DIY Heating Plumbing Tips 5 - Central Heating Systems Problems

DIY Heating Plumbing Tips 5 - Looks at some of the common problems associated with hydronic central heating systems.

Loud sounds inside the boiler and pipework

This is because the boiler is possibly overheating, and this may be caused by:

  • Hard Water.
  • In hard water areas lime-scale can build up in the boiler's heat exchanger. The build up of lime-scale can cause excessive overheating within the heat exchanger.

The picture below indicates the areas of accumulated sludge in a central heating system.

  • This overheating can produce loud hissing and banging sounds within the boiler and the heating system's pipework and radiators.

Solution - Power Flushing, Power Flush the heating system and treat with a descaler and corrosion inhibitor.

  • A blocked chimney due to the heavy build-up of soot can cause overheating in solid fuel boilers.

Solution - Have the chimney swept to remove and clear away the heavy soot.

  • A faulty boiler thermostat.


- Turn the boiler off and allow the system to cool down. Most boilers have a 'pump overrun' feature, this simply means that the water 'circulating pump' will continue to circulate the heated water around the system for a few minutes after the boiler has been shut off.

This is to help cool down the water in the boiler's heat exchanger. When the system has cooled down try operating the thermostat for the boiler, if no clicking sound can be heard then there is a problem with the thermostat, and you will need to call a heating engineer.

Many of the new boilers have an onboard management system and will display an error code if a problem is detected. Refer to the boiler's user manual for error code definition and solution.

If you can hear the thermostat click when operated then you may just need to reset it to a lower temperature.

It is a better practice however, to have the boiler 'interlocked'. This is a wiring arrangement that only allows the boiler to fire-up if there is a demand for space heating from the room thermostat, or a demand for hot water from the hot water cylinder thermostat.

Interlocking the boiler completely prevents 'boiler short cycling', the unnecessary firing-up of the boiler when there is no demand for space heating or hot water.

  • Circulating pump not working.


- With the older traditional boilers the pump is installed separately on the heating circuit and not within the boiler, unlike the new system boilers where the pump is now part of the boiler.

Check that the pump is running, if not, switch off power and check the wiring connection. If the pump is running but the pipe on the pump outlet side is cool, then there may be an air-lock.

Switched off the pump and unscrew the the bleed screw on the center of the pump face plate to remove any trapped air, tighten up screw when water starts to come out.

If the pump is still not working close off the isolating valves on either side of the pump and remove the pump. Check for blockages like the build-up of black sludge within the pump and clean, or if need be, replace the pump.

  • Lack of water in the heating system.


- In open vented systems check the feed & expansion tank. If empty then there may be a problem with the 'ball float valve', it may be stuck. Try moving the the float arm up and down to free it and to restore the flow.

Check that the mains water supply to the feed & expansion tank has not been turned off. Check that there are no frozen pipes.

With sealed heating systems check the water pressure, there should be at least 1 bar (100 Kpa) of pressure when the system is cold. If the pressure gauge reads less than 1 bar, connect the water 'filling-loop' for the heating circuit and top up (the pressure gauge may be located on the boiler or connected to the pipework on the primary circuit).

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Boiler has stopped working

The boiler may not be working for a number of reasons:

  • No fuel getting to the boiler - fuel has run out, or has been turned off.


- Check fuel storage or mains supply. check that the main fuel supply valve has not been turned off.

  • The boiler's gas pilot light has gone out.


- If the boiler has a permanent pilot light and it has gone out, try re-lighting the pilot light by following the manufacturers instructions. The re-lighting instruction is generally written on the back of the boiler's front panel.

If the pilot light fails to ignite it may be faulty and need to be replaced. Turn off the gas supply to the boiler and call a central heating gas engineer.

Most new gas-fired boilers no longer have a permanent pilot light, instead they have a electronic pilot ignition - meaning that the pilot only comes on and lights the burner when there is a demand for heating or hot water. Refer to the user manual for definition of error code - if it's an ignition fault call a central heating gas engineer..

  • The timer and/or programmer is not working.


- Check that the timer and/or programmer is turned on and that the heating times are set correctly. If it is switched on and correctly set then it may be faulty and needs to be replaced.

  • The thermostat is set too low


- Check that the room thermostat, the hot water cylinder's thermostat and the boiler's thermostat are not set too low. The boiler will not fire-up if the is no heating demand from any of the thermostats.

