Difference between revisions of "Central Heating Repair"

Revision as of 14:58, 20 February 2007

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?

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?

@@ Line 11: / Line 11: @@
 == Fault-finding ==
-{| border=1; cellpadding=2
+=== No CH or DHW ===
-! colspan=2; align="left"| No CH or DHW
+; All systems
-|-
+: boiler working? - boiler fault-finding guide (Ed?)
-| All systems
+; sealed system
-| boiler working? - boiler fault-finding guide (Ed?)
+: pressure OK?
-|-
-| sealed system
+=== No CH, DHW OK ===
-| pressure OK?
+; conventional system
-|-
+: programmer, room stat or zone valve prob
-! colspan=2; align="left"| No CH, DHW OK
+; combi boiler
-|-
+: programmer or room stat, (diverter stucK?)
-| conventional system
-| programmer, room stat or zone valve prob
+=== CH OK, no DHW ===
-|-
+; conventional system
-| combi boiler
+: programmer, cylinder stat or zone valve prob
-| programmer or room stat, (diverter stucK?)
+; combi boiler with PHE & diverter valve
-|-
+: diverter valve
-! colspan=2; align="left"| CH OK, no DHW
+; combi boiler with combined heat exchanger
-|-
+: flow switch or scaling
-| conventional system
-| programmer, cylinder stat or zone valve prob
+=== DHW OK, some parts of CH not working ===
-|-
+; some rad(s) compeletely cold
-| combi boiler
+:
-| diverter valve if PHE design, flow switch or scaling if combined heat exchanger
+* balancing - try turning off other rads
-|}
+* stuck TRV - take head off pull up pin (carefully - can pull out and leak on some)
-* DHW OK, some parts of CH not working
+* completely full of air - bleed & find cause
-** rad(s) compeletely cold
+* only upstairs working? pump failed?
-*** balancing - try turning off other rads
+* only downstairs working? vented system? loss of water?
-*** stuck TRV - take head off pull up pin (carefully - can pull out and leak on some)
-*** completely full of air - bleed & find cause
+; some rad(s) lukewarm
-*** only upstairs working? pump failed?
+:
-*** only downstairs working? vented system? loss of water?
+* balancing
-** rad(s) lukewarm
+* faulty TRV
-*** balancing
-*** faulty TRV
+; rad(s) cold at top
-** rad(s) cold at top
+: bleeding
-*** bleeding
-** rad(s) cold at bottom
+; rad(s) cold at bottom
-*** sludge
+: sludge
-* CH on when should be off
-** All rads on
+=== CH on when should be off ===
-*** timer? room stat? zone valve?
+; All rads on
-** Some rads on
+:
-*** reverse circulation
+* timer?
-*** rad on DHW circuit - often in bathroom: may be design "feature"
+* room stat?
+* zone valve?
+; Some rads on
+: reverse circulation
+: rad on DHW circuit - often in bathroom: may be design "feature"
+----
 === rads constantly need bleeding ===