mains water pressure and boiler

[Johan]

Hi,

Not a regular poster, but often read this newsgroup.

I have just replaced some of my cold water plumbing, and now seem to have a problem with the combi boiler: whenever I close a cold water tap, the boiler sounds for a few seconds as if it is about to turn itself on. This also happens when a toilet has been flushed, and the water flow stops. The boiler is on the first connection to the mains water. The mains pressure is fairly high (lead pipe from the street to the house replaced last summer). I cannot think what I have changed that caused the boiler to do this. I used speedfit pipe and fittings (yes, I have read lots about those on this newsgroup; I find them very easy to use).

I was old a 'hammer arrestor' would solve my problem, but this has not worked (the one I got is made by Sioux and on the packaging it states it will keep the pressure below 10 bar; seems a bit high to me).

Is it possible that cold water is forced into the boiler when I close a tap, which makes the boiler 'think' a hot tap has been opened? At what pressure would this happen?

Any suggestions on how to fix this problem?

Many thanks in advance,

Johan

[Doctor Evil]

It could be that through the combi when a tap is opened the pressure becomes negative in relation to the rest of the cold water pipework. When you close the tap the pressure increases in the pipework and moves across the combi and activates the flow switch, or the 3-way valve. What make of combi, they don't all work the same way?

You could

1. Insert a none return valve just before the combi. Then this will not allow pressure to lower inside the combi and hot water pipework.

1. Take the combi cold feed right back to the stop c*ck, making this supply dedicated to the combi only. Have the cold water taps tee off this pipe at the stop c*ck.

Try No. 1 first.

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[Johan]

The boiler is a Vokera Linea. Is there any point in putting a non return valve in the hot water pipe after the boiler, as well as in the cold water feed to the boiler?

Johan

Doctor Evil wrote:

[John Stumbles]

I'm not familiar with this model but if it's a type that doesn't have a diverter valve then you absolutely must NOT put a non-return valve in the supply side (from the mains): it should say this in the manual.

[Johan]

The manual says there is a diverter valve, but this appears to direct the water either through the central heating, or through the Domestic heat exchanger (to produce hot water). How does this affect putting a non-return valve in the hot water pipe after the boiler? It does not say anywhere in the manual that a non-return valve should not be used in the water supply, it also doesn't say that it is ok..... Sorry to keep asking questions, but I'd really like to understand this problem now.

Johan

[Doctor Evil]

I'm pretty sure it has a 3-way valve and a flow switch, hence the momentarily turning on of the fan. Then you can. I would check with makers first though. When the DHW is heated via the main gas heat exchanger (no

3-way valve) it uses the cold supply as an expansion route. Using a plate heat exchanger the temperatures will get nowhere near as hot. I would put a shock arrestor on the hot draw-off side as well. As near to any lever tap as possible, as these can cause clunks when turned off, and the arrestor will absorb any expansion from the combi too.

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[Doctor Evil]

Johan,

No problem, you can put one on. See my other post.

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[Pete C]

Hi,

Sounds like an airlock in the HW pipe after the boiler, the air will expand and then compress as a cold tap is opened and closed due to fluctuation in water pressure, making water flow momentarily in the HW pipe and causing the boiler's flow switch to operate.

cheers, Pete.

[Ed Sirett]

When you had the cold water off air may have got into the Hot pipework. If you have a 'dead' branch somewhere this could harbour some air. The effect of the air would be for a small flow to occur through the boiler when the cold taps are worked.

This boiler is able to dump residual heat after doing HW into the CH pipework so this does not precluded the fitting of a non-return valve (NRV).

However since this boiler did work correctly before searching for the trapped air is likely to be more fruitful. A NRV would solve problems. A shock arrestor won't help IMHO.

Some boilers (e.g Ferolli) a NRV would be bad news, and the manual will forbid it. Since residual heat would lead to expansion of the water in the HW pipes with nowhere to go.

[Johan]

I have just run water through the 'dead' branch I added in preparation for the en-suite I need to put in (sorry, didn't mention that in my original post). There was indeed air in the pipe, and getting rid of this seems to have cured the problem. Hope it stays away now. And many thanks for all the suggestions!

Johan