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Boiler's working but Radiators remain cool

If the boiler is working but the radiators remain cool then it is probably a problem with the.

  • Water circulating pump.


- See above Click here

  • The pump's thermostat/programmer or timer's, and/or wiring connection may be faulty.


- Check and adjust if needed the the setting on the thermostat/programmer or timer. Switch off the electrical power if the pimp is still not working and check the wiring for any loose connections. If the wiring looks in good order but the pump still fails to work you will have to call a heating engineer.

When there is a demand for heating or hot water the thermostats, either the room thermostat or the hot water cylinder's thermostat will firstly switch the motorized valve(s) to heating or hot water, then turn on the circulating pump, and then finally, switch the boiler on.

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Radiators in one area of the house do not heat up

This could be because there is a problem with:

  • A faulty motorized zone valve.


- Switch off the electrical power to the boiler and drain the heating circuit, then remove and replace the motorized valve.

  • The circulating pump not working


- See above Click here

  • The room thermostat or timer is set too low, or is faulty.


- See above Click here

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A single radiator does not heat up

A radiator may not heat up because:

  • The radiator is not turned on


- Check that the radiator's handwheel valve is open. turning the handwheel valve in an anti-clockwise direction will open the valve, fully turning it in a clockwise direction will close the valve.

  • A faulty thermostatic radiator valve.


- Check the valve is turned on and not set too low. If the radiator still does not heat up, close the the valve and remove the head and check that the plunger pin moves freely.

If there is a problem with the thermostatic radiator valve, the system will have to be drained and the valve replaced.

  • The lockshield valve is closed.


- Remove the cover cap on the lockshield valve and release the locking nut around the valve shank, turn shank in a anti-clockwise direction, to open use a screwdriver or spanner.

Some types of lockshield valves are operated (open and closed) with a special key.

  • The radiator valves are blocked - by installation debris such as 'wire-wool', or corrosion.

Power Flushing, Power Flush the heating system and treat with a descaler and corrosion inhibitor.

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Top, Inc.

Top of the radiator is cool while the bottom is warm

The reason a radiator remains cool at the top while the bottom is warm is because there is:

  • An airlock


- Bleed the radiator. To bleed a radiator turn off the heating system and wait for the circulating pump to stop running. Open the bleed valve which is located at one of the top ends of the radiator with special bleed valve key. Close the valve after the air has been released from the radiator and the water begins to come out. Catch water in bowl.

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Center of the radiator is cool while top and ends are warm

The reason why the center (and bottom) of the radiator is cool while the top and the ends are warm is because:

  • Corrosion.
  • The build up of rust deposits at the bottom of the radiator has restricted the circulation of the water within radiator


- Power Flushing, Power Flush the heating system and treat with a descaler and corrosion inhibitor.

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The overflow pipe in the feed & expansion tank keeps dripping

If the overflow pipe from the feed & expansion cistern/tank is continually dripping it could be due to:

  • A faulty ball valve.


- Close off water supply to feed & expansion tank and bale out the water to a level below the valve, then remove the ball valve and check the washer. If the washer is no longer seating correctly replace it with a new one.

  • The 'ball float' has a leak.


- Bale out the water in the feed & expansion tank to below the valve level and then unscrew the 'ball float' from the 'float arm' and fit a new 'ball float'.

  • A leak in the hot water cylinder's coil (the heat exchanger).


- Replace hot water cylinder if coil cannot be easily repaired or replaced.

If the coil is split, then the overflow pipe in the feed & expansion (F & E) tank will only drip if the F & E tank is positioned below the cold water storage cistern (CWSC) and therefore exerts less static pressure (head) than the CWSC.

To test this (if possible), turn off the boiler and close off the water supply to both the cold water storage cistern and the f & e tank, and allow the system to cool down. Measure the water level in both cisterns using a dip-stick, do not use water overnight.

Take another measurement the next day, and if the water level in the f & e tank has`increased and the water level in the cold water storage cistern has decreased, then the coil most probably has a leak. You will need to call in an heating engineer to further test the system.

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Water leaking from system

Water may be leaking from pipe`connections:

  • Loose pipe unions and joints.


- Turn off the boiler and then tighten up the leaking joints.

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