loud hammering from ch system

[sailor]

my central heating system has recently begun to hammer loudly when the boiler switches off. this only happens when all radiators are off i.e heating water only. if one or more radiators on everythings fine.(boiler is potterton suprima 30L)
anyone shed any light please?
I should add that I have bled the system and there does not appear to any obvious problems with air locks. Thanks.

 

[masona]

To help you when the plumbling guru get in tonight to answer your question Can you tell us where about the noise is coming from?

 

[sailor]

Thanks for reply.
It's difficult to say but it appears to be from the vicinity of the boiler itself,
as the vibration from the banging can be clearly felt on the boiler front panel, though of course this could simply be transmitted via the pipework.

 

[corgiman]

below entered twice D'OH

 

[corgiman]

could be that boiler is over heating because it either has no bypass or the bypass fitted is closed

the suprima accordibng to manufacteres ints must be fitted with an automatic by pass as it is a pump over run boiler

this means the pu8mnp keeps running for a time after everythings heated up to reduce the temp in the flow and returns pipes

the symptons you describ suggest this to me because the boiler dont make any noise when the heatings on

between the pump and the valve(s) shuold be a pipe teed in, probably to a gatevalve (tut tut but there you go) if so try opening it up a bit and see what happens

 

[masona]

I just remembered I've had this problem and found my boiler stat was too high and only did it on the hot water side which have a shorter circ's so the boiler was overheating in the short time the boiler was on,you could try turning the stat down to see if it make any difference before you take anything apart

 

[sailor]

Thanks.
I assume the valve you are referring to is the motorised valve (Danfoss
hs3) above the T piece. There are gate valves in both sections of the Horizontal section of the T and both are fully open. The pump is beneath the Vertical leg of the T.
Could there be a problem with the Motorised valve? It is in 'auto' position
and moves to 'manual' on occasions with no resistance felt, but on other
occasions resistance from a motor is felt. If it is left in 'manual' it reverts
to 'auto' after a while by itself so I assume the motor is ok.

Have tried Boiler thermostat at various levels but problem still persists...

 

[sailor]

Have tried lowering Thermostat level (as recommended by Masona) and
although I had tried this before, lowering the level to about 20-25% of maximum appears (touchwood) to have done the trick! I obviously didn't go low enough before. So as long as the water/radiator temperature is OK
I'm happy.
Thanks again to Masona and Corgiman for your efforts, it's been most appreciated.

 

[corgiman]

I just remembered I've had this problem and found my boiler stat was too high and only did it on the hot water side which have a shorter circ's so the boiler was overheating in the short time the boiler was on,you could try turning the stat down to see if it make any difference before you take anything apart

its because of this effect why you MUST have a bypass valve

A bypass is not the three port valve but bringes the primary flow and return usually between the the pump and the three port valve and the cylinder return

Sailor if you ever have to drain down ask your plumber to pop one on if you have not got one already.

Just remeber all the time the boiler is making a noise is building up scale TUM TUM TUUUUUUUUUUUUUUUUUUUUUUUUUUUUUMMM

 

[sailor]

ok mate, thanks for the info and well noted.
all has been quiet for the last few hours............