Central Heating Repair

This article is about Central Heating systems using hot water as a heat-carrying medium. (Warm-air systems are sometimes found in the UK but their design and installation is not covered here. There is a discussion on updating existing warm-air systems here)

The article is about diagnosing and fixing a faulty central heating system.

There are separate articles about:

• Central Heating Design which will help you understand what sort of system you have
• Central Heating Operation for help operating a working CH system.

This article is a skeleton: the main points to be covered are listed but some or all still need to be expanded

Fault-finding

No CH or DHW

All systems

• power to system? check FCU fuse
• boiler working? - boiler fault-finding guide (Ed?)
• controls working? - programmer, thermostats etc

Sealed system

pressure OK?

No CH, DHW OK

Conventional system

• programmer?
• room stat?
• zone valve?

Combi boiler

• programmer?
• room stat?
• boiler - diverter stucK?

CH OK, no DHW

Conventional system

• programmer?
• cylinder stat?
• zone valve?

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 pull up pin (carefully - can pull out and leak on some)
• 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

bleeding

rad(s) cold at bottom

sludge

CH on when should be off

All rads on

• timer?
• room stat?
• zone valve?

Some rads on

reverse circulation

rad on DHW circuit - often in bathroom: may be design "feature"

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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)

• leaking Pressure Relief Discharge (PRD) Valve
• problem with expansion vessel:
  • lack of air/gas pressure in vessel
  • ruptured diaphragm
• leak in pipework

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

noises

• 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?