Converting to a combi boiler:the cost, and when it is the wrong choice

How much does it cost to convert to a combi boiler?

A conversion is priced as the boiler tier you choose plus a fixed amount for the conversion work. The conversion amount is the same at every tier, because the work of removing a cylinder or tanks does not change with the boiler brand.

Fitting the combi where the old boiler is, or in the airing cupboard the cylinder leaves, costs nothing extra. A new position, such as another room, the loft or the garage, adds +£850 for the extra gas, heating and condensate pipework and the new flue route. Other published uplifts can apply to any job, such as +£150 for a second bathroom or separate shower, +£395 for a vertical flue through the roof, or +£250 to replace a floor-standing boiler with a wall-hung one.

What do some real conversions add up to?

The quickest way to see your own total, with every line itemised, is the instant boiler quote. For what sits inside the tier price, see how much a new boiler costs.

What does a combi conversion involve?

• System to combi. The hot water cylinder is drained and removed, and its pipework capped. Hot water pipes to the taps are re-connected so they are fed from the combi, which heats water from the cold mains as you use it.
• Regular to combi. Everything above, plus the cold water storage tank and the feed and expansion tank in the loft come out or are disconnected, and the heating becomes a sealed, pressurised system.
• Back boiler to combi. The back boiler sits behind a fire in the chimney breast, so the fire, the old boiler and its pipework are removed and the new combi is fitted on a wall with its own flue.
• Regular to system. The loft tanks go, but a cylinder stays, now fed from the mains. This keeps stored hot water for homes that need it.

Every job also gets the same commissioning as a straight swap: a chemical flush and inhibitor to BS 7593, the manufacturer's system filter and a Boiler Plus compliant control. An open-vented system has only ever run at the low pressure of its loft tank, so a weak joint or tired radiator valve can show itself once the system is sealed and pressurised. If sludge is heavy and a chemical cleanse cannot recover the system, a deep powerflush is a £225 add-on.

What is not included in a conversion price?

• Chimney lining or capping when a back boiler is removed.
• Asbestos survey, testing or removal. HSE says asbestos may still be present in any building built or refurbished before 2000, and lists pipe and plant insulation and fireproof insulation panels among the places it was used.
• Gas supply work beyond the boiler, such as a meter upgrade or a larger supply pipe beyond 3 metres.
• Buried or concealed pipework that needs floors, walls or ceilings opened, and making good beyond a workable standard: no plastering, tiling, decorating or flooring.

If we find work outside the price, we stop, price it in writing and wait for your approval. If you decline, we leave the system safe and refund your deposit, less the call-out.

When is a combi the wrong choice?

A combi saves space and gives mains-pressure hot water without a tank, but it can only heat so much water at once. Keep a cylinder, or move to a system boiler, if any of these describe your home:

• Two bathrooms in use at the same time. Worcester Bosch describes system boilers as ideal for homes with more than one bathroom, supplying any number of taps at once. A combi shares its flow between outlets, so two showers both weaken.
• Low mains flow. A combi cannot deliver more than the incoming main supplies. Baxi's installer guidance gives the example that if the cold main supplies only 12 litres per minute, any combi above 28kW is out of the question.
• A heat pump in your plans. Vaillant states that all types of heat pump need a hot water cylinder. Removing the cylinder now can mean finding room for one again later. See heat pump costs and the Boiler Upgrade Scheme.
• Solar panels with a diverter. A diverter sends surplus solar electricity to an immersion heater in a hot water cylinder, so with no cylinder there is nowhere for it to go. Solar water heating also needs a cylinder: Worcester Bosch lists system and regular boilers as the types compatible with it.

How do you measure your mains water flow rate?

You need a measuring jug of 1 to 2 litres and a timer. Glow-worm's method for homeowners is:

1. Put the jug under the tap you want to test. For a combi, use the kitchen cold tap, which is normally fed straight from the main.
2. Turn the tap on fully and start a 6-second timer at the same moment.
3. Turn the tap off when the timer ends, and multiply the litres in the jug by 10. For example, 0.8 litres is 8 litres per minute.

Glow-worm treats above 15 litres per minute as ideal, 10 to 15 as satisfactory, and below 10 as low. Then compare the result with the combi's rated hot water flow, which manufacturers quote at a 40°C temperature rise. Worcester Bosch lists 9 litres per minute for its Greenstar 4000 25kW and 12 to 16 litres per minute across its Greenstar 8000+ combi range. A combi rated above what your main supplies will not perform as advertised. Test at a busy time of day as well, because Glow-worm notes mains pressure can vary with demand.

Is converting to a combi worth it?

For a one-bathroom home with good mains flow and no plans for a heat pump or solar diverter, usually yes. You free the cylinder cupboard, clear the loft tanks and get hot water at mains pressure, which Worcester Bosch notes can mean a more powerful shower. Every new combi in an existing home in England must also be fitted with one extra efficiency measure, explained in Boiler Plus controls.

For a busy two-bathroom house, or a home heading towards a heat pump, keeping stored hot water is often the better long-term answer even though it costs more today. The trade-offs are set out in combi vs system vs regular boilers.

Common questions