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Discuss Sizing expansion vessels in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Matt0029

Gas Engineer
Messages
804
Has anyone got a good rule of thumb for sizing an expansion vessel for a central heating system. A combi has a 8 litre vessel. And will deal with the expansion of say 8-10 rads. Could one use this as a ball park figure and multiply up from there Obviously pipe work to take in to account 22mm or if any 28mm on the system. Thanks for any input
 

EvilDrPorkChop

Esteemed
Plumber
Gas Engineer
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972
I think @John.g has a useful spreadsheet for sizing expansion, that i've found useful in the past.
 

John.g

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2,488
Unfortunately cannot post "live" calculations, but here is a screenshot of a system with a 8 litre EV + 110 litre system contents. Final pressure still OK at < 2.5 bar.

Add the total radiator contents + say 25 litrs for a oil fired boiler or ~ 5 litres if G.fired + 0.3litres/meter for 22mm piping and 0.14litres/meter for 15mm piping.

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ShaunCorbs

Staff member
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Plumber
Gas Engineer
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Over 1.2 yes won’t harm if you oversize expansion
 

Matt0029

Gas Engineer
Messages
804
That is the system has 34 rads on it. They want 4 single rads swapping for doubles and an extra rad. Adding to the system. The system currently has a 35 litre expansion on it. Wondering whether to recommend up sizing to a 50 litre.
 

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John.g

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Using the same criteria as in the S.sheet above, a 35 litre EV will accomodate 480 litres with a final pressure of 2.46 bar, a 50 litre EV assuming the same 480 litres gives a final pressure of 2.1 bar.
A 50 Litre EV will accomodate 683 litres with a final pressure of 2.46 bar.

The boiler(s) in addition, may have their own internal expansion vessels?.
 

John.g

Messages
2,488
You can probably get a general handle on it from the various dimensions knowing single or doubles but a possibly easier solution below.

You need the EV capacity (which you have) , the EV precharge and filling pressures (at the boiler, cold) then with all rads etc on and system up to normal temperature note the boiler pressure, and the flow and return temperatures, easy then to calculate the system volume reasonably accurately.

It would/will be quite interesting to see what you come up with but you certainly won't go far wrong with the 50 litre EV as you can't oversize it.
 

Matt0029

Gas Engineer
Messages
804
You can probably get a general handle on it from the various dimensions knowing single or doubles but a possibly easier solution below.

You need the EV capacity (which you have) , the EV precharge and filling pressures (at the boiler, cold) then with all rads etc on and system up to normal temperature note the boiler pressure, and the flow and return temperatures, easy then to calculate the system volume reasonably accurately.

It would/will be quite interesting to see what you come up with but you certainly won't go far wrong with the 50 litre EV as you can't oversize it.
Expansion vessel pre charge is 1 bar. And the cold fill pressure 1 bar. When the boiler / pump kicks in the pressure drops to 0.5 bar. So finding it hard to get an actual gauge of the pressure.
 

ShaunCorbs

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Gas Engineer
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Jon means

Pre charge vessel this is 1 bar
System charge 1 bar
System charge when hot / upto running temp eg 65dc lets says 2.5 bar
System flow temp 65dc
System return 50dc
 

John.g

Messages
2,488
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

Gas Engineer
Messages
804
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

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2,488
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

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John.g

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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

Gas Engineer
Messages
804
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

Messages
2,488
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.
 
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