Expansion vessel size?

[Nigel R]

I have a large closed system with 29 rads over 3 floors plus cellar, total dissipation 38kW, what would you suggest for a pressure vessel? It's a heat-only boiler with a 300l megaflow hw tank. The boilers coming out have been fine for pressure with only their built in 20 litres, but they may never have fully heated the system. On the other hand the new boiler is condensing of course so only averaging 60 degrees. I'd be grateful for your thoughts.

 

[fixitflav]

I have a large closed system with 29 rads over 3 floors plus cellar, total dissipation 38kW, what would you suggest for a pressure vessel? It's a heat-only boiler with a 300l megaflow hw tank. The boilers coming out have been fine for pressure with only their built in 20 litres, but they may never have fully heated the system. On the other hand the new boiler is condensing of course so only averaging 60 degrees. I'd be grateful for your thoughts.

Boiler being condensing doesn't automatically mean the temperature will be low. If there is plenty of rad area vs boiler output it will be, but if not the temperature might need to be higher (at least in cold weather) to reach required room temperature.

But a more typical 10-rad system has water volume about 100 litre, so as an estimate yours is 300 litre. Worst-case expansion of water, between 4° and 100°C is about 4%. In practice 3% or 9 litre expansion is probably OK and according to my estimate total vessel volume 20 litre as currently should be fine, but I wouldn't go any smaller.

 

[Gasguru]

A 9 litre vessel will be hopeless...all the vessel calculators are useless as they expect the vessel to reach a ridiculous hot pressure (that shortens the diaphragm life) and leaves no margin before the safety valve opens.

Oversize the vessel and it then prevents an excessive pressure rise when the rads are hot.
If you have the space, the 35 litre floor standing vessels are not too expensive and you'll get a minimal pressure rise (I shove them on a braided hose so I can move them if necessary for access).

 

[Nigel R]

Thanks @fixitflav and @Gasguru. Good point about the water temperature when it's really cold, and the 35l nice and big concept to give the diaphragm an easy time, and making it movable. I think I have room for the 35l so I'll go for that. Thanks again

 

[fixitflav]

A 9 litre vessel will be hopeless...all the vessel calculators are useless as they expect the vessel to reach a ridiculous hot pressure (that shortens the diaphragm life) and leaves no margin before the safety valve opens.

Oversize the vessel and it then prevents an excessive pressure rise when the rads are hot.
If you have the space, the 35 litre floor standing vessels are not too expensive and you'll get a minimal pressure rise (I shove them on a braided hose so I can move them if necessary for access).

The 9 litre is the estimated expansion, not total vessel volume. OP said the existing 20 litre has been OK, and I didn't want to overdo things. 300 litre system volume could be on the high side. In-built vessels in system boilers and combis tend to be on the skimpy side, for obvious reasons. But I agree that going for a big vessel if space and £££ allow is always a good idea.

 

[Nigel R]

I really lacked a reference, as the existing two Vaillant 628e could have either 6 or 10l expansion each (depending on round or square which I can't see yet) but in any case only have their standard pumps which were inadequate and never heated the farthest rads - so I wasn't trusting the system specification to start with! So yes a calculation of 9l actual expansion as you say @fixitflav plus some margin and experience has been perfect for me to move forward with confidence. And as soon as I looked, there was a 35l with legs on special offer! Perfect

 

[Gasguru]

And remember vessels are complete garbage these days with paper thin diaphragms so the nitogen/air volume after just a year is often very
depleted (of course they should be checked yearly) but vessels 20 years ago were much higher quality.

System boilers are nearly always a bad idea unless fitted to new build with small heating loads...the integral pumps/vessels are generally undersized for you typical UK house.

 

[Nigel R]

Yes, thanks I'll keep an eye on it, and indeed @Gasguru this system has been in this house for 20 years and I don't think it has ever worked properly. So now it's getting a separate 32-100 Magna1 pump and your 35l expansion tank and let's see.