Expansion after buffer tank

gigglingfish

Hi all,
Looking for some advice.
I have a biomass 28kw boiler with a 400L buffer tank.
When installed and commissioned there was a 36L expansion vessel installed.
It's always had a problem with getting the pressure right. The installers left after trying over a couple of days but to be honest they really didn't seem like heating engineers, so I was reluctant to keep flogging a dead horse with them.

I went away for a week and the pressure dropped very low with no use.
I then discovered that all the AAVs (3 in total) in the system were broken and old, so replaced them.

I filled the system and checked the air was escaping during the fill. I got it to 1 bar on a completely cold system. Then turned on all the pumps to let any extra air release.

I then set the boiler to heat.
The pressure went from 1 bar when cold to 2.2bar when hot and the buffer was heated.

I have been lead to believe that cold pressure should be 1.5 bar - but with the 1.2 bar increase in pressure that would take it up near the 3 bar limit of the system and I'm therefore concerned.

I did have an almost same system in place before with a 24L expansion vessel, but with the new boiler we added the 400L buffer.

So here's me finally getting to the point:
Is 1.2 bar of pressure increase normal?
If not and I am supposed to set cold pressure at 1.5 then why would there be so much pressure increase?

I've checked that the expansion vessel was set at 1.5 bar with the system under no pressure, so think that is set as it should.

It seems a 36L vessel is big enough, but I'd like someone to confirm that.

Any advice or guidance on this please?

For reference: the system is UFH with 5 rooms (approx 110 sq mtrs total), the DHW tank and 6 rads.

Cheers

John.G

Have to make a few assumptions here. Say total system contents of 500 litres at 70C, expansion will be 10 litres, assume E.vessel pre charge pressure at 1.5 bar and filling pressure (cold) at 1.5 bar, final pressure at 70c will be 2.46 bar.
Ideally to give a small reserve (4.7 litres) of water in the E.vessel to make up for any air release etc then pre charge pressure should be 1.0 bar, filling pressure (cold) to 1.3 bar, final pressure at 70c will then be 2.38 bar.

You can do your own calcs in the attachment.

gigglingfish

Thanks John.

So looks like I will have to take some pressure out of the exp.vessel because right now the vessel is set with higher pressure than the system (when cold). So I've probably left myself with no water reserve (as far as I'm interpreting).

That calc is really useful.

So just a quick one. Would you recommend that I drain the system again, release that vessel pressure and then re-pressurize cold at 1.3 bar? Or is there a quicker way without a full drain and refill?

Thanks again, this is really helpful. I've not been able to get a heating engineer to turn-up to any agreed times for some reason, so this will give me the info I need to self-help.

John.G

IF the pressure is = or < 2.5 bar when everything is fully heated up, I would suggest just adding 0.1 bar (water side) when cold and re check the hot pressure again, then repeat this until the hot pressure is not > 2.7/2.75 bar when hot. I would be happy enough with this as its a bit of a dose having to drain the whole system down.

You could probably also do this with system fully hot, as long as you have enough mains pressure to do so.