Combi Boiler Installation Glossary

Combi Boiler Installation terms explained in plain English

Definitions of the technical terms, parts and standards that come up on quotes and service sheets.

Combi Boiler (Combination Boiler): A compact single-unit gas or oil boiler that provides both space heating for radiators and instantaneous domestic hot water directly from the cold mains, removing the need for an attic cold tank or hot water cylinder.
Plate Heat Exchanger: The secondary stainless-steel heat exchanger inside a combi boiler where hot primary circuit water transfers heat across thin metal plates to incoming cold mains water on demand, without the two water streams mixing.
Diverter Valve: A motorised internal valve in a combi boiler that switches heated primary water between the central heating radiator loop and the domestic hot water plate heat exchanger whenever a hot tap or shower is opened.
DHW Flow Rate (Litres per Minute): The volume of hot water a combi boiler can deliver at a 35C temperature rise, typically 10 to 12 L/min on a 24 kW unit, 12 to 14 L/min on a 30 kW unit, and up to 16 L/min on a 35 kW unit.
Dynamic Mains Water Pressure: The working pressure of the incoming Irish Water cold supply measured in bar while a tap is open and flowing, as opposed to static pressure when all taps are shut; combis require at least 1.5 bar dynamic pressure.
Sealed Pressurised System: A closed central heating loop filled from the cold mains via a filling loop to around 1.0 to 1.5 bar cold, using an internal expansion vessel and pressure relief valve instead of an open attic feed-and-expansion tank.
Cylinder-to-Combi Conversion: The pipework upgrade that decommissions an aired-cupboard copper cylinder and attic cold storage tanks, reroutes bathroom cold taps onto direct mains pressure, and connects hot outlets to the new combi boiler.
Condensate Drain and Plume Pipe: The 21.5 mm plastic drain pipe that carries slightly acidic condensed water vapour from a high-efficiency condensing combi heat exchanger to an internal soil stack, sink waste, or insulated external drain.
Magnetic System Filter: An inline canister fitted on the 22 mm heating return pipe immediately before the boiler to capture black magnetite sludge and metallic debris before it can foul the combi plate heat exchanger or pump.
Weather and Load Compensation: Smart modulating boiler controls that adjust the radiator flow temperature up or down based on outdoor sensor readings or room temperature deficit, keeping the combi in condensing mode below 55C return temperature.
Flue Terminal Clearances (IS 813): Mandatory minimum distances under Irish Standard IS 813 between a balanced room-sealed boiler flue outlet and any openable window, door, air vent, gutter, boundary wall, or opposite surface.
Gas Line Sizing (22 mm vs 15 mm): Upgrading the copper gas supply pipe from the meter to the boiler to 22 mm (or 28 mm on long runs) so that a high-output 30 kW or 35 kW combi maintains full working gas pressure with less than 1 mbar pressure drop.
RGII Conformance Certificate: The numbered safety certificate issued by a Registered Gas Installer in Ireland after commissioning a new gas combi boiler, verifying soundness testing, flue gas analysis, and compliance with IS 813.
Power Flush and Chemical Cleanse: A high-flow cleansing process using a flushing machine, heated dispersant chemicals, and magnetic filtration to strip iron oxide sludge from existing radiators and pipework prior to connecting a new combi boiler.
Thermostatic Mixing Valve and Shower Compatibility: Matching shower valves to high-pressure mains hot and cold feeds; a combi conversion eliminates gravity-fed shower booster pumps and works directly with thermostatic mixer showers rated for balanced mains pressure.

Combi boilers, by job

What each job costs and what's involved.

A kitchen with a white wall-hung boiler fitted on the wall between the units

Combi boiler costEvery price, by size and job.More on combi boiler cost →

A wall-hung boiler with its front cover off, on a workshop test rig

Converting to a combiCylinder and tanks out, combi in.More on converting to a combi →

A white wall-hung gas boiler on a grey kitchen wall beside wooden slatted units

Oil combi boilersFor homes off the gas network.More on oil combi boilers →

Combi Boiler Installation guides & price breakdowns

Straight answers to what people ask most: itemised costs, specifications, planning or grants, and what to check before you book.

Comparison GuideCombi or system boiler: which do you need?A combi heats water on demand from the mains. A system boiler keeps a cylinder. Which suits which home, and the prices.Read the guide →Specification GuideCombi boilers for apartmentsReplacing a combi in an apartment: size, flue rules, management company and price (€2,240–€3,000 for a 24 to 25 kW combi).Read the guide →Specification GuideCan a combi heat a large house?A combi in a 4 or 5-bed house: when it works, when a system boiler is better, and the sizes and prices.Read the guide →Grants & RegulationsIs there a grant for a combi boiler?No SEAI grant pays for a new combi. What SEAI does pay: heating controls, heat pumps and boiler scrappage.Read the guide →Cost & Pricing GuideWhat does a combi boiler cost to run?Gas costs about 13.0c a unit of energy, kerosene 12.6c, LPG 17.67c and electricity 34.38c, on SEAI's figures. What that means for a combi.Read the guide →Specification GuideElectric combi boilers: when they make senseAn electric combi has no flue and no gas, but electricity costs about 34.38c per kWh. Which homes suit one, and the alternatives.Read the guide →

More guides & resources: Combi boilers in every county · All guides & calculator hub · Technical & trade glossary · Frequently asked questions · About this site

Combi Boiler Installation across all 26 counties

Requests from all 26 counties go to one vetted local specialist who covers your county — from Dublin, Cork, Galway and Limerick to Kildare, Meath, Wicklow, Waterford and Donegal. Pick your county below for local prices, specifications and advice.