Back boiler replacement

Figures checked against installers' published price pages and SEAI's fuel cost comparison on 29 October 2026.

What a back boiler is

A back boiler is a small boiler built into the back of a fireplace, behind a gas fire or a solid fuel fire. The fire heats the water in it, and the hot water rises by gravity, or is pumped, to a cylinder and the radiators. They were fitted in Irish houses for decades, usually with a hot water cylinder in the hot press and a cold tank in the attic. Many are still working in houses built before 1980, which is 49% of Dublin's homes (CSO Census 2022, county tables).

Why people replace them

• Efficiency. SEAI's Domestic Fuels comparison gives a typical seasonal efficiency of 40–50% for an open fire with a high-output back boiler and 85–97% for a condensing boiler (SEAI, 1 July 2026, read 29 October 2026). Put simply, around half the fuel's heat goes up the chimney.
• Parts and servicing. Gas back boilers still need a yearly service by an RGI, and parts for old units are harder to find each year.
• The fire has to be on. Heating and hot water depend on the fire being lit, which no longer suits most households.
• Safety. An old unit, an old flue and a chimney that's been altered are the combination engineers worry about. An RGI who finds a hazard must issue a Notification of Hazard and may isolate the appliance (rgi.ie).

What replacing it involves

1. Survey.The installer checks the gas supply, the chimney, the cylinder and the pipe runs, and decides where a new boiler can go: a kitchen wall, a utility room, the hot press or the attic.
2. The back boiler is disconnected and capped.The gas supply to the fire is made safe. If the old unit stays in the fireplace, it's isolated; many people have it removed and the fireplace made good or blocked up.
3. New boiler, new flue.A wall-hung condensing boiler with a flue through the wall. The old chimney isn't used for the new boiler.
4. Pipework and the cylinder.A gravity-fed system becomes a pumped, sealed one. Keep the cylinder for a system boiler, or take it and the attic tank out for a combi.
5. Flush, fill, commission.A chemical or power flush, inhibitor, a magnetic filter, and the RGI's commissioning. You get a CERT 2 completion certificate for the boiler replacement.
6. Controls.A timer and thermostat, often zoned. This is where the €700 SEAI heating controls grant can apply, if the home was built and occupied before 2011 and approval comes first.

What it costs: the nearest sourced figures

Back boiler replacements are priced on survey, and on 29 October 2026 no Dublin installer page we could re-read gave a back-boiler figure. The closest published prices are for the job it most resembles, a gravity-fed system with a cylinder converted to a sealed system or a combi:

A back boiler job usually costs more than those, because of the extra work at the fireplace, the new flue route and the new pipe runs. Treat the table as the floor, not the price. Get the price in writing, with VAT, after a survey.

Combi or system boiler to replace it

If the house has one bathroom and decent mains pressure, a combi lets you take out the cylinder and the attic tank as well, which is often the point of the job. If two showers run at once, or a heat pump is likely in the next few years, keep a cylinder and fit a system boiler. The switching to a combi section covers what to ask, and our sister site has the full guide to converting to a combi boiler.

Grants

None for the new gas boiler: "The Better Energy Homes Scheme does not cover replacing a gas or oil boiler with a new gas or oil boiler." (Citizens Information, page edited 22 October 2026, checked 29 October 2026). The €700 heating controls grant may apply, and a house that scraps its back boiler for a heat pump instead can get up to €14,500. See boiler grants.

Before you ring

• Is the back boiler gas or solid fuel? Gas means an RGI for the whole job.
• Is there a hot water cylinder, and is there a tank in the attic?
• How many bathrooms and radiators?
• Where could a new boiler go, with an outside wall for the flue?
• Do you want to keep the fireplace, or block it up?

Questions people ask