Constant knocking noise in pipework, NOT BOILER!

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I say not boiler as when I searched this forum, boiler noise is common. I can't see my problem anywhere.

I know when the joists heat up, they "creak" and this can only be resolved by reseating the pipework etc. What I'm hearing is another problem. When running, the CH is relativly quiet. When the thermo cuts out, then begins a knocking from under the bedroom floor. It is a "clanging", every 2 or 3 seconds. This continues for several minutes and drives me mad! I've flushed the CH with acid, put in an inhibitor but still the noises are there. Is it scale? Is it air? Any ideas please.
 
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Does your boiler have a pump overrun on it if so it sounds like that could be your problem. If the pump overrun stat is not working properly, then this will cause the problem you described, it circulates water through the heat exchanger after the burner has stopped. If the pump goes off straight away when the flame goes out the residual heat in the HE will cause an overheat situation.
 
Pipes "holding on " to notches as they contract- :?:
 
The boiler's a Baxi WM 51/3RS. I think I understand here.

You're saying the water should circulate for a few moments after the flame has gone out to allow the system to cool down a little, so as not to have red hot water static in the pipes or boiler.

Does a Baxi WM 51/3RS have such a facility? I guess I check this by turning off the CH whilst standing by the pump. If the pump stops when the CH is turned off, I've got a problem?
 
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Hi Robert Im sure that boiler won't have a pump over run Do you know how old it is?.What temperature r u running it on. If its as old as i think it should be suitable for gravity hot water when it was made so it could the the boiler thermostat so try running it on minimum or it could need strong descaling (Fernox DS40) for example.
 
robertbullen said:
The boiler's a Baxi WM 51/3RS.

I haven't worked on that type of boiler but by process of elimination WM= wall mounted & RS = room sealed it could well be a low water content boiler & as such would need a pump overrun. I'm sure you will get a definite answer tomorrow when everyones hangover has cleared.
 
been out havin agood time 4 5 hrs but wm does mean wall mounted & rs room sealed but it also means it was a boiler defore the solo 1 launched around 1992 so its probably ballanced flue ,permanent pilot. green button honeywell gas valve with the ability to gravity hot water at a guess.
 
So, we've established the boiler doesn't have a pump overrun. The boiler could be up to 18 years old. I don't know what temperature it running at, only 22 on the room thermo and number 3 on the boiler "knob".

I could try some Fernox DS40 but I'm pretty sure the knocking isn't coming from the boiler. It's hard to tell but I think the noise that's waking me up is coming from an upstairs rad. I've tonight taken it off the flushed it through. I've never seen so much sludge, but then I don't flush that many rads (IT Engineer by trade). Can excessive sludge cause this knocking noise?

I'll try and let you know if this flushing has fixed the problem. If so, I guess the others will need doing too.
 
rackin me brains on this one. is it gravity hot water. when the heatin goes off ar u getting a surge of hot water up the primary flow or feed/vent pipe?
 
Pretty sure it's not gravity, the pump still runs when HW only on, there's an electronic valve near the tank.

I don't have a clue if there's any rush of water. I can still here the knocking right now.
 
Sorry im all out on ya problem at present. Its a lot easier when on site.
 
As an interim measure try turnng off the rad(s) you suspect of causing the noises. Eliminate the obvious first, then you'll have a clear idea of "when" and "when not".

Have you tried doing a search on "ghosts"?
 
Ok, I've located the rad causing the noise, flushed it and refilled it. It's still making this clonking/knocking noise. Could the noise be in the pipes and transmitting throught the metalwork?
 
Now I'm confused. I've turned off both valves on the rad I thought was causing the noise and it still makes the noise. Again, this knocking happens every seven seconds for several minutes after the CH has gone off.

Today, the thermo turned the CH on and the knocking stopped. A second or two after, the timer turned the CH off. I didn't hear the knocking again for a few minutes, but then I had to go out.
 

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