Here is the mechanism, because it explains almost everything else about these systems. A furnace creates heat by burning fuel or running electricity through resistance coils. A heat pump does neither. It uses refrigerant to absorb heat from one place and release it in another, exactly like your air conditioner does. The difference is that a heat pump can run that cycle in either direction. In summer it pulls heat out of your house and dumps it outside. In winter it pulls heat out of the outdoor air, even air that feels cold to you, and brings it inside.

Moving heat takes far less energy than making it, which is why heat pumps dominate in mild climates. Northwest Florida winters sit right in the range where a heat pump does its best work, so the same equipment cools you all summer and heats you through the short cool season without a separate furnace in the picture at all.

The part that gets misunderstood is the auxiliary heat. Most heat pumps here include electric strip heat as a backup for genuinely cold mornings and for defrost cycles. Strips are expensive to run. When a heat pump underperforms, the system quietly leans on those strips instead, the house stays warm, nothing seems wrong, and the power bill is the only evidence that anything is off.

The reversing valve is the component unique to heat pumps and the one most people have never heard of. It is a valve that redirects refrigerant flow to switch the system between heating and cooling. When it sticks partway or fails to shift, the classic symptom is a system that works perfectly in one mode and poorly or not at all in the other. Because our heating season is so short, a reversing valve that failed in spring often is not discovered until December.

Defrost is the second area, and it is normal operation that people frequently mistake for a fault. In heating mode the outdoor coil gets colder than the outside air, so on cool damp mornings frost forms on it. The system periodically reverses itself briefly to melt that frost off, which is why you see steam rolling off an outdoor unit in winter and hear the system change its tone. That is correct behavior. The problem is when defrost does not initiate, so ice builds up and blocks airflow, or when it initiates constantly and the system spends its time defrosting rather than heating. Both usually trace to a sensor or a control board.

Then there is auxiliary heat staging. Strip heat should come on only when the heat pump genuinely cannot keep up or during defrost. If low refrigerant charge, a dirty coil, or a thermostat configured wrong causes the system to call for strips constantly, you get a warm house and a shocking bill. This is one of the most common things we find and one of the least noticed, because nothing about it feels like a malfunction.