Central Heating Repair

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The article is about diagnosing and fixing faulty Central Heating systems. (This is about systems using hot water as a heat-carrying medium: warm-air systems are sometimes found in the UK but they are not covered here.)

There are separate articles about:

This article is still partly a skeleton: some points need to be expanded


No CH or DHW

All systems
  • Is boiler firing? On older boilers flames can be seen through a window in the combustion chamber: some have a permanent small blue pilot light when the boiler is idling, and all have a mass of flames (often flecked orange/yellow) when the main burner is alight. On newer boilers often there is only an indicator light which, depending on the model of boiler, may not accurately indicate that the boiler is actually running. Whirring and clicking noises from within the boiler usually indicate that the boiler is attempting to light, but not necessarily that it has succeeded in doing so.

    Check the pipes connected to the boiler: at least one should be getting hot (too hot to hold, only to touch for a second or two) when the boiler is alight. The flue should also be emitting hot gas. WARNING! flue gases on conventional boilers are hot enough to burn! Even if the flue is not physically accessible (e.g. high up) it should be possible to see a heat haze when the boiler is alight. Condensing boilers (which can usually be identified by having a 22mm plastic pipe connected to them as well as 3, 4 or 6 copper ones) have flues which emit mist when running (although under some conditions the mist may be barely visible).

Boiler not firing

  • lockout? / other fault? - boiler fault-finding guide (Ed?)
  • power to system? check FCU fuse
  • controls fault? - programmer, thermostats etc
Sealed system
pressure low? (much below 1 bar on pressure gauge)

Boiler firing then going out again

  • fault with boiler flame-detection system?
the boiler is liable to stay alight for only a few seconds and the flow pipe may not even get hot
  • pump not running or other fault preventing circulation?
the boiler is likely to run for tens of seconds or some minutes (depending on how much gravity-driven circulation is possible). Very little or any of the system heats up but the flow pipe from the boiler will typically be very hot (depending on the boiler's thermostat setting) at the time the boiler cuts out.

Boiler firing but no CH or DHW

  • pump not running?
this is likely to result in the boiler firing up briefly then shutting down again


Conventional system
Is DHW being heated electrically by an immersion heater? If so check whether the DHW can be heated by the boiler and, if not, treat as for No CH or DHW
  • programmer?
  • room stat?
  • zone valve?
    • Y-plan system (3-port mid-position valve)
      Does valve move correctly between HW-only <-> mid/both <-> CH-only positions as controls are operated?
      No: valve stuck in HW-only position
    • S-plan system (2 or more 2-port valves)
    • Gravity hot water, pumped CH system
  • pump not working?
Combi boiler
  • programmer?
  • room stat?
  • boiler - diverter stucK?

CH only when DHW on (DHW OK)

IF Y-plan system (3-port mid-position valve) AND valve moves correctly between HW-only <-> mid/both <-> CH-only positions as controls are operated BUT boiler only fires when in HW-only or mid-position THEN faulty microswitch in valve head.


Conventional system
Combi boiler with PHE & diverter valve
diverter valve?
Combi boiler with combined heat exchanger
flow switch or scaling?

DHW OK, some parts of CH not working

some rad(s) compeletely cold
  • balancing - try turning off other rads
  • stuck TRV - take head off and pull up pin (carefully - can pull out and leak on some models)
  • part of system completely full of air - bleed & find cause
  • only upstairs working?
pump failed?
  • only downstairs working? vented system? loss of water?
some rad(s) lukewarm
  • balancing
  • faulty TRV
rad(s) cold at top
rad(s) cold at bottom

CH on when should be off

All rads on
Some rads on
  • reverse circulation
  • rad on DHW circuit - often in bathroom: may be design "feature"

rads constantly need bleeding

  • air or hydrogen in system? (lighter test - with care!)
    • corrosion:
      • inhibitor?
      • vented system? - pumping over? - check feed & expansion connections (<150mm apart), header tank level, vent pipe rise & termination height, pump speed setting (old system pump replaced with newer model?)
    • air
      • pinhole leaks?
      • vented system? - pumping over?

system pressure dropping

This applies to sealed systems only.

Possible causes (see also Ed's Sealed System FAQ)

leak in pipework or fittings.
This will cause a more-or-less steady continuous gradual drop in pressure. A common source of leaks is radiator valves: there can be leaks at the compression fittings connecting the pipework to the valve, at the junction between the valve and the radiator or the threaded tail fitting into the radiator itself. Another place where leaks may occur, particularly on older systems, is at the shaft or spindle of the radiator valve. This is usually not immediately visible, being covered by a cap or knob of some sort.
leaking Pressure Relief Discharge (PRD) Valve
This will cause a continuous gradual drop in pressure, similar to a leak in other parts of the system.
leaking boiler heat exchanger.
Unless large, this will be evaporated and lost via the boiler flue. Even large leaks in condensing boilers will probably just go to the condensate drain and therefore be difficult to spot.
problem with expansion vessel
* lack of air/gas pressure in vessel
* ruptured diaphragm
This will manifest as a rapid rise of pressure when the system heats up followed by a discharge through the PRD resulting in a sudden drop of pressure.

Leakage which is only detectable as a dampness at a fitting when touched with a finger or piece of tissue paper can result in gradual loss of system pressure over weeks or months. When the system is hot leakage can evaporate before it accumulates enough to form a drop so may never be noticed, especially if the leak only occurs when the system is hot.

See discussion on uk.d-i-y on Tracing tiny leaks in sealed system


  • screeching from pump: vented? loss of water? float valve stuck or feed pipe blocked - sludge or scale
  • kettling from boiler: scale - X200 etc
  • clicking, knocking from floors & walls: pipework expanding & contracting - expose and free up
  • rushing noises from pipes and radiators - air in system?

See Also