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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3573
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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My hot water pipes and gas boiler make a loud knocking sound

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My hot water pipes and gas boiler make a loud knocking sound (like a gun shot) when the boiler fires up for both hot water (with no heating) and also the central heating. I've put noise reducer in the expansion tank but this has not cured it. I've bled the radiators but still no luck. Is there anything else I can try?
Hi my name is XXXXX XXXXX lets see if we can get you sorted in the shortest possible time. You could try altering the pump speed to see if this can quieten it down. The only other thing is to turn down the boiler thermostat so that it turns the boiler off just as the noise starts. It will be a longer warm up time but you won't get a headache doing so.
Customer: replied 4 years ago.
Hello Andrew,
Thank you for this. I've tried turning the boiler thermostat down but it made no difference. I'll turn the pump speed down a notch to see and will come back to you if I may.
You will need to turn the pump speed up if you can - you are trying to get the water through the boiler faster to stop it heating to fast and "kettling". When you put the silencer in the tank, did you drain some water from the system to ensure it is fully mixed and not just sat in the tank?
Customer: replied 4 years ago.
Thank you Andrew. No, I did not empty the expansion tank and will try again this weekend. I'll also turn the pump up and get back to you early next week.
Let me know how you get on.
Customer: replied 4 years ago.

Hello Andrew,

I've switched off the cold feed to the expansion tank and drained down a living room radiator. Then put in the additive. However, I seem to have lots of air in the system which I now cannot get rid of - despite bleeding all rads several times. The rads are cold and the boiler now won't fire up. The pump is activated and I've tried venting it from the screw at the back. I've now twice successfully re-set the boiler but it switches off after about 2 minutes and I have to wait until I can re-set it again. It's a Potterton. Now I have no heating and it's not even firing for hot water. I think I'd rather have had the banging!

Can you leave the bleed screw out of the pump while you are filling the system and make sure the header tank has refilled itself.
Customer: replied 4 years ago.

Hello again, Andrew.

I've left the vent screw out of the pump and re-set the boiler (a little reddish button underneath). The boiler burned for exactly one minute before tripping again and shutting down. The pump is extremely hot but the pipes leading down from the loft (pump and boiler are in loft) are luke warm only and the rads water is cold - with no air. The pump is running (humming) even when all the system is turned off at the programmer. The expansion tank is still full. I'm wondering if the pump has packed in on me. It's cerainly a bit noisy. Or is that trapped air? However, each time I manage to fire up the boiler, I get more air being released out of the system vent upstand pipe near the pump. It has a hexagonal nut at the top. And, I can't hear the expansion tank filling up after bleeding - it's right up in the loft apex so I can't see inside it.

I am wondering if the pipe between the tank and the system is blocked, therefore preventing all the pipework filling with water. If you partly drain the system - does the tank start refilling?
Customer: replied 4 years ago.
Thanks Andrew. I shall try that tomorrow and let you know. It's impossible to see inside the expansion tank so I'll have to just try and hear to know if it's filling or not.
Take a torch and let me know.
Customer: replied 4 years ago.
I wish I could. The tank is right up tight against to apex of the loft and it's only just about possible to put your arm inside to feel the water level! It really is that tight.
Ok, you will hear the water running into it, if it is filling. Let me know what you find.
Customer: replied 4 years ago.

Yep, will do. Speak tomorrow.

Thanks very much for your help and patience so far.

no problem - just answer tomorrow once you have had a look
Customer: replied 4 years ago.
Morning Andrew. I've drained the central heating system down completely and the water level in the expansion tank has not moved. I guess that indicates a blockage somewhere and that's beyond my capability to trace and repair, I guess.
Where the 15mm pipe comes down from the tank and joins the rest of the pipework (follow it down) is usually where it gets blocked over time. You could try a few taps with a hammer all around the area of the tee and see if it shifts it. If not then it may need cutting out and replacing.
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