Central heating expansion vessel pressure calculation

10-10-20

Hi, I have a Vokera Mynute 28e with an integrated 10l expansion vessel. The system size is 14 rads with an approx capacity of around 100l plus another ~30l for pipes. I fitted an additional expansion vessel on the basis of advice from a Vokera engineer, so the expansion capacity is now 20l. The system is a low-pressure sealed system fed from the attic tank at 5.4m height from the ground floor.

The initial system pressure has always registered as 0 or 0.1 bar on the dial at the boiler, rising to ~0.4 bar if the whole system is heated. Another pressure gauge at the upstairs hotpress reads similar, but just sligltly lower than the boiler pressure (which is 4m lower).

Question is - I know that JohnG posted an Excel spreadsheet in the past for pressure calculations, but I can't find it now - but I want to know what the expected pressure should be with a sealed system like this if it's gravity fed off the attic tank? I have set the expansion vessels to between 0.6 and 0.75 bar on the basis of what I think I calculated off that spreadsheet in the past.

Basically what's on my mind is considering moving to a mains pressure system with a automatic filling valve/regulator instead by modding the piping in the hot-press. Is this a good plan?

John.G

THere was no need for that extra 10L EV, with a precharge pressure of 0.7bar then the final pressure will only be 1.15bar and only 0.9bar with 2X10L EVs. The "filling pressure" gravity head is only ~ 0.45bar as even though you can precharge to any pressure you like, the only effect it will have is that the final pressure will be higher. When doing the calc (gravity system) just make the filling pressure the same as the pre charge pressure, the water will almost instantly reach the precharge pressure as soon as heating commences so the calc will then be correct. You can install a mains fed system but under no circumstances whatsoever would I advise using a auto fill PRV as it only masks any leakage, if you precharge to say 1.0 bar and fill to 1.5 bar then you will have a nice little reserve of 4.0L before the pressure falls to 1.0bar (cold) which means "Never" topping up (maybe once every 2 years or so) you can also monitor any leakage by looking occasionally at the cold boiler pressure. (The final pressure with precharge/flling pressures of 1.0/1.5bar will be 1.88bar (20L EV).

I forget to protect the spreadsheet but you can do it yourself or just make changes in the yellow cells.

Expansion Vessel Calculation 10-10-20.xlsx

10-10-20

https://www.boards.ie/discussion/comment/121147707#Comment_121147707

Many thanks John, I really appreciate the advise you provided here and the general advice that both you and others provide on the forum in the past.

Thanks for the point about the PRV. There was a significant leak on the system when we bought the house and I think that's why the system wasn't mains connected as the leak was in the concrete and they couldn't locate it, so connected it as a gravity fed system instead to keep the loss somewhat hidden. I found and fixed it since.

I'll chew over my options here and play with that spreadsheet. 👍️

10-10-20

https://www.boards.ie/discussion/comment/121147707#Comment_121147707

Well @John.G, I got it converted over to mains pressure last night with a double check-valve etc. All straight forward and working now at 1.5bar. Thanks again.

John.G

And filling valve shut I hope??

10-10-20

https://www.boards.ie/discussion/comment/121168227#Comment_121168227

Oh the whole shebang - double ball-valve. And a shiny pressure gauge to match.