System Boiler sizing

[colin_]

Hi

I am going to have my current system boiler replaced with a Worcester and want to ensure the correct size (had different sizes from differenet sales personel)

I have 3 zones
* 8 radiators ~ 15KW
* 10 radiators ~ 10KW
* Unvented cyclinder ~ 17KW

Q1: Half the year there is only one zone on at a time (2 can overlap and in winter all 3 can for around an hour whilst the water heats) so I was wondering what size is required (I have been told anywhere between 30KW and >40KW!)

Q2: The current boiler does not have an external expansion vessel but I have been kindly advised on this forum that I should also have an external expansion vessel for the amount of radiators, it was mentioned 18L but would this be a total figure (so ~10/12L as the boiler also has an expansion vessel) or a full extra 18L (space where boiler is is a bit tight).

Thanks
Colin

 

[ShaunCorbs]

25kw and stagger hot water times

yes add an external 18l expansion vessel this can go anywhere eg airing cupboard etc

 

[colin_]

25kw and stagger hot water times

yes add an external 18l expansion vessel this can go anywhere eg airing cupboard etc

Hi

Thank you for the quick reply, I shall ask for the expansion vessel to go into the same room that holds the cylinder as that has space to accommodate it.

Thanks
Colin

 

[SJB060685]

The sizing of an expansion vessel can be quite complicated, if you want it to be but in short is down to system water volume and the temperatures the system runs at.
Ideally if the system incorporated two or more expansion vessels then they should be manifolded and tied into the system at a common neutral point.
Remember the point at which the expansion vessel tee's into the system is the point of no pressure change and the pump should be installed shortly after that point. If the vessel was on the discharge side of the pump this would create a negative system pressure which causes a host of problems.

 

[colin_]

The sizing of an expansion vessel can be quite complicated, if you want it to be but in short is down to system water volume and the temperatures the system runs at.
Ideally if the system incorporated two or more expansion vessels then they should be manifolded and tied into the system at a common neutral point.
Remember the point at which the expansion vessel tee's into the system is the point of no pressure change and the pump should be installed shortly after that point. If the vessel was on the discharge side of the pump this would create a negative system pressure which causes a host of problems.

I am slightly confused, I am going to have the boiler replaced and from earlier posts it is suggested that I need an external expansion vessel as the boilers one alone is insufficient for the number of radiators. The only pump (I think) is the one in the boiler, where would be the best place for the expansion vessel in this case?

Thanks
Colin

 

[SJB060685]

Sorry if I confused you. The system should work fine wherever the additional vessel is located. My comment was really to point out that ideally all vessels should manifold in at the same point, with the pump shortly after.

 

[colin_]

Sorry if I confused you. The system should work fine wherever the additional vessel is located. My comment was really to point out that ideally all vessels should manifold in at the same point, with the pump shortly after.

Hi

No problem, I appeciate you taking time to post a reply and the more information I have the better. I have some sales people coming to see me next week and I am in a much better position to understand what they may propose.

Thanks
Colin

 

[John.g]

The sizing of an expansion vessel can be quite complicated, if you want it to be but in short is down to system water volume and the temperatures the system runs at.
Ideally if the system incorporated two or more expansion vessels then they should be manifolded and tied into the system at a common neutral point.
Remember the point at which the expansion vessel tee's into the system is the point of no pressure change and the pump should be installed shortly after that point. If the vessel was on the discharge side of the pump this would create a negative system pressure which causes a host of problems.

Here's one for you SJB, if you connect two Evs into any system, one close to the pump suction and one close to the pump discharge what happens to the pump head since both are points of no pressure change?

 

[Vee]

EV fitted on common return as close the boiler as possible.

 

[SJB060685]

Here's one for you SJB, if you connect two Evs into any system, one close to the pump suction and one close to the pump discharge what happens to the pump head since both are points of no pressure change?

