"Heat pump" is one of those phrases that sounds like a single product but hides at least three quite different machines. They all work on the same clever trick — moving heat from one place to another rather than burning fuel to make it — but the amount of work involved in fitting one, and what you get at the end, varies enormously. Before you spend anything, it's worth understanding which is which. This is the short version.
The one idea behind all of them
A heat pump is a fridge run backwards. A fridge takes heat out of its inside and dumps it into your kitchen; a heat pump takes heat out of the cold outdoors and pumps it into your home. Because it's moving heat rather than making it, it delivers far more energy than it uses. Put one unit of electricity in and you typically get three to four-and-a-half units of heat out. That ratio, measured across a whole year, is the SCOP (Seasonal Coefficient of Performance) — the single most useful number for comparing systems. A gas boiler, by contrast, can never beat 1.0: it can only turn fuel into slightly less heat than the fuel contained.
Where the three types differ is (a) where they grab their heat, and (b) how they deliver it into your house. Those two choices decide everything about cost, complexity and how easy the install is to get right.
1. Ground (and water) source — the gold standard, and the big dig
The most efficient option grabs its heat from the ground or from a body of water rather than the air. Below about a metre down, UK soil sits at a near-constant 10–12 °C all year round, even during a February cold snap. Because the pump is always drawing from that steady warmth, its efficiency barely dips in winter — a well-designed ground source system holds a SCOP of roughly 4.0 to 4.5, sometimes higher.
The catch is getting at that heat. You either lay a long loop of pipe in trenches across a big garden, or you drill one or more boreholes tens of metres straight down. That's serious groundworks — diggers, drilling rigs, or a lot of land. It's why ground source runs to roughly £18,000–£35,000 installed, before the government grant. If you have the space and you're staying put for decades, it's a beautiful bit of engineering. For most homes, it's overkill — which is why the air source machines below dominate the market.
2. Air-to-water — the "proper" heat pump most people mean
This is the type you picture when someone says "we're getting a heat pump": a fridge-sized box on an outside wall, humming quietly, feeding your radiators and hot water. It pulls heat from the outdoor air and hands it to water, which is then circulated around the house. That water-based delivery is called a hydronic system, and it's where the real work lives.
A gas boiler is happy to blast water round your pipes at 70 °C, so it doesn't much care whether your radiators are the right size. A heat pump is most efficient running much cooler water — often 35–45 °C. To get a warm house out of cooler water, you need bigger emitters and a well-balanced circuit. In practice an air-to-water install can involve:
- Resizing radiators — many need swapping for larger ones, or underfloor heating, so they give out enough heat at a lower flow temperature.
- A hot-water cylinder — usually 150–300 litres — because, unlike a combi boiler, the pump can't heat water instantly on demand.
- Pipework, buffer tanks and controls — getting the flow rates, weather compensation and controls tuned so the system runs low and slow rather than short-cycling.
- A proper heat-loss survey — room by room, to size the whole thing correctly.
Done well, it's superb: whole-house heating and hot water from one efficient machine, a SCOP around 3.5–4.5, and eligibility for the full £7,500 Boiler Upgrade Scheme grant, which drops the typical net cost to somewhere around £500–£6,500. Done badly — undersized radiators, a system never balanced — it runs hot, guzzles electricity and gives heat pumps their occasional bad reputation. The heat-side design is the hard part, and it's where installs succeed or fail.
3. Air-to-air — the simple one (and yes, it's basically air conditioning)
Here's the bit most people don't realise: an air-to-air heat pump is air conditioning. It's the same reverse-cycle technology in the split-system air conditioners fitted in millions of homes and offices worldwide — an outdoor unit and one or more wall-mounted indoor units (the "splits") that blow warm air in winter and, run the other way, cool air in summer.
There's no water, no cylinder, no radiators, no hydronic balancing act. Refrigerant runs from the outdoor box straight to the indoor units, which blow conditioned air directly into the room. That single fact removes almost all the complexity that makes air-to-water installs tricky:
- No wet plumbing. Nothing to resize, no cylinder to plumb in, no flow temperatures to tune. You're mounting units and running a small refrigerant line.
- Fast, low-disruption fit. Often a day or so, not a week of pipework.
- High raw efficiency. Without water-circuit and cylinder losses, air-to-air SCOPs are excellent — commonly 3.5–5.0.
- Cooling included, free. The same box that heats you in January cools you in July. None of the other options do that.
The one genuine limitation: air-to-air doesn't make hot water. It heats and cools rooms, full stop. So you still need something else for your taps and shower — most commonly a hot-water cylinder on an immersion (ideally soaking up cheap overnight or solar electricity), or an existing water heater. That's the trade-off for the simplicity.
Figure 1 — Rough SCOP ranges by type. All three comfortably beat a gas boiler's ceiling of 1.0. Ground source is steadiest through winter; air-to-air posts high raw numbers because it skips the water circuit and cylinder losses entirely. Bars show typical UK ranges, not a guarantee for any one install.
Side by side
| Ground source | Air-to-water | Air-to-air | |
|---|---|---|---|
| Heat from | Ground / water | Outside air | Outside air |
| Delivers heat via | Water (radiators/UFH) | Water (radiators/UFH) | Warm air (wall units) |
| Typical SCOP | 4.0–5.0 | 3.5–4.5 | 3.5–5.0 |
| Makes hot water? | Yes | Yes | No |
| Cooling in summer? | No | Rarely | Yes |
| Install complexity | High (groundworks) | High (hydronic) | Low |
| Rough cost before grant | £18k–£35k | £8k–£14k | £3k–£8k |
| Boiler Upgrade Scheme grant | £7,500 | £7,500 | £2,500* |
*Air-to-air only became grant-eligible in the April 2026 Boiler Upgrade Scheme overhaul, and only for homes currently on direct electric heating (storage heaters, electric panels) — not homes swapping out a gas, oil or LPG boiler. Figures are indicative UK ranges; your quote will vary with home size and site.
Why we chose air-to-air at Rose Cottage
The honest reason is that the simple thing tends to be the thing that actually works. An air-to-water system lives or dies on getting the heat side right — radiator sizing, flow temperatures, cylinder, controls, all balanced by an installer who really knows what they're doing. Get any of it wrong and you're chasing gremlins for years.
Air-to-air sidesteps all of that. No hydronic system means nothing to mis-size and very little to go wrong: you mount the units, run the refrigerant line, and it heats beautifully — and cools in summer as a bonus. It doesn't need the sophistication, which is exactly why it's so likely to succeed. We keep hot water separate. For a cottage like ours, it was the obvious call — and it's the option far more people should at least be looking at.
So which should you get?
There's no single winner — it depends on your house and what you're replacing. As a rough guide: if you have land, a long horizon and want the steadiest efficiency, ground source is the premium choice. If you want one machine to do whole-house heating and hot water and you're prepared for the emitter and cylinder work (and want that £7,500 grant), air-to-water is the mainstream answer. And if you want simplicity, summer cooling, a fast low-fuss install and high efficiency — and you can sort hot water separately — air-to-air is the quietly brilliant option most people have never had explained to them. It's air conditioning that happens to be one of the cheapest ways to warm a room too.