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Discuss Hot Water Tank Expansion Vessel Question in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Am I correct in saying that the capacity of an expansion vessel as written on the appliance data label is the capacity of the water compartment alone and excludes the air compartment capacity?

Thanks
 
It's the total capacity of the E.vessel, if you disconne t the water end and pressurise the air end u tail no .ore water then the full capacity f the E.vessel is full of air and vice
Vers, pressurising,e the water end until o Ir
Pressure is the same capacity, the labeled capacity.
press
 
It's the total capacity of the E.vessel, if you disconne t the water end and pressurise the air end u tail no .ore water then the full capacity f the E.vessel is full of air and vice
Vers, pressurising,e the water end until o Ir
Pressure is the same capacity, the labeled capacity.
press
Ok

Reason for asking is if you do the math using the physical dimensions of the tanks to calculate the volume of the tank, even allowing for the schrader valve and the connection, the calculated volume comes out several litres above the stated value.

So when the system is cold there is no water in the vessel at all. Is that the case?
 
The actual volume should be slightly less than the measured capacity of the E.vessel, the EV must then be able to accommodate the expanded volume which is - 1.7 litres in a 100 system volume.
The EV is normally pre pressurised to say 1 bar and you don't have to have the cold filling pressure greaterthan this but you should fill to 1.5 bR S it will then give water reserve of 2 litres cold to make up for any time you losses etc to avold equent top ups
 
The actual volume should be slightly less than the measured capacity of the E.vessel, the EV must then be able to accommodate the expanded volume which is - 1.7 litres in a 100 system volume.
The EV is normally pre pressurised to say 1 bar and you don't have to have the cold filling pressure greaterthan this but you should fill to 1.5 bR S it will then give water reserve of 2 litres cold to make up for any time you losses etc to avold equent top ups
Cheers. I tend to go by the appliance data on the hot water tank for its EV settings
 
The actual volume should be slightly less than the measured capacity of the E.vessel, the EV must then be able to accommodate the expanded volume which is - 1.7 litres in a 100 system volume.
The EV is normally pre pressurised to say 1 bar and you don't have to have the cold filling pressure greaterthan this but you should fill to 1.5 bR S it will then give water reserve of 2 litres cold to make up for any time you losses etc to avold equent top ups
Does it make any difference if the EV is on a hot water tank or a CH system as far as pressure is concerned?

I'm reading that it should be set to the same as the supply pressure when cold which is 1 bar in the UK.

Is that for both usage types HW and CH?

They both have the same water supply. The HW has a max temp of 70C and the CH a max temp of 85C.

The EV I have bought has the same spec as the existing one and comes pre-charged to 3 bar as does the original.

The hot water tank itself specifies EV pre-charge of 3 bar too.

So do I install it at 3 bar or 1.5 bar?

I have had a good hunt around the net and it's really not clear at all.

I'm inclined to pre-charge it to 1.5 bar and keep an eye on it. 1.5 bar makes sense.

What do you think?

Cheers
 

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It is recommended that the pre charge pressure for a unvented HW CyliNDer EV is set 0.2 bar less than the cold cylinder pressure so a 220 litre HW cylinder with cold pressure of 2.5bar and a 18 liter EV will have 3.4 bar hot pressure.
 
Does it make any difference if the EV is on a hot water tank or a CH system as far as pressure is concerned?

I'm reading that it should be set to the same as the supply pressure when cold which is 1 bar in the UK.

Is that for both usage types HW and CH?

They both have the same water supply. The HW has a max temp of 70C and the CH a max temp of 85C.

The EV I have bought has the same spec as the existing one and comes pre-charged to 3 bar as does the original.

The hot water tank itself specifies EV pre-charge of 3 bar too.

So do I install it at 3 bar or 1.5 bar?

I have had a good hunt around the net and it's really not clear at all.

I'm inclined to pre-charge it to 1.5 bar and keep an eye on it. 1.5 bar makes sense.

What do you think?

Cheers
Just got your post, a unvented cylinder is generally intalled with a PRV which can be
set to any pressure as long as its approx 0.5 to 1.0 bar lower than the mains pressure so the above still applies, set it to the cold HW Cylinderr pressure minus 0.2bar.
 
It is recommended that the pre charge pressure for a unvented HW CyliNDer EV is set 0.2 bar less than the cold cylinder pressure so a 220 litre HW cylinder with cold pressure of 2.5bar and a 18 liter EV will have 3.4 bar hot pressure.
Wouldn't the cold cylinder pressure in the UK be 1 bar? The supply pressure?
 
It is recommended that the pre charge pressure for a unvented HW CyliNDer EV is set 0.2 bar less than the cold cylinder pressure so a 220 litre HW cylinder with cold pressure of 2.5bar and a 18 liter EV will have 3.4 bar hot pressure.
Ignore previous question.

Just been on to United Utilities and they say their supply pressure is between 2.4 and 4 bar.

So I am going to install it at 3 bar in line with the cylinder and EV pre-charge.

I don't know what this Google query is all about.

Thanks again for your input.
1650565576952.png
 
Just got your post, a unvented cylinder is generally intalled with a PRV which can be
set to any pressure as long as its approx 0.5 to 1.0 bar lower than the mains pressure so the above still applies, set it to the cold HW Cylinderr pressure minus 0.2bar.
8
Wouldn't the cold cylinder pressure in the UK be 1 bar? The supply pressure?
Th UK mains can be anything up to 5 bar but 1bar is only guaranteed AFAIK. if you look at the setting on the PRV then suggest setting the EV pressure slightly below this, IMO all unvented cylinders should have a pressure gauge installed but in any event even if the precharge pressure is set too low it won't cause a problem.
 
8

Th UK mains can be anything up to 5 bar but 1bar is only guaranteed AFAIK. if you look at the setting on the PRV then suggest setting the EV pressure slightly below this, IMO all unvented cylinders should have a pressure gauge installed but in any event even if the precharge pressure is set too low it won't cause a problem.
See previous post. The 1 bar Google search result is b0ll0x.
 
Ignore previous question.

Just been on to United Utilities and they say their supply pressure is between 2.4 and 4 bar.

So I am going to install it at 3 bar in line with the cylinder and EV pre-charge.

I don't know what this Google query is all about.

Thanks again for your input.
View attachment 75092
As I said have a look at the PRV setting and set to this slightly less.
 
The PRV I am referring to is the pressure reducing valve, the cylinder PRV is the pressure relief valve (safety valve) the other PRV that you refer to is probably the expansion valve and as its set to 4.5bar it might lift if you set the EV prechargeto 3bar. Have you got a pressure reducing valve installed? and if so check its setting.
 
Are you g3 registered as your working on an unvented?
 
The PRV I am referring to is the pressure reducing valve, the cylinder PRV is the pressure relief valve (safety valve) the other PRV that you refer to is probably the expansion valve and as its set to 4.5bar it might lift if you set the EV prechargeto 3bar. Have you got a pressure reducing valve installed? and if so check its setting.
Ah. The one in the combination valve. Got it. There are 3 valves in total then.
Two in the combination valve. A PRV at 4.5 bar (blue cap) and the reducing valve which connects to the supply.
I presume that's limited to the supply pressure or there about.
One PRV in the tank at 6 bar.
 

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I'm not. But the guy who is supervising the job is.

And what does he say set the pre charge to ? Also why are you changing an expansion vessel on an newer cylinder?
 

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