Installing a gas condensing boiler

Installation means the heating engineer's work fitting and hydraulically connecting one condensing boiler, most often a wall-hung boiler in a flat or house. It typically involves:

• fixing the mounting bracket or pre-piping frame to the wall and hanging the boiler level,
• connecting the heating flow and return, the cold mains supply and, for a combi boiler, the hot water outlet,
• fitting isolating valves, a magnetic system filter on the return and the filling loop,
• running the pressure relief valve discharge pipe to a safe point,
• draining the condensate via a trap to the foul drain,
• fitting the flue from the boiler through the wall, through the roof or into a lined chimney according to the design,
• filling, venting and pressure testing the heating circuit,
• cooperation during first firing and basic setup.

What installation does not normally include: the boiler itself, the flue as material, a chimney liner, a hot water cylinder, controls, removal of the old boiler and building work. The gas connection is gas work that must be done by a Gas Safe registered engineer and is described separately on the page gas boiler installation and connection. The electrical supply to the boiler, usually via a fused spur, and the wiring of the thermostat or programmer follow the manufacturer's instructions.

In the UK the installing Gas Safe engineer normally commissions the boiler and completes the Benchmark commissioning checklist in the boiler manual. Manufacturers usually require this, together with warranty registration, for the guarantee to be valid. Before ordering, check whether commissioning and warranty registration are included in the quote.

Most often it replaces a boiler at the end of its life. The old boiler stops working, spare parts are no longer available, or the owner decides to replace it during a renovation. In a new build, the boiler is fitted after plastering and screeding, once the heating and water pipework is in place and the flue route is prepared.

When replacing a boiler it pays to think beyond the boiler itself. A condensing boiler only condenses fully when the water returning from the radiators is cool enough. With old radiators run at a high temperature it condenses less and the saving is smaller than the brochure suggests. A lower flow temperature, weather compensation and a balanced system all help. If insulation work on the house is planned, it is worth choosing the boiler output with that in mind.

In England, a replacement boiler must also be fitted with time and temperature controls, and a replacement combi boiler needs one additional efficiency measure such as weather compensation, load compensation, smart controls with automation and optimisation, or flue gas heat recovery. Rules in Scotland, Wales and Northern Ireland differ in detail, so the installer should confirm what applies.

Before the installer arrives, the following should be settled:

• Boiler type and output. Output is chosen according to heat loss and hot water demand. The old boiler is often oversized.
• Flue route. Whether the flue goes horizontally through the wall, vertically through the roof, into an existing chimney with a new liner, or another solution. The terminal position must respect clearances from windows, doors, boundaries and the ground.
• Condensate disposal. An internal foul drain within reach of the boiler is preferred, ideally the soil stack or a waste pipe with a trap. If there is none, a condensate pump is used.
• Electrics. A fused spur or supply according to the instructions, with accessible isolation.
• Access and space. The manufacturer specifies clearances around the boiler for servicing.

Removing the old boiler and preparation

If the boiler is being replaced, the gas is isolated first, the system or part of it is drained and the old boiler is disconnected and taken down. This is a separate item described on the page removal of a gas or electric boiler. It is worth checking the condition of the system water at this point. Black water full of sludge is a sign that the system needs cleaning before the new boiler is hung.

Cleaning the system

Condensing boilers have a fine heat exchanger with narrow waterways. Sludge, magnetite and debris from years of use can block it within the first heating season. Manufacturers therefore require the old system to be cleaned and flushed, dosed with inhibitor and fitted with a filter on the return, in line with the British Standard code of practice for heating system water. Depending on the condition of the system, a chemical flush or a power flush is used. In practice the clean is sometimes skipped to make the job cheaper, and a year later there is a warranty claim the manufacturer rejects.

Hanging the boiler

The mounting bracket or pre-piping frame is fixed to the wall using the template. The wall must be able to carry the boiler. On plasterboard or a timber stud wall, a load-bearing support or noggins are used. The boiler is hung level, with clearances as stated in the instructions. During a renovation the question is often whether to keep the boiler in its original position or move it, for example from the kitchen into a cupboard or the loft. Moving it means new water, heating and gas pipework and a new flue solution.

Hydraulic connection

The boiler is connected to the heating flow and return and, for a combi boiler, to the cold mains and hot water outlet. Isolating valves are fitted on the connections so that the boiler can be isolated for servicing without draining the whole system. A magnetic filter belongs on the return. The pressure relief valve discharge pipe is run to a safe, visible point, usually outside, so that water does not end up on the floor in an overpressure situation. The filling loop is disconnected after filling. If the system has a separate hot water cylinder, it is connected via a diverter or zone valves according to the manufacturer's diagram.

Condensate disposal

Condensate drains from the boiler continuously, in a typical family home several litres a day during the heating season. It is mildly acidic. It is run in plastic pipe with a constant fall to the foul drain, through a trap or air break so that drain smells cannot reach the boiler. An internal route is always preferred. Where the pipe has to run outside, it should be as short as possible, upsized to at least 32 mm and insulated with weatherproof lagging. If the condensate freezes, the boiler locks out and shows a fault, a common winter call-out in the UK. With a soakaway, larger boilers or where the drainage owner requires it, a neutraliser is added.

Flue

A condensing boiler is room sealed and fan assisted. The fan draws in combustion air and pushes out the flue gases. The most common solution is a concentric flue, where the flue gases travel through the inner pipe and fresh air through the outer one. Flue gases also condense inside the flue, so horizontal sections must rise slightly away from the boiler, in other words fall back towards it, so that the condensate runs back into the boiler and not out of the terminal. The flue must be the system approved for that boiler, and joints are sealed and supported so they cannot pull apart. Condensing boilers produce a visible plume, so where the terminal would discharge close to a window, a neighbouring property or a walkway, a plume management kit is often fitted to redirect it.

