Boiler shuts down

[michael190]

My CH makes a load banging noise just as the hot water gets up to temp. The noise comes from around the area of the pump and hot water cylinder. It always start at this time of year and hasn't done it all through the summer.
It's as if there is air in or hot and cold water suddenly meeting each other.
It is a Myson Apollo boiler and Myson unit 3 pump.
I have tried, changing the pump speed from 1 to 2. Have changed the pump overun stat, the hi temp stat, the thermocouple and the hot water stat on the tank. The problem is, when it happens it puts the pilot out on the boiler. It may be coincidence but, it seems to happen after some bathwater has been run.
HELP

 

[kevplumb]

boiler is overheatint and tripping the high limit
have you got a bypass fitted ?

 

[michael190]

Is that the one accross the pump

 

[kevplumb]

between the flow and return

 

[michael190]

what do I need to do with it?

 

[ChrisR]

Make sure it isn't shut, for a startoff!

 

[michael190]

Bypass definately not shut, how far open should it be, and what else could be my problem?

 

[ChrisR]

How far is the bypass valve fromthe boiler?
Next time you have a bath try it with the pump on max.

 

[namsag]

You say it has not done it all summer, do you by any chance have the stat set to LOW in summer and switch it to HIGH when you put rads on

 

[michael190]

Bypass is upstairs with pump and hot tank, boiler is downstairs in garage.
Hi - Lo stat not altered from summer to winter.
Thinking of cleaning system out and putting some Sentinel X300 in, I have had rads off in the past for decorating etc but no signs of furring up or sludge. Could it be that the heat exchanger in the boiler is clogged up?

 

[ChrisR]

It could. Worth trying the pump on max though, as a test if not a cure.

 

[kevplumb]

and try opening the bypass a bit more

 

[aquabunny]

err...sorry to be a pain...but what's a 'bypass' ?...where should I have one ?

 

[ChrisR]

Oi, get back to yer own thread It's for when that third thermostat comes into action - you've got enough trouble with the ones you know a bit about. Ask your mum.

 

[kevplumb]