Back boiler removal: what is actually involved?

If you have a gas fire in the living room with a boiler hidden behind it, you have a back boiler. Plenty are still working.

When one finally has to go, the quotes come back further apart than for any other boiler job. That is not installers guessing. A back boiler replacement is two jobs stacked on each other: fitting a new boiler somewhere, and dealing with the hole in the wall where the old one lived. The second is the one that varies, and the one nobody explains before the price lands.

The decision that moves the price most

There are two legitimate ways to deal with the old unit and they are not the same amount of work. Full removal means the fire comes off, the boiler behind it comes out, and you are left with an opening to make good. Decommissioning in place means the gas is disconnected and capped and the appliance made permanently safe, but the casing stays, usually because getting it out means taking the fireplace and part of the chimney breast apart.

Both are done by a Gas Safe registered engineer. The Gas Safety (Installation and Use) Regulations control who may touch that pipework, so there is no version of this job a general builder can start.

One quote may have priced removal and the other decommissioning. Unless the document says which, the two are not describing the same day's work. It is the single most useful question you can put to a back boiler quote.

Source: HM Government, Approved Document J, paragraphs 3.4 and 3.5

The wildcard: what is inside the old appliance

This is the part no comparison site prints, and the reason a careful installer will not price a back boiler job over the phone. The Health and Safety Executive lists asbestos rope seals and gaskets as something found in gas and electric heating appliances, and says they may be found in some older domestic boilers. It also lists asbestos cement flues as something sometimes found in boiler systems, including domestic ones.

That is not a crisis. HSE treats removing intact rope seals and gaskets as lower risk, non-licensed work, provided the person doing it is trained to recognise asbestos and work safely with it. What changes it is the material being broken up during removal, at which point it becomes notifiable non-licensed work.

Nobody can tell you what is behind your fire until it comes off. A quote that has thought about it says what happens if asbestos is found. One that never mentions it has assumed the best case, or has not looked.

Source: Health and Safety Executive, Locations of asbestos and taking the right action

The chimney stops being a flue

A back boiler and its fire vent through the chimney. A modern boiler is room sealed: it draws combustion air from outside and sends the products of combustion back out through its own flue, so it has no use for your chimney at all.

Closed off completely at top and bottom, a redundant flue can trap moisture, which is how people end up with damp patches on a chimney breast a year after a boiler swap. The usual answer is a cap that keeps rain and birds out while still letting air through, plus a vent in whatever closes the opening at the bottom, so the flue can dry.

That is builder's work rather than gas work, which is where it gets missed. It can sit on the boiler quote, on a separate builder's quote, or on nobody's quote at all and become your problem in the spring.

One thing you must not do to the air vents

Older houses with an open flued appliance usually have permanent ventilation in the room, often an air brick, put there so the appliance had air to burn. If everything gas in that room is going, it is no longer needed. If anything gas remains, it very much is.

This is not opinion. Approved Document J sets out that no alteration is allowed to premises in which a gas fitting is fitted where that alteration would adversely affect the safety of the fitting and stop it complying with the regulations. Blocking an air brick another appliance depends on is exactly that. So the person deciding what happens to the vents is the Gas Safe engineer, not the plasterer.

Source: HM Government, Approved Document J, paragraph 3.5(h)

The heating system changes shape too

Back boilers were almost always part of an open vented system: a cold water tank in the loft, a feed and expansion tank beside it, hot water moving partly by gravity. A new boiler runs sealed and under pressure, so the job includes emptying the loft, capping the pipes that fed those tanks, and converting the system.

Go to a combi and the cylinder goes too, along with the airing cupboard, and every hot tap is then fed from the mains. A combi cannot give you more water than the street gives you, so here the mains flow rate reading is not a formality. It decides whether a combi suits the house at all.

Where the new boiler actually goes

Almost nobody puts the new boiler back into the chimney breast. It goes to a kitchen, utility, garage or cupboard: a new flue through an outside wall, a new gas run from the meter, a condensate drain that did not exist, and far more pipework than a straight swap.

Each brings its own constraint. The flue has to terminate a set distance from windows, doors, boundaries and air bricks. The condensate has to reach a legitimate drain. And the gas pipe has to be sized so the pressure lost between the meter outlet and the boiler stays inside the manufacturer's limit, which on a long run means a bigger pipe and a floor coming up.

Why we still see so many locally

Back boilers went in through the late 1960s, the 1970s and the early 1980s, and both towns have a lot of housing from exactly that window. Northampton was designated a New Town in 1968 and the development corporation estates at Lumbertubs, Moulton Park, Round Spinney and Camp Hill followed. Milton Keynes is a new town entire, and the first grid squares such as Netherfield, Beanhill and Fishermead went up through the 1970s. The older terraces in Semilong, St James and Far Cotton got them too, retrofitted into fireplaces already there.

So a local installer has almost certainly done your version of this job.

What to ask before you accept a price

You do not need to understand the work. You need the quote to say which version of it you are buying.

• Is the old back boiler being removed, or decommissioned and left in place?
• What happens if asbestos is found, and who carries that?
• Who caps and vents the chimney, and is it on this quote or a separate one?
• Who makes good the fireplace opening, the hearth and the wall?
• Are the loft tanks and cylinder being removed and the pipework capped, or left?
• What was the mains flow rate, if a combi is being proposed?
• How many days without heating and hot water, and do they run together?

The honest thing about the price

A back boiler removal is priced on unknowns, so the gap between two genuine quotes is wide for a real reason rather than a commercial one. What separates a good quote from a bad one is not the number at the bottom. It is whether those unknowns are named on the page or left to be found on the day.

Get three, and read them for what they say about the chimney, the asbestos and the making good rather than for the total.