Position of pump in sealed multizone CH system

[dobad]

In a sealed multizone CH system, does it matter whether the pump is between the pressure vessel and the boiler or whether the pressure vessel is between the pump and the boiler? (On the hot side in all cases)

I know it matters in an open vent system but not sure in a sealed system.

Thanks in advance

 

[cross thread]

Pressure vessel is preferred on the return,pump on the flow

 

[dobad]

Thanks @cross thread. Could you explain why?

 

[cross thread]

Look at the boiler you have installation advice

 

[dobad]

I did, it has boiler, then vessel, then pump (on the hot side) but unlike for for open vent systems the manual doesn't explicitly state that the order is important.

 

[Johntheo5]

Thanks @cross thread. Could you explain why?

When the EV is installed on the inlet to the pump the pump discharge head will be the EV pressure + the pump head, a EV cold head of 1.0bar + a pump head of say 5M or 0.5bar will result in discharge head of 1.5bar. If the EV is installed on the pump discharge side then the inlet head will be the EV head - the pump head, 1.0bar - 0.5bar, or 5M, a system with very restrictive small bore pipework might tend to run at a negative head especially if the EV head was allowed to fall to say 0.5bar before recharging. Obviously its pretty vital to connect the cold feed, ~ 2.5 to 4M head, to the pump suction on a OV system otherwise, practically the whole system will be running at a negative head.

 

[dobad]

In my case the EV is also ~12M higher than the boiler and the pump so hopefully that helps keep the system pressurized..

So what I'm understanding from your last sentence is that the extra 1bar or so of pressure that the EV affords the system is why it's more critical in an open vent system to have the pump behind the tank whereas it might be OK in an EV system as long as the pressure is high enough in the EV?

 

[Johntheo5]

Yes, not very critical in a sealed system as the EV is generally precharged at 0.7bar to 1.0bar with a cold filling pressure of 1.3bar to 1.5bar, i have a OV system with the pump pumping ito the oil fired boiler ground floor and a co. Ined vent and cold feed in the attic resulting in my sttic rad running with a vey very slight negative head but has never caosed a problem over the past 40 years or so.

 

[dobad]

Thank you.