Will a Heat Pump Give You Enough Hot Water?

Most people worrying about heat pumps are worrying about radiators. But once the survey is done, the question that comes up again and again is the quieter one: what happens to my hot water? If you’ve spent twenty years with a combi boiler producing endless hot water on demand, the switch does involve a genuine change in how your home works. It’s worth understanding properly before you commit.

The big change: you’ll have a cylinder

A combi boiler heats water instantly as it flows through, which is why it can run indefinitely. A heat pump can’t do that — it produces heat gently and steadily rather than in a sudden burst, so it needs somewhere to store it. That means a hot water cylinder, usually a well-insulated unvented one, sitting in an airing cupboard, loft or utility space.

For homes that already have a cylinder and a conventional boiler, this is barely a change at all. For combi households it’s the single biggest practical adjustment, and it’s the first thing we look at on a survey: is there a sensible place to put one? Most homes have somewhere, but it’s an honest deal-breaker in a handful of very tight flats and terraces, and we’d rather tell you that at survey stage than after a deposit.

Will you run out?

Only if the cylinder is undersized — which is a design failure, not a heat pump failure. Sizing is based on how many people live in the house and how they actually use hot water: how many bathrooms, whether you run baths or showers, whether two people shower at the same time each morning. A properly specified cylinder covers a household’s normal daily use with headroom.

What does change is recovery. A combi refills its capacity instantly; a heat pump reheats a cylinder over a period of an hour or two. In practice this rarely matters, because the system is programmed to have the tank hot before you need it — typically overnight and again in the afternoon. Where people do get caught out is with an unusually heavy day: three long showers plus a bath, back to back, on a cylinder specced for two people. That’s a conversation to have at survey rather than a limitation to discover later.

Water temperature and the weekly cycle

Heat pumps are most efficient producing water at moderate temperatures, so cylinders on a heat pump typically sit a little cooler than a boiler would run them. That sounds alarming until you remember what you actually do at the tap: you mix it with cold. Very few people bathe in water anywhere near cylinder temperature, so a slightly lower stored temperature is unnoticeable in daily use.

The one thing that isn’t optional is the periodic high-temperature cycle. Stored water needs to be raised well above normal storage temperature on a regular schedule — usually weekly, usually overnight — to keep it safe from legionella bacteria. Your installer programmes this in and the immersion element handles the final push. It runs while you’re asleep, costs very little, and you never think about it again.

Efficiency, honestly

Producing hot water is harder work for a heat pump than warming radiators, because the target temperature is higher. Your system’s efficiency when making hot water will be lower than its space-heating efficiency, and that’s reflected in the seasonal figure — a well-designed system typically lands somewhere between an SCOP of 2.5 and 4 across the year. Even at the bottom of that range you’re getting two and a half units of heat for every unit of electricity, which is why running costs stack up as well as they do against direct electric immersion heating.

There’s a neat trick here too. If you have solar panels, hot water is one of the best places to put surplus generation — heating the cylinder mid-afternoon with free electricity from the roof rather than exporting it at around 12p/kWh. Pair that with an overnight heat pump tariff and most of your hot water can be made when electricity is cheapest.

Getting the specification right

None of this is complicated, but all of it depends on the survey. Cylinder size, where it goes, pipework, the reheat schedule and how it dovetails with your tariff are decisions made before anything is installed. Get them right and you’ll never think about hot water again; get them wrong and you’ll notice every morning.

If you’re weighing up a heat pump and want a straight answer about whether your home has the space and the pipework for it, our survey is free and there’s no obligation — we’ll tell you honestly if it isn’t the right fit.