Combi or cylinder -
which suits your home?

A combi suits flats and smaller to medium size homes. A cylinder with a system boiler suits homes where more than one shower or bath runs at once - especially larger homes that have the space for the cylinder. A combi is only designed to supply one hot water tap at a time - that is the limit of it, and there is no way round it. We install both, and we will tell you at the survey which your home actually needs.

Two ways of making hot water
and why the difference matters

A combi heats water the moment you open the tap. There is no tank and no waiting for anything to heat up, which is why it suits smaller properties so well - one compact unit on the wall does the heating and the hot water, and every airing cupboard in the house comes back to you as storage.

A system boiler works with a hot water cylinder - a Megaflo, for instance. The cylinder stores a large volume of hot water, heated and ready, at mains pressure. When two showers run at the same time, both get hot water at proper pressure, because the water is already there rather than being made to order.

That difference decides most of it. Brands, controls and efficiency all matter, but they matter less than whether your household's hot water is made on demand or stored, and that is the first thing we work out on a survey.

The limit nobody tells you about
until you are in the shower

A combi is designed to supply one hot water outlet properly at a time. Run a second shower and both drop - less flow, swinging temperature. Nothing is faulty when that happens. Heated on demand means the boiler can only heat so much water per minute, and two outlets end up sharing it.

For one bathroom, that limit never shows itself. For a household where the family gets ready at the same time each morning, it shows itself daily. This is why for multiple bathrooms we suggest an unvented cylinder - it is the answer built for exactly that situation.

When the right answer
does not fit in your home

This is the situation we see all the time in London - a house with two or three bathrooms and nowhere to put a cylinder.

Some engineers deal with it by fitting a very high output combi to feed multiple bathrooms. It can work to an extent, and we understand why it gets done - but it is not the best solution. You are asking an on-demand appliance to behave like stored hot water, and at busy times the limit above is still there.

Where there is space, a system boiler with a cylinder is the best answer for a multi-bathroom home. The boiler and the cylinder do not need to be placed in the same space - as long as the pipework and wiring can be done they can be in different rooms. We have often placed one or both in either the loft (even in the eaves if there is a loft conversion, as long as the space is big enough), in external structures, garages and other spaces. They can even be placed in an outhouse if there is no other option.

Where there genuinely is no space, a combi storage boiler can help to some extent - it is a combi with a small hot water store built inside the casing, so it copes better with a second outlet than a standard combi, without needing a separate cylinder. It does not turn the house into a stored-hot-water house, but it softens the limit.

Which of those is right for your home comes out of the survey, once we have seen the water pressure, the pipework and the space, and we will tell you there, including when the cheaper option is enough.

What each route costs
as a rough guide

As a rough guide, a combi boiler installation usually comes in between £2,500 and £4,500, and a system boiler with a cylinder is a bigger job - approximately £7,000 to £8,000 - but both can vary with the job, the brands and models used, and other factors. The full breakdown is in our new boiler cost guide, and every DGN quote is a written fixed price after a survey, so the number you are given is the number you pay.

When we visit a client we firstly ask the client exactly what their requirements are, their budget if they have one and any particular brand or model preferences. We also ask if they specifically want a longer warranty. We assess the site and see what is feasible and make a judgement on the work involved. Then we send a no obligation quote to the customer for them to consider. They are free to get in touch if they have any follow up questions.

The short version

One bathroom, or a smaller home - a combi, and you would be spending money for nothing going further. More than one shower or bath running at once - a cylinder with a system boiler, if the space exists. No space - there are ways to soften the combi's limit, and we will tell you at the survey which one actually fits your home and your budget.

Quick answers