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A new boiler in an Eltham home is a regulated installation, not just a swap. Efficiency rules, a Building Regulations notification and a mandatory carbon-monoxide alarm all apply — and on Eltham’s suburban and conservation streets, where the flue and condensate can go matters too.
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Fitting a boiler is gas work — a legal Gas Safe job — and a carbon-monoxide alarm now comes with it.
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On this page
What a boiler installation legally involves · Boiler Plus: the efficiency rules · The CO alarm the law requires · Combi, system or heat-only for an Eltham home · Flues, condensate and conservation areas · Hard water and protecting a new boiler · What it costs in Eltham — 2026 · How a compliant installation runs · Why verified engineers · FAQ
What a boiler installation legally involves in Eltham
Replacing a boiler is not a job anyone can legally do. The Health and Safety Executive states that anyone working on gas appliances in domestic premises must be Gas Safe registered and competent in that specific area of gas work¹ — the competencies are printed on the back of the engineer’s Gas Safe ID card, so a boiler ticket and a cooker ticket are not the same thing.
A boiler installation is also notifiable work under the Building Regulations. A Gas Safe registered installer self-certifies the installation as compliant and notifies it, which is what produces your Building Regulations compliance certificate — the document a solicitor asks for when you sell. After installation, the HSE advises that gas appliances, flues and pipework are serviced at least annually by a Gas Safe registered engineer¹, and most manufacturer warranties are conditional on that annual service being logged.
The practical takeaway for Eltham homeowners: a cheap quote from an unregistered fitter isn’t a bargain — it’s uncertified, uninsurable work that can surface as a problem at resale, and it’s illegal.
Boiler Plus: the efficiency rules for a new boiler
Since April 2018, England has had a minimum standard called Boiler Plus, set out in the government’s Boiler Plus factsheet and implemented through the Domestic Building Services Compliance Guide². It sets three things every new Eltham installation must meet:
- Minimum efficiency. A new gas boiler must have an ErP efficiency of at least 92%² — in practice, a modern condensing model.
- Time and temperature controls. Timers and room thermostats are an explicit requirement for all gas and oil systems² where not already present.
- A combi extra. When a combination boiler is installed, one additional energy-efficiency measure is required² — a choice of flue gas heat recovery, weather compensation, load compensation, or a smart control with automation and optimisation.
Watch this on a quote: a basic programmable thermostat does not satisfy the combi requirement. Ask the engineer to confirm which of the four measures they’re fitting and that it qualifies — a compliant install names it explicitly.
The carbon-monoxide alarm the law now requires
Fitting a new or replacement boiler now triggers a carbon-monoxide alarm requirement under Building Regulations Approved Document J. The government’s Approved Document J guidance sets out where the alarm goes: in the same room as the appliance, between one and three metres horizontally from it, and high on a wall but not within 150mm of the ceiling³ (or on the ceiling at least 300mm from any wall). Where the boiler sits in a cupboard people don’t normally enter, the alarm goes just outside it.
Separately, the HSE recommends CO alarms as a precaution and advises they comply with BS EN 50291 and carry a Kitemark or equivalent approval mark¹ — and stresses an alarm is a second line of defence, never a substitute for correct installation and annual servicing. A good engineer supplies and sites the alarm as part of the job; if yours doesn’t mention it, ask.
Combi, system or heat-only — matching the boiler to an Eltham home
Eltham’s housing is mostly interwar and post-war suburban — semis and terraces around Well Hall, Eltham Park, Avery Hill and New Eltham — alongside older cottages near the High Street and Eltham Palace. That stock shapes the right choice:
- Combi. No cylinder, no loft tanks, hot water on demand. Ideal for smaller homes and flats, and the most common conversion. But a combi’s flow rate is finite — in a three-or-four-bedroom Eltham semi with two bathrooms, simultaneous showers can disappoint.
- System boiler + unvented cylinder. Stored hot water at mains pressure, good for higher simultaneous demand — often the better fit for larger family homes. An unvented cylinder is itself notifiable work (Building Regs G3) and needs a suitably qualified installer.
- Heat-only (regular). Keeps the traditional cylinder and feed tanks. Sometimes retained in older properties where a full re-pipe isn’t wanted.
Converting a heat-only system to a combi removes the cylinder and loft tanks — freeing space but changing your hot-water behaviour. A good survey sizes the boiler to the home’s actual heat loss rather than “the same size as the old one,” because an oversized boiler short-cycles and wastes fuel (more on that below).
Flues, condensate and conservation areas
Two Eltham-specific issues catch installations out.
