How to Switch Your AC to Heat
Most of the switch is the thermostat: mode to Heat, fan on Auto, and a setpoint above the room. What you do next depends on whether the house has a furnace or a heat pump.
Switching from cooling to heat is a thermostat change. You do not flip a seasonal valve on a standard central air conditioner. Set the system mode to Heat, set the fan to Auto, and raise the setpoint a couple of degrees above the room temperature. Then wait. A furnace and a heat pump do not start the same way.
The thermostat steps
- Mode: Heat. Not Cool, not Off, and not Emergency or Aux unless you mean to use backup heat on purpose.
- Fan: Auto. On runs the blower all the time and can feel like weak heat even when the equipment is fine.
- Setpoint: above the current room temperature. If the setpoint is already below the room, the system is not being asked to heat.
- Hold: if a schedule keeps putting the setpoint back down, a hold is what keeps your new setting. That is a schedule feature, not a repair.
If the thermostat screen is blank, the switch is not the issue yet. Power to the thermostat is a separate problem: batteries on some models, or the furnace switch and the breaker on others.
Furnace versus heat pump
A gas furnace should light, the blower should come on after a short delay, and the supply air should turn warm. A heat pump moves heat. The outdoor unit runs in heating, and the supply air can feel cooler than furnace air even while it is still warming the house. Give it time before you decide it is broken. Emergency or auxiliary heat turns on electric strips or the furnace as backup. Leaving a heat pump in emergency heat is a common reason a winter electric bill jumps, because strips make heat instead of moving it.
Ice on a heat pump outdoor coil can be a normal defrost cycle. A solid block of ice that never clears, or a unit that never leaves auxiliary heat, is not a thermostat setting.
Checks that stay outside the cabinet
- Replace a dirty filter only if it is the user-serviceable one, with power off the way the manual describes. A packed filter can cut airflow enough that a furnace overheats and shuts down, or a heat pump cannot deliver the air you feel at the register.
- Make sure supply and return vents are open. Closing a lot of them does not make the remaining rooms stronger in a useful way.
- Look at the outdoor heat pump. Leaves packed into the coil, or something blocking the fan, will not show up as a thermostat error.
Do not open the furnace burner compartment, jumper any safety, or hose down a heat pump that is iced into a block. If the house smells like gas, leave and call the gas utility from outside. If a carbon monoxide alarm is going off, get everyone out and call emergency services.
If the mode is right, the filter is clean, and the house still does not heat, the next step is a diagnosis of the system you actually have. Heating repairs in Vancouver are scheduled from heating repair, and heat pumps from heat pump repair.
Common questions
Why does the air feel cool after I switch to heat?
A heat pump's supply air is often cooler than furnace air. Cool air that never warms the house, or air that stays at room temperature, is a different problem than the first few minutes of a heat pump cycle.
Should I use Emergency Heat?
Only as a temporary backup when the heat pump cannot run. Leaving it on makes the house with electric resistance heat.
How long should I wait after changing the mode?
A few minutes for a normal start. A furnace blower is often delayed on purpose. If nothing at all runs after that, and the setpoint is above the room, it is time to look past the thermostat.
