DENVER GUIDE

What a heat pump is doing on a cold Denver night

Frost feathers the outdoor coil, the fan changes its sound, and a plume of vapor rolls off the cabinet. Indoors, the supply air may cool for a few minutes or a second heat source may kick on. That cluster is a heat pump meeting a cold night. It is easy to log as a breakdown before anyone has named defrost or backup heat.

By Mike Maziarz · Sources checked October 6, 2026 · How we write these guides

What the outdoor unit is doing

A heat pump outdoor unit can both heat and cool. In heating season it is gathering heat from outdoor air that may be well below comfortable room temperature. The fan runs. The coil gets cold enough to grow frost. None of that requires you to decide the compressor is failing. It requires you to say what you saw and when.

Write the time, the feel of the outdoor air, and whether the fan was turning. A unit that is silent when the thermostat is calling for heat is a different note from a unit that runs for a long stretch. A Central Park closet may pair that outdoor cabinet with an air handler you can actually walk around. An older house may tuck the indoor half into a basement corner. Name the room.

Day-to-night temperature swings make the same machine look busy. A mild afternoon and a sharp night are a normal Front Range pattern. The heat pump may coast in the afternoon and run hard after dark. That change of pace is a weather note. Check the National Weather Service if you want the overnight picture.

Defrost, steam, and a brief cool blow

Defrost is the outdoor coil clearing frost. The cycle can stop the outdoor fan, reverse the machine, and send a cloud of vapor off the cabinet that looks like smoke if you have never stood next to it. It is vapor from melt, not a flue. You should still trust your nose. A hot electrical smell or a gas odor is not defrost. Vapor on a cold morning, with no acrid smell, often is.

Indoors, supply air can feel cool for a short stretch during defrost because the cycle has borrowed heat to melt the coil. If the cool blow is brief and heat returns, write the length as best you can remember. If the air stays cool and the house temperature falls, that is no longer a normal defrost story.

Do not pour hot water over a frozen outdoor coil, and do not chip frost with a tool. Clear snow away from the cabinet so the fan and the drain holes are not buried. Leave the ice that is part of the coil itself for the cycle or for a technician.

Backup heat on the coldest nights

Backup heat is the second source that carries the house when the heat pump alone is not keeping up, or while defrost needs support. In some houses that second source is electric strips at the air handler. In others it is a gas furnace in the same cabinet, sometimes called dual fuel by the people who installed it. Both show up. Do not treat either one as the citywide default.

You will hear or feel the change. Electric backup can come on with a different blower note and a hotter supply temperature. A gas furnace as backup has its own ignition sequence. If you smell gas when that furnace tries to light, leave the building, avoid the switches, and from outside use the emergency number on the gas bill. Xcel Energy is the utility named on many of those bills. Defrost vapor does not cancel a gas odor.

Ask the technician how backup is supposed to behave on a night like this one, on this equipment. There is no single outdoor temperature on this page at which every heat pump must hand off. If they name a setup for your thermostat, write their words. If backup never comes on and the house is still sliding colder, that failure belongs in the visit, not in another hour of waiting.

Long runtimes and dry rooms

Heat pumps often run longer than a gas furnace that blasts and stops. A long, steady runtime on a cold night can be the design, not a stuck relay. The question is whether the rooms are holding. A house that holds temperature with a long run is a different report from a house that loses a degree an hour while the outdoor unit never rests.

Those long heating hours land in air that was already dry outdoors. Rooms get drier. Static, scratchy sinuses, and a humidifier you have not cleaned since last winter are a comfort topic, not a refrigerant symptom. The indoor humidity notes stay with winter dryness and what a humidifier discussion is for.

A thermostat setback that looks thrifty can ask the heat pump and the backup to recover the whole house at the coldest hour. Note the schedule. Note whether emergency heat or auxiliary heat was selected by hand.

When cold stops being a curiosity

A noisy defrost on a night the house still holds heat can wait until a normal appointment. No heat, a gas smell, a carbon monoxide alarm, or symptoms that ease outdoors cannot. Get people out for the alarm or the symptoms and call emergency services. Do not search the air handler while everyone stays inside to 'see if it passes.'

The no-heat emergency page sorts what has to happen tonight from what can wait until morning: vulnerable people, a house still falling in temperature, pipes on an outside wall, and the difference between a heat pump that is defrosting and a system that has quit.

Space heat, if you use it while you wait, has to be listed for indoor use and plugged in without an extension cord coiled under a rug. Do not run a fuel-burning heater or a grill in the kitchen to imitate backup heat.

What the visit is allowed to become

A repair on a heat pump can stay small: a sensor, a defrost board, a contactor, a thermostat setting. It can also grow into a refrigerant leak search, a failed compressor, or a recommendation to change the outdoor unit and the air handler together. Those are different scopes. The cost factors page lists what changes the work, access and after-hours calls included, without putting a price on any of it.

Replacing the outdoor unit, changing the indoor coil, or adding a heat pump where a furnace stood are the kinds of jobs that raise a permit question inside Denver. Denver Community Planning and Development is the office that can say what this address needs. The permits and inspections guide is the place for that conversation, including the reminder that Aurora, Lakewood, and Arvada do not share Denver's counter. This page does not invent a license class for the person who pulls the permit.

Leave the visit with three sentences: what the outdoor unit was doing, what backup heat did or did not do, and what the technician changed. If they adjusted a cold-weather setting, ask them to say it back in ordinary words.