A replacement boiler should be sized from the building’s heat loss and hot water demand, not from the old appliance’s output. It is notifiable work, so you should receive a certificate. The system must be cleaned before the new appliance goes on it, and Boiler Plus sets requirements for the controls fitted alongside.
Oversizing, which is the norm
Read the output off the old boiler, fit something similar – that is how most replacements are specified. The problem is that the old boiler was very likely oversized when it went in, the house has probably had insulation and glazing since, and the replacement therefore compounds the error.
An oversized boiler does not make the house warmer. It short-cycles: fires, satisfies demand, shuts down, cools, fires again. Every cycle has a purge and an ignition, the appliance spends its time in the least efficient part of its modulation range, and on a condensing boiler it frequently runs at return temperatures too high to condense – which is the specific thing you paid for.
The right answer comes from a room-by-room heat loss calculation plus your actual hot water demand. On a combi, hot water demand usually dominates and sets the output; on a system boiler with a cylinder, space heating usually does. Either way it is a calculation, and it takes less time than choosing the brand.
Combi, system or regular
| Type | Suits | Limitations |
|---|---|---|
| Combi | One or two bathrooms, no cylinder, limited space, lower simultaneous demand | One outlet at a time in practice. Flow rate limited by the boiler. In hard water the plate exchanger scales |
| System boiler with unvented cylinder | Multiple bathrooms, simultaneous use, high hot water demand | Needs cylinder space and a good incoming main |
| System boiler with vented cylinder | Keeping an existing gravity hot water arrangement | Retains the loft tank and gravity pressure upstairs |
| Regular (heat only) | Older systems with loft tanks, or where the existing arrangement suits | Most components outside the boiler, more parts in the loft |
The combi question is usually decided by how many people want hot water at the same time. A combi heats water as it passes through, so two showers at once is beyond what any of them can do comfortably. A cylinder stores it, so simultaneous use is a matter of capacity rather than instantaneous power. See combi, system or regular.
Boiler Plus, and what has to be fitted with it
Since 2018, a replacement gas boiler in England has to meet a minimum efficiency and be fitted with time and temperature control. For a combi, one additional efficiency measure is also required, chosen from a defined list – typically weather compensation, load compensation, flue gas heat recovery, or smart controls with automation and optimisation.
The point is that a boiler alone does not deliver efficiency; how it is controlled does. Of the options, compensation is the one that makes the most difference, because it lets the boiler run at the lowest flow temperature that will heat the house – which is exactly where a condensing boiler is most efficient.
Ask what is being fitted to meet the requirement and why. See heating controls and Boiler Plus explained.
The things that separate quotes
Two boiler quotes rarely differ on the appliance. They differ on these:
- System cleaning to BS 7593. Required by manufacturers, and the most common reason a warranty claim gets declined when it is skipped.
- A magnetic filter, which several manufacturers now require.
- Controls. Whether what is fitted actually modulates the boiler, or just switches it on and off.
- Flue route and terminal position. Governed by minimum clearances to openings, boundaries and surfaces. Often the constraint on where a boiler can go, and always worth establishing at survey.
- Condensate routing. An external condensate run in small-bore pipe is a boiler that will shut down in January. Internal termination wherever possible; oversized, insulated and short where not.
- Gas supply and pipe sizing. A new boiler may need more gas than the existing pipework can deliver at the required pressure. This check is skipped remarkably often and produces a boiler that underperforms from day one.
- Electrical work, making good, and removal of the old appliance.
- Certification and notification.
What you should be left holding
- A certificate confirming the installation was notified under the Building Regulations. Keep it – a solicitor will ask for it.
- The manufacturer’s installation and user instructions.
- A completed benchmark or equivalent commissioning record, with the gas rate and combustion readings filled in.
- Warranty registration, done rather than left for you to do.
- A written record of the system clean and the inhibitor used.
- A demonstration, in person, of how the controls work and where the isolation is.
Getting quotes? Ask each one what size boiler and why. The answer tells you a great deal about the quote.
Questions we get asked about this
How long does a boiler replacement take?
A straight swap in the same position with no system problems is usually a day. Changing type, moving position, altering the flue or cleaning a badly sludged system all add time. We will give you a realistic figure at survey.
Should I switch to a combi?
It depends on hot water demand. One bathroom and modest use – probably yes, and you gain the cylinder cupboard. Two or more bathrooms used at the same time – probably not, and you will regret losing the stored water.
Does my gas pipe need upgrading?
Sometimes. A modern boiler needs a specific gas rate at a specific pressure, and older or long pipe runs cannot always deliver it. It is a straightforward check with a gauge and it should be part of every survey. See gas supply and pipe sizing.
Can the boiler go somewhere else?
Often, but it is governed by flue terminal clearances, condensate routing, gas supply and electrics. It is usually the largest single variable in the price, so it is worth discussing at survey rather than late.
Do I have to have the system cleaned?
Every manufacturer requires it, and they will decline warranty claims on a heat exchanger fouled by system debris. It is not an upsell – it is the condition on which the warranty rests.
What warranty will I get?
It varies by manufacturer and sometimes by installer accreditation. What matters as much as the length is the conditions: annual servicing, system cleaning, inhibitor and often a filter. A long warranty with unmet conditions is not a long warranty.
Regulations and standards referenced on this page
- Approved Document L: conservation of fuel and power (the Boiler Plus measures apply in England) — Ministry of Housing, Communities and Local Government
- BS 7593: Code of practice for the preparation, commissioning and maintenance of domestic central heating systems — BSI. Not free to read; available from BSI.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.