Electric Combi Boiler: When It Makes Sense and When It Really Does Not

Draft insight by Vector Heat

An electric combi boiler sounds beautifully simple: no gas, no flue, hot water on demand. Unfortunately, heating has a habit of making simple ideas expensive once the electricity bill arrives.

The short answer: an electric combi boiler can make sense in a small property where there is no gas, no practical flue route, no outdoor unit option, and no room for a cylinder. In most other homes, it should be treated as a last resort because there are usually cheaper-to-run options. If a heat pump is physically possible, that should normally be looked at before an electric combi.

What Is an Electric Combi Boiler?

An electric combi boiler provides heating and hot water directly using electricity.

Like a gas combi, it heats hot water on demand rather than storing it in a cylinder. The difference is that it uses electric elements instead of burning gas.

That means:

• no gas supply needed
• no gas flue needed
• no stored hot water cylinder
• compact installation
• high electricity demand while running

When an Electric Combi Boiler Can Make Sense

The strongest case is a difficult flat.

For example:

• a middle flat with no outside wall
• no practical route for a flue
• no roof route
• no space for a cylinder
• no suitable outdoor position for air conditioning or a heat pump unit
• very tight cupboard space

In that situation, an electric combi boiler might be the only realistic option.

It solves a practical problem: how to provide heating and hot water where other systems are hard or impossible to install.

Why Electric Combi Boilers Are Usually a Last Resort

The problem is running cost.

Electricity is usually much more expensive per unit than gas. A direct electric boiler turns electricity into heat, but it does not multiply the heat like a heat pump can.

So while the boiler may be compact and simple, the running cost can be high.

That is why I would not usually recommend an electric combi unless the property has boxed you into a corner. Sometimes the corner is real. Sometimes it is just bad design wearing a hi-vis.

What Are the Alternatives?

If someone is searching for an electric combi boiler, there is a decent chance gas is not available or not practical. So I would not put a gas boiler at the top of the alternatives list. If gas and a flue route were simple, most people would probably be looking at gas combis instead.

Depending on the property, the alternatives to check first are:

• air source heat pump, if there is a suitable outdoor location and room for the right hot water setup
• air conditioning used for heating, especially for flats or rooms where air-to-air heating makes sense
• hot water cylinder with immersion heater, if there is space to store hot water
• time-of-use electricity tariff, so stored hot water or storage heating can use cheaper electricity periods
• storage heaters, where space heating needs to stay electric
• direct electric heating in limited cases
• gas boiler only if there is a gas supply and a legal, practical flue route

The heat pump point is important. A heat pump is usually far cheaper to run than an electric combi because it moves heat rather than making heat directly from electricity. The Boiler Upgrade Scheme on GOV.UK currently lists a £7,500 grant towards an eligible air source heat pump in England and Wales, so in some homes it may be cheaper to install as well as cheaper to run.

That does not mean every flat can have one. Outdoor space, lease rules, noise, planning, hot water storage and pipe routes all matter. But if a heat pump is possible, it deserves a proper look before choosing an electric combi.

If you can store hot water, you may be able to heat it on a cheaper overnight or smart tariff. That can be better than using an electric combi for all hot water instantly at peak electricity prices.

Storage heaters can also make use of cheaper time-of-use tariffs for space heating.

Electric Combi Boiler vs Cylinder

An electric combi saves space because it does not need a hot water cylinder.

But a cylinder gives you storage and usually much better hot water performance. An unvented cylinder can give strong flow rate and pressure for showers, baths and more than one outlet, as long as the incoming mains supply is good enough. That is a completely different experience from many electric combis.

Storage can also be useful if you can heat water when electricity is cheaper and use it later.

So the decision is often:

• no space for a cylinder: electric combi may be considered
• space for a cylinder: stored hot water may be cheaper to run and much better to use
• high hot water use: electric combi may struggle, become expensive, or disappoint on flow rate
• low hot water use: it may be more acceptable, but still needs careful thought

Electric Combi Boiler Hot Water Performance

Hot water output is limited by the power of the boiler and the incoming water temperature.

This is one of the biggest problems with electric combis. The flow rate is usually poor compared with a good gas combi or an unvented cylinder. They can be fine for modest use, but they are not what I would choose for someone who wants powerful showers, fast baths, or more than one hot tap running at once.

That means an electric combi may not suit homes with:

• multiple bathrooms
• high shower demand
• frequent baths
• several people needing hot water at once

For a small flat with one bathroom and modest demand, it can be workable.

How Many kW Is an Electric Combi Boiler?

Domestic electric combi boilers are commonly somewhere around 9 to 24 kW, depending on the model. Smaller units can be very limited for hot water. Larger units still usually do not match the hot water performance of a good gas combi or a correctly designed unvented cylinder.

The bigger issue is the electrical load. A 24 kW electric boiler is not like plugging in a kettle. Before recommending one, the incoming electrical supply needs checking properly. That means looking at the main fuse, consumer unit, available capacity and whether the supply can safely support the boiler alongside the rest of the property.

Sometimes the answer may need input from an electrician, the electricity network operator, or both.

What I Would Check First

Before recommending an electric combi boiler, I would check:

• is a heat pump physically possible?
• would the Boiler Upgrade Scheme grant apply?
• is there any space for a cylinder?
• would an unvented cylinder give the customer much better hot water?
• is the incoming electrical supply adequate for the kW size needed?
• does the main fuse, consumer unit and available electrical capacity allow it?
• what is the hot water demand?
• could a time-of-use tariff help?
• could storage heaters work?
• could air conditioning provide heating?
• is there any outdoor space for a heat pump or AC outdoor unit?
• is there any possible gas supply and flue route, or is that already ruled out?

Only after that would I call an electric combi the best answer.

FAQ

Is an electric combi boiler expensive to run?

Usually, yes compared with a heat pump. An electric combi uses direct electricity, so running cost can be high unless usage is low or tariff conditions are favourable.

When should I fit an electric combi boiler?

Usually only when other options are impractical, such as a small flat with no gas, no flue route, no outdoor unit location and no room for a cylinder.

Does an electric combi boiler need a flue?

No. It does not burn gas or oil, so it does not need a combustion flue.

Can an electric combi boiler run a shower?

It can, but hot water flow is often limited. It is usually better suited to smaller properties with modest hot water demand, not homes where people expect a powerful shower or more than one hot outlet at a time.

Is an electric combi boiler better than a cylinder?

Not usually if there is space for a cylinder. An unvented cylinder is normally much better for flow rate, showers, baths and multiple outlets. It also gives storage, which can be useful if you can heat water during cheaper electricity periods.

Does my electrical supply need checking for an electric combi boiler?

Yes. Electric combis can be a very large electrical load. The incoming supply, main fuse, consumer unit and available capacity should be checked before deciding whether one is suitable.

Is a heat pump better than an electric combi boiler?

Often, yes, if the property can physically have one. A heat pump is usually cheaper to run and may qualify for a £7,500 Boiler Upgrade Scheme grant, but it needs a suitable outdoor location and the right hot water design.