System boiler
Heats a hot water cylinder, usually in an airing cupboard. Mains-fed, so no loft tanks.
Best for: Bigger homes with two or more bathrooms in use at the same time.
Read more →Combi conversions · Croydon
System to combi conversion in Croydon
Swap the cylinder, the tanks and the waiting for hot water for one boiler on the kitchen wall. A combi conversion can free up a whole cupboard, but only works well with the right water supply, so we measure it before we recommend it.
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Before and after
From a cylinder to hot water on demand
Whether you start with a system boiler or a regular one with loft tanks, you end up in the same place: one boiler, no stored hot water.
Regular boiler
Also called heat-only or conventional. A cylinder plus cold water tanks in the loft.
Best for: Older homes with low mains pressure, or where the pipework is built around it.
Read more →Combi boiler
Heats water on demand, straight from the mains. No hot water cylinder, no tanks in the loft.
Best for: Flats and most 1 to 3 bedroom homes with one bathroom, or two with a larger model.
Read more →The deciding factor
Your mains water decides whether a combi will work
A combi heats cold mains water as it passes through. So the hot water you get can never be more than the cold water coming into the house. Pressure is part of it, but flow, how much water arrives each minute, matters more.
Most Croydon homes have enough for a combi. Some don’t, especially high up a hill, on an old lead or narrow supply pipe, or where several homes share a supply. Fitting a combi there means weak showers that no boiler can fix.
That’s why we measure flow and pressure before we recommend a conversion. If yours is borderline, we’ll tell you, and a system boiler with an unvented or vented cylinder may serve you better.
The work
What a conversion involves
Usually 2 days, sometimes 3 if the boiler moves rooms or there’s a lot to take out of the loft.
Cylinder drained and removed
The cylinder comes out of the airing cupboard along with its valves and controls, and is taken away.
Loft tanks out, if you have them
On a regular system, the cold water and feed and expansion tanks are drained, disconnected and removed.
Hot and cold re-piped
Hot water now comes from the combi and cold from the mains. Redundant pipes are capped or taken out.
Shower pumps removed
A pump can’t be connected to the mains, so it comes out. Some older shower valves need changing to suit mains pressure.
Gas supply checked
A combi can need more gas than the boiler it replaces. If the pipe from the meter is too small, it’s upgraded.
Sealed and pressure-tested
An open-vented system becomes a sealed one at higher pressure. We flush and test it, and tell you about any weak radiator or valve.
Sizing guide
What size boiler do I need?
A rough guide for Croydon homes. Bigger isn't better: an oversized boiler cycles on and off and wastes gas. We confirm the size by checking your water pressure and flow rate on the day of the survey.
| Home | Bathrooms | Usually suits | Output |
|---|---|---|---|
| Flat or 1 to 2 bed house | 1 bathroom | Combi | Around 24 to 28kW |
| 3 bed house | 1 bathroom | Combi | Around 28 to 32kW |
| 3 to 4 bed house | 1 bathroom + en-suite | Larger combi | Around 32 to 38kW, if mains flow allows |
| 4 bed or more | 2+ bathrooms used at once | System boiler + cylinder | Sized to the home’s heat loss |
When not to convert
Sometimes keeping the cylinder is the better call
We’d rather talk you out of a conversion than fit one you’ll regret. These are the usual reasons.
2 or more showers at once
A combi shares its hot water between every tap open. Busy households with 2 bathrooms often find that frustrating.
Weak mains flow
If your incoming flow is low, a combi’s showers will be too. A larger combi doesn’t change that.
You want a hot water backup
A cylinder with an immersion heater gives you hot water if the boiler stops. A combi doesn’t.
You have solar water heating
Solar thermal panels heat water in a cylinder. Remove the cylinder and you lose what the panels do.
You’re thinking about a heat pump
Heat pumps work with a hot water cylinder. If you’re considering one in the next few years, think twice before giving up the space.
You love a deep bath
A combi fills a bath at the rate your mains allows. A cylinder can often fill it faster, at least until the stored water runs out.
FAQ
Combi conversion questions
Will my showers be better with a combi?
With good mains flow, usually yes, because you get mains pressure instead of gravity from a loft tank. With poor flow, they can be worse. We measure it first so you know before you commit.
What happens to my electric shower?
An electric shower runs from the cold mains and heats its own water, so it isn’t affected by the conversion. Pumped showers and power showers are the ones that change.
Will my old radiators cope with a sealed system?
Most do. Running at higher pressure can show up a weak valve or a pinhole in an old radiator. We pressure-test the system and tell you about anything that needs attention before we finish.
Can I convert a regular boiler with loft tanks?
Yes. It’s the same job with more to remove: both loft tanks come out as well as the cylinder. Choose “Regular (heat only)” in the quote form.
Can the combi go where the cylinder was?
Sometimes, if the airing cupboard has an outside wall or roof route for the flue, and room for the boiler. Many people prefer the kitchen. We’ll look at the options from your photos.
Guides
Read before you decide
- Choosing a boilerCombi boilers: water pressure and flow rate explainedWhy your mains water flow rate matters for a combi boiler, the difference between pressure and flow, a simple home test, and what to do if it’s low.Read the guide
- Choosing a boilerCombi vs system vs regular boiler: which is right for your home?Combi, system or regular boiler? How each one works, which homes each suits, and when changing type makes sense. A plain guide from Gas Safe engineers.Read the guide
- InstallationHow long does a boiler installation take?How long does a new boiler installation take? Like-for-like swaps are usually done in a day, moves and conversions take 1 to 3 days. What makes a job longer.Read the guide
Everything we do
New boilers
Conversions
Repairs & servicing
Find out if a combi will suit your home
Tell us about your bathrooms and showers in 2 minutes. We’ll check your water supply before recommending anything.