Gas Boiler Installation and Connection

People often mix up the terms, so it helps to know exactly what you are ordering. Fitting means mounting the boiler in place, including brackets and the connection kit. Connecting means linking the mounted boiler to each of the services. A replacement also includes taking out the old appliance.

The job usually covers:

• Mounting the boiler and connecting it to the gas pipe through a gas isolation valve
• Connecting the heating flow and return
• Connecting cold and hot water on a combi boiler
• Connecting the flue and the air intake
• Running the condensate drain to the waste pipe on a condensing boiler
• Filling the system, bleeding air, testing for gas leaks and first start-up

Before you sign anything, check whether the quote includes removing and disposing of the old boiler, pipe alterations, a flue liner and the safety inspection. What is included varies a lot between installers.

For a replacement, the engineer first turns off the gas, drains the system, isolates the electrics and removes the old boiler. They take care not to damage pipes that will stay in use. Then they check the wall, the connections and the condition of the chimney or flue.

The new boiler is mounted according to the manufacturer's instructions, keeping the required clearances from walls and combustible materials. The gas connection is made with approved fittings and seals, never with a random hose from a DIY shop. A magnetic filter or dirt separator is usually fitted to the heating circuit so that sludge from old radiators does not block the new boiler's heat exchanger. A pressure relief valve, a pressure gauge and a way to bleed the system must also be in place.

Next come the flue and electrical connections, filling, bleeding and the tightness test on the gas joint. At first start-up the engineer sets the output, temperatures and controls, and measures the flue gases with an analyser. A straightforward like-for-like swap usually takes one day. More complex alterations take longer.

New domestic boilers are now almost always condensing boilers. They also recover heat from the flue gases, so the exhaust is cool and condensate (water) forms inside. This affects the installation in three ways:

• The condensate has to be drained to the waste pipe, sometimes through a neutralising unit if the manufacturer requires it
• An old masonry chimney is usually not suitable, so a plastic or stainless steel liner is fitted, or a concentric pipe-in-pipe flue through the wall is used instead
• The boiler only runs efficiently when it is set up properly, especially with weather compensation that adjusts the water temperature to the outdoor temperature

So if your home has an old open-flue boiler on a traditional chimney, swapping it for a condensing one is rarely a simple unscrew-and-replace job. Expect changes to the flue and a new condensate drain. If the boiler heats a separate hot water cylinder, connecting the cylinder adds more pipework.

The simplest case is replacing a boiler with a similar type in the same spot, where the connections line up, the flue is ready and a drain is close by. These things make the job longer and more expensive:

• Moving the boiler to a new position, which means new pipe runs
• Needing a flue liner or a new flue through the wall or roof
• No drain near the boiler for the condensate
• A dirty heating system that needs flushing first
• Electrical changes or wiring a new thermostat
• Tight space, for example a boiler inside a kitchen cupboard

In a block of flats, any change to a shared chimney or a flue through the facade must be agreed with the building management or owners' association. If the flue route or the look of the facade changes, ask your local building authority what you need to arrange under local building regulations. Do not assume a simple notification will be enough.

Gas work is regulated in practically every country. Only a qualified, registered gas engineer may connect the boiler to the gas supply. Commissioning is often done by a technician approved by the manufacturer, because otherwise the warranty may not apply. Any electrical work, such as a dedicated circuit or a fused spur, is done by a qualified electrician.

Once the job is finished, you should have:

• A record of the gas tightness test
• A safety inspection report or certificate for the gas installation
• Confirmation that the flue or chimney has been checked, where local rules require it
• A commissioning record with the measured flue gas values

Your insurer and the manufacturer may ask for these documents if there is a claim. In some countries the installer must also register the new boiler with a national body or notify building control. Ask your installer what applies where you live. The boiler is installed according to the manufacturer's instructions and the relevant EN standards. It then needs regular servicing, usually once a year, and safety checks at the intervals set by local rules.

In practice the same problems come up again and again:

• Uncertified seals or hoses on the gas line and no tightness test
• A condensing boiler connected to an old unlined chimney, which then gets damp and deteriorates
• A condensate drain without a trap, so drain smells come into the room
• No filter on the return, so sludge from old radiators clogs the heat exchanger
• The boiler left on factory settings with the water too hot, so it hardly condenses at all
• Room-sealed ventilation ignored: on boilers that take air from the room, air vents blocked after new windows are fitted

If you smell gas, do not switch the boiler on, do not use light switches or naked flames, turn off the gas at the meter, open the windows and call your gas emergency line or 112 from a safe place outside.

Before the engineer leaves, go through these points together:

• There is a gas isolation valve at the boiler and it is easy to reach
• The system has a pressure gauge, a pressure relief valve and a filter
• The water pressure on the gauge is within the range the manufacturer specifies
• The boiler responds to the thermostat and provides heating and hot water without error codes
• The condensate drain has a trap and nothing drips
• The flue is firmly fixed and the joints are secured
• You have the tightness test record, the safety certificate, the flue confirmation and the commissioning record
• The engineer has shown you how to top up the pressure, bleed radiators and read the fault codes
• You have the service contact and know when the first annual service is due