expansion vessel for potable water

[Jeramy]

Hi, I have a massive 310 litre expansion vessel connected to a glowworm 300 litre unvented hotwater tank which i think is there to maintain pressure when 2 or more showers are being use simultaneously. Can anyone tell me what the pressure should be in the expansion vessel. I have a water pressure of 2bar and the factory set pressure of the tank is 2.6bar? The expansion vessel has currently 1.6 bar?
Thanks in advance.
Jeramy

 

[Johntheo5]

What you have I think is a accumulator. If the pressure when using no water is 1.6bar then the accumulator will contain 300L of air at 2.6bar and not one drop of water. If you require say a minimum of 1.6bar pressure when using two showers then the accumulator pre charge pressure must be set to 1.6bar (or very slightly lower), when little or no draw off of water then if the mains pressure is 2.0bar, the accumulator will contain 40 litres of water which will flow back into the system in falling from 2.0bar to 1.6bar, it will contain 77 litres at 2.5bar and 105 litres at 3.0bar.
So what is the water pressure with no water usage and what do you think is the minimum showering pressure you require?

Do you have a (charging) pump supplying this accumulator?

 

[Jeramy]

Hi thanks for the reply. What i would like is for the water pressure not to drop when more than one shower is being used. When i put a gauge on a tap i get just under 2bar. I have an electric car tyre pump which i can use to change the pressure.

 

[Johntheo5]

If you are getting 2.0bar pressure (known as the static pressure) at a tap with no flow then the pressure (dynamic) must be lower once you start using water, its no good increasing the accumulator air end pressure, this is why some accumulators have a pump to increase the water end pressure to give a reserve (accumulation) of water. If you leave that pressure gauge attached to the tap note the pressure when you put the 2 showers on and post back.

If you had a pump on the accumulator to increase the pressure from 2.0bar to say 4.5bar then the water volume in the accumulator will be 136 litres, sufficient to give almost 7 minutes of showering at 2.0bar with 2 showers pumping at say 10LPM each.

 

[Jeramy]

Hi, With 2 showers on the pressure drops to 1.8 bar. There are four showers in the house but only 2 would likely be used simultaneously.

 

[Jeramy]

I cant see how the accumulator can fill up with water if the ballon pressure is greater than the mains pressure with out a pump being involved?

 

[Johntheo5]

I cant see how the accumulator can fill up with water if the ballon pressure is greater than the mains pressure with out a pump being involved?

Thats quite correct, as I said originally, if the air end (baloon) precharge pressure is 2.6bar and the mains pressure is less than 2.6bar then you will have a accumulator full of AIR at 2.6bar and no water.
Which tap have you the PG on?, I'm surprised it only falls from 2.0bar to 1.8bar with the two showers on, the PG should be attached to a outside tap or a tap that is fed directly from the mains to get the correct pressures.

A accumulator without a charging pump is of little or no use unless there is a substantial pressure difference between the minimum required dynamic pressure (say 2.5bar) and the mains static pressure, if the mains static pressure is say 4.0bar then the accumulator will eventually be filled and contain 90 litres of water at 4.0bar.

 

[Jeramy]

Hi the gauge is attached to an outside tap. I suppose i will just reduce the pressure in the accumulator to just below that of the mains?

 

[Johntheo5]

Have you checked if there is a PRV installed somewhere close to the stop valve as its difficult to imagine just a 0.2bar static/dynamic differential with 2 showers in action.

(Any changes to a unvented cylinder should really be carried out by a G3 qualified person)

 

[Jeramy]

hi, there is PRV installed

 

[Johntheo5]

hi, there is PRV installed

Is the supply to the outside tap taken off before or after this PRV, is the supply to the accumulator taken after this PRV and is this PRV also supplying the 300L HW cylinder. If the accumulator is fed after this PRV then no good as its the direct mains that should supply it.