Converting a conventional boiler to a combi: what's actually involved

A conventional system heats water slowly and stores it: a cylinder in the airing cupboard, one or two tanks in the loft, and gravity doing a lot of the work. A combi does away with all of it and heats water straight off the mains the moment you open a tap. Converting from one to the other is the biggest version of a boiler job there is, and it is worth understanding before anyone quotes for it.

It is also not right for every house. A good installer will sometimes talk you out of it, and this article explains why.

What physically happens

This is why a conversion takes two to three days where a straight swap takes one. The system is not being replaced so much as redesigned.

• The heating system is drained down completely.
• The old boiler comes out, and so does the hot water cylinder. The cold water storage tank and the small feed-and-expansion tank come out of the loft.
• Pipework is rerouted. Hot water now comes from the boiler on demand rather than from the cylinder, and the cold side switches from tank-fed to mains-fed, which changes the pressure every tap and shower sees.
• The gas supply pipe is checked and usually upgraded to 22mm, because a combi draws far more gas in short bursts than an old conventional boiler ever did.
• The new combi goes in with a new flue, a condensate run, a magnetic filter and compliant controls, the system gets a proper flush, and everything is commissioned and certified.

Source: Gas Safe Register

The question that decides everything: your mains supply

A combi is only as good as the cold main feeding it. It has no stored water to fall back on, so if your mains flow rate is poor, no combi on the market will give you a good shower, and the conversion is money spent making your hot water worse.

This is the first thing a competent installer checks on a survey, with a flow gauge on an outside tap or the kitchen cold. It takes two minutes. If someone quotes you for a conversion without measuring it, that quote is a guess. Older houses on long, narrow or shared supply pipes are the usual failures, and some of those can be fixed with a new supply pipe, which is a real cost that belongs in the quote rather than in a surprise later.

When a conversion makes sense

The honest pattern from the jobs we see around Northampton and Milton Keynes:

• One bathroom, or two that are rarely used at once. A combi serves this pattern well.
• You want the airing cupboard and the loft space back, and an end to overflow pipes and tank maintenance.
• The old cylinder is unlagged or ageing, so stored heat is leaking into the house all day whether you need hot water or not.
• The whole system is old. If the boiler, cylinder and tanks are all near end of life, replacing the lot once is cheaper than replacing them one failure at a time.

When to keep what you have

• Two or more showers genuinely running at the same time, most mornings. Stored hot water handles simultaneous demand; a combi shares its output between outlets.
• Weak mains flow that cannot be economically fixed.
• A large house with long pipe runs, where you would wait a long time for hot water at distant taps.
• You rely on an immersion heater as backup. A combi has no cylinder, so when it is down, there is no hot water at all.
• In these cases the better modernisation is often a system boiler with an unvented cylinder: mains pressure showers and stored volume. That is a different article, but it is the comparison worth asking any installer to price.

Why conversion quotes vary more than swap quotes

Because the houses vary more. One conversion needs a new gas run halfway across the house; another has the meter next to the boiler. One has accessible loft tanks; another has a boarded loft and a hatch the tank will not fit through. One needs the cold side repiped; another barely needs touching. Labour is the biggest line in any conversion quote, and labour depends on your house, not on a price list.

So compare quotes the same way as any boiler job: make, model and output of the boiler, what pipework and gas supply work is included, the type of system flush, who makes good the holes, and how many days on site. Two quotes that both say conversion can describe very different amounts of work.

The short version

A conventional to combi conversion strips out the cylinder and tanks, reroutes the pipework, usually upgrades the gas supply, and takes two to three days. Whether it is right for you comes down to your mains flow rate and how much hot water your household uses at once. Get the flow measured before anything else, and treat any quote that skipped that step with suspicion.

If you want it assessed properly, we arrange quotes from local Gas Safe registered installers who survey the house and measure the mains before they put a number on paper.