Worcester Bosch banging noise when DHW cold inlet presurised

[BigG]

A problem strarted occuring a week or so ago and is as follows :

Whenever the cold mains water supply re-pressurises (e.g. after the washing machine, dishwasher or toilets have finished taking in water) there is a banging noise followed by some pipe rattle upstairs.

At first it was suspected that there was air in the system somewhere that was causing the problem.

Further investigation has found the pipework to be sound and no evidence of air. The banging noise is coming from the boiler (Bosch 28Si) and this then causes the rattle back down the DHW cold inlet pipe.

Having looked at the boiler more closely I can see that the DHW cold inlet pipe simply runs in to the water flow switch, on to the heat exchanger and then out to the DHW flow. if I turn off the DHW cold inlet on the boiler banging no longer occurs.

From deduction I can only assume its the water flow switch thats on its way out. I don't think its its the heat exchanger as if it was surely I'd see a leak of some sort or I would see the pressure of the closed system gradually rise (internal break in the exchanger).

I can see as the hot taps are turned (i.e. the DHW flow increases) the spring (on the top of the switch) does correctly move and have the desirved effect on the micro-switch that is attached.

I've looked for kits for the water flow switch but it appears to only be sold as a single unit.

1) Can anyone advise if I am along the right line?
2) In addition to this, from a heat-exchanger perspective am I ok to run the central heating with the DHW cold inlet turned off in the meantime to prevent banging (but get the missus moaning at me about no hot water) or could that cause damage?

Apart from this the boiler is running fine.

 

[ChrisR]

am I ok to run the central heating with the DHW cold inlet turned off

No, turn it back on.

I may be wrong but suspect the problem is caused by the expansion of the tap water in the boiler when the heating comes on. There should be a small pressure vessel on the CW inlet pipe, often missing.
OFten the water will expand up the mains pipe, but iif it can't (modern installations have a valve to stop that) then the only place the water can go is into the WC cistern.
As a test, when the thing is banging, flush the loo or run a cold tap - it'll stop the noise.
With nowhere at all to go when you shut the CW inlet, something could burst.

 

[BigG]

am I ok to run the central heating with the DHW cold inlet turned off

No, turn it back on.

I may be wrong but suspect the problem is caused by the expansion of the tap water in the boiler when the heating comes on. There should be a small pressure vessel on the CW inlet pipe, often missing.
OFten the water will expand up the mains pipe, but iif it can't (modern installations have a valve to stop that) then the only place the water can go is into the WC cistern.
As a test, when the thing is banging, flush the loo or run a cold tap - it'll stop the noise.
With nowhere at all to go when you shut the CW inlet, something could burst.

Thanks ChrisR.

Firstly...I have turned back on the DHW cold inlet back on in the boilder.

To answer your question....if I run the cold (or hot) water taps whilst the WC or other appliances are filling up, when they shut off there is no bang from the boiler. If I then gently close off the cold or hot water taps there is still no banging. I don't think its so much the pressure but possibly the rapid increase in pressure thats causing the problem?

The heating has not been on for several days and I can get the banging to happen each time by simply flushing a WC. Each time the bang happens I see the direct flow switch vibrate. I'm not sure what you mean about the pressure vessel on the cold water inlet pipe. I can confirm there is none however unless it is built in to the direct flow switch? The only pressure vessel on the boiler that I am aware of is the expansion vessel at the back of the boiler for the closed system central heating.

 

[boilerdoktor]

i,ve had this sort of problem after a water meter and non return valve were fitted. A mini expansion vessel on cold inlet will sort it! do you have a water meter??

 

[BigG]

i,ve had this sort of problem after a water meter and non return valve were fitted. A mini expansion vessel on cold inlet will sort it! do you have a water meter??

Thanks boilerdoktor.

No water meter but about 2 months ago I turned off the mains water supply to the house from the stop-cock on the street to repair a pipe to the downstairs loo. Turned it back on and everything was running fine.

I didn't mark the position of the stop cock before I turned it off. I guess it could be possible I have unknowingly increased the pressure flow to the house and this is causing the problem but I would have thought the problem would have become apparent immediatly after turning back on the stop cock.

I could try closing off the stop cock a little to see if this resolves the problem.

If not then I'll whack on on a mini expansion tank. How do I know which one will be correct for myself and of course what would it need to be pressurised to?

 

[boilerdoktor]

they come pressurised pal and you want a MINI exp vessel. a little bit bigger than a tennis ball. plumbcentre/base should have them with fittings!

 

[Agile]

I would suggest that before you fit anything you turn the stopcock fully on ( then back a 1/4 turn as good practice ) and see if it still makes the noise.

Tony

 

[Gavin10]

What's the pressure gauge reading,sounds like the bi-flow heat exchanger passing gavin

 

[BigG]

What's the pressure gauge reading,sounds like the bi-flow heat exchanger passing gavin

There's only one pressure reading and thats on the closed CH system. It reads constantly at 1 bar unless I turn on the CH and then it can rise to just above 2 bar so nothing unusual there and it drops back down to 1 when the heating is off.

I plan to reduce adjust the stop cock outside tomorrow (in daylight) to see if this resolves the pressure problem with the DHW cold inlet.

 

[BigG]

Wahoooooooooo!

This morning went outside and fully closed the stopcock then opened it back up. It appeared that when opened (about 360 degrees) you could keep turning anti-clockwise but it would not increase the flow rate. Therefore closed it again and opened it again to about 90 degrees. This produced a flow rate of roughly 7-8 ltrs per minute.

This stopped all the banging and pipe shuddering but the flow rate appeared too slow. Then opened it to 0 degrees. The flow was now very fast and produced the banging again. Then closed it back to 70 degrees (as shown in the picture).

The flow rate is now roughtly 12 ltrs per minute which appears more than adequate for the boiler, dishwasher, washing machine and taps.....and no more banging

I guess there's a moral to this story. ....when adjusting the stopcock either ascertain the current flow rate or mark the stop-cock positioning before doing so. I'm just suprised we didn't notice the banging many, many weeks ago.

Many thanks to all who have helpded.

btw - anyone know what the thing is circled in blue?

Untitled

• BigG
• 20 Nov 2010
• 1