Changing to a Combi Boiler: Cost Factors and What's Involved

Changing from a regular or system boiler to a combi costs more than a like-for-like combi boiler replacement because it is more work: the hot water cylinder, the loft tanks and their pipework come out, the heating circuit is converted to a sealed, pressurised system, and the gas supply pipe, flue and condensate route often need changing too. How much more depends on how much of that your home needs, which is why a conversion is priced after a survey rather than read off a list. The upside is a cleared airing cupboard, a cleared loft and mains-pressure hot water.

What actually changes in a combi conversion

• The old boiler comes out, along with the hot water cylinder in the airing cupboard.
• The loft tanks go. On a regular (gravity-fed) system that means the cold water storage tank and the small feed and expansion tank, plus their feed, vent and overflow pipes. A system boiler already has a sealed heating circuit, so there is less to remove.
• Hot and cold pipework is reconnected so the taps and showers are fed directly from the mains through the new boiler.
• The heating circuit is sealed and pressurised. An open-vented system becomes a sealed one, running at a set pressure with an expansion vessel inside the boiler.
• A new combi is fitted, often in a different place from the old boiler, with a new flue and condensate pipe.
• The system is cleaned and a magnetic filter fitted, then the boiler is commissioned, the Benchmark checklist completed and the work notified under Building Regulations.

What drives the cost of changing to a combi

Two conversions in two similar-looking London houses can be priced quite differently. The main factors are:

• Where the new boiler goes. Keeping it close to the old one is cheapest. Moving it to a kitchen or utility means new gas, water and heating pipe runs and a new flue position. See moving your boiler location.
• The gas supply pipe. A combi heating hot water on demand draws far more gas at peak than an old heat-only boiler did. Many older homes have a gas pipe too small to feed it, and upgrading the run from the meter is a common addition.
• How much redundant pipework there is. Cylinder and tank pipework is capped off or removed. Accessible runs are quick; runs under floors or boxed into walls take longer.
• The condition of the existing heating system. Old radiators, valves and joints that were fine on a low-pressure open system can start weeping once the system is pressurised. The survey looks for this, and anything that needs replacing should be on the quote.
• System cleaning. Older open-vented systems often carry a lot of sludge. A cleanse or power flush before the new boiler goes on protects it and its warranty.
• Making good. Filling the old flue hole, and what happens to the airing cupboard once the cylinder has gone.
• Showers. A power shower with its own pump cannot stay as it is on a combi. That pump comes out and the shower is replumbed or replaced, which is a cost to include.

The wider list of what moves any boiler price is in what affects the cost of a new boiler installation, and our pricing page explains how the survey-then-fixed-quote process works.

Check these before you decide to convert

Your mains flow rate. A combi can only deliver the hot water your incoming mains supplies. In older London terraces with long or narrow supply pipes, and in upper-floor flats, the flow can be too low for a combi to feel like an upgrade. It needs measuring at the property, not guessing from the pressure alone.

How many bathrooms you use at once. A combi serves one outlet well at a time. If two showers run together every morning, a conversion can leave you worse off than the cylinder did. In that case a system boiler with an unvented cylinder usually gives mains pressure and enough stored hot water; see system boiler replacement and choosing a replacement cylinder.

Losing the backup. A cylinder usually has an immersion heater, so you keep hot water if the boiler fails. A combi has no backup.

Solar thermal or a second heat source. If the cylinder is also heated by solar panels or a stove, removing it removes those too.

Homes where changing to a combi makes sense

A conversion is usually worth it in a flat or a one-bathroom house with a reasonable mains flow, where the loft tanks and airing cupboard are space you would rather have back, and where the old cylinder and boiler are both near the end of their lives anyway. It is also a sensible moment to do it during a kitchen or loft project, while floors are up and pipe runs are easy to change. The guide to how a combi boiler works covers the everyday trade-offs.

How long a conversion takes

A straight combi-to-combi swap in the same position often takes a day. Converting from a regular or system boiler usually takes one to two days, depending on how much pipework changes and whether the boiler is moving. Expect the water and heating to be off for parts of both days, and the engineer to need access to the loft, airing cupboard and gas meter. What happens on installation day walks through the order of work.

Rules that apply to a conversion

The work has to be done by a Gas Safe registered engineer and notified under Building Regulations. In England, fitting a combi also brings in the Boiler Plus requirements for controls. The new flue has to meet current positioning rules, which is sometimes why a boiler cannot simply go back where the old one was.

Getting a conversion priced

Because the cost depends on your mains flow, gas pipe, pipe runs and system condition, the only honest price is one that follows a look at the house. Our combi boiler replacement survey measures the flow rate, checks the gas supply and the existing system, and tells you plainly if a combi is the wrong call before anything is quoted. The quote you get is fixed and in writing. Book a free survey to find out what changing to a combi would involve in your home.