Replacing a boiler in a flat: what is different

A flat is not a small house as far as a boiler replacement goes. Two questions come up that never come up in a house, and both of them get answered before anyone can sensibly price the work.

It affects more people around here than you might expect. The 2021 census put purpose-built flats at 17 per cent of households in Milton Keynes and just over 15 per cent in Northampton, and in central Milton Keynes they are close to all of it.

Source: ONS, Census 2021, TS044 accommodation type

Your lease comes first, not the boiler

Most flats are leasehold, which means the building belongs to somebody else and you own the inside of yours for a long fixed period. Whether you need permission to replace the boiler depends on what your lease says, and leases differ, so the honest answer is to read yours or ask the managing agent.

As a rule of thumb, a like-for-like swap in the same place, using the same flue hole, is the version least likely to need anyone's consent. The moment the job involves a new hole through an external wall, a flue terminal in a new position, moving the boiler to a different room, or altering pipework that runs outside your own flat, you are into alterations, and alterations are usually the landlord's to agree.

The Leasehold Advisory Service, which is the free government-funded service for this, makes two points worth carrying into the conversation. Where the alteration is an improvement, consent should not be unreasonably withheld. And do not rely on somebody saying yes on the phone: put the request in writing, keep a copy, and get the answer in writing too.

Source: The Leasehold Advisory Service, alterations and home improvements

Start that early. Managing agents work to their own timetable and a consent letter has stopped more boiler jobs in their tracks than any technical problem. If you are replacing a working boiler by choice, ask before you book anybody.

The flue decides the job

In a house the flue usually goes through whichever wall the boiler is nearest. In a flat the choices narrow quickly. You may have one external wall, or none you are allowed to use. The wall you do have may look onto a shared balcony, a walkway or a light well, all of which bring clearance rules into play. If the boiler sits in an internal cupboard, the flue has to travel to daylight, and any part of a flue running out of sight inside a ceiling void needs inspection hatches so an engineer can see it.

None of that makes the job impossible. It does mean the flue route, rather than the boiler, is what an installer is working out on a survey, and it is why two quotes for a flat can differ far more than two quotes for a semi.

Shared flues: the one that stops a job dead

This is the part almost nobody writes down for homeowners, and it matters because getting it wrong is dangerous rather than merely expensive.

Some multi-storey blocks, mostly built from the 1960s onwards, were constructed with a shared flue duct running the height of the building. Every flat's appliance draws its combustion air from that duct and sends its combustion products back into the same duct. The two common designs are called the SE-duct and the U-duct. The Energy and Utilities Alliance puts the number of UK homes with gas appliances connected to shared flue systems at around 350,000.

They were designed around the appliances of the day, which sent hot, buoyant gases up the duct. A modern condensing boiler does the opposite. Its flue gases are much cooler, so they rise less willingly, and they carry far more water vapour. Fit the wrong appliance and the flow inside the duct can break down, which means combustion products can be drawn into the boilers of the flats above and below yours along with the air they need to burn safely.

Source: Energy and Utilities Alliance, high-efficiency boilers and shared flue ducts

Which is why the standards are blunt about it. IGEM/UP/17, the industry standard covering shared chimney and flue systems, states that a condensing appliance shall not be fitted on a shunt-duct, SE-duct or U-duct system. BS 5440-1 states that only appliances designated by the manufacturer as suitable for the application may be used on SE-duct or U-duct installations. Gas Safe Register publish a technical bulletin listing appliances suitable for these systems, which is where an installer will look.

In plain terms: on a shared duct you cannot have any boiler you fancy. You can have one from a short list, and your installer has to confirm your flat is on such a system before choosing anything.

Source: Energy and Utilities Alliance, quoting IGEM/UP/17 clause 6.1.4 and BS 5440-1 clause 12.4.1; Gas Safe Register, Technical Bulletin 123, replacement room-sealed appliances for SE-duct and U-duct chimney systems

How to find out whether you are on one

Three things point to it, and none of them needs a tool:

• The block is multi-storey and was built somewhere between the 1960s and the early 1980s.
• Your boiler or water heater sits against an internal wall and there is no flue terminal visible on the outside of the building anywhere near it.
• Neighbours above and below have their appliance in the same position as yours.

The way to settle it is to ask the managing agent, freeholder or landlord for the shared flue inspection record. A shared duct is supposed to be inspected annually, and the certificate belongs to whoever is responsible for the building rather than to you. Ask for a copy before any new appliance is connected, not after. If nobody can produce one, that is a finding in itself and your installer will want to know.

Source: Energy and Utilities Alliance, high-efficiency boilers and shared flue ducts

If you are on a shared duct, you have three routes

None of them is a disaster, but they are not equal and only two of them are yours to choose.

• Fit a replacement appliance specifically approved for SE-duct or U-duct use. Simplest, and the usual answer.
• Take your flat off the shared duct entirely by fitting a conventional room-sealed boiler with its own balanced flue through an external wall. That opens up the whole market again, but it depends on your flat having a usable wall and on the freeholder agreeing to a new terminal. The old appliance has to be removed and its connection into the shared duct sealed properly.
• A coordinated replacement of every appliance in the block onto a proper shared chimney system. Genuinely the best long-term answer for an old duct, and not a decision one flat owner makes. It needs a survey of the whole system and the building owner behind it.

One warning that follows from all this. If an appliance has been fitted to a shared duct that was never approved for it, an engineer who spots it is required to treat it as an unsafe situation, and that can end with the boiler turned off. That is not a threat, it is the procedure, and it is a good reason to get the appliance selection right rather than accept the cheapest boiler somebody has in the van.

The smaller things that still change the price

Beyond the lease and the flue, a few practicalities show up on a flat quote and not on a house quote:

• The condensate drain. A combi produces acidic condensate that has to go somewhere. In a house that is often an outside drain. Several floors up there may be no drain to reach, so it has to be run internally to a soil stack, or lifted by a pump.
• Getting the old one out and the new one in. Stairs, lifts, narrow landings and no parking outside all add time, and time is what an installer is pricing.
• The building's rules. Many blocks set working hours, require notice, or want the contractor's insurance details in advance.
• The gas meter. In a lot of blocks it lives in a communal cupboard rather than in your flat, which matters if the supply needs any work.

What to settle before you accept anything

A short list, in the order they should be answered:

• Does my lease require consent for this, and has it been asked for in writing?
• Is this flat on a shared flue duct, and has the installer confirmed that rather than assumed it?
• If it is, is the appliance being quoted one that is approved for that duct?
• Where exactly is the flue terminating, and where is the condensate going?
• Has anyone seen the shared flue inspection record?

Everything on the gas side of this is a Gas Safe registered engineer's work, and in a flat it is not only your safety that depends on it being done properly. That is worth saying out loud, because a shared flue means your appliance choice reaches into your neighbours' homes.

If you want a flat priced honestly, ask for quotes from installers who have worked in blocks before and who will say the words shared flue without being prompted. We pass details to vetted Gas Safe registered firms around Northampton and Milton Keynes, with no obligation.