If the flue gases are routed into an existing chimney, a new liner is almost always installed. A liner left from an old conventional open-flued boiler is not suitable for a condensing boiler, because it was not designed for acidic condensate or positive pressure. An existing liner can only stay if it is demonstrably rated for condensing operation. The chimney is usually swept first and then lined with a plastic or suitable flexible flue system approved by the boiler manufacturer.

Filling, testing and commissioning

The system is filled, vented and pressure tested. All joints, the pressure relief valve and the condensate route are checked, and the condensate trap is filled with water. Once the gas connection is complete and tightness tested and the flue has been checked, the engineer commissions the boiler. This includes a flue gas analysis, setting the maximum heating output and the heating curve, and showing the owner how to use the controls. Fine-tuning the controls is sometimes done as a separate job, see boiler control and adjustment.

Combi boiler

The boiler heats water for the radiators and also provides instantaneous hot water. It is compact and the most common choice for flats and smaller houses. With several bathrooms, or when a shower and the kitchen tap run at the same time, the hot water flow rate may not be enough.

System or regular boiler with a cylinder

The boiler only heats the radiators and the hot water is stored in a separate cylinder, unvented or vented. This suits homes with higher hot water demand. It takes more space and the installation is more involved because of the cylinder connection.

Flue types

• Horizontal concentric flue through the wall. The most common solution in the UK. Technically simple, but the terminal must keep clearances from windows, doors, gutters and boundaries.
• Vertical concentric flue through the roof. Common when the boiler is in the loft or on an internal wall below the roof.
• Into a chimney with a liner. Combustion air is drawn through the gap between the liner and the chimney or through a separate pipe. Used in older houses where a wall terminal is not possible or not wanted.
• Shared flue in a block of flats. If boilers are connected to a common flue system, any change is decided by the building owner and a specialist for the whole block. Nothing is altered individually.

As a guide, UK cost guides put a typical gas combi boiler for a flat or house at roughly £600 to £2,000 for the unit alone, depending on brand and output. A new flue route or chimney liner adds to the cost depending on its length and the solution chosen.

• New boiler on an old liner. In practice a boiler is sometimes connected to the original chimney without assessment. The condensate gradually attacks the liner and flue gases or condensate leak into the masonry.
• No clean and no filter. Sludge from the old system blocks the heat exchanger and the boiler stops working properly, often in the first winter.
• Condensate without a trap or exposed to frost. Drain smells in the room, or a frozen pipe and a boiler in lockout.
• Flue without a fall back to the boiler. Condensate drips from the terminal onto the wall and in winter icicles form beneath it.
• Oversized boiler. It cycles on and off frequently, wears faster and condenses less.
• High flow temperature. If the boiler is left permanently at 75 to 80 °C, it condenses little and the saving compared with the old boiler is small.
• Relief valve without a discharge pipe. At the first overpressure there is water on the floor or in the cupboard.
• Boiler boxed into a cupboard without clearances. The service engineer cannot reach the boiler and the manufacturer may dispute the warranty.

By law, anyone doing work on a domestic gas installation or gas appliance must be registered with Gas Safe Register for that type of work. In the UK the same Gas Safe registered heating engineer usually does both the hydraulic installation and the gas connection, then tests and commissions the boiler. You can check the engineer's registration number and the categories on their ID card before work starts. More on testing and certificates can be found on the page gas safety certificate and inspection. A carbon monoxide alarm in the room with the boiler is strongly recommended and in many situations required.

Fitting a new boiler is notifiable work. In England and Wales the registered engineer notifies it through Gas Safe Register, and the owner should then receive a Building Regulations Compliance Certificate, which you will need when selling the property. For a new flue position on a listed building, in a conservation area or through the external wall of a leasehold flat, check with the local authority and the freeholder whether consent is needed.

At the end of the job, ask the installer for the completed Benchmark checklist, confirmation of the Building Regulations notification, the gas test results and the warranty registration. You will need these documents for any warranty claim and when selling the flat or house.

• the boiler hangs level, is securely fixed and has the service clearances stated in the instructions,
• isolating valves are fitted on the connections and a magnetic filter on the return,
• the pressure relief valve has a discharge pipe to a safe point,
• the condensate pipe has a constant fall, runs through a trap, is not kinked and any external section is upsized and insulated,
• the horizontal flue section falls back towards the boiler and the terminal is firmly fixed,
• the penetration through the wall or roof is sealed and does not leak,
• the system pressure after filling matches the manufacturer's figure and does not drop noticeably over a few days,
• the connections do not drip and the boiler does not switch on and off every few minutes when heating,
• you have the Benchmark checklist, the gas test results and the Building Regulations notification.

Watch the pressure gauge for the first few weeks. A slow drop indicates a water leak or air in the system that still needs venting. If condensate drips from the flue terminal onto the wall, the flue does not have the correct fall.

The current labour price for installing a gas condensing boiler can be found in the NUMIQ price list under heating work. It is quoted per unit and covers the heating engineer's labour including small consumables, without the boiler itself, the flue, the chimney liner or other main materials. Labour prices vary by region and city.

The final cost is most affected by:

• whether the boiler stays in its original position or moves and needs new pipework,
• the condition of the system and the type of clean needed,
• the flue route, its length and whether a chimney has to be lined,
• the distance and height of the drain for the condensate, or the need for a condensate pump,
• connection of a hot water cylinder or several heating zones,
• related work such as removing the old boiler, the gas connection, new controls and commissioning.