Flue position. The HSE notes that the Gas Safety (Installation and Use) Regulations 1998 and Building Regulations require flues to be installed in a safe position at specified distances from the boundary¹. On tightly-spaced Eltham terraces and semis, a like-for-like flue position isn’t always compliant — relocation or a plume management kit may be needed near a neighbour’s window or a boundary.
Conservation control. Parts of Eltham sit in designated conservation areas. Royal Greenwich lists Eltham Palace, the Progress Estate, Well Hall Pleasaunce and Sun in the Sands among its conservation areas, and notes an Article 4 direction applies to the Progress Estate⁴. An Article 4 direction removes some permitted-development rights, so alterations that would normally be automatic — including a visible flue terminal on a front elevation — may need planning permission. On a Progress Estate house, don’t assume a front-facing flue can simply be drilled; check with the council first.
Condensate. Every condensing boiler produces acidic condensate that must drain away. Where the pipe runs externally — common in suburban Eltham garages, side returns and lean-tos — it can freeze in a cold snap and lock the boiler out. A correctly sized, insulated internal route (or a properly lagged, adequately sized external one) is part of a compliant install, not an afterthought.
Hard water and protecting a new boiler
Eltham is in Thames Water’s supply area, and Thames Water is clear that all the water in its region is hard because it passes through chalky limestone⁵. Hard water scales the inside of heat exchangers and cylinders, and scale on a heat exchanger reduces efficiency and drives the “kettling” rumble over time.
A new boiler is only as protected as the system it joins. Best practice on installation is to clean the existing system, dose it with a corrosion-and-scale inhibitor, and fit a magnetic system filter to catch circulating debris — many manufacturers make warranty cover conditional on it. Our London hard water guide explains what scale does to a system over time, and whether a scale reducer is worth it on your mains.
What boiler installation costs in Eltham — 2026
Editorial estimate only — not official or regulated pricing, and no substitute for written quotations. Ranges reflect typical London 2026 supply-and-fit work; your actual price depends on boiler make and warranty length, access, flue route and any conservation-area constraints.
| Scenario | Typical supply-and-fit range |
|---|---|
| Combi swap — like-for-like, straightforward | £2,000–£3,200 |
| System/heat-only → combi conversion | £2,800–£4,500+ |
| New system boiler + unvented cylinder | £3,200–£5,500+ |
| Heat-only (regular) boiler replacement | £2,000–£3,500 |
| Back-boiler removal + combi | £3,000–£4,800 |
| Flue relocation / extension | £150–£500+ |
| Magnetic system filter (supply + fit) | £120–£250 |
| System clean + inhibitor at install | £150–£350 |
| Power flush (pre-install, where needed) | £400–£800 |
| Boiler Plus control (smart/compensating) | £150–£350 fitted |
| CO alarm (supply + fit) | £30–£80 |
| Condensate reroute / frost protection | £80–£250 |
Always get more than one written quote, and ask each engineer to itemise boiler, controls, flue, system clean, filter and CO alarm separately — our how to read a plumbing quote guide shows what a complete quote looks like.
How a compliant installation actually runs
A good Eltham install is a sequence, and understanding it helps you tell a thorough job from a rushed one.
Survey and sizing. The engineer measures the home’s heat loss and hot-water demand rather than copying the old output. Oversizing matters: an oversized boiler reaches temperature quickly and then short-cycles — firing and stopping repeatedly — which wastes fuel and wears components. Sizing to load is the difference between an efficient system and an expensive one.
Why condensing needs low return temperatures. A condensing boiler recovers latent heat by cooling flue gases enough to condense the water vapour in them. That only happens when the water returning from the radiators is cool enough — which is why controls that lower flow temperature in mild weather (weather and load compensation) genuinely save fuel, and why a well-balanced system outperforms a hot-and-fast one.
System cleanliness. A brand-new boiler plumbed onto an old, sludged system is a warranty claim waiting to happen — circulating magnetite scours the new heat exchanger. That’s why cleaning, inhibitor dosing and a magnetic filter aren’t upsells; they protect the appliance.
Commissioning. On handover the engineer runs a flue gas analysis, measuring the CO/CO₂ ratio to confirm combustion is within the manufacturer’s tolerance, and records it on the commissioning (Benchmark) checklist. Those analyser readings are the technical proof the boiler is burning safely — keep the record.
