Second Expansion Vessel Position

[Johntheo5]

The question of where a second EV should be installed if required due to extra rads etc being installed is often asked, I did a few calcs based with a single EV installed just before the pump (recommended) and just after the pump, I just used a simple pipe loop to do the calcs to simplify them. But if a second EV is installed after the pump, I just can't calculate the resultant pressures at the various points in the system, does this mean that no circulation will occur if a EV is installed just before and just after a circ pump??.

 

[Johntheo5]

With 2 EVs, and the circ pump running, the water can be pushed from one to the other resulting in different EV pressures so might look something like below from 2 EVs pressurized to ~ 1.5bar, these pressures will change with different pump heads, pressures when system is hot etc.

 

[Madrab]

If an EV's pre-charge is balanced to the system pressure then there should be an cancelling out effect, therefore there shouldn't be a massive impact on the circulators effective flow. There can be an impact on pressures just given the dynamics of the pump and the behaviour of the EV membrane of course

As far as absorbing expansion is concerned though, the way it was explained to me was that the EV should be on the suction side of the pump to maximise effectiveness of the vessel's ability to absorb the pressure fluctuations. If placed on the positive side of the pump then when the pressure increases when the circulator kicks in then that sudden increase could potentially bypass the vessel, reducing it's ability to absorb that bounce in pressure efficiently. What used to (and still does) confuse me is that the other comment was, ideally the EV should also be on the system prior to any of the of the zones, that therefore allows the maximum protection to those zones from the effects of expansion and that's how a lot of combi's vessels are connected - after the pump/HEX.

As far as stopping flow though I'm not sure the impact of the pressure fluctuations would actually hinder circulation greatly

 

[Johntheo5]

From a negative pressure/air ingress problem In a closed system, it normally should not be a problem with the EV positioned after the circ pump since the cold EV pressure is rarely below 1.0bar so the minimum pump inlet pressure with a pump head of 0.35bar (3.5M) is still, 1.0-0.35, 0.65bar, (1.0bar if positioned before the pump), a OV system like mine can quite easily run with negative pressure depending on where the cold feed is teed in, mine is teed in at the cylinder flow pipe on the second floor, I reckon that even though the cold head height from the F&E to the pump outlet is 4.5M the dynamic head is probably around 2.5M due to pipe resistance, I run with a 3.5M pump head so the pump inlet will run at, 2.5-3.5, -1.0M as my converted attic rad has allways run with a very very slight negative pressure but has never caused the slightest problem.