Switching from cooling to heating is a two-second change on most thermostats: flip the mode switch from Cool to Heat, set a setpoint 3 degrees F above current room temperature, and the first heating cycle runs. That’s 90% of it. The rest is verifying it works.
The basic changeover
- Set mode to Heat (not Auto).
- Set setpoint 3-5 degrees above current room temp. If the room is 68 degrees, set 72 degrees. The thermostat needs a real demand before the heat will kick on.
- Wait 5-10 minutes. A heat pump may take a minute to reverse; a gas furnace lights within seconds.
- Verify warm air at the registers. Gas furnace: 90-110 degrees F supply air. Heat pump: 90-100 degrees F in mild weather (heat pumps blow cooler than gas at moderate outdoor temps, which is normal).
If you get no heat after 10 minutes, check the O/B setting below.
About the smell
The first heating cycle of the season will smell like something is burning. This is normal: dust settled on heat exchangers and electric resistance elements burns off in the first cycle. It clears in 15-30 minutes.
Stop and call a technician if: the smell persists past 30 minutes, you see smoke, or you smell burning plastic or sharp electrical odor. That is not dust.
Heat pump O/B reversing valve: the fall changeover gotcha
Heat pumps use a reversing valve to switch refrigerant flow between heating and cooling. The thermostat controls it via the O/B wire.
- Most systems (Carrier, Trane, Lennox): O/B energizes in Cool mode (called “O”).
- Some systems (Rheem, Ruud, older Bryant): O/B energizes in Heat mode (called “B”).
If this is configured backwards, the heat pump blows cold air in Heat mode. If you replaced a thermostat this year and now get cold air on a Heat call, check the O/B setting in the installer menu. Full guide at What is the O/B Wire on a Thermostat.
Auto mode vs Heat mode
Auto switches between heating and cooling based on setpoint and can behave unexpectedly during fall transition weeks (cooling on a warm afternoon, heating at night). Use manual Heat mode until the shoulder season is over.
Setting a fall schedule
Once heat is confirmed working:
- Home/awake: 68-70 degrees F. The DOE winter recommendation is 68 degrees.
- Away/asleep: 60-65 degrees F setback. Heat pumps handle this well on mild days; setback recovery is slower at very low outdoor temps, which is expected.
- Balance point: if outdoor temp drops below roughly 25-35 degrees F, auxiliary heat activates automatically. This is correct. See Aux Heat vs Emergency Heat.
If you’re still on central AC only
Fall changeover is a natural moment to ask whether adding a heat pump makes sense. A heat pump handles both cooling and heating, and modern cold-climate models maintain full output down to 5 degrees F or below.
Before comparing installation quotes, know this: every heat pump requires a dedicated 240V circuit with a 40-50A breaker. If your current panel is 100A, it likely needs an upgrade to 200A, which adds real cost. That’s exactly the kind of line item some contractors quote and others quietly omit for the same job scope, making bottom-line price comparisons misleading. Use BidSmart to compare quotes and check whether electrical scope is itemized consistently across bids. Fall and spring are the windows when contractors have capacity; shopping now avoids the spring rush lead times.
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Related: Aux Heat vs Emergency Heat · What is the O/B Wire · Best Thermostat Settings for Summer
