How does a heat pump work?

The principle, in one sentence

This Old House's heating expert puts it better than we can:

We're trying to move it. We're not trying to make it.

A furnace makes heat by burning something. A heat pump finds heat that already exists and carries it to where you want it. That is the whole idea, and everything else on this page follows from it.

In summer the machine takes heat from inside the house and lets it dissipate outside. In winter it runs the other way, scavenging heat from the outdoors, even in temperatures as low as zero degrees, and pumping it into the house. This Old House names the one requirement: enough electricity to run it.

Source side and load side

This is the vocabulary that makes the rest make sense, and it is This Old House's framing:

The machine does not change. The direction does. Our note, marked as ours: that is why a heat pump replaces a furnace and an air conditioner together, and why our heat pump vs furnace page argues that comparing it against a furnace alone is not a fair comparison.

Why it can beat 100% efficiency

A furnace is rated on AFUE, the percentage of fuel it turns into heat. This Old House's comparison guide puts the best of them at up to 98%, and 100% is the hard ceiling, because you cannot get more heat out of a fuel than is in it.

A heat pump is rated on SEER for cooling, up to 30, and HSPF for heating, up to 13, on the same guide. Those are ratios of useful output to electricity consumed, and because the machine moves heat rather than creating it, the ratio is not capped at one.

Our reading, marked as ours. This is the single most misunderstood thing about heat pumps and the reason the two rating systems cannot be compared side by side. A 98% furnace is nearly perfect at what it does. A heat pump is doing something else entirely, and that is why it can deliver more energy into the house than it draws from the meter.

What the two ratings measure, and why a SEER number and a SEER2 number for one machine differ by about 4.5%, is on our SEER rating page.

Fixr prices that efficiency directly:

The four types

This Old House groups them by what is on each side. The names tell you the answer: the first word is the source, the second is what carries the heat into the house.

Air to air is the one most people have seen, and This Old House says so: it is the most common.

Air to water is the one most people have not, and the guide is enthusiastic about it: no drilling, the efficiency of hydronics, and the hot water can go to a zone manifold for heating, through a heat exchanger for domestic hot water, or elsewhere. Our radiant floor page covers what the floor half of that costs.

The two water-source types need geothermal drilling, which is where the price goes. Our geothermal page sets out why, and the short answer is that you are paying for excavation rather than for a machine.

Fixr prices the practical version of the same split:

Fixr notes that a ductless mini-split is itself a type of air-source heat pump using the same technology, which our mini split page covers in full.

The limit that gets left out

This Old House names one that most explainers skip. Geothermal systems max out at around 120 degrees, where boilers can go much higher.

Our reading, marked as ours. That matters for an old house with cast iron radiators sized for water far hotter than 120 degrees. The radiators will not put out their rated heat at that temperature, so the system either needs larger emitters, a heated floor with more surface area, or a back-up. Nobody is going to tell you this from a price table, and it is the reason a heat pump retrofit in an old hydronic house is a design job rather than a swap. Our boiler replacement page is the alternative if the answer turns out to be no.

The other limit is temperature on the air side. The system still works at zero degrees on This Old House's own figure, but efficiency falls as it gets colder, which is why its comparison guide says furnaces are often the better choice in cold climates.

What it costs to buy and how long it lasts

Fixr: $5,000 to $15,000 for an air-source system, average $8,350 across types, and a lifespan of 10 to 15 years. Our heat pump page has the figures from a second publisher, and our lifespan page sets that 10 to 15 against the 15 to 20 of a furnace.

Fixr also gives the conversion worth memorising: one ton is the equivalent of 12,000 BTUs, which is the unit every quote you receive will be written in.

What to take from this

1. It moves heat, it does not make it. That is why its efficiency ratings look impossible next to a furnace's and why they cannot be compared.
2. The type is decided by your house, not by preference: ducts or water, and whether drilling is possible.
3. Air to water is worth asking about if your house is heated by water and you assumed a heat pump meant ducts.
4. Ask about the output temperature if you have existing radiators. 120 degrees is not the same water your boiler was sending them.
5. Price it against both machines it replaces, not just the furnace, which our heat pump vs furnace page works through, and against what each type costs to repair, on our heat pump repair page.