Matching Unvented Hot Water Cylinder Dynamic Flow Pressure To EV Pressure

[Matchlessman]

Hi,

Can anyone advise me where the best point is on a unvented hot water cylinder system to measure DF water pressure? Would it be the balanced cold outlet on the combination valve? The unvented HW system was recently installed and I've noticed that it makes a shuddering noise when the hot water taps are first turned on in the morning after the boiler has heated up the HW. After reading similar forum postings I think there may be a mis-match between the lower DF system water pressure and the 3 Bar EV factory setting resulting in the increased system pressure trapped in the pipework.

Any help would be much appreciated.

 

[Madrab]

The dynamic pressure would be in one of 2 places, depending on what the mains is delivery and what the UV Pressure Reducing valve is set to. If the dynamic mains is greater than 3 bar and the PRV is set to 3bar then yes on the balanced supply, if not then it would be on the mains at a full flow outlet, say an outside tap with one or 2 other outlet running.

 

[Johntheo5]

Hi,

Can anyone advise me where the best point is on a unvented hot water cylinder system to measure DF water pressure? Would it be the balanced cold outlet on the combination valve? The unvented HW system was recently installed and I've noticed that it makes a shuddering noise when the hot water taps are first turned on in the morning after the boiler has heated up the HW. After reading similar forum postings I think there may be a mis-match between the lower DF system water pressure and the 3 Bar EV factory setting resulting in the increased system pressure trapped in the pipework.

Any help would be much appreciated.

In a perfect world, the PRV would be set to 3.0 bar and the EV precharge pressure set slightly lower at say 2.8bar with mains pressure sufficiently high to maintain 3.0 bar at the PRV under all flow conditions, the PRV also contains a NRV which closes when the cylinder is heating and the EV absorbs the expansion of a few litres so the cylinder pressure rise will depend on the cylinder/EV vol ratio, as soon as a few litres of HW are drawn off then the cylinder pressure will fall to 3.0bar.
If the PRV and EV pressures are both set to say 3.0bar then even if the mains DF pressure is only say 2.0bar then the cylinder will operate at very close to 2.0 bar but once heating starts then the the clinder pressure at the end of the heating period will be almost exactly the same as it would have been had the DF pressure been 3.0bar since water is largely incompressible.
I,m allways surprised that all UV cylinders don't have a pressure gauge and if I had a UV cylinder I would have it on the EV outlet, its easy then to allow for elevation pressure differences.

But to answer your question on its location, I would not install it on the balanced cold water outlet as that will only reflect the cylinder pressure while water is being drawn off because of the NRV location, the cylinder pressure could rise to 6bar and lift the expansion safety valve if the EV had failed or the air end pressure had fallen for whatever reason but your PG would still only indicate 3.0bar or whatever. So anywhere on the cylinder or as suggested, the EV outlet.

What is your cylinder and EV capacities? the final pressures can be calculated quite easily and the effects of a low cold water inlet pressure etc.

 

[Matchlessman]

Many thanks Madrab and Johntheo5 for your helpful replies. The UV cylinder is a Gledhill ES 250 with a nominal capacity of 248 litres and the EV volume of 25 litres pre-charged to 3 Bar pressure.

I'm not sure about the exact mains inlet pressure, but I think it was measured at just under 3 Bar prior to the installation. As the system pipe noise is only apparent when either hot water is first drawn and the boiler on or just with the boiler on leads me to think the EV set at 3 Bar is not absorbing the increased heated water volume, resulting in a system pressure increase that is only released when the hot taps are opened.

 

[Madrab]

You need to get the unvented (UV) serviced by a qualified UV specialist - part of that service will be to test the EV for it's pre-charge in relation to the mains pressure/pressure through the PRV and then should be set appropriately.

