Central Heating Repair

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:

• Central Heating Design which will help you understand what sort of system you have
• Central Heating Operation for help operating a working CH system.
• Boilers describes the main types of boiler.

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 firing? lockout? / other fault? - boiler fault-finding guide (Ed?)
• pump running?
• controls working? - programmer, thermostats etc

Sealed system

pressure OK?

No CH, DHW OK

Conventional system

Is DHW being heated electrically by an immersion heater? In that case treat as for No CH or DHW

• 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?