A combi can’t beat your mains
A combi can only heat the water that comes into the house. If the incoming flow is modest, a bigger boiler just heats the same trickle.
System boilers · Croydon
System boiler installation in Croydon
When 2 showers run at once and nobody wants to wait their turn, a system boiler with a hot water cylinder is usually the answer. We fit them across Croydon, sized to your home and your water supply, with the cylinder chosen to match.
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How it works
A boiler that heats a store of hot water
A system boiler heats your radiators and a hot water cylinder, usually in an airing cupboard. The pump and expansion vessel are built into the boiler, so the heating runs as a sealed system with no small tank in the loft to top it up.
Because the hot water is made in advance and stored, several taps and showers can draw on it at the same time. That’s the whole point. A combi heats water as it flows, so it has to share what it makes between every tap that’s open.
The trade-off is space and planning. You need room for a cylinder, and once the stored hot water is used up the cylinder takes time to reheat. Size it properly and most households never notice. Get it wrong and the last person in the queue gets a cool shower.
The obvious question
Why not just fit a bigger combi?
It’s what plenty of people ask, and for some homes a larger combi is fine. Here’s why it often isn’t in a bigger house.
Two showers split the flow
Open a second shower on a combi and both lose flow, and often temperature. A cylinder has hot water ready for both.
Larger homes, longer pipe runs
In a big house the furthest bathroom can be a long way from a combi in the kitchen. A cylinder near the bathrooms can shorten the wait for hot water.
A backup for hot water
Most cylinders have an immersion heater. If the boiler stops, you can still heat water with electricity while it’s fixed.
The cylinder
Vented or unvented cylinder: the plain version
A system boiler works with either. The difference is where the cold water comes from, which decides how your showers feel.
| Vented cylinder | Unvented cylinder | |
|---|---|---|
| Fed from | A cold water tank in the loft | The cold mains, directly |
| Shower pressure | Set by the height of the loft tank. Upstairs showers often need a pump | Mains pressure at every tap, if your supply is strong enough |
| Loft tanks | The cold water tank stays. The small heating tank can go | Both loft tanks can go |
| Safety devices | Simple. It’s open to the tank, so pressure can’t build up | Relief valves, an expansion vessel and a discharge pipe to a safe place |
| Who can fit it | A heating engineer | Someone with an unvented hot water qualification (G3) |
| Upkeep | Very little | An annual check of the safety devices, which manufacturers ask for |
An unvented cylinder depends on your mains flow just as a combi does. We measure it before recommending one, and we’ll confirm on your quote who will fit and sign off the cylinder.
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 |
What good looks like
What goes into a proper system boiler installation
The boiler gets the attention, but the cylinder, controls and clean system water decide how well it works. Check your quote covers all of these.
- 1
The right boiler, properly sized
Chosen for your hot water demand and the home’s heat loss, with mains pressure and flow measured on the survey. Not just the biggest one on the van.
- 2
A new flue, in a legal position
Flue terminals must sit clear of windows, doors, boundaries and eaves. A new flue kit goes in with every boiler.
- 3
Magnetic system filter
Catches the rust and sludge that circulate in older systems before they reach the boiler’s heat exchanger.
- 4
Gas supply sized for the boiler
Combis can need more gas than the boiler they replace. The pipe from the meter is checked and upgraded if it’s too small.
- 5
Condensate drain that won’t freeze
Routed inside to a drain where possible. Outside runs are kept short, insulated and wide enough not to block in a frost.
- 6
Controls that meet Boiler Plus
Time and temperature control as a minimum, and on a combi an extra energy-saving measure such as a smart thermostat with optimisation.
- 7
System cleaned and protected
Radiators and pipes flushed, then dosed with inhibitor to slow new corrosion. Required by the regulations and by every manufacturer.
- 8
Commissioned and signed off
Gas pressure and combustion checked with an analyser, the Benchmark checklist completed, Building Control notified and the warranty registered.
Near you
System boilers across Croydon
What the job usually looks like in the homes of each area, from the people who do it there.
FAQ
System boiler questions
What’s the difference between a system boiler and a regular boiler?
Both heat a hot water cylinder. A regular boiler also relies on a small tank in the loft to fill and top up the heating. A system boiler has the pump and expansion vessel built in, so the heating is sealed and that tank isn’t needed. See regular boilers.
Do I need an unvented cylinder with a system boiler?
No. A system boiler works with a vented cylinder fed from a loft tank, or an unvented one fed from the mains. Unvented gives mains-pressure showers but needs good mains flow and a G3-qualified installer. We’ll tell you which suits your home.
Is a system boiler right for a big house in Sanderstead or Purley?
Often, yes. Many homes in Sanderstead, Purley and Shirley have 2 or 3 bathrooms, and that’s where stored hot water earns its keep. It depends on how many showers run at once, not just bedrooms.
Will I run out of hot water?
A properly sized cylinder covers a normal busy morning. If everyone has long baths back to back, it can run low and then needs time to reheat. We size the cylinder to your household, not the other way round.
Can I switch from a combi to a system boiler?
Yes, if you have room for a cylinder. It means fitting the cylinder and new pipework to it, so it’s a bigger job than a straight swap. Tell us in the quote form message box and we’ll price it.
Guides
Read before you decide
- 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
- Choosing a boilerWhat size boiler do I need?What size boiler do you need? How kW, mains flow rate, bathrooms and heat loss decide it, why bigger isn’t better, and the rough sizes for flats and houses.Read the guide
Everything we do
New boilers
Conversions
Repairs & servicing
Get a price for a system boiler
Tell us how many bathrooms you have and where the cylinder is. Photos help us price it without a visit.