Why does my combi boiler fire up when I run cold water?

[joe90]

To be more specific, the boiler fires up briefly when I turn the sink cold water tap off. I do have high water pressure, reduced to the kitchen tap and outside tap but nearly 8 bar everywhere else (brilliant shower 🤔). Do I need to reduce the pressure elsewhere?

[Temp]

Perhaps when you turn off the kitchen sink cold tap a shock wave travels back through the cold main to the boiler and through the flow sensor. 

 

Does it do it if you turn off the tap really slowly? If that's OK perhaps fit an anti thump device on pipe to the tap?

[joe90]

4 minutes ago, Temp said:

Perhaps when you turn off the kitchen sink cold tap a shock wave travels back through the cold main to the boiler and through the flow sensor. 

That makes sense, tried fast and slow but it does not happen all the time so difficult to see. Yes when I turn it off fast there is an audible click from the boiler. Never heard of an “anti thump” device, I suppose it’s similar to water hammer 🤷‍♂️

Edited June 27, 2023 by joe90

[JohnMo]

Mine clicks on, then off when I was filling the central heating, the fill valve is in the cold water line into the combi. Think the flow switch in the boiler must see a slight reverse flow.

 

Maybe we should have a check valve in the line?

[Temp]

6 hours ago, joe90 said:

Never heard of an “anti thump” device, I suppose it’s similar to water hammer 🤷‍♂️

 

Yes that exactly what I mean.

[PeterW]

At 8 bar I would fit an inline PRV and then a small potable expansion vessel somewhere to reduce the pressure to 3-5-4 bar  max in the house. It won’t reduce the flow that much, and you’ll not have the pressure bounce on the flow switch in the combi that you’re getting today.

[Conor]

Is your pipework mostly copper or plastic?

 

By shutting off a high flow under high pressure, you create a pressure wave (surge) the propogates back downstream. In the water industry we use either surge vessels (bit like a large expansion vessel) or a surge relief valve. These deal with the shock wave by allowing it to push the water in to an empty space. In a closed system like yours, there's nowhere for the water to go so the energy is transferred in to the pipework and attached applicances.

 

Not normally needed in a domestic setting tho, either turn your tap off slowly or reduce your pressure down to 3bar. I'd be doing the latter as you're also reducing the risk of a leak by a factor of several (there's a graph somewhere with failure rates and pressure.)

 

[joe90]

I think I will move the PRV that I fitted to the kitchen tap and outside tap only to the incoming supply and find a balance of pressure that gives a good shower but controllable elsewhere, I have noticed when the loo finishes filling there is a real thump within the pipe work.

4 minutes ago, Conor said:

you're also reducing the risk of a leak.

Yes I agree 👍.

[joe90]

16 minutes ago, Conor said:

Is your pipework mostly copper or plastic?

Sorry didn’t see this, copper.

[joe90]

Not got around to this but putting this before my boiler was suggested by a plumber friend.   https://www.screwfix.com/p/sioux-chief-dw660-2j-water-hammer-arrestor-1-2-bsp/1492r
 

Opinions!

[SteamyTea]

41 minutes ago, joe90 said:

Opinions

Looks similar to the medicine the French Doctor gave me.

 

Is it just a tube with some air in it that acts as a spring?

[joe90]

1 hour ago, SteamyTea said:

Looks similar to the medicine the French Doctor gave me.

 

Is it just a tube with some air in it that acts as a spring?

Yes the French love suppositories.

 

https://southendplumbingllc.com/what-you-need-to-know-about-water-hammer-arrestors/

[PeterW]

2 hours ago, joe90 said:

Not got around to this but putting this before my boiler was suggested by a plumber friend.   https://www.screwfix.com/p/sioux-chief-dw660-2j-water-hammer-arrestor-1-2-bsp/1492r
 

Opinions!

not that good if you have very high pressure even though it says good to 250psi. Better to put a 4 litre potable expansion vessel in and it will do the whole system. 

[Nickfromwales]

4 minutes ago, PeterW said:

not that good if you have very high pressure even though it says good to 250psi. Better to put a 4 litre potable expansion vessel in and it will do the whole system. 

+1, and will soften the hammer off the WC's and any 1/4 turn taps which 'slam' shut.

 

At 8bar, you defo need a PRedV on the main as that will be causing all kinds of havoc 'behind the scenes'!!

Set it at 4bar and the flow will still be stonking. A 3bar PRedV actually strangles the flow quite a bit, so make sure you can define the set pressure. Remember that you need to have water flowing through the PRedV to set it accurately, so leave a tap on somewhere just running a couple of l/p/m whilst you set it.

[Nickfromwales]

And then.........

 

 

"Peckham Spring". 

[joe90]

7 hours ago, PeterW said:

not that good if you have very high pressure even though it says good to 250psi. Better to put a 4 litre potable expansion vessel in and it will do the whole system. 

 

7 hours ago, Nickfromwales said:

+1, and will soften the hammer off the WC's and any 1/4 turn taps which 'slam' shut.

 

At 8bar, you defo need a PRedV on the main as that will be causing all kinds of havoc 'behind the scenes'!!

Set it at 4bar and the flow will still be stonking. A 3bar PRedV actually strangles the flow quite a bit, so make sure you can define the set pressure. Remember that you need to have water flowing through the PRedV to set it accurately, so leave a tap on somewhere just running a couple of l/p/m whilst you set it.

 

7 hours ago, Nickfromwales said:

And then.........

 

 

"Peckham Spring". 

Ok guys, will do, I just wanted to consider all my options. As I often say “every day is a school day” (even at my age 😳).

[joe90]

Right chaps, finally got around to moving the PRV to the incoming supply (it was wet on Saturday so indoor jobs a must) even at 2 bar (which I set it too fir the outside tap) shower flow is still very good and no more boiler hiccups and toilet filling quieter. Thanks guys 👍