I seek the advice and opinions of the experts in the forum!

Three weeks ago I had my boiler replaced with an Intergas Xclusive 36. I use the Evohome control system and OpenTherm using the dedicated bridge. My house has 12 radiators if I include towel rails. One radiator is never on. All the rest have HR92s apart from one, a tiny one in the downstairs loo, a throwback to when I left one radiator “open”.

I am running the central heating at a flow rate of 65c as opposed to 80c. 65c is fine. DHW is currently set to 55c. All works well. All rooms reach their set point and several rooms having the same schedule, all do so more or less together.

However, reading the Intergas installation manual it says that the inbuilt 6 litre expansion vessel is good for up to 8 radiators or more specifically, 100 litres in the system. The manual states that Intergas’s advice must be sought if the system exceeds these parameters. On that point I raised it with Intergas and they have so far not responded at all! Seek their advice and you don’t get it! Richard from the Intergas Shop has been much more helpful.

From enquiries made it seems many Intergas appointed installers are not aware of this,

Now for the questions!

The 100 litre maximum is I understand based on a flow temperature of 80C. It is implied that above that volume the 6 litre expansion vessel needs an additional external one. Does anybody know what the formula is if the system is run at a lower temperature? The volume level must go up but to what? In my case I am running at 65c. So far perfectly adequate.

Next is the effect of Evohome. The benefit of Evohome is that only zones one wants heating are heated. Thus some, possibly most radiators, are closed off. Therefore a 12 radiator system does not mean 12 radiators of volume. Is there any way of calculating the “Evohome effect” on the capacity limit?

So far I have had no indication at all that there is any issue with the volume of water in my system.