closed or blocked supply registers and a return grille covered by furniture can starve the system of air. Most houses should have nearly all registers open; closing more than a few can strain the blower.
Still nothing?
Write down what you checked and what you saw, including any blinking light codes on the furnace control board (visible through a small window on many units). It helps the technician and shortens the visit.
Common questions
Should I restart the system by cutting power?
Turning the service switch off for a minute and back on resets many furnace control boards. If it immediately fails again, do not keep cycling it.
How often should the system be serviced?
Once a year for the furnace before winter and once for the AC before summer, or a combined visit for a heat pump.
Our guides are general information for typical homes. Codes, permits and conditions vary, so check local rules, and leave gas, main electrical panel and structural work to a licensed professional.
