Converting to a Combi Boiler: Should the Cylinder Go?
Converting to a combi boiler means taking out the hot water cylinder, the tanks in the loft and the pipework that served them, fitting a combi that heats water straight from the mains, and turning the heating into a sealed, pressurised system. It frees up a cupboard and the loft. It works well in a home with one bathroom and a strong incoming mains supply. It is a poor swap in a house with two bathrooms, a weak main, or showers that regularly run at the same time, because a combi can only heat as much water as the mains delivers. The cost is set by how much has to change around the boiler, not by the boiler itself: moving it, upsizing the gas pipe, a new flue route, removing the loft tanks and making good. Our hourly rate is on the pricing page. This guide covers what the job involves, what adds to it, and the questions to answer before you commit.
What a combi conversion involves
Most homes that ask about converting have either a regular boiler with a vented cylinder and two tanks in the loft, or a system boiler with a cylinder. The regular boiler setup is the bigger job, because more comes out.
On a regular system, the cold water storage tank that feeds the cylinder comes out of the loft, along with the small feed and expansion tank for the heating, and the vent and feed pipes that run between them. The cylinder comes out of the airing cupboard. The motorised valves, the cylinder thermostat and often the programmer are taken out or replaced. The new combi goes in, the hot water pipework is connected to the mains through the boiler, and the heating is filled and pressurised as a sealed system.
On a system boiler with a cylinder, the heating side is already sealed, so the loft work is smaller or not needed at all. The cylinder, its valves and its controls come out, and the new combi replaces the old boiler.
- Old boiler, cylinder and loft tanks drained and removed
- Redundant pipework capped or removed, including feed and vent pipes
- Motorised valves and cylinder controls taken out, new controls fitted
- Gas supply checked and upsized if needed
- Flue and condensate pipe routed for the new boiler
- Heating system flushed, filled, pressurised and tested as a sealed system
- Installation registered by the Gas Safe engineer
Check the mains before anything else
A combi does not store hot water. It heats the cold mains as it passes through, so the amount of hot water it can deliver is limited by the flow coming into the house. If the main is weak, the combi gives a weak shower, and a bigger boiler will not change that.
The simple test is to time how long the kitchen cold tap takes to fill a bucket with everything else in the house turned off, and work out litres a minute. Our guide on combi or system boiler with a cylinder explains what the result means and what flow a combi needs to be a real option. Test in the morning and in the evening too, because in a flat or a converted house what the neighbours are drawing affects your flow.
Older London houses often have a long run of narrow or lead pipe from the stopcock in the pavement. If that is what limits your flow, the conversion may need a new supply pipe first, which changes the job and the cost.
What drives the cost of a combi conversion?
Every conversion includes the same basic work: removing the old equipment, fitting and commissioning the boiler, and flushing and pressure testing the system. What makes two conversions on the same street very different is everything around the boiler.
Moving the boiler is the biggest variable. Leaving it where the old one was, if the flue can stay close to the same route, keeps the job simple. Moving it from a cupboard to the kitchen, or from the loft to the ground floor, means new gas, water, heating and condensate runs, and more making good.
The gas supply is the one people do not expect. A combi can draw much more gas than a regular boiler when it is heating water at full output, and a gas pipe that was fine for the old boiler may be too small. Upsizing it back to the meter is common and adds time.
Old radiators and pipework are the other risk. A vented heating system runs at very low pressure. A sealed system runs at a higher pressure, and old joints, radiator valves or corroded radiators that never leaked before can start to weep. A good installer pressure tests the system and tells you before the boiler goes on the wall.
- Boiler staying in place, or moving to a new position
- Gas pipe size from the meter to the new boiler
- Flue route and whether it meets clearances from windows and boundaries
- How much comes out of the loft and how easy it is to reach
- Condition of the radiators and pipework once the system is pressurised
- Making good the airing cupboard, loft hatch and any lifted floorboards
Any figure given before someone has looked at the gas route, the flue and the mains flow is a guess. Ask what each quote includes, and whether the gas pipe and the system pressure test are in it.
When converting to a combi is a mistake
A combi is a good fit for a flat or a small house with one bathroom and a strong main. It is a poor fit where more than one hot tap regularly runs at once, because the flow is shared and each outlet gets less. Two showers running together is the classic complaint after a conversion that should not have happened.
Think about the future too. If a heat pump is a realistic prospect in the next few years, it will need a hot water cylinder, and removing the airing cupboard now means finding space for a cylinder again later. Our guide on the heat pump hot water cylinder explains why. Solar thermal needs a cylinder as well.
If the reason you want to convert is weak pressure from a gravity fed cylinder rather than a need for more space, there is another route. Keeping stored hot water but switching to an unvented cylinder at mains pressure gives you strong showers from every outlet at once, and keeps the capacity for a busy bathroom. Unvented vs vented cylinder explains the difference, and unvented cylinder installation covers what that job involves.
- Good fit: one bathroom, one to three people, strong mains flow, space is tight
- Poor fit: two or more bathrooms, showers used at the same time, weak mains
- Think twice: heat pump or solar likely in the future
- Consider an unvented cylinder instead if the real problem is low pressure
How long does a combi conversion take?
A straightforward conversion with the boiler staying in the same place is usually a couple of days. Moving the boiler, running a new gas pipe, clearing a difficult loft or dealing with radiators that leak under pressure all add time. Expect to be without heating and hot water for at least part of the job, and plan for it in winter.
Getting advice before you decide
If you are weighing up a combi against keeping a cylinder, call 020 4652 5111. Tell us how many bathrooms you have, how many people live there and whether the showers run at the same time. We can check the mains flow, look at where the boiler and the flue would go and tell you honestly which setup suits the house. If the answer is a new boiler, boiler replacement covers what we look at, and our questions on what to ask before agreeing to a new boiler are worth reading before any quote.
Our hourly rate and call-out are on the pricing page and agreed before we attend. The rate is fixed; the total depends on how much has to change.
Questions we get asked
- How much does it cost to convert to a combi boiler?
- It depends mostly on what changes around the boiler, not the boiler itself: whether it moves, whether the gas pipe needs upsizing, the flue route, how much comes out of the loft and how the old radiators cope with a sealed system. Get a survey that checks those before you compare quotes. Our rate is on the pricing page.
- Can I remove my hot water tank and get a combi?
- Yes, if the mains flow into the house is strong enough and you do not regularly run more than one hot tap or shower at once. Test the flow at the kitchen cold tap first. In a home with two bathrooms, a combi usually gives worse showers than the cylinder did.
- Is it worth converting to a combi boiler?
- It is worth it in a small home with one bathroom, a strong main and a real need for the cupboard or loft space. It is not worth it where hot water demand is high, the mains is weak, or a heat pump is likely later, because you would need a cylinder again.
- How long does a combi conversion take?
- Usually a couple of days if the boiler stays in the same place. Moving the boiler, upsizing the gas pipe or fixing leaks that appear once the system is pressurised can add more time.
- Will my radiators leak after converting to a combi?
- They can. A vented system runs at low pressure, and a sealed combi system runs higher, so weak joints or corroded radiators may weep. A good installer pressure tests the system and deals with any weak points before the new boiler is commissioned.
- Is there an alternative to a combi if my shower pressure is poor?
- Yes. An unvented cylinder runs at mains pressure, so you keep stored hot water for several outlets at once and get strong showers. It suits homes with more than one bathroom better than a combi does.