Plain answers
What is a heat pump and how does it work?
One machine that heats and cools by moving heat from one place to another.
Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking
The short version
A heat pump is an electric system that moves heat instead of making it. In winter it pulls heat from outdoor air and pumps it inside.
In summer it runs the same cycle in reverse and works like an air conditioner. DOE notes that because it transfers heat rather than burning fuel, it can deliver more heat than the electricity it uses.
How can a heat pump heat a house when it is cold outside?
Outdoor air holds heat even when it feels cold. Refrigerant inside the outdoor coil is colder still, so heat flows into it. The compressor then squeezes that refrigerant, which raises its temperature well above room temperature.
The hot refrigerant travels to the indoor coil. A fan blows house air across it, the air warms, and the refrigerant cools and heads back outside to collect more heat. Nothing is burned. The electricity runs the compressor and the fans.
This is why DOE describes a heat pump as transferring heat rather than converting it from a fuel. An electric resistance heater, by contrast, turns one unit of electricity into one unit of heat and no more.
What are the main parts of a heat pump?
| Part | Where it sits | What it does |
|---|---|---|
| Compressor | Outdoor unit | Raises refrigerant pressure and temperature. The heart of the system. See compressor. |
| Outdoor coil | Outdoor unit | Picks up heat from outside air in winter; dumps heat outside in summer. |
| Indoor coil | Air handler or furnace cabinet | Releases heat into the house in winter; absorbs it in summer. |
| Reversing valve | Outdoor unit | Flips the direction the refrigerant flows, which switches between heating and cooling. |
| Expansion device | Between the coils | Drops refrigerant pressure so it gets cold again. See expansion valve. |
| Backup heat strips | Air handler | Electric resistance heat for very cold hours and defrost. |
The refrigerant loop is sealed. Opening it needs an EPA Section 608 certified technician.
How does a heat pump switch to cooling?
A reversing valve changes the direction the refrigerant flows. DOE describes this valve as what switches the system between cooling and the winter defrost cycle.
In cooling mode the indoor coil becomes the cold coil. It pulls heat and moisture out of house air, and the outdoor coil rejects that heat.
That is the same job a central air conditioner does. A heat pump is, in effect, an air conditioner that can also run backwards. See heat pump vs AC for when the difference matters.
Why does the outdoor unit steam or go quiet in winter?
In cold, damp weather frost builds up on the outdoor coil. Frost blocks airflow, so the system runs a short defrost cycle. It briefly switches to cooling mode to warm the outdoor coil and melt the ice. You may see steam and hear a whoosh. That is normal.
DOE recommends a heat pump with demand-defrost controls, which defrost only when needed. Fewer defrost cycles mean less use of backup heat. Read more on the defrost cycle.
What is backup or auxiliary heat?
Most ducted heat pumps have electric resistance heat strips in the air handler. They help on the coldest hours, during defrost, and when you raise the thermostat a lot at once. DOE notes that energy codes require controls so these strips do not run when they are not needed.
The strips cost more to run than the heat pump itself, so you want them to run rarely. If your thermostat shows "aux heat" often in mild weather, have the controls checked. The "EM heat" setting is a different thing; see what EM heat means.
Is a heat pump right for your house?
The heat pump vs furnace calculator uses your state’s prices.
Some are tested at 5°F. See cold-climate heat pumps.
Ask for an ACCA Manual J load calculation, not a rule of thumb.
DOE’s field review found duct leakage is common. A good heat pump on leaky ducts still underperforms.
The 25C credit does not apply to systems placed in service after December 31, 2025. See heat pump rebates for what remains.
Questions
Does a heat pump work in freezing weather?
Is a heat pump the same as an air conditioner?
Why does my heat pump blow lukewarm air?
Can I add refrigerant myself?
How long does a heat pump last?
Written and audited by
HYRE HVAC Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the federal microdata file, the statute or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify.
The counts below are generated from the published pages themselves, last counted October 26, 2026, and they are what we have actually published rather than what we intend to.
- 18
- studies published
- 15
- federal sources read and cited
- 15
- studies published with their full dataset as CSV
- 51
- jurisdictions reproduced against EIA’s own tables
How this desk works
- Primary sources only. Federal data comes from the agency that collects it, in the file that agency publishes. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- We validate against the agency before we publish. First, we use each federal microdata file to reproduce the agency’s own published tables. Our cooling research reproduces EIA’s state estimates and standard errors for all 51 jurisdictions. That check caught a variance formula that was off by a factor of four.
- Every estimate carries its uncertainty. These are survey figures, not counts. Standard errors are computed from the replicate weights the federal file supplies and printed beside the estimate. An estimate too imprecise to publish is reported as such rather than printed.
- Nothing is typed by hand. Prose, tables and charts all read from one dataset built by script, so a number in a sentence and the same number in the table below it cannot disagree.
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Data as of Sources checked October 26, 2026. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
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