DHW Cylinder Expansion Vessel Pre-Charge

[belfastbob]

Hi guys

I have a quick question regarding the correct pre-charge pressure for an expansion vessel (24 ltr) on the cold supply of a DHW cylinder (300 ltr).

As measured after a combined 3 bar PRV + 6 bar safety relief valve assembly, the static pressure of the cold supply is 2.5 bar. The dynamic pressure is 1.7 bar with a few cold taps open.

What pressure should the expansion vessel be charged to when it is disconnected from the cylinder?

Rest assured, I have no intentions of recharging the vessel myself as I am not qualified to work on unvented cylinders, but it seems every plumber I ask has a different answer. Some say charge it to the PRV limiting pressure, others say charge it to the dynamic (working) pressure of the cold feed.

So, can some kind (competent) chap on here provide me with the genuine answer?

Thank you in advance

Bob

 

[IDCHAPPY]

MI's would usually tell you mate, Usually it set to roughly 3 Bar. Again you do realize that it should only be looked at by an unvented qualified engineer 🙂

 

[belfastbob]

Hi Chappy

Yes, I realise that the plumber should have his G3 ticket. I bought the cylinder from one supplier and had it fitted by another. One is telling me the pressure should be set to 3 bar to match the PRV (as you have said). The other has said that if the incoming mains is actually less than 3 bar, that it should be pressurised to match the mains static pressure. This got me confused so I did some hunting, on this forum and others, and found yet another suggestion - that the vessel should be pressurised to match the dynamic pressure of the mains.

I'm no plumber so I wont be touching it, but I'd have more confidence in my installer and supplier if they agreed on what to do.

The way I see it, if the water cylinder is empty and I fill it from the cold mains, when the water stops flowing into it, the tank will be pressurised to the static mains pressure. If I then heat the tank, the water needs to expand to I need an expansion vessel fitted. I don't want water to go into the vessel when the tank is cold, so the vessel needs to be charged to the same pressure as a full, cold water tank i.e. the mains static pressure. If the mains static is less than 3 bar, then the vessel should be charge to less than 3 bar. Does that make sense, or have I misunderstood?

Thanks again,

 

[chris watkins]

Simple, because the main pressure changes with demand or whatever the undertaker provides at the time, the vessel should always be set to just above that of the PRV in the composite valve set.
If your plumber seems uncertain of that you need a new one, plumber that is not vessel!

 

[belfastbob]

Simple, because the main pressure changes with demand or whatever the undertaker provides at the time, the vessel should always be set to just above that of the PRV in the composite valve set.
If your plumber seems uncertain of that you need a new one, plumber that is not vessel!

Hi Chris

Thanks for replying. If my static pressure never seems to go above 2.5 bar, I guess it would be safe to reduce the pressure on the expansion vessel to 2.5 bar if (and only if, from what you've said) the PRV limiting pressure was also reduced to 2.5 bar?

B

 

[kay-jay]

The precharge pressure will be stated on the data badge of the exp vessel

 

[chris watkins]

Hi Chris

Thanks for replying. If my static pressure never seems to go above 2.5 bar, I guess it would be safe to reduce the pressure on the expansion vessel to 2.5 bar if (and only if, from what you've said) the PRV limiting pressure was also reduced to 2.5 bar?

B

Bob I think I have provide more than enough information (which was clear) to someone who is going to employ the services of someone who can legally work on these systems don't you?

 

[Dotty]

Sorry Bob

It is illegal under DFPNI Technical booklet P regulation 88 for you to be attempting any work at all on your UV cylinder.

As stated, get someone in who is. And let them worry about it.

Thread closed.