Noisy heating pipes & intermittent hot water

Hi,

I need help desperately! I have an Alpha 280e boiler which feeds my excessively noisy central heating. I have replaced the divert valve as it was inoperative and I've also cleaned the heat exchanger. When I first refill the system and bleed the radiators, it's great, quiet, smooth hot hot water etc. After a couple of days however, the pipes start to thud or hammer repeatedly while the heating is on. This noise gets progressively worse until the noise is pretty constant while the heating is on. The only way to stop it is to drain the system and start again and again and again.... Also, once the knocking starts, the boiler swithes off briefly when heating the hot water leaving me with hot/cold/hot/cold water at the taps although the water pressure is constant.

Please help, I haven't had a decent nights sleep in weeks!

Did you change the boiler auto air vent? There may be air trapped in the heat exchanger causing noise. The old vent may be blocked.

You may have a problem with the flow thermister on the left hand side pipe coming down from the heat exchanger.

Check the burner flame height reduces as you turn the hot tap flow down, if it doesn't the thermister may be duff. These can be difiicult to check with the meter - I find many sensors with intermittant faults.

Cheap part and easy to change as its in a dry pocket (ie no need for drain down).

The heating may be noisy due to scale in the heat exchanger (despite your effort at cleaning).

Check the return filter is not blocked and the heating is range rated down to a sensible level.

Lazy installers don't match the boiler heat output to the radiator loading. This is OK until the heat exchanger scales up and then becomes noisy.

Hi Gasguru,

Thanks for the reply. Have located the automatic air vent cleaned it etc. no change, whats a thermister and how will I find it? There is no mention of it in the Installation & Servicing Instrctions.

The thermister is a device to tell the circuit board the temperature of the water, the flow thermister is on the left hand flow pipe from the heat exchanger, however I think you may want to consider calling in a boiler engineer.

The manual probably just calls them temperature sensors. Easy to spot, 13mm hex and a black plastic bit with a wee plug on it. 10k ohms cold and a lot less, like 3k, hot, but yes, hard to be sure, to be sure.
If you change the hot water sensor(which is the same) turn the CW input off first or you'll get cross.
Did you clean the secondary heat exchanger - that's the shiny plate one??
If all that doesn't work it might be that the gas is going out instead of down, when it should - corgi territory.

Yeah, looks like the flame actually cuts out rather than drops. That means I need a Corgi man then, just what I need 2 weeks after having a baby. Oh well, thanks for all your help fellahs!