Under normal circumstances, your HVAC system is designed to be an “either-or” operation. If both systems are active simultaneously, they are fighting each other, wasting an enormous amount of energy, and placing dangerous stress on your equipment. If your heat is going on at the same time as your AC, it is almost always an electrical communication issue. Here is how to identify the problem and what you should do to protect your system.
Immediate Action Step
If you verify that both the outdoor condenser and the furnace burners (or heat strips) are running at once, turn the system off at the thermostat immediately. Running both simultaneously can break your AC’s compressor or make your furnace overheat, both of which are expensive repairs.
3 Reasons Your Furnace Heats When the AC Is On
When your system’s heat and AC are running at the same time, it’s usually because of a breakdown in the communication line between your thermostat and your equipment. Since 1969, our technicians have seen this “cross-talk” happen for three main reasons:
Faulty Thermostat Wiring
If you recently installed a new smart thermostat, the wiring is the first place to look. If the Y (Cooling) and W (Heating) wires are touching or are accidentally jumped together on the back of the thermostat, the system will send power to both units whenever it calls for air.
A “Stuck” Relay on the Control Board
Think of your control board as the “brain” of your HVAC system. It uses small switches called relays to send power to the right place. Over time, these relays can physically “weld” shut due to electrical surges. If the heat relay is stuck in the closed position, your furnace will stay on even when the thermostat is requesting air conditioning.
Shorted Thermostat Cable
Your thermostat is connected to your furnace by a multi-colored cable, usually hidden behind your walls. If a nail was driven through the wall or a rodent chewed on the wire, the heating and cooling wires can “short” together. This creates a bridge that activates the furnace every time the AC kicks on.
How Allied Handles AC and Furnace Running at the Same Time
When you call Allied for an air conditioning repair, we use the logic of the system to trace the electrical path. If a $20 wire repair is all you need, that’s exactly what we’ll tell you.
We aren’t owned by a private equity firm, so our technicians don’t have sales quotas. Our only goal is to ensure your home is comfortable and your equipment is running efficiently. If your system is trying to heat and cool at the same time, contact us. We’ll clear up the communication lines and get your home back to a single, consistent temperature.
Common Questions About Furnace and AC Running at the Same Time
Is it safe to switch between heat and AC on the same day?
Yes, it is safe to switch between heat and AC on the same day–provided you give your system a 5-minute breather. Most modern digital thermostats have a built-in delay to protect the compressor. When you flip the switch, the refrigerant pressures need a few minutes to equalize. If you force the system to switch instantly, you risk “short-cycling” the compressor, which is a leading cause of premature equipment failure.
Does the furnace control the AC?
No, the furnace does not control the AC, but they share the same blower fan. If the fan is running, it doesn’t always mean the furnace is “on.”
Why is my furnace fan always running?
Check your thermostat. If the fan setting is on “ON” instead of “AUTO,” the blower will run 24/7 regardless of the temperature.
Can a power surge cause my heat to go on at the same time as my A/C?
Yes, a power surge can cause your heat to go on at the same time as your A/C. A surge can scramble a thermostat’s logic or fry a relay on the furnace control board.