That's a very good question John and one I have been thinking about recently.
I believe the pump would suck a little out of the vessel on suction side and lower the pressure slightly and in turn would pump the water into the vessel on discharge increasing the pressure.
The point of no pressure change would then be somewhere between the two vessels, most like the pump?
That's the conclusion I've come to but if it's wrong then please correct me.

 

[John.g]

Will try and balance them later.

 

[SJB060685]

Will try and balance them later.

If you could come up with a different conclusion to me then please do. Consider vessels both with and without reserve water.
This has been a topic I've been trying to understand inside out for a little while.
Its good to talk to someone like you because you're a lot like me, you have a passion to understand on more than just a basic level.

 

[John.g]

That's a very good question John and one I have been thinking about recently.
I believe the pump would suck a little out of the vessel on suction side and lower the pressure slightly and in turn would pump the water into the vessel on discharge increasing the pressure.
The point of no pressure change would then be somewhere between the two vessels, most like the pump?
That's the conclusion I've come to but if it's wrong then please correct me.

I think that's what will happen alright.

If you consider 1 EV (12 litre) on the pump suction p.pressurised to 1 bar and filled to 1.5bar (=2.4 litre water reserve) (and a pump head of 0.5bar) then the pump discharge will be 2.0bar.

If you install the second EV on the pump discharge then my calcs shows suction side EV with pressure of 1.28 bar and 1.45 litres water reserve and discharge side EV with pressure of 1.78 bar and 3.35 litres water reserve.

If you have 1 EV (12 litre) on the pump suction p.pressurised to 1 bar and filled to 1.0bar (no water reserve) (and a pump head of 0.5bar) then the pump discharge will be 1.5bar.
If you install the second EV on the pump discharge then there is no water (reserve) to be shunted from one EV to the other so I'm inclined to think that the pump suction will be 0.5 bar and the pump discharge will be 1.0 bar.

 

[SJB060685]

This is why I like discussing things with you mate, not everyone is passionate about the finer details. Many are happy with the basics

 

[SJB060685]

How did you calculate this out of curiosity mate?

 

[John.g]

From a spreadsheet I put together years ago, the bottom two are linked for the above exercise but I can't post excel s.sheets (or other files, even if zipped) for some time on here now.

System or HWC capacity 0.00 Litres
Vessel Capacity 12.00 Litres
Pre Charge Pressure 1.00 Bar.G
Pre Charge Pressure 2.00 Bar.A
Filling/supply Pressure 1.50 Bar.G
Filling Pressure 2.50 Bar.A
Air Space Vol 9.60 Litres
Reserve Water Vol 2.40 Litres
Expansion % 0.00 %
Expansion 0.00 Litres
Water Space Vol 2.40 Litres
Air Space Vol 9.60 Litres
Final Pressure 2.50 Bar.A
Final Pressure 1.50 Bar.G

System or HWC capacity 0.00 Litres
Vessel Capacity 12.00 Litres
Pre Charge Pressure 1.00 Bar.G
Pre Charge Pressure 2.00 Bar.A
Filling/supply Pressure 1.2746 Bar.G
Filling Pressure 2.27 Bar.A
Air Space Vol 10.55 Litres
Reserve Water Vol 1.449 Litres
Expansion % 0.00 %
Expansion 0.00 Litres
Water Space Vol 1.45 Litres
Air Space Vol 10.55 Litres
Final Pressure 2.27 Bar.A
Final Pressure 1.2746 Bar.G

System or HWC capacity 0.00 Litres
Vessel Capacity 12.00 Litres
Pre Charge Pressure 1.00 Bar.G
Pre Charge Pressure 2.00 Bar.A
Filling/supply Pressure 1.7746 Bar.G
Filling Pressure 2.77 Bar.A
Air Space Vol 8.65 Litres
Reserve Water Vol 3.350 Litres
Expansion % 0.00 %
Expansion 0.00 Litres
Water Space Vol 3.35 Litres
Air Space Vol 8.65 Litres
Final Pressure 2.77 Bar.A
Final Pressure 1.775 Bar.G

 

[SJB060685]

Thanks mate. Watching the boxing at the moment. Will go through it tomorrow.