Boiler pressure changes

[colin_]

Hi

When cold the boiler pressure reads approx 1 bar.

When the heating is on it rises to around 2.2-2.4 bar which I think is quite a large rise (still a way from the 3 bar limit).

I was wondering if this was due to the expansion vessel in the boiler not being big enough for the system (~15 rads + unvented cylinder + 3 levels high on a zoned system).

I am presuming that the vessel is correctly presurised as it has been recently serviced.

Assuming that it can be fitted whilst still having the boilers own expansion vessel, would an external expansion vessel be of any help and if so what like sizes as not much space for one where the boiler is fitted.

Thanks
Colin

 

[ShaunCorbs]

2-2.5 bar when hot is normal

you should have an external expansion vessel for the heating could be around 12-18l capacity

 

[colin_]

Thanks for letting me know 2.5 bar is OK (just means I cannot pressurise to 1.5 bar when cold as then it would go to around 3 bar).

Would an externel expansion vessel reduce the upper bar when the system is hot?

I will be changing the boiler soon to a Worcester (30 or 35KW), would it make sense to add an external expansion vessel or just rely on the boilers (given the number of radiators and the fact that the boielr is in the kitchen and top radiators in roof conversion)?

 

[ShaunCorbs]

Add / allow for one when you change the boiler normally Worcester include a 7 or 9 L but tbh I would size the extra expansion vessel to match the system eg 18l

 

[Scott_D]

The expansion vessel inside the boiler will be undersized for that many radiators.
Fluctuating pressure put more strain on boiler components.

 

[colin_]

Many thanks for the replies, I will ask for an external expansion vessele to be fitted when the boiler is changed out.

Forgive my ignorance but ShaunCorbs mentioned 18L for the number of radiators I have but would that be the external ones size or does it need 18-7L (that the boiler has) and so ~11L external?

 

[Knappers]

Yes 🤭