Can a tankless water heater supply a whole house in the UK

An electric one realistically cannot, because a shower alone needs around twenty kilowatts of heat and a single phase domestic supply cannot sustain that alongside everything else. A gas combi boiler is a whole house tankless water heater and works well in homes with one bathroom and a good mains supply.

Is an instant water heater cheaper to run than a cylinder

Not usually for a whole house, because it runs on electricity at the moment you use it, often at a daytime rate. It can be cheaper for a single distant basin, because it avoids heating a long pipe run that cools down between uses and it has almost no standing loss.

Do undersink water heaters need servicing

Unvented undersink units carry safety valves that should be checked, and in hard water the element and valves scale up over time. Follow the maker's maintenance instructions, and have the relief valve and its discharge checked if you ever see water coming from it.

Do I need building regulations approval for an undersink heater

Unvented storage heaters of fifteen litres or less fall outside the notifiable unvented rules, though they must still be installed to the maker's instructions with a safe discharge. A new electrical circuit for any heater is separate electrical work and needs a qualified electrician.

Why is my electric shower flow so much weaker than a mixer shower

Because an electric shower has to heat the water as it passes, and the power available limits how many litres a minute it can lift to a comfortable temperature. In winter, when the cold main is colder, the flow drops further. A mixer shower draws from water already heated by a cylinder or a combi.

Should I replace a failed cylinder with instant heaters

Rarely. For a home with a bathroom you would need more electrical capacity than most properties have, and the showers would be weaker. The usual choices are a new cylinder or a conversion to a combi, and which suits depends on the number of bathrooms and the strength of the mains supply.

Instant Water Heater or Hot Water Cylinder: Which to Choose

For a whole house in the UK, a hot water cylinder or a combi boiler is almost always the better choice, and an electric instant water heater is the right answer only for a single outlet: a cloakroom basin, a kitchen sink a long way from the cylinder, or a small office. The reason is not marketing, it is physics. Heating water as it flows takes a great deal of power at once, and an ordinary single phase domestic supply cannot deliver enough of it to run a shower and a kitchen tap together through one electric unit. The tankless water heaters you read about in American and European guides are usually gas fired, and in Britain that appliance already exists under another name: the combi boiler. Below is how each option works, where each one earns its place, and how to decide for your property.

What "tankless" means in a British home

A tankless or instantaneous water heater stores nothing. Cold water passes through it, gets heated on the way through, and comes out of the tap hot. There are three versions you will actually meet in London.

The first is the gas combi boiler, which is a tankless water heater for the whole house that also runs the central heating. The second is the electric instantaneous heater, a small box fitted under or above a basin or sink that heats water for that one outlet. The third is the electric shower, which is the same idea built into a shower unit. Undersink storage heaters are often sold alongside these but are a different thing: they hold a few litres of hot water, like a miniature cylinder, and reheat it with a small element.

A hot water cylinder, by contrast, heats a store of water in advance, from the boiler, an immersion element or a heat pump, and draws on that store when you open a tap. If you are not sure which type you have now, our guide to unvented vs vented cylinder systems shows how to tell them apart by the pipework.

Why an electric instant heater cannot run a whole house

It takes about 4.2 kilojoules to raise one litre of water by one degree. In winter the cold main in London arrives cold, and a comfortable shower sits around forty degrees, so every litre has to be lifted by roughly thirty degrees. Do the arithmetic and one litre a minute at that rise needs a little over two kilowatts of continuous power.

A decent shower runs at eight to ten litres a minute. That is somewhere near twenty kilowatts of heat, all the time the shower is running, and more if someone opens the kitchen tap at the same moment. A typical London house or flat has a single phase supply that has to carry the oven, the kettle, the washing machine and everything else on the same main fuse. That is why electric showers are built to run at a much lower flow than a mixer shower, and why nobody sensible fits a single electric unit to supply a whole home.

Stored hot water gets round this by spreading the work out. A cylinder can take an hour or more to heat, using a modest immersion element or the boiler coil, and then hand over a large volume at high flow when you need it. A combi gets round it by burning gas, which can deliver far more power through a flue than a domestic cable can carry.

Any new electric water heater that needs its own circuit is electrical work that has to be installed and certified by a qualified electrician. Do not run one from a plug socket or an existing spur unless the manufacturer says it is designed for that.

Where an instant or undersink heater is the right answer

There are jobs where a point of use heater beats running hot water from the cylinder, and in London they come up regularly.

A cloakroom or utility basin at the far end of the house, where the pipe run from the cylinder is so long that you run a sink full of cold water before anything warm arrives.
A kitchen that has moved during a renovation and now sits a long way from the airing cupboard.
A small office, shop or consulting room that only needs hand washing water.
A garden room, outbuilding or loft conversion where extending the hot pipework would mean lifting floors.
A flat where the cylinder has failed and you need hot water at one sink while a replacement is arranged.