Why verified engineers in Eltham
A boiler installation is high-value, safety-critical and notifiable — exactly the work where credentials matter most. Every engineer listed here is checked before they appear: identity, insurance, trading presence and Gas Safe registration where gas work is involved, and re-checked annually. Because plumbers pay a flat monthly listing fee, there’s no customer-side introduction fee and no pay-to-play ranking deciding who you see first. Workmanship-guarantee badges show whether the installer backs the work for 1, 3, 6 or 12 months — and you should still confirm current Gas Safe registration and insurance on the day, and check the engineer holds the right boiler category for your appliance.
Listing checks are administrative and don’t replace your own checks or guarantee workmanship. For the full method, see how we verify plumbers.
Frequently asked questions
Yes.
A boiler installation is notifiable work.
A Gas Safe registered installer self-certifies and notifies it, producing your Building Regulations compliance certificate.
That’s the document you’ll be asked for when you sell.
Unregistered work has no certificate.
Keep three things.
Keep the Building Regulations compliance certificate.
Keep the commissioning record, usually the Benchmark record, showing the flue-gas-analysis readings.
Keep the manufacturer’s warranty registration.
You’ll need them for resale and for warranty claims.
Under Building Regulations Approved Document J, a carbon-monoxide alarm is required when a new or replacement combustion appliance is installed, other than a gas cooker.
It should be sited in the same room, one to three metres from the boiler and high on the wall.
A good engineer fits it as part of the job.
Roughly, one bathroom and modest demand suits a combi.
Two bathrooms or a busy family home often does better on a system boiler with an unvented cylinder.
That holds hot water at mains pressure.
A proper survey sizes it to your home rather than guessing.
Not automatically.
The Progress Estate has an Article 4 direction, which removes some permitted-development rights.
A visible external alteration like a front-elevation flue terminal may need planning permission.
Check with Royal Greenwich before the install is designed.
For most systems, yes — and many warranties require it.
Hard water and old pipework mean scale and magnetite circulate.
A system clean, inhibitor and magnetic filter protect the new heat exchanger and keep the warranty valid.
The HSE advises annual servicing by a Gas Safe registered engineer.
Manufacturers also make warranty cover conditional on a logged annual service.
Skip it and you risk both safety and cover.
Boiler installation across Greenwich — areas we cover
Boiler Installation Eltham · New Eltham · Mottingham · Kidbrooke · Blackheath · Greenwich · Charlton · Woolwich · Plumstead
Related services in Eltham
Central Heating Repair · Boiler Repair · Emergency Plumber · Leak Detection
Related guides
Combi vs System Boiler · Boiler Repair or Replace · London Plumbing Costs Guide · London Hard Water Guide · New Homeowner Plumbing Guide
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Source provenance
Regulatory and safety guidance on this page is drawn from primary UK sources: the Health and Safety Executive (Gas Safe registration required for domestic gas work; annual servicing; CO alarms to BS EN 50291; safe flue positioning under GSIUR 1998 and Building Regulations), the government’s Boiler Plus factsheet (minimum 92% ErP efficiency, time and temperature controls, and an additional efficiency measure for combi boilers in England), Building Regulations Approved Document J (carbon-monoxide alarm required on installation of a new combustion appliance, and its placement), Thames Water (hard-water region), and the Royal Borough of Greenwich (Eltham conservation areas, including the Progress Estate Article 4 direction). Cost figures are an editorial estimate only — not regulated rates and not official market data — and Eltham-specific housing observations are local editorial context, not official data.
Sources & further reading
¹ HSE — Gas safety: home owners: https://www.hse.gov.uk/gas/domestic/faqownerocc.htm
² GOV.UK — Boiler Plus factsheet: https://assets.publishing.service.gov.uk/media/5b2cc1e2ed915d586e2d8fe9/Boiler_Plus_Factsheet_v3.pdf
³ GOV.UK — Building Regulations Approved Document J (carbon-monoxide alarm amendments): https://assets.publishing.service.gov.uk/media/63355536d3bf7f34f3359673/220927_Divisional_Circular_letter_ADJ_alarm_amends.pdf
⁴ Royal Borough of Greenwich — Conservation areas: https://www.royalgreenwich.gov.uk/info/200194/conservation_areas_and_listed_buildings/28/protecting_the_character_of_local_areas/2
⁵ Thames Water — Hard water: https://www.thameswater.co.uk/help/water-and-waste-help/water-quality/hard-water
Last reviewed: July 2026 by Adiel Khan — SFEDI-accredited business advisor, 20+ years’ experience (South East Enterprise Ltd) and operator of VerifiedPlumbers. LinkedIn ↗. This page is checked for compliance and regulatory accuracy against HSE, Gas Safe Register, GOV.UK legislation, Thames Water and Royal Borough of Greenwich guidance. Source links are provided within the page where relevant.