Alpha combi boiler - DHW expansion tank?

[Neil Williams]

I've got an Alpha CB25 combi boiler. Had a bit of hassle with it this morning (pressure in the heating system was down probably due to a leaky radiator which I'll have to find). In doing so I was reading the manual and noticed that it says if a water meter is fitted, a DHW expansion device needs to be fitted in the cold water supply line.

Having just had a water meter fitted, I guess I need to sort this - however, all the information I've found on sizing such a device has been based on a specific size of water heater tank - except an on- demand combi boiler doesn't have such a tank as such.

Does anyone have any information as to whether such a device is indeed required for a combi boiler (the diagram that mentions it seems to show a traditional system with a header tank rather than a combi boiler, despite being in the manual for a combi), and if so how I determine the correct size of tank? Also, does it have to be fitted near the boiler or can it go anywhere in the cold supply after the meter?

Thanks

Neil

[Bob Minchin]

In true usenet tradition - i'll not answer your question but....

This could be related to the electricity meter requirement that it must not be made to run backwards. I've just discovered this rule/law(?) as I just had a solar electricity system fitted. It could be that there is a similar rule for water meters. If your water board did not ask if you had a combi boiler than I would leave well alone. I only discovered the electricity rule as the form I had to fill in for the leccy board, asked the specific question. As it happens my digital meter cannot run backwards so it is not an issue.

Bob

[Neil Williams]

I believe it is - and it's got around by water meters generally having a non-return valve, which means that when you get a temperature change (caused by running the hot water) you get pressure building up in your cold supply, which can cause a burst pipe.

But any documentation I can find about this refers only to water heaters with a tank of some sort - not on-demand combi boilers.

Neil

[YAPH]

They're usually the little mini-expansion vessels sometimes used as surge arresters. I think Toolstation do one. They're only required for the sort of combi where domestic hot water is heated in the gas-to-water heat exchanger alongside primary water for the radiators, rather than combis with a secondary (usually plate) heat exchanger for DHW. I assume your Alpha is one of the former if it's telling you to do this in the manual.

[Neil Williams]

This I guess?

They're only required for the sort

a diagram on page 6 that shows very clearly a secondary heat exchanger for hot water. Odd.

Neil

[Neil Williams]

place it's mentioned is the diagram on p11, nowhere in the text.

Neil

[YAPH]

that's the badger...

The mini expansion vessel should only be needed with the SY25, not the CB model.

[Neil Williams]

That would seem to make sense to me.

Thanks

Neil