Fitting a wall-hung electric water heater

The price list item covers hanging and connecting one wall-hung electric water heater with a capacity of up to 120 litres. It typically includes:

• checking the wall and choosing suitable fixings for the wall material and the weight of the full heater,
• fitting the wall bracket or hanging bolts and hanging the heater,
• connecting the cold feed and hot outlet with flexible hoses or fixed pipework,
• fitting the manufacturer's unvented safety kit on the cold feed (pressure reducing valve, check valve, expansion relief valve and expansion vessel),
• fitting the tundish and connecting the safety valve discharges to it,
• filling the heater, purging the air and checking all joints for leaks,
• connecting to a prepared electrical supply and the first heat-up,
• setting the temperature and explaining operation and maintenance to the user.

The installation does not include the heater itself, a new circuit from the consumer unit, chasing walls, new water pipework or a new discharge pipe run from the tundish to the outside if one is needed. Removing and disposing of the old heater is a separate item.

A new heater is usually fitted during a bathroom or flat refurbishment, once the pipework and tiling are finished. Before tiling, it must already be clear where the heater will hang, so that the water and electrical connections end up in the right place, the discharge pipe has a route to a safe termination and the wall can take secure fixings.

An old heater is replaced when the tank starts leaking, the water comes out rusty or discoloured, or the heater takes too long to heat even after descaling. It often happens that the owner only notices a problem when water starts dripping from the bottom of the unit or appears at the tundish. If the tank itself is corroded through, repair is no longer worthwhile and the whole heater is replaced.

Before installation, the following should be ready:

• A load-bearing wall. Brick, concrete or dense block. Plasterboard, aircrete blocks or hollow partitions need reinforcement or special heavy-duty fixings.
• Water connections. Cold mains and hot outlet with isolating valves at the correct height. An unvented heater also needs adequate mains pressure and flow, which the installer should measure.
• A route for the discharge pipe. The tundish must sit in the same space as the heater and the pipe below it must run with a continuous fall to a safe, visible termination, often outside.
• An electrical supply. A dedicated circuit sized for the heater's rating, ideally prepared by an electrician during the refurbishment.
• Space for maintenance. There should be room below or beside the heater to remove the element flange and inspect the anode, otherwise servicing means taking the whole heater down.

Checking the wall and fixings

Manufacturers only allow the heater to be hung on a wall that can carry it when full of water. Some installation manuals state that the wall fixings must be rated for around three times the weight of the full heater. The installer drills holes to match the bracket spacing, sets the fixings and hangs the heater. With larger heaters this is a two-person job.

Cold feed and safety kit

Wall-hung unvented heaters are supplied with a safety kit that goes on the cold feed. It normally combines a pressure reducing valve, a check valve and an expansion relief valve, plus an expansion vessel that takes up the extra volume of water as it heats. The tank itself carries a factory-fitted temperature and pressure relief valve. Manufacturers state that no valve may be fitted between the expansion relief valve and the heater. If anyone closed such a valve, the pressure created during heating would have nowhere to go. The isolating valve belongs upstream of the safety kit, never between the kit and the heater. Only the components supplied or approved by the manufacturer should be used.

Mains pressure and the pressure reducing valve

The pressure reducing valve keeps the incoming pressure below the opening pressure of the relief valves. Without it, high mains pressure would make the expansion relief valve discharge regularly. The expansion vessel's pre-charge must match the setting of the pressure reducing valve, as given in the manual. At the other extreme, many UK properties have low mains pressure or poor flow, and then an unvented heater will not perform as expected. The pressure and flow can be measured with a gauge and flow cup at a tap before the heater is chosen.

Tundish and discharge pipe

The relief valves discharge through a short metal pipe into a tundish, an open funnel that creates an air gap and shows any discharge. The tundish should be vertical, close to the valves, in the same space as the heater and clearly visible. Below it, the discharge pipe should be at least one size larger, run vertically for a short section and then fall continuously to a point where hot water or steam cannot hurt anyone. The pipe must be made of a material that can withstand very hot water. The discharge must never be capped or blocked.

