The Best Tankless Water Heater for UK Homes (2026): Combis and Instantaneous Units, Honestly Assessed

A combi boiler is a tankless water heater. Mains cold water passes through a plate heat exchanger, comes out hot, and nothing is stored. The American-style 24kW electric whole-house unit that dominates the search results is a different animal, and on a typical UK single-phase supply with an 80A main fuse it is close to unusable — 24kW at 230V is over 100 amps on its own.

So this guide covers the two things that genuinely work in British homes: gas combis for whole-house hot water, and electric instantaneous heaters for one outlet at the end of a long pipe run. We have been equally blunt about the households that should keep a cylinder, because for roughly a third of the homes we survey, going tankless makes the showers worse.

On-demand heating suits a specific shape of household: sequential rather than simultaneous demand, and an incoming main that can actually deliver. Where two people shower at once, a stored system still wins.

• •Good fit: one bathroom, one to three occupants, an incoming main delivering 15 litres a minute or better at 1.5 bar or above.
• •Good fit: a flat or terrace with no airing cupboard to spare, where losing a cylinder frees genuinely useful floor area.
• •Poor fit: two bathrooms in simultaneous use — a combi cannot split its output twice without both showers going tepid.
• •Poor fit: a weak main. Time a bucket of known volume filling from the kitchen cold tap — under 12 litres a minute, a combi will disappoint whatever the box claims.
• •Poor fit: an outbuilding or annexe with no gas run — here an electric point-of-use unit, not a whole-house heater, is the sane answer.

We have not run these units on a calibrated test rig, and we will not pretend otherwise. This assessment is drawn from installing and servicing these appliances in UK properties, from manufacturer flow data quoted at a 35°C temperature rise, from ErP efficiency figures, and from what heating engineers report about spare-part availability three and seven years in.

Four criteria decide it: hot water flow at a realistic winter temperature rise; whether the warranty is honoured or hedged with annual-service conditions; how quickly a plate heat exchanger or diverter valve can be sourced on a Saturday; and total installed cost, not box price.

Manufacturers quote hot water output at a 35°C rise. In a British January the incoming main sits around 5 to 8°C, so a 40°C shower needs closer to a 35°C lift with nothing to spare, and any unit rated at a smaller rise is being flattered. The arithmetic is simple: litres per minute equals kilowatts times 60, divided by 4.18 times the temperature rise.

• •A 30kW combi: 30 × 60 ÷ (4.18 × 35) = roughly 12.3 litres a minute. Comfortable for one shower and a kitchen tap.
• •A 35kW combi: roughly 14.4 litres a minute. Fills a 70-litre bath in about five minutes and copes with a rain head.
• •A 9.5kW electric instantaneous unit at a 30°C rise: roughly 4.5 litres a minute — a hand basin or a slow trickle at a utility sink, nothing more.
• •Any electric unit big enough to run a real shower needs 40 amps or more on a dedicated circuit. Before ordering, work through how to calculate your home electrical load; two such units on one domestic supply is rarely possible.

This is where imported American buying advice actively misleads people. Every gas appliance in the UK — installation, replacement, servicing, even moving it 300mm along the wall — must be worked on by an engineer on the Gas Safe Register, under the Gas Safety (Installation and Use) Regulations 1998. There is no competent-DIYer exemption. Ask to see the ID card and check the back for the specific appliance categories.

The electrical side has its own gate. In England and Wales, Part P of the Building Regulations makes a new dedicated circuit notifiable, and work in a room containing a bath or shower falls under scrutiny too: use a registered competent person under NICEIC, NAPIT or ELECSA, or notify building control first. Scotland and Northern Ireland work to their own regulations.

• •Boiler Plus (England): a replacement combi must be at least 92% ErP efficient and fitted with time and temperature control, plus one of load compensation, weather compensation, flue gas heat recovery or smart controls. A quote that omits this is not compliant.
• •Part G applies to unvented hot water storage. A pure instantaneous heater stores nothing, so it sidesteps G3 — one of the genuine reasons combis got popular here.
• •Landlords: the annual gas safety check and CP12 certificate covers every gas appliance and flue in the property, and must be done within 12 months of the last one. Swapping to electric instantaneous heating does not remove the duty if a gas hob remains.
• •Flue termination distances from openings and boundaries are set by the manufacturer and Gas Safe. A flue that clears the wall but sits 200mm from a neighbour's bedroom window will be failed.
• •Prices are broken down in our boiler replacement cost guide.

Almost every disappointing installation we are called to inspect traces back to one of five decisions, all made before anyone lifted a spanner.

• •Oversizing on kW to fix a flow problem that is actually a weak incoming main or a partly closed stopcock. The bigger boiler simply idles more.
• •Fitting a 35kW unit on the existing 15mm gas run. Above about 30kW a 22mm supply from the meter is usually required, and skipping it causes lockouts under load.
• •Ignoring water hardness. Across the South East and East Anglia, scale on a plate heat exchanger is the main killer of combis — our limescale removal guide explains the chemistry, and a scale reducer on the main is £120 to £200 well spent.
• •Removing the cylinder in a two-bathroom house. We have refitted more than one of these at the owner's expense within 18 months.
• •Assuming no hot water means a dead boiler. Half the callouts are a diverter valve, a low-pressure lockout or a failed thermistor — work through the no hot water troubleshooter before you agree to a replacement.
• •Skipping the running-cost comparison. The tank versus tankless 10-year cost analysis is the honest arithmetic for UK tariffs.

For most UK homes with one bathroom and mains gas, a 30kW condensing combi is the tankless water heater, and the differences between the three above matter far less than whether the installer sizes the gas run, flushes the system and commissions it properly. Budget £2,000 to £3,500 all-in and spend the extra on the fitter rather than the badge.

With no gas, be realistic about scope. Electric instantaneous units are outbuilding, cloakroom and utility-sink appliances here, not whole-house solutions. And with two bathrooms in daily use, keep the cylinder and put the money into a decent unvented system instead.