Yes, you can usually swap an old conventional, regular or system boiler for a combi. Just don’t treat it like a like-for-like swap. A combi heats water straight from your mains, so the job only works if your water supply, pipes and layout can handle it. An engineer should check those things before anyone gives you a price.
The quick version:
- A combi needs no hot water cylinder and no loft tank.
- It suits flats and homes with one or two bathrooms best.
- Weak mains water pressure is the most common deal-breaker.
- A conversion costs more than replacing one combi with another.
Can You Replace an Old Boiler With a Combi Boiler?
How much work is involved depends on what you have now. Most older London homes have one of three setups.
| Your old boiler | What usually comes out | How involved |
|---|---|---|
| Conventional (regular) | Boiler, hot water cylinder, loft tanks | Medium to high |
| System | Boiler, hot water cylinder | Medium |
| Back boiler | Boiler, fire surround, flue, often old pipes | High |
Conventional or regular boilers
This is the most common conversion. A regular boiler works with a cylinder in the airing cupboard and tanks in the loft. A combi replaces all of that with one wall-mounted unit, so the engineer removes the tanks and cylinder and reworks the pipes.
System boilers
A system boiler has no loft tank, but it still feeds a hot water cylinder. Going to a combi means removing the cylinder and changing the hot water pipework. It is often a little simpler than a regular boiler conversion.
Back boilers
A back boiler sits behind a living room fire. Taking it out means dealing with the fireplace, the flue and usually old pipes. The combi normally goes somewhere new, like a kitchen cupboard, so expect more time and cost.
Will a combi suit your home?
A combi suits homes with modest hot water demand and a healthy mains supply. Three things decide it.
How much hot water you use at once
A combi heats water as you use it, so it has a ceiling. Think about a normal Monday morning. If one person showers while someone fills the kitchen sink, a combi copes fine. If two showers and a washing machine run together in a three-bathroom house, it may struggle.
Your mains water pressure and flow
This is the big one. A combi takes cold water straight from the mains, so weak mains means weak hot water. Two numbers matter: pressure (in bar) and flow (in litres per minute).
As a rough guide, many combis want about 1 bar and around 10 litres per minute. Bigger models want more. Your engineer should measure both at your property, ideally with other taps running. A loft-tank system often gives weak showers, so a combi can feel like a big upgrade, but only if the mains can deliver.
Your property type
- Flats: Usually a good fit. Removing a cylinder frees up a cupboard. Check your lease and the flue route first.
- Victorian and Edwardian terraces: Very common candidates, though older pipework and narrow access can add work.
- Larger houses: Fine for small households. Big families often do better with stored hot water.
What happens on installation day
Expect the old boiler, tanks and cylinder to come out, the pipework to change and the new combi to go on the wall. Here is the usual order.
- The engineer surveys the system, mains supply, gas pipe and flue route.
- The heating system is drained.
- The old boiler is removed.
- The cylinder and loft tanks are removed or safely capped.
- Heating and hot water pipes are altered and old connections are removed.
- The combi is mounted and connected to gas, water and heating.
- The flue and condensate pipe are fitted. The condensate pipe carries acidic water to a drain and needs a sensible route so it doesn’t freeze.
- The system is cleaned, and a magnetic filter goes on to catch sludge.
- The engineer commissions the boiler and completes the Benchmark record.
A Gas Safe registered engineer must do the gas work. New boilers in England also have to meet Building Regulations on efficiency and controls, and your installer should handle that paperwork.
How much does it cost to replace an old boiler with a combi?
Most conventional-to-combi conversions run from about £2,500 to £5,500 or more, and London jobs often sit at the upper end. Published prices vary a lot, so treat these as a guide and get a written quote after a survey.
| Conversion | Complexity | Indicative cost |
|---|---|---|
| Combi to combi | Lower | £2,000–£3,500 |
| System to combi | Medium | £2,800–£4,500 |
| Conventional to combi | Medium to high | £3,000–£5,500 |
| Back boiler to combi | High | £3,500–£6,000+ |
What moves the price most:
- Boiler size and brand
- How much pipework changes
- Whether the boiler moves to a new spot
- Loft access and removing the tanks and cylinder
- A tricky flue route
- Smart controls or weather compensation
- Parking and access on your street
How long does the conversion take?
Most conversions take about two days. A straight combi-for-combi swap is often done in one. Back boiler jobs, boiler moves and badly sludged systems can stretch to three or four days. You will usually be without heating and hot water for part of that time, so ask how your installer plans to limit the gap, especially in winter.
Pros and cons of switching to a combi
A combi gives you hot water on demand and frees up space, but it has limits. Here is the honest picture.
What you gain
- Hot water whenever you need it, with no cylinder to reheat
- A cupboard back, and no loft tank
- Mains-pressure showers if your supply is strong
- A newer, usually more efficient boiler
What you give up
- Hot water for several outlets at once can dip
- Performance depends on your mains supply
- Conversion costs more than a straight swap
- No stored hot water if the boiler breaks down
Savings on bills depend on your old boiler, your controls and your habits, so be wary of anyone who promises an exact number.
