The short answer
- Like-for-like combi swaps are the quickest type of boiler installation
- Converting from a tank-fed system adds time for removing cylinders and tanks
- Combis heat water on demand, so there is no stored hot water to run out of
- They can struggle when two showers run at once in larger homes
- Mains water pressure and flow need checking before a combi is recommended
Is a combi the right choice?
A combination boiler heats water directly from the mains as you turn on a tap, so there is no cylinder to warm up and no tank in the loft. That frees up cupboard space and suits flats, terraces and smaller semis with one bathroom very well.
The trade-off is flow. A combi can only heat so many litres a minute, and if two people shower at the same time the flow at each outlet drops. Homes with two or more bathrooms that are regularly used together often find a system boiler with an unvented cylinder a better fit. Your incoming mains pressure also matters; if it is weak, a combi will be disappointing no matter how powerful the boiler is.
Before installation day
A survey confirms the boiler position, the flue route, the size of the gas pipe back to the meter and the route for the condensate drain. The engineer checks water pressure and flow at the kitchen tap and works out what output the boiler needs. If the boiler is moving to a new wall, extra pipe runs and making good will be discussed at this point.
- Clear the cupboard or wall area around the existing boiler
- Make sure there is access to the gas meter and stopcock
- Let the engineer know about any upcoming kitchen or loft work
- Plan for no heating or hot water for at least part of the day
What happens on the day
The engineer isolates gas, water and electricity, drains the heating system and removes the old boiler and flue. On a conversion, the hot water cylinder and cold water tanks are taken out and the pipes that fed them are capped or rerouted.
The new mounting frame is fitted, pipework adapted to suit the new connections, and the flue installed through the wall or roof. Gas, heating flow and return, cold mains in and hot water out are connected, along with the condensate pipe. A magnetic system filter is normally fitted on the return pipe to catch sludge before it reaches the boiler.
The system is then cleaned, refilled, treated with inhibitor and fired up. Gas rates and combustion readings are checked, controls are set, and the boiler is registered with the manufacturer so its warranty starts.
How long does a combi installation take?
A simple swap in the same position is at the quick end, typically a day and occasionally spilling into a second. Moving the boiler to a new room adds time for new pipe runs and flue work. A full conversion from a regular or system boiler is the longest, often two to three days, because of the tank and cylinder removal plus the alterations to pipework in the airing cupboard and loft.
If the heating system is heavily sludged, a powerflush beforehand may be recommended, which adds to the programme but protects the new boiler.
After the installation
You should receive the commissioning checklist, building regulations notification and warranty paperwork. Most manufacturers require an annual service to keep their warranty valid, so it is worth putting a reminder in the calendar straight away. We install Viessmann and Grant boilers, which can carry manufacturer's warranties of up to 10 years on qualifying installations.
Frequently asked questions
- Will I lose my hot water cylinder when I switch to a combi?
- Yes. A combi heats water on demand, so the cylinder and loft tanks are no longer needed and are usually removed, freeing up space.
- Can a combi run two showers at once?
- It can, but flow drops at each outlet. Households that regularly use two showers together often suit a system boiler with stored hot water better.
- Does the new boiler have to go where the old one was?
- No, it can move, though relocating it adds pipework and flue work and therefore time. The flue position must meet regulations for distance from windows and boundaries.
- How soon after installation should it be serviced?
- Around twelve months after installation, then every year, which is what most manufacturers ask for to keep the warranty valid.
Want a straight answer for your own home?
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