How do I work out if I need an additional expansion vessel on a Condensing combi boiler?

[Stephen H]

The manual for the combi condensing boiler we have bought does not
mention anything about fitting additional pressure vessels if specific
circumstances apply.

There are 14 rads in total, 7 double panels with double convectors (type
22), 5 towel type radiators, one double panel single convector (Type 21)
and a single panel single convector (type 20)

7 of the rads are on the upstairs zone, the other 7 on the downstairs zone.

Clearly the flow pipework has been duplicated twice in 22mm to permit
the upstairs and the downstairs zoning arrangement and to allow for
additional rads to be added should the house be extended so there is a
lot more copper tube than usual.

Regards,

Stephen

[Newshound]

Obviously, it depends on the size of the rads, but that *sounds* similar
to my house. I did the sums (I'm surprised there is no "spec" in your
manual) and decided that I needed the smallest additional vessel from
BES, but in the end put in the second smallest (about the size of a
watermelon) and this has been fine. For ease of checking and
repressurising, you need an isolating valve between the vessel and the
system, and a drain tap on the vessel side. When I changed the boiler I
found that the internal vessel had failed without my knowledge. These
are often very inaccessible and inconvenient to check, and are, I
suspect, often neglected.

[Roger Mills]

On 11/01/2012 10:00, Stephen H wrote:

Does the boiler manual say what size of expansion vessel is included
and/or what system volume it can cater for?

Do you know your system volume? This will expand by up to 2.5% between
cold and hot - and the expansion shouldn't more than half fill your
expansion vessel in order to limit the pressure rise to something sensible.

So, to give an example: System volume 100 litres. Expansion 2.5 litres.
Expansion vessel 5 litres.

You should be able to work out from this whether or not you need any
additional expansion capacity.
--
Cheers,
Roger
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[John Rumm]

On 11/01/2012 10:00, Stephen H wrote:

> The manual for the combi condensing boiler we have bought does not
> mention anything about fitting additional pressure vessels if specific
> circumstances apply.
>
> There are 14 rads in total, 7 double panels with double convectors (type
> 22), 5 towel type radiators, one double panel single convector (Type 21)
> and a single panel single convector (type 20)
>
> 7 of the rads are on the upstairs zone, the other 7 on the downstairs zone.

That does not sound like an excessively large system - given the towel
rads are usually relatively water content in comparison to proper rads.

> Clearly the flow pipework has been duplicated twice in 22mm to permit
> the upstairs and the downstairs zoning arrangement and to allow for
> additional rads to be added should the house be extended so there is a
> lot more copper tube than usual.

Try it and see what happens to the operating pressure. If it gets too
high, create some temporary space by letting some air into a rad, and
add another expansion vessel when you get the chance.

--
Cheers,

John.

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