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	boiler working? - boiler fault-finding guide (Ed?)
sealed system	pressure OK?
No CH, DHW OK
conventional system	programmer, room stat or zone valve prob
combi boiler	programmer or room stat, (diverter stucK?)
CH OK, no DHW
conventional system	programmer, cylinder stat or zone valve prob
combi boiler	diverter valve if PHE design, flow switch or scaling if combined heat exchanger

• DHW OK, some parts of CH not working
  • 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?
  • 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"

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?