Hello,

I want to add an external expansion vessel to my combi boiler as I
think the built-in one has failed. The built-in one is fed off a
15mm port on the DHW exchanger. I think it will be difficult to
run 15mm copper to this, so could I use a short length of 10mm or
will the reduced diameter cause problems?

Thanks in advance!
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On 26/08/2020 15:50, Stephen wrote:
Hello,

I want to add an external expansion vessel to my combi boiler as I
think the built-in one has failed. The built-in one is fed off a
15mm port on the DHW exchanger. I think it will be difficult to
run 15mm copper to this, so could I use a short length of 10mm or
will the reduced diameter cause problems?

Thanks in advance!

Shouldn't be a problem, if you can find all the suitable adaptors.

But the expansion vessel can go anywhere on the supply or return
pipework to the radiators. There's a lot to be said for fitting them on
a flexible connection (like the filling loop). One thing I would
recommend is fitting a ball valve for isolation, so that you can test
and when necessary recharge the vessel easily without depressurising the
main system. (You should fit a drain point downstream too, otherwise you
will have to release pressure by cracking a compression fitting). And
obviously, remember to de-isolate afterwards.

Thank you. What is the advantage of using a flexible hose? I
assumed this would perish over time due to the heat? I thought
most boiler manufacturers specified copper only within a metre of
the boiler? Thanks again!
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On 26/08/2020 19:20, Stephen wrote:
Thank you. What is the advantage of using a flexible hose? I
assumed this would perish over time due to the heat? I thought
most boiler manufacturers specified copper only within a metre of
the boiler? Thanks again!

But as I said, you don't need to be that close to the boiler. External
vessels usually hang on a bracket to keep the weight off the pipework.
Having a flexible hose makes it easier to connect. Mine is tucked up
quite close to the boiler so there is not much room around the Schraeder
valve. By unhooking it, it becomes easy to attach a pressure gauge or a
hose from a bicycle pump. Also, if I do want to depressurise it to check
the pressure, by unhooking it from the bracket and swinging it out from
the wall it is easier to direct the drain valve outlet into a suitable
bucket.

I'm not sure what type of rubber is used in these hoses but I've never
had one fail. They are mechanically well protected by the mesh which
stops them getting kinked; a common failure mechanism for rubber is
attack from UV or ozone which is always worse at stressed regions. And a
dead leg like this (or a filling loop) will never get very warm from the
water side even if it goes into the boiler outlet pipe.

Thank you, that's reassuring. So if the 10mm microbore is not too
narrow, I assume I can also use a "standard" isolating valve and
don't need to use a full bore one.

What is the advantage of using a flexible hose? I was thinking the
heat might make it perish and leak? I thought boiler
manufacturers usually recommend copper pipes only within a metre
of the boiler?
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On 27/08/2020 13:10, Stephen wrote:
Thank you, that's reassuring. So if the 10mm microbore is not too
narrow, I assume I can also use a "standard" isolating valve and
don't need to use a full bore one.

What is the advantage of using a flexible hose? I was thinking the
heat might make it perish and leak? I thought boiler
manufacturers usually recommend copper pipes only within a metre
of the boiler?

There's no continous flow. You could use the type of 2mm or so OD
capilliary tube used for some types of thermometer or thermostat!