heat pump defrost control board - or something else?
I live in north central Florida, where it does get below freezing from time to time, but our HVAC systems are optimized for cooling. I have a Payne heat pump system that wasn't heating when the outside temps got very cold (below freezing), but would heat at more mild outdoor temps and didn't have a problem cooling. We also noticed that the outdoor unit had frost over it.
I called a local HVAC company and they diagnosed the problem as a faulty defrost control board. They quoted us at $870 to install a new one, but I found the part online for $200, so we went ahead and installed it ourselves. The problem is that this didn't fix the problem. The heat still doesn't work below freezing and the outside unit frosts up when it gets cold (see photo). For a variety of reasons, I don't have a lot of confidence in the HVAC tech.
My question is: is the problem that the defrost control board wasn't the issue in the first place (perhaps it was a temp sensor?), or is the problem that we didn't do the install correctly? To install the new board, we simply removed the wires from the old one and then plugged them into the new one; is there something else we should have done?
Posted Mar 8, 2026, 6:06 PM UTC