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
  • boiler working? - boiler fault-finding guide (Ed?)
  • sealed system? - pressure OK?

• No CH, DHW OK
  • conventional
    • programmer, room stat or zone valve prob
  • combi
    • programmer or room stat, (diverter stucK?)
• CH OK, no DHW
  • conventional
    • programmer, cylinder stat or zone valve prob
  • combi
    • 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?

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