 

[Johntheo5]

I can assure you 100% that unless the outlet from the EV is blocked or the precharge pressure has fallen to 0.52bar that your EV can certainly absorb the expansion.
Assuming precharge pressure at 3.0bar and that the whole 248L is heated from 15C to 60C then the EV in absorbing 4.09L of expanded water will result in a final pressure of 3.78bar which shouldn't cause the slighest problem. The precharge pressure would have to fall to 0.52bar to cause the final pressure to rise to > 6.0bar and expansion valve lifting.
You might, and only very might get a noise problem if the DF pressure is say only 2.0bar and the cylinder starts heating as the cylinder pressure will rise almost instantly to 3.0 bar but will still then only rise to 3.78bar on reaching 60C, opening a hot tap should then not cause the slightest of problems but unfortunately, Gledhill Lite cylinders suffer from all sorts of noises, you could fill a small book with complaints, Gledhill have replaced cylinders, they also said that one batch of cylinders were manufactured because the curing time for the sprayed on insulation wasn't long enough.

 

[Madrab]

Apologies, I missed the part about recently installed. You need to get the installer back to set the system up properly. EV's don't always ship with the correct pre-charge, that or the pre-charge may not be set up correctly that the system is running at. They are very rarely just plug and play.

 

[Johntheo5]

Would agree that its best to set the precharge properly, normally 0.2bar below the running prssure, if the above running pressure is 2.0bar and the precharge is set to 1.8bar then the final pressure is still only 2.64bar with 248L/25L Cyl/EV and only 2.59bar if precharge set to 2.0bar whereas if set to 2.8bar will result in a final pressure of 3.83bar or 3.78bar if precharge set to 3.0bar, so if the running pressure is only 2.0bar then the pressure drop on opening a hot tap is 0.64/0.59bar in the first case but 1.83/1.78bar in the second case which may cause some noise in some systems.

 

[Matchlessman]

Many thanks for taking the time for your detailed inputs to my enquiry - I will contact the G3 installer and ask him ensure the EV pressure is correctly set-up for the incoming mains pressure. Hopefully that will stop the shuddering noise that seems to coming from EV when the hot taps are first turned on.

 

[Madrab]

Do you get a small surge in HW pressure when it then makes the noise and then it settles down? As suggested, the noise is usually down to the initial release of excess pressure after the cylinder has been heated fully. The EV is the safety component that regulates that pressure and it needs to be closely matched with the cylinder pressure and not just left as it arrived.

Hopefully your installer is well versed on setting it up, not all are unfortunately, which is probably why you are now experiencing this issue.

 

[Matchlessman]

Yes, that's exactly what happens when a hot tap is first opened after the cylinder has heated up - a short bust of increased water pressure accompanied by a shuddering noise from the EV before settling down to the DF pressure.

 

[Johntheo5]

I would suggest that your plumber installs a pressure gauge teed off at or from the EV outlet, you dont' really know what the cylinder/EV pressure is after heating up. Also IMO the outlet pipe from the EV should be 22mm.
As stated previously if the static pressure is 3.0bar and the DP is 2.0 bar then there will be 4.09L of expanded water in the cylinder at the end of a whole cylinder reheat to a final pressure of 3.61bar, when you open a hot tap then 4.09L of water will be expelled from the cylinder before the pressure drops to 2.0bar, if you assume the HW tap flow at ~ 10LPM, then it will take 60*4.09/10, 24.5secs for the pressure to fall by 1.61 bar from 3.61 to 2.0bar. But you don't know if the PRV on the combination valve is drop tight so over a period of time the cylinder cold pressure can creep up to whatever the mains pressure is. You say the static pressure is 3.0 bar but its very likely that it will increase at night, if it increased to 3.85bar and the PRV passed 4.38L into the cylinder over a period of time then the final pressure on reheating will be 5.0bar, the expanded vol is still the same at 4.09L but now the pressure must fall by 3.0bar, from 5.0 to 2.0bar in that 25secs or so. In normal circumstances I still don't think that should cause a problem except a defective EV diaphragm thats not moving smoothly inside the EV.
I would also ensure that the diaphragm is never bottomed out by ensuring that the precharge pressure is 0.2/0.3 bar lower than the expected minimum DF pressure, so if 2.0bar is the expected lowest, set the precharge to 1.7/1.8bar.

 

[Matchlessman]

Many thanks for your further thoughts - EV outlet pipe is 22mm and I'll ask the installer to fit a pressure gauge teed off it.