Hot outlet and filling

The hot outlet is connected to the hot water pipework. Then the cold supply is opened together with the hot tap on the nearest outlet. The heater fills until water runs from the tap with no air. Only then is the heater full. All joints are checked and none may drip.

Electrical connection and first heat-up

Power is connected only after filling. A heating element switched on in an empty tank can be destroyed within minutes. The electrical work is done by an electrician. The heater is connected to its own protected circuit with an earth conductor and a double-pole means of isolation, usually a fused connection unit or double-pole switch, with RCD protection. Manufacturers specify the cable size according to the heater's rating. After switching on, the electrician checks that the heating indicator lights up, and the temperature is set.

Unvented heaters over 15 litres must be installed by a competent installer with recognised unvented (G3) training, so replacing a typical 50 to 120 litre wall-hung heater is not a DIY job. A capable householder can replace a small point-of-use heater of up to 15 litres with a similar model that uses the same connections and the same existing electrical supply. Even then, the electrical connection is best left to an electrician, particularly in a bathroom. Steps:

1. Switch off the heater's supply at the consumer unit and confirm that there is no voltage at the terminals.
2. Close the cold supply and open a hot tap on the nearest outlet.
3. Drain the old heater through the drain point into a drain or bucket.
4. Disconnect the hoses and take the heater off the wall with a helper.
5. Check the fixings and fit new ones to suit the new heater's bracket if necessary.
6. Hang the new heater and connect the cold feed through the new safety valve supplied with it.
7. Connect the hot outlet and check the washers and seals.
8. Fill the heater until water runs from the hot tap without air.
9. Check the joints and only then have the electrician reconnect the power.

The safety valve is normally replaced together with the heater. An old valve is often furred up with limescale and may not open when it should.

Vertical and horizontal heaters

Vertical heaters are the most common. Horizontal models are chosen where height is limited, for example above a door or under a ceiling. Not every heater can be mounted both ways, and the manufacturer states this in the manual.

Capacity

A flat for two people often has an 80 litre heater, while a family with both showers and a bath tends to need 100 to 120 litres. A larger capacity means longer heating times, higher standing losses and a heavier unit on the wall. Slim twin-tank models such as the Ariston Velis Evo are popular where depth is limited. At UK retailers an 80 litre model with its unvented kit typically costs around £430 to £580.

Point-of-use heaters up to 15 litres

Small undersink and oversink heaters supply a single basin or kitchen sink. Up to 15 litres they are generally outside the unvented notification rules, provided the electrical work is not notifiable either, but they still have their own safety valves that must be fitted as the manufacturer specifies.

Off-peak heating

On a two-rate tariff such as Economy 7, the heater can be set to heat only during the cheaper night hours. Switching is usually handled by a two-rate or smart meter with an off-peak supply, or by a dedicated off-peak timer with a boost function. This needs the correct wiring at the consumer unit and the right tariff agreed with the electricity supplier. The capacity must be large enough to last until the next heat-up.

Where to put the heater

Manufacturers recommend placing the heater as close as possible to the outlet that uses the most hot water. With a long pipe run, a lot of water cools in the pipe and users wait for hot water to arrive. A heater in the bathroom or in a cupboard next to it is therefore better than one at the far end of the flat. In a bathroom, however, the safe distances from the bath and shower set by the wiring regulations for special locations must be respected, and the heater must have a suitable IP rating for its zone. When fitting into a narrow cupboard or above a lowered ceiling, it is worth checking in advance that the heater can later be taken down and replaced without breaking anything out.

