Combi boiler vs heat pump hot water: what actually changes

Quick answer: On price they’re close: hot water costs about 8.9p per kWh from a 90% combi at October–December 2026 gas rates, and roughly 9.7p from a heat pump cylinder. The difference you’ll feel is the cylinder itself. Stored mains-pressure water runs two showers at once; a combi never runs out but slows when a second tap opens.

Hot water is where combi owners get nervous about heat pumps, and fair enough: you’re trading on-demand water for a tank. Here’s what actually changes, cost first, experience second.

The running cost maths

Gas costs 7.97p/kWh at the current cap and a modern combi turns it into hot water at around 90% efficiency, so call it 8.9p per kWh of heat in the water (Ofgem cap rates, October to December 2026).

A heat pump pays 26.32p/kWh for electricity but multiplies it. The catch: its headline seasonal figures come from low-temperature space heating. Pushing a cylinder to 50°C works the compressor harder, so the hot water COP sits below the headline SCOP. Samsung’s MCS-listed figures for one current 8kW unit show the slide: a SCOP of 4.66 at 35°C flow falls to 2.89 at 55°C (retailer listing, checked July 2026). Assume 2.7 for stored hot water and you get 9.7p per kWh.

For a family using around 2,000kWh of hot water heat a year, that’s £178 on gas against £194 on the heat pump. Sixteen pounds. Not the decider.

The decider might be off-peak. A cylinder can reheat overnight on a smart tariff; at 7p/kWh (EV-style overnight rates ran 5 to 10p when we checked in July 2026) the same water costs £52 a year. A combi can’t time-shift anything. It burns gas the moment you open the tap.

Flow rates: the shower test

A combi makes hot water live, and output depends on boiler size. A Worcester Bosch Greenstar 4000 manages about 12 litres a minute at a 35°C temperature rise; the 40kW Greenstar 8000 Life reaches 16.4 (manufacturer figures via retailer listings, checked July 2026). That’s one good shower. Open a second hot tap and both outlets slow, which anyone in a busy combi household has felt at 7.45am.

A heat pump system stores water in an unvented cylinder at mains pressure. Two showers can run at once without the stutter. The limit is volume, not flow: when 200 litres of hot water are gone, they’re gone until the reheat finishes, typically an hour or two depending on cylinder size and heat pump output. We haven’t tested either setup ourselves; figures are published specs, checked July 2026.

Sizing settles most of the fear. Installers work from rules of thumb, often around 40 litres of hot water per person per day, rounded up for bath households, so a family of four commonly lands on a 200 to 250 litre cylinder. Get the sizing conversation in writing. It will matter more to your mornings than the brand on the heat pump does.

What you give up, honestly

The cylinder needs a home: typically an airing-cupboard footprint for 150 to 250 litres. Combi households often gave that cupboard up years ago, and finding the space is the single biggest snag in flats.

There’s also a weekly legionella cycle, where the system (usually the immersion element) pasteurises the tank. It runs automatically, but it’s a running cost gas owners never think about.

Temperature isn’t the problem people expect. Modern R290 machines can push flow temperatures to 75°C when needed; the Samsung EHS Gen 7 we reviewed is one. You’ll store water around 50°C for efficiency, which is what most combis deliver to the tap anyway.

Solar owners hold one extra card. A diverter sends surplus generation into the cylinder instead of out to the grid. And stored hot water is the cheapest battery money buys. A combi has nowhere to put that surplus. If panels are on your roof or on your list, the cylinder quietly becomes the better long-term partner.

Our verdict

Two bathrooms and three or more people: the cylinder wins. Simultaneous showers, off-peak reheating and solar-diverter potential beat on-demand convenience, and the running costs are level pegging. One bathroom, one or two people, no spare cupboard: the combi experience is genuinely hard to better, and hot water alone is no reason to switch. Weigh it against the full heating picture in our heat pump guide and boiler guide, or put your own usage into the calculator.

FAQs

Will a heat pump run out of hot water?

It can, in the way any cylinder can: a properly sized tank covers a normal day with margin, and the reheat runs quietly in the background. Chronic shortages mean the cylinder was sized wrong, not that the technology failed.

Is heat pump hot water hot enough?

Yes. Cylinders typically store at around 50°C with a regular sterilisation cycle above that, and R290 models can reach 75°C flow when required. For comparison, bath water is comfortable near 40°C.

Can I keep a combi and add a heat pump?

Hybrid systems exist and suit some hard-to-treat homes, but you keep the gas standing charge and two sets of maintenance. Most households do better committing one way. Our costs methodology shows how we model the comparison.