· Michael Blair
A heat pump in heating mode pulls heat out of the outdoor air, and to do that the outdoor coil runs colder than the air around it. On a damp Middle Tennessee morning that coil frosts over. The defrost cycle clears it, and from inside the house that cycle looks and sounds alarming the first time you notice it.
What defrost actually does
When the outdoor coil ices up, the system briefly switches itself into cooling mode, sending hot refrigerant to the outdoor coil to melt the frost. The outdoor fan stops, and the melting ice turns to a cloud of steam that can look like smoke. Inside, the air coming from the registers goes cool for the few minutes it runs, which is why most systems fire the backup heat strips during defrost to cover the gap.
The whole thing lasts a few minutes and ends with a whoosh or hiss as the reversing valve switches back. On a humid morning in the thirties and forties it may happen every hour or so. That frequency is normal for our climate, where winter air carries far more moisture than in a dry cold region.
Normal versus not normal
Normal: a coating of white frost on the outdoor coil that clears during defrost, a steam cloud while it runs, cool supply air for a few minutes, water and thin ice on the pad beneath the unit afterward.
Not normal: a solid block of ice encasing the coil that never clears, ice building on the top or the fan blades, a defrost that runs constantly or never runs, or supply air that stays cool for much longer than a few minutes. Those point at a failed defrost control or sensor, a low refrigerant charge, an outdoor fan problem, or a coil packed with debris that has stopped air moving through it.
- Light frost that clears: normal
- Solid ice that stays: a problem
- Steam during defrost: normal
- Ice on the fan or the top of the unit: a problem
Where the water goes
Every defrost cycle melts ice that has to drain out of the base of the unit. If the pad has settled so it tilts, or leaves have blocked the drain holes, that water refreezes in the base and builds up until it reaches the fan. A unit sitting in a low spot that collects runoff has the same problem.
Check that the unit is level, raised off the ground on its pad, and clear of leaf litter and mulch at the base. Gutters that drip onto the outdoor unit are a common cause of ice buildup that has nothing to do with the refrigeration circuit at all.
Auxiliary heat and the bill
The backup heat strips that cover the defrost gap are electric resistance heat, the most expensive way to warm a house. They are meant to run briefly. If the thermostat shows aux or emergency heat for long stretches, or the winter bill jumps, the heat pump may not be keeping up on its own and is leaning on the strips.
That can be a thermostat set to raise the temperature too quickly, a system losing capacity, or simply a very cold snap. Set the thermostat and leave it rather than turning it up and down, because every large setpoint jump calls the strips. If aux heat is on most of the time in ordinary winter weather, have the system checked.