• Heater on an unsuitable wall. It happens that a heater is hung on plasterboard or an old hollow partition and gradually starts pulling away.
• A valve between the heater and the safety kit. The relief valve then becomes useless and dangerous overpressure can build up during heating.
• No proper discharge pipe. The tundish is missing, hidden in a cupboard or drains onto the floor or behind the tiling.
• Capped discharge. Someone stops water appearing at the tundish by blocking it. The correct fix is to find the cause, usually a flat expansion vessel or a faulty pressure reducing valve.
• Switching on without water. The heating element is destroyed before the heater is ever used.
• Supply through an extension lead or undersized cable. The cable overheats, especially during long heat-ups.
• No space for servicing. The heater is boxed in so that the anode and element cannot be reached without taking it down.
• Neglected servicing. After a few years the anode is used up or the expansion vessel loses its charge, and the tank or valves start to fail.

The plumbing is done by a plumber or heating engineer. For an unvented heater over 15 litres, the installer must be competent in unvented systems, which in practice means holding a recognised G3 qualification, and the installation falls under the building regulations for hot water safety. An installer registered with a competent person scheme can self-certify the work, otherwise check with your local building control body before work starts. The electrical connection must be made by a qualified electrician. A new circuit, and electrical work within the zones around a bath or shower, are notifiable in England and Wales, so it pays to use an electrician registered with a scheme such as NICEIC or NAPIT, who will issue an Electrical Installation Certificate and arrange the compliance certificate. Scotland and Northern Ireland have their own building regulations. Some manufacturers make the warranty conditional on professional installation and registration, so ask the installer for written confirmation and the completed commissioning record.

• the heater hangs vertically and does not move when pushed,
• the safety kit is on the cold feed and there is no valve between it and the heater,
• the tundish is visible, in the same space as the heater, and the discharge pipe runs to a safe point and is not blocked,
• the joints do not drip, even after the first heat-up when the materials expand,
• no water appears at the tundish during normal heating,
• the heating indicator lights up and the water is hot after heating,
• the heater is on its own protected circuit with a double-pole isolator and the cable is properly fixed,
• there is space to remove the element flange and inspect the anode,
• you have the manual, the warranty registration, the commissioning record, the electrical certificate and, for unvented heaters over 15 litres, the building regulations compliance certificate.

Check the safety valves regularly as the manual instructs. Manufacturers typically advise twisting the cap of the expansion relief valve briefly and making sure water flows freely into the tundish. If the water does not stop after the cap is released, or water appears at the tundish without the valve being operated, have the installer check the valve and the expansion vessel.

A correctly installed heater lasts for many years if it is looked after. Maintenance is not demanding, but it is often forgotten.

Anode and safety devices

Where the heater has a magnesium anode, it gradually dissolves and in doing so protects the tank. Once it is used up, corrosion starts attacking the tank itself. Manufacturers set out how often the anode and the unvented safety devices should be checked by a competent person, and many link the warranty to regular servicing. In hard water areas the anode is used up sooner. Replacing an anode costs far less than a new heater.

Limescale

Limescale builds up on the heating element and at the bottom of the tank, which is a particular issue in hard water areas. The heater then takes longer to heat, may rumble or hiss during heating and uses more electricity. During a service the tank and element are usually descaled as well.

Winter shutdown

If a heater in a holiday home or unheated space is switched off over winter, it can freeze. In that case it is drained. Manufacturers recommend fitting a drain valve on the cold feed between the safety kit and the heater, at a low level, so that draining is possible without undoing joints.

The labour price in the NUMIQ price list is €190 per unit for hanging and connecting a wall-hung electric water heater of up to 120 litres, plus €8 for small consumables. The price does not include the heater itself or any extra valves or discharge pipework that need to be bought. Labour prices vary by region and city.

The final cost is affected by:

• the type of wall and the need for reinforcement or special fixings,
• whether the water and electrical connections are ready in the right place,
• the length and route of the discharge pipe from the tundish to a safe termination,
• removing and disposing of the old heater,
• access to the installation point, for example a heater above a door or in a narrow cupboard,
• a new electrical circuit and the related certification.

Related work can be priced separately, for example removal of a water storage tank, terminal valve installation or an electrical installation condition report. More items can be found under plumbing work.