Defrost is a short cycle, usually a few minutes, when your heat pump reverses to melt frost off its outdoor coil. Auxiliary heat is backup, from electric strips or a furnace, that kicks in when the heat pump alone falls short, most often near DC’s 14°F design temperature. At Perennial DC Heat Pumps we install ducted heat pumps across Washington, DC, and we walk you through both at handover.
Why does your outdoor unit stop and steam in winter?
It’s clearing frost. In heating mode the outdoor coil runs colder than the outside air so it can pull heat from it. When the coil drops below 32°F and the air is damp, water vapor freezes on the fins. Frost blocks airflow and cuts output, so the control clears it every so often.
Frost builds fastest in the high 20s to high 30s °F with damp air, a common DC winter day, rather than on the coldest dry ones. During defrost the reversing valve briefly switches the system into cooling mode, sending hot refrigerant to the outdoor coil. The outdoor fan stops, you may hear a hiss or whoosh as the valve shifts, and a plume of vapor can rise.
That meltwater has to drain away from the unit. The National Weather Service puts DC’s average seasonal snowfall at about 13.8 inches, so on our cold-climate service page we set stand height and drainage for your site from the manufacturer’s instructions, not from one fixed number.
Why does your thermostat say “aux heat”?
Auxiliary heat runs for three main reasons. During defrost your indoor coil is briefly cooling, so backup warms the supply air to spare you a cold blast. During a big recovery, such as raising the setpoint several degrees after a setback, the control may call backup to get there faster. And below the balance point, your house loses heat faster than the heat pump can supply it.
Electric strips and a kept furnace behave differently. Strips have a COP of 1, so each kWh becomes 3,412 Btu; a dual-fuel system burns gas instead. Our electrical guide explains why a 10 kW strip, drawing about 42 amps at 240 volts, can drive your circuit and service work.
Is it normal, or does it need a service call?
A normal defrost lasts minutes, ends with the coil clear and the fan restarting, and may come back every half hour to a couple of hours in frosty, damp weather, depending on the control. Ice that keeps thickening across cycles, a coil buried in solid ice, rooms losing temperature for hours, or water pooling where people walk all need a technician.
Don’t chip ice, pour hot water on the coil, reach through the fan guard or open the electrical panel. We install and replace heat pumps; we don’t dispatch repairs. A hazardous condition goes to your equipment’s service provider or to emergency services, not to an estimate request.
Put these questions into a project brief
Describe what you know about the property. Technical unknowns can stay unknown.
Why does it defrost more on some days than others?
Two days at the same temperature can look very different. At 34°F in freezing drizzle the coil frosts quickly and defrosts often; at 20°F with dry air it may hardly defrost at all. A neighbor’s unit on a different control, or in a different spot, can behave differently without either one being faulty.
Setbacks matter too. A deep overnight setback followed by a quick morning recovery can bring on resistance heat, which costs about 2.5 times as much per unit of heat as a heat pump running at a COP of 2.5. Many heat pump thermostats offer smart recovery or an outdoor-temperature lockout for backup; our thermostat guide explains which settings are yours and which we set at installation.
What should you write down before you call anyone?
Note the date and time, the outdoor temperature, the thermostat mode and setpoint, whether the display showed auxiliary or emergency heat, and how long it lasted. A photo taken from where you normally stand can show frost or drainage without going near the fan.
Our first-week article gives a simple log format. Change one thing at a time. If winter performance is why you want a replacement, bring us the log: long stretches of backup on mild days point us to a capacity or control problem, not just an old outdoor unit.
What should we hand you when the job is done?
At handover we give you the installed model pair, the backup type and size in kW or Btu/h, the backup and heat pump lockout temperatures if we use them, and how defrost is controlled. Those settings explain most of what your thermostat will show in January, which a quick warm-air demonstration on install day can’t.
Our sizing guide explains the balance point that decides how often backup runs. Use our quote checklist to get these settings in writing before you buy, and our running-cost guide to price backup hours on your own bill. Planning a new system? Request an estimate with your current setup and the winter problems that worry you.
Questions before you decide
Does steam-like vapor during defrost always indicate trouble?
No. During defrost the outdoor coil warms quickly and melts its frost, and the meltwater can flash into a visible plume of vapor for a minute or two. What needs a service call is ice that keeps building between cycles.
Can I remove ice with a tool?
No. The coil's aluminum fins bend and puncture easily, and the fan can start without warning. Leave the ice to the defrost cycle, note how long it stays, and have a qualified technician look at ice that keeps building or stops the unit heating.
Should auxiliary heat always be disabled to save energy?
No. Auxiliary heat warms the supply air during defrost and covers the gap between your heat pump's output and your home's heat loss on the coldest days. Locking it out can leave you short of heat near 14°F; we treat the lockout temperature as a design setting, not an owner shortcut.