System boiler service: what is checked

Ealing was nicknamed the Queen of the Suburbs, and its Edwardian family houses are still some of the most common homes in W5 and W13. Many have had their heating modernised to a system boiler with an unvented cylinder. It gives strong hot water to more than one bathroom, without tanks in the loft.

How a system boiler works

A system boiler heats water for radiators and a hot water cylinder. Unlike a regular boiler, it has the pump and an expansion vessel built in, so the heating is sealed and pressurised. There is no small feed and expansion tank in the loft.

The cylinder can be:

• Vented. Fed from a cold water tank in the loft. Lower pressure.
• Unvented. Fed straight from the mains, sealed and under pressure. Stronger showers.

What the service covers

The boiler

Check	Why
Gas supply pressure	Correct burning
Burner and heat exchanger	Clean and working
Seals, electrodes, fan	Catch wear
Condensate trap	Clean, to prevent lockouts
Flue and terminal	Safe removal of fumes
Combustion test	Measure what the boiler is burning

The sealed heating side

The system pressure gauge should read what the maker sets when cold, often around 1 to 1.5 bar, but check your manual. The engineer checks:

• The built in expansion vessel charge. A flat vessel causes pressure to rise when hot and drop when cold.
• The pressure relief valve. It lets water out if the heating pressure gets too high.
• The pump.

Controls

Motorised valves that switch between heating and hot water are checked, along with the cylinder thermostat, room thermostat and timer.

Unvented cylinder checks

Unvented cylinders hold hot water at mains pressure. They have safety devices that stop the cylinder over pressurising or overheating. Cylinder makers usually ask for these to be checked regularly, typically yearly, by someone competent in unvented hot water systems.

Part	What it does
Pressure reducing valve	Keeps incoming mains pressure at a safe level
Expansion relief valve	Releases water if pressure gets too high
Temperature and pressure relief valve	Releases water if the cylinder gets too hot or too pressurised
Expansion vessel or air bubble	Takes up the expansion of heated water
Tundish and discharge pipe	Lets you see a discharge, and carries water safely outside

If the tundish drips, a valve may be letting by or the expansion vessel may have lost its charge. That needs checking.

Ask for the unvented checks when you book, so an engineer competent in unvented systems is sent and the time is planned in.

Ealing houses

Ealing’s Edwardian houses often have airing cupboards on the first floor landing, which suits a cylinder. Many have had loft conversions, which sometimes moved the cylinder or added a bathroom. Our Ealing area page covers the housing, including the Brentham Garden Suburb conservation area, where outside changes like moving a flue may need consent.

Hard water and cylinders

Ealing is supplied by Thames Water, which says all the water in its region is hard. Scale builds up on the coil inside the cylinder and in the valves. Some unvented cylinders have scale protection on the incoming water. A cylinder that takes much longer to heat than it used to may have a scaled coil.

Sludge

Sealed systems take in less fresh water than open vented ones, but they still produce sludge, especially if the system has been drained and refilled during building work. A magnetic filter on the return to the boiler helps. Clean it at the service. Our magnetic filter clean page explains more.

Common faults

Symptom	Possible cause
Pressure rises when hot, drops when cold	Flat expansion vessel
Pressure keeps dropping	Leak or pressure relief valve letting by
Hot water but no heating	Motorised valve sticking
Tundish dripping	Cylinder valve or vessel problem
Slow hot water recovery	Scaled coil or thermostat fault

See our post on boiler pressure keeps dropping for more on pressure faults.

Warranties

Most boiler makers ask for a yearly service to keep the guarantee valid. Cylinder makers often have their own conditions about yearly checks on unvented safety devices. Check both sets of paperwork.

During the visit

The boiler and hot water are off for part of the visit, and some cylinder checks need the hot water supply isolated for a short time. Plan showers before the engineer arrives.

Booking

When you book a system and regular boiler service, tell us:

• Boiler make and model
• Whether the cylinder is vented or unvented, if you know
• Where the boiler and cylinder are
• Any problems, such as a dripping tundish or pressure changes

Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see the ID card and check it on the Gas Safe Register.