Sizing expansion vessels

[John.g]

I believe it drops as the expansion vessel is on the suction side of the pump?

It actually means that the EV is on discharge side of the pump, the EV pressure is the point of no pressure change so the pressure is the EV pressure minus the pump head, if EV on pump suction then pressure is the EV pressure plus the pump head. So stop pump after noting flow/return temps then note pressure.

 

[Matt0029]

It actually means that the EV is on discharge side of the pump, the EV pressure is the point of no pressure change so the pressure is the EV pressure minus the pump head, if EV on pump suction then pressure is the EV pressure plus the pump head. So stop pump after noting flow/return temps then note pressure.

Thanks so when I'm looking at the pressure on the gauge. When it drops to 0.5 bar from 1 bar when pump kicks in. Rises to 1.5 bar when up to temperature at the moment. Is the 1.5 bar actually 2 bar?
Hope that makes sense

 

[John.g]

Yes, would expect to see 2 bar if stopped now, what were the flow/return temps?

 

[Matt0029]

Yes, would expect to see 2 bar if stopped now, what were the flow/return temps?

Flows set at 66

 

[Matt0029]

It's returning around 55 small delta tee that's with the pump on minimum

 

[John.g]

Did/can you confirm that the pressure was 2 bar with boiler/circ pump off? This is important.

That gives a system contents of 720 Litres

 

[John.g]

The 66/55C flow/return will only be valid if the boiler was running constantly, ie a gas boiler, if not, or an oil boiler which always cycles then the system dT and the boiler dT will be completely different.

 

[Matt0029]

Thanks the boiler is pretty much running constantly there is huge heat loss as one wall of the whole building is nearly completely glass. Sorry I would have to check the pressure when the pump is off. This is seen as the true pressure?

 

[John.g]

Yes, it will give the true reading then.
If all the readings are correct, the spreadsheet will be correct, 720 litres seems a huge systems content though,
the exact pre charge pressure also has a huge influence on the results, for example if it was 1.25 bar (or 0.5bar) the system contents will be 540 Litres.
One way or the other though. I think you should be quite confident that a 50 litre EV will have no problem in dealing with the extra rads water content.