How I regret my heat pump lacks air conditioning

…and how air-to-water heat pumps could bounce back against heatwaves

I bought my heat pump in the wrong month

I bought my heat pump in the spring of 2022, which is to say I bought it during a war. Russia had just invaded Ukraine, the price of gas was doing things the price of gas is not supposed to do, and I was not sleeping — not because of the news, but because I had a twelve-month-old who had decided that sleep was for other families. So there I was at three in the morning, awake anyway, reading about wholesale gas curves and deciding that the sensible thing to do — the hedge, the geeky thing, the right thing — was to get off gas. A few months later a shiny Mitsubishi Ecodan appeared on the side of the house.

I don't regret it. I want to say that clearly, because heat pumps have somehow become a culture war, and I refuse to file a grievance I don't have. For about nine months of the year mine is quietly excellent. It runs at a seasonal efficiency close to 4 — nearly four units of heat out for every unit of electricity in — and because I've a home battery that buys something like 99% of my power at the cheap overnight rate, my annual heating bill lands somewhere between £100 and £200. I have spent more than that on a single annual service. (That figure takes a battery, a canny tariff and more attention than any sane person gives their heating — a point I'll come back to.) It heats a slightly draughty house on flow temperatures that would embarrass a gas boiler, it drinks my own solar when the sun is out, and — far from forgetting it exists — I treat the thing as a private science project, watching its every degree of flow temperature for the sheer pleasure of the data it throws off.

And then it's July, and I'm awake at three in the morning again. Different baby-shaped problem this time — the problem is that the house is too hot, the windows are open onto still, warm Dartmoor air that isn't helping, and I am lying there acutely aware of one thing: bolted to the side of my house is a large, sophisticated, refrigerant-filled machine whose entire purpose is to move heat from one place to another, and on the one night I would give anything for it to move some heat out, it sits there doing nothing.

You have to picture where this is. My house sits in one of the highest villages in Britain, close enough to the Atlantic to take its weather full in the face — cold, wet, the kind of place where the rain tends to arrive sideways. On paper it is close to the last spot in the country you would ever think to fit air conditioning. In 2022, the idea that I'd be lying awake up here sweating — and in May, not the depths of August — would have seemed faintly absurd. And yet the data tells it plainly. By the only measure that makes sense this high — three days running above 23 °C, which for a village on the roof of Dartmoor is a genuine heatwave — my village has had eight since I fitted the heat pump: three of them this year alone, and none at all in 2024. They seem to be getting both more frequent and more fierce, up here on the high, wet roof of the south-west. That is how fast the ground has moved.

On the worst of those days — the 24th of June — I worked out at the time that, on a "feels-like" basis, it was probably the hottest day my corner of Dartmoor has ever recorded: 31.6 °C on the thermometer, but with the air so thick it felt closer to 36. That gap is the tell. What made it punishing wasn't the heat alone, it was the humidity riding along with it — which, as we'll see, is exactly why a cooling system worth the name up here has to wring the moisture out, not just take the edge off the temperature.

That's the regret. Not that I got a heat pump. That nobody — me least of all — spent a single second thinking about summer.

It's dated in a quieter way, too. Mine runs on R32; the propane units that have arrived since reach hot-water temperatures mine never comfortably will — so where older systems still lean on a resistance immersion to sterilise the tank, a newer one simply wouldn't. Two refrigerants behind, and it can't cool. I bought at the wrong moment in more ways than one.

The three things that don't work

Octopus has started trialling a gadget it's calling, with a completely straight face, the Cosy Cooler — a cooling add-on for its heat pump. Greg Jackson trailed it as one of the most-requested features they get, "for obvious reasons," and he's right, because a heat pump is a reversible machine that already contains everything you need to make cold. So of course I went looking for how to add cooling to what I've got. There are three obvious answers. I tried, or seriously costed, all three. They all fail, and they fail in instructive ways.