Hot water pump problem

The hot water pump occasionally makes a rattling sound when the boiler fires up.

The noise can be stopped by turning the pump on and off (its plugged into a standard wall socket) which sometimes works first time but on occasions it can take numerious attempts. Its only an intermittent fault and the system seems to work ok whether the pump makes a noise or not. There appears to be no obvious pattern.

It seems to be unrelated to the central heating as it occurs whether the heating is on or off.

Other than the basic principles of the system I have no real knowledge of boilers, pumps etc and assumed that maybe the bearings in the pump were worn.

However, someone suggested that there may be air in the system. There is a bleed valve on the pipe between the pump and the cylinder and a small amount of air was expelled when this was opened.

The pump did not rattle for about a week, but then the rattling returned and again I opened the bleed valve and a small amount of air escaped. This happened a number of times with the interval between the noise returning varying from a few days to over a week. The last time only water came out, although you can still hear rushing water when the system first comes on which I assume you wouldn't get if the system had no air in it.

It's a fairly old system which I think dates from the early eighties. It's a gas fired Glow Warm 105-120 open flue boiler and the hot water pump is a SMC Commodore Controller. If its of any relevance the central heating pump is an SMC Commodore 130 which immediately follows the hot water pump in the circuit. I'm not sure how efficient the system is, but it works well whether the pump is rattling or not in that we have plenty of hot water and the boiler heats 20 plus radiators without a problem.

It has not rattled since the last time it was bled, but I am not hopeful that it wont and therefore has anyone any idea what it could be.

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geoffr
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