Where a cylinder or combi is the right answer

For a whole house with a bathroom, a cylinder or a combi is the only realistic way to get a proper shower and a bath that fills in reasonable time. Which of those two suits you depends on how many bathrooms you have, how many people shower at once and how strong the incoming main is. We cover that choice in detail in our guide to a combi or system boiler with a cylinder.

In a flat with no gas, the alternative to an electric instant heater is not doing without, it is an electric hot water cylinder heated by immersion elements, often on an off peak tariff. That gives full flow at every tap without drawing huge power at the moment of use, because the heating has already happened overnight.

Where hot water already reaches every tap but you run out too early in the evening, a point of use heater is the wrong fix. The problem is the size or recovery of the store, and it is worth reading about why hot water runs out quickly before buying anything.

Undersink storage heaters and the unvented rules

Small undersink storage heaters come in two kinds. Vented or open outlet units need a special tap that leaves the outlet open to atmosphere, because the heater itself is not built to hold pressure. Unvented units connect to the mains and to a normal mixer tap, and they carry their own safety valves because they are, in effect, a tiny pressurised cylinder.

The building regulations for unvented hot water storage apply to systems holding more than fifteen litres. Units at or below that size fall outside the notifiable rules, but they still need their relief discharge taken somewhere safe and visible, and an installer who follows the maker's instructions. Anything larger is an unvented cylinder in law and has to be fitted by an engineer qualified for that work.

In hard London water, all of these small heaters scale up. The element and the valves need the same attention as a full size cylinder, just on a smaller scale.

Running cost: instant versus stored

Heating a litre of water by the same number of degrees takes the same energy whichever box does it. The differences in running cost come from three places. The first is the fuel: gas is generally cheaper per unit of heat than electricity at a daytime rate, which is why an electric instant heater is rarely the cheap way to run a whole house. The second is standing loss: a cylinder loses some heat through its insulation all day, while an instant heater loses almost none. The third is waste in the pipework: water left cooling in a long pipe run is heat you paid for and never used, which is exactly why a point of use heater at a distant basin can make sense.

For a detailed look at the stored side, our guide to immersion heater running cost works through what an element uses and how timers and tariffs change it.

How to decide for your property

If you are weighing up a new system, the useful first step is measuring what the incoming main actually delivers in litres per minute. That single figure rules options in or out faster than any brochure.

One basin or sink a long way from the hot supply: an electric instant or small undersink heater.
A flat with one bathroom and a good incoming main: a combi is usually simplest.
Two bathrooms, or showers used at the same time: a cylinder, usually unvented if the main can support it.
No gas at all: an electric cylinder on an off peak tariff, not a whole house instant heater.
Hot water runs out early: look at the size and recovery of the existing store first.

Talk it through with an engineer

We fit and repair cylinders, combis and point of use heaters, so there is no reason for us to steer you towards one over another. If your current hot water has failed, we will tell you whether the existing system is worth repairing before anyone talks about replacing it, and if a cylinder does need to go, our hot water cylinder replacement service covers the whole job.

Call 020 4652 5111 to book. Our rates are on the pricing page and are agreed before we attend.

Questions we get asked

Can a tankless water heater supply a whole house in the UK: An electric one realistically cannot, because a shower alone needs around twenty kilowatts of heat and a single phase domestic supply cannot sustain that alongside everything else. A gas combi boiler is a whole house tankless water heater and works well in homes with one bathroom and a good mains supply.
Is an instant water heater cheaper to run than a cylinder: Not usually for a whole house, because it runs on electricity at the moment you use it, often at a daytime rate. It can be cheaper for a single distant basin, because it avoids heating a long pipe run that cools down between uses and it has almost no standing loss.
Do undersink water heaters need servicing: Unvented undersink units carry safety valves that should be checked, and in hard water the element and valves scale up over time. Follow the maker's maintenance instructions, and have the relief valve and its discharge checked if you ever see water coming from it.
Do I need building regulations approval for an undersink heater: Unvented storage heaters of fifteen litres or less fall outside the notifiable unvented rules, though they must still be installed to the maker's instructions with a safe discharge. A new electrical circuit for any heater is separate electrical work and needs a qualified electrician.
Why is my electric shower flow so much weaker than a mixer shower: Because an electric shower has to heat the water as it passes, and the power available limits how many litres a minute it can lift to a comfortable temperature. In winter, when the cold main is colder, the flow drops further. A mixer shower draws from water already heated by a cylinder or a combi.
Should I replace a failed cylinder with instant heaters: Rarely. For a home with a bathroom you would need more electrical capacity than most properties have, and the showers would be weaker. The usual choices are a new cylinder or a conversion to a combi, and which suits depends on the number of bathrooms and the strength of the mains supply.