How a back boiler works
A back boiler heats water for your radiators and hot water cylinder from inside the fireplace opening, while a gas fire on the front heats the room. The two usually have their own gas controls but share the space and the chimney. The Baxi Bermuda is the best known example: Baxi's installation instructions for the Bermuda Inset 2 describe it as a combined central heating boiler and gas fire, fitted within a builder's opening in the living space.
Back boilers are open flued. They draw combustion air from the room and rely on the chimney to carry the fumes away, which is why the chimney and the room's air vents matter so much. Most are part of an older open vented system, with a hot water cylinder in an airing cupboard and a small feed and expansion tank in the loft. In some homes the hot water still circulates to the cylinder by gravity rather than with a pump.
You probably have one if there is a gas fire in the living room, no boiler on a kitchen or utility wall and a panel by the fire hiding a thermostat dial and ignition button.
Does a back boiler have a pressure gauge?
Most back boilers have no pressure gauge, because they run on an open vented system that is not pressurised. The feed and expansion tank in the loft keeps the system topped up through a float valve, so there is no filling loop to use and nothing to repressurise.
If your system does have a gauge, it has been converted to a sealed system, which some models allowed with an extra overheat thermostat kit. In that case the usual guide is about 1 to 1.5 bar when cold, but check with your engineer. Our guide to boiler pressure explains how to read the gauge.
Common back boiler faults and what they look like
Back boilers have no digital display, so there are no fault codes like those on modern boilers. You go by what the boiler, fire and pipes are doing.
| What you notice | Likely cause | What to do |
|---|---|---|
| Pilot light will not stay lit | Worn thermocouple, draught, or a flue or ventilation problem | Stop relighting after one try and call an engineer |
| Pilot lit but no heating or hot water | Programmer, room thermostat, boiler thermostat or pump | Check the timer and thermostats are calling for heat |
| Banging or kettling from the fireplace | Scale or sludge in the heat exchanger, poor circulation | Engineer |
| Water in the loft tank rising or overflowing, steam in the loft | Pumping over or overheating | Turn the boiler down or off and call an engineer |
| Loft tank empty, radiators cold at the top | Stuck float valve or a leak | Engineer |
| Yellow flames, soot, stained fire front, more condensation | Poor combustion or fumes not clearing | Stop using both appliances and call an engineer |
Noises are covered in more detail in our guide to a boiler making a banging noise, and most of the same causes apply to an old back boiler.
Relighting the pilot light safely
If the pilot has gone out, you can usually relight it yourself by following the user instructions for your model. For example, Baxi's instructions for the Bermuda Inset 2 tell you to hold in the grey button on the gas control, press the piezo igniter until the pilot lights, then hold the button for a further 15 seconds. If the pilot goes out, you turn the control off and wait 3 minutes before trying again.
- Do not relight anything if you can smell gas.
- If the pilot goes out again soon after lighting, stop. Gas Safe Register lists a pilot light that keeps going out as a warning sign of an unsafe appliance.
- Never take off the fire front or the boiler's inner panels to get at the burner. That is a job for a Gas Safe registered engineer.
Are back boilers dangerous?
A back boiler is not dangerous simply because it is old, but it carries more risk than a modern room sealed boiler if it is not checked every year. The main risk is carbon monoxide. If the chimney is blocked by a nest or debris, if a liner has failed, or if an air vent has been covered, fumes can spill back into the room instead of going up the chimney.
Gas Safe Register lists the warning signs: a floppy yellow flame instead of a crisp blue one, black or brown marks on or around the appliance, a pilot light that keeps going out and more condensation than usual on the windows. The NHS lists headaches, dizziness, feeling sick, tiredness, confusion and shortness of breath among the symptoms of carbon monoxide poisoning, and notes that they tend to ease when you leave the building.
Fit an audible carbon monoxide alarm in the room with the back boiler and test it regularly. If it sounds, or you feel unwell and suspect fumes, turn the fire and boiler off if you can, open doors and windows, get everyone outside and call the National Gas Emergency Service on 0800 111 999. Our guide to a carbon monoxide alarm going off explains what happens next. If you smell gas, follow the steps in smell gas near a boiler.
In England, landlords must fit a carbon monoxide alarm in any room used as living accommodation that has a fixed combustion appliance, other than a gas cooker, under the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022, which came into force on 1 October 2022.
What to do about the gas fire
The gas fire and the back boiler share a chimney, so treat them as one installation. A fault that stops fumes clearing affects both.
- Have both checked at the same visit. The fire front usually has to come off before the engineer can reach the boiler, so ask for the fire to be serviced and safety checked too.
- If an engineer says either appliance is unsafe, do not use it until it is repaired. If the problem is the chimney or ventilation, assume neither appliance can be used until it is put right.
- Using the fire on its own in summer, with the boiler off, is normal, as long as both passed their last check.
- When the boiler is eventually replaced, the fire in front of it usually comes out too, because it was made to sit on the boiler unit. You can fit a standalone gas fire once an engineer has checked the chimney, choose an electric fire or close off the fireplace.
Servicing and upkeep
Book a service and safety check every year, which is Gas Safe Register's general advice for any gas appliance. Baxi's instructions for the Bermuda Inset 2 also call for annual servicing. Ask for an engineer whose Gas Safe ID card covers the appliance types involved, and ask them to test that the chimney draws properly with a spillage test. Our guide to the annual boiler service explains what the visit includes.
- Have the chimney swept or inspected if the engineer recommends it.
- Never block the air vents fitted for the fire or boiler, even in cold weather.
- Look in the loft tank once in a while: it should hold some water, not be empty or overflowing.
- Ask whether the system contains inhibitor. Old open vented systems can build up sludge, which makes radiators cold at the bottom.
Parts availability
Parts are the biggest practical problem with a back boiler. Traditional back boilers have not been made for many years, and specialist suppliers report that while common spares such as thermocouples and pilot assemblies can still be found, manufacturer stock of older parts is shrinking. When a major part such as the gas valve or heat exchanger fails, there may be no replacement at all.
You cannot buy a new one either. New gas boilers fitted in England and Wales have had to be condensing since April 2005, apart from exceptions, and traditional back boilers are not. Baxi made a condensing version, the Bermuda BBU HE, but that has also been discontinued. Keeping your existing back boiler is legal as long as it is working safely.
At the annual check, ask the engineer two questions: can they still get parts for your model, and what is its condition. The answers tell you how long you are likely to have before a replacement becomes necessary.
Replacing it is a later decision
If the back boiler is working and passes its safety checks, you do not have to replace it now. Planning ahead means you are not choosing in a hurry in mid winter. Our guide to whether to repair or replace a boiler covers the questions to ask.
When the time comes, it costs more than a simple boiler swap because the new boiler usually goes somewhere else and pipework has to be rerouted. Checkatrade puts the average back boiler to combi conversion at around £4,000 supplied and fitted. That is a typical figure; the new boiler, its location and the pipework needed all change the price. See new boiler cost for wider pricing.
When to call an engineer
Call a Gas Safe registered engineer if the pilot will not stay lit after one careful relight, if you see yellow flames, soot or staining, if the loft tank is overflowing or empty, or if the fireplace is banging or leaking. Stop using both the fire and the boiler and get everyone outside if your carbon monoxide alarm sounds, and call the National Gas Emergency Service if you smell gas.