Combi vs conventional boiler
Neither is better for everyone. It comes down to your home and how you use hot water.
| Combi | Conventional | |
|---|---|---|
| Hot water | On demand | Stored in a cylinder |
| Cylinder and loft tank | Not needed | Needed |
| Space needed | Less | More |
| Several taps at once | Can be limited | Handled better |
| Water pressure | From the mains | Often gravity-fed |
| Best for | Flats, small and medium homes | Larger homes, high demand |
A system boiler with a cylinder sits in the middle. It gives stored hot water and mains pressure without needing a loft tank.
Will you need new pipework?
Often some, yes. How much depends on what you have now.
- Gas pipe: It may need upgrading if it is too narrow for the new boiler.
- Heating pipes: Flow and return pipes usually need reconnecting or altering.
- Hot water pipes: These change to take their supply from the combi.
- Old tank and cylinder connections: They must be removed or properly capped.
- Mains water pipe: Some older London homes still have narrow or lead supply pipes that limit flow.
- Condensate pipe: It needs a safe route to a drain.
Your quote should list all of this so nothing comes as a surprise.
Can you do it in a London home?
Yes, in most London homes, but the property shapes the job.
- Flats and maisonettes: Popular and space-saving. Check shared walls, flue routes and any freeholder rules.
- Victorian and Edwardian terraces: These often have loft tanks and airing cupboard cylinders, so expect some pipework changes.
- Top-floor flats: Mains pressure can be lower, so a flow test matters.
- Tight access: Narrow stairs and street parking affect how the work is planned.
Mains pressure varies between boroughs and even between streets, so test it on site rather than assuming.
When a combi is the wrong choice
Keep stored hot water if your household often uses several outlets at once. A combi is probably not right if:
- You have three or more bathrooms in regular use
- Mains flow or pressure is low and can’t be improved
- Your current system already works well
- There is no practical flue route
- You plan to add a heat pump soon, since you will need a cylinder anyway
In these cases a system boiler with a cylinder, or an efficient regular boiler, is usually smarter.
What size combi do you need?
Pick the size from your home’s heat loss and hot water demand, not by buying the biggest one. As a rough guide, 24 to 27kW suits flats and one-bathroom homes, 28 to 32kW suits many two- and three-bedroom homes, and 33 to 40kW suits larger homes. An oversized boiler can switch on and off too often, which wastes energy. A proper heat loss calculation is the best way to get it right.
What a good survey checks
A good survey looks well beyond the boiler. Your engineer should check:
- Your current boiler type and age
- Where the boiler sits now and where it could go
- Property layout and radiator sizes
- Hot water demand
- Mains pressure and flow rate
- Gas pipe size
- Flue route and condensate drainage
- Existing pipework and system condition
- How the cylinder and tanks will come out
- Access and heating controls
If a company quotes for a combi without visiting or asking detailed questions, be cautious.
Frequently asked questions
Can I replace my old boiler with a combi boiler?
Yes, in most cases. A conventional, system or back boiler can be replaced with a combi if your mains supply, pipework and flue route allow it. An engineer must survey first, because it is a conversion, not a straight swap.
Can I change a conventional boiler to a combi?
Yes. The cylinder and loft tanks usually come out, the pipework is altered and the combi connects to the mains. It is common in London flats and terraces, but it takes longer and costs more than combi-to-combi.
Can I replace a system boiler with a combi?
Yes. A system boiler already has a sealed heating circuit, so there is no loft tank to remove. The cylinder usually comes out and the pipes change. It is often simpler than converting a regular boiler.
Can I replace my boiler and hot water tank with a combi?
Yes, if your mains flow and pressure are good enough. The combi heats water on demand, so the cylinder is no longer needed. Large households may find one combi can’t meet peak demand.
How much does it cost to change an old boiler to a combi?
Typically £2,500 to £5,500 or more, depending on boiler size, pipework, flue and access. London labour and access can push costs higher. Always get a written quote after a survey.
How long does a boiler-to-combi conversion take?
Usually about two days. A like-for-like combi swap can take one. Back boiler conversions, boiler moves and sludged systems can take three days or more.
Do I need new pipework for a combi boiler?
Often some. Heating and hot water pipes usually change, and old tank connections must be removed or capped. The gas and mains water pipes may need upgrading if they are too narrow.
Is a combi suitable for a two-bathroom house?
Often yes, if mains flow is strong and two showers rarely run together. If they do, a larger combi or a system boiler with a cylinder may work better.
Is changing to a combi worth it?
For many small and medium homes, yes. You gain space and hot water on demand, and a modern boiler is usually more efficient. It is less worthwhile in large homes with high demand or weak mains supply.
Can I fit a combi in a London flat?
Yes, combis are very popular in London flats. Check the flue route, building rules and your mains supply first. Top-floor flats can have lower pressure, so a flow test is important.
So, is a combi right for your home?
In most London homes the answer is yes, as long as your water supply and hot water habits suit it. The boiler itself is rarely the problem. Mains flow, pipework and household size usually decide it.
Before you commit, book a survey with a Gas Safe registered engineer. They can test your water pressure, check your pipes and flue route, and tell you plainly whether a combi, a system boiler or a regular boiler fits best.
Thinking about a combi? Book a boiler assessment and get a clear, tailored quote for your London home.