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 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"
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?
- corrosion:
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?