IDEAL ICOS M3080 OVERHEATING/PUMPING OVER

[derica]

Ideal icos condensing boiler fine when both central heating and hot water in use. When calling for hot water only, the boiler fires and after about 4 minutes choking and clunking noises are heard from boiler in addition to occasional slurping sounds from the hot water circuit. Water is then violently forced up the open vent pipe into the expansion tank. Boiler fires up again and heats water normally till cylinder stat is satisfied. A new pump was fitted at the beginning of the year and is working fine.

The problem occurs about 1 in 4 times when calling for hot water only - the boiler stat is set to the halfway position. I'd very much appreciate any help with this annoying intermittent fault. Many thanks in advance.

 

[twgas]

A restriction in the HW circuit is quite likely, possibly a valve turned, almost, to off.

 

[derica]

A restriction in the HW circuit is quite likely, possibly a valve turned, almost, to off.

Do you mean a zone valve or gate valve? Thanks.

 

[twgas]

One would not turn a zone valve nearly off, although it might be partially blocked, so a gate valve or something similar is likely to be turned down.

 

[derica]

One would not turn a zone valve nearly off, although it might be partially blocked, so a gate valve or something similar is likely to be turned down.

Thanks for your help Mick. There are two gate valves - one at the cold feed to the expansion tank and one at the bottom of the hot water cylinder. Both valves are turned fully to the anti-clockwise position. I'm reluctant to fiddle with them in case they jam shut. Does the gate valve at the bottom of the cylinder act only as an Isolating valve for the cylinder or does this also form part of the HW circuit from the boiler?
Derek.