Unvented Cylinder Pumped Secondary Hot Water Circuit EV Sizing Consideration

[Matchlessman]

I'm aware that there is an empirical method to calculate the required EV volume based on the Unvented Cylinder capacity its operating pressure and temperature, however does the volume of the hot water outlet and pumped secondary return circuit also need to factored into the calculation? If the secondary hot water return pump runs when the cylinder is being heated then doesn't the volume water in this circuit is also need to be taken into account in determining the overall increase of water volume that needs to accommodated by the EV?

 

[Johntheo5]

A EV might typically be sized to 10% of the UV cylinder capacity so a 200L cyl with a 19L EV and with the PRV pressure set to 3.0bar will result in a final pressure of 3.82bar in heating from 15C to 60C, if you allow say 3L for the secondary hot water (20M of 22mm piping) then the final pressure will be 3.84bar.

 

[Matchlessman]

Is my assumption correct then, in that the unvented cylinder pumped hot water circuit volume needs to be included in the EV volume calculation?

 

[Johntheo5]

Yes, IMO.

 

[Matchlessman]

Thanks for confirming my thoughts, however my installation is larger than the average property with 3 bathrooms so there likely to be a longer than 20m secondary hot water circuit.

 

[Johntheo5]

If your cylinder capacity is 120L (as per your other thread) then even though unlikely that the EV is "only" 12L, it would take a total volume of 182L to give a final pressure of 4.3bar (less than the expansion valve relief pressure) so you still have, 182-120, 68L for secondary volume.

 

[Madrab]

I may be wrong but your other thread shows a 250L cylinder does it not?

 

[Johntheo5]

I was looking at a reply to @Matchlessman.

If cylinder is 250L with a 24L EV then the final pressure will be 3.81bar after heating to 60C, it would take a total volume of 360L to give a final pressure of 4.3bar so you still have, 360-250, 110L for secondary volume.

 

[Matchlessman]

Yes, its a 250L cylinder with a 25L EV.

 

[Matchlessman]

The reason behind my inquiry is that I'm still getting a loud bang from the EV when opening a hot tap whilst the water is being heated and wondered if a larger EV would help eliminate the noise given the increase in effective hot water volume with a pumped secondary circuit, see attached video -

EV_Noise.mp4

• Matchlessman
• 6 Sep 2023

Noise from EV when hot tap opened

If the EV pre-charge is 2.9 Bar, Static Mains pressure = 3.2 Bar and Dynamic pressure = 3 Bar I don't understand why the EV should still make a discharge bang. The severity depends on how quickly the pressure is relieved, i.e. load bang when tap or shower opened quickly to small shuddered for a very slow release.

FYI, I have attached a picture of the unvented cylinder installation in case it shows any clues to the problem.

 

[Johntheo5]

A search, above for "Gledhill Lite Problems" will give lots of posts re noise problems like yours.

Its one of the worlds greatest mysteries (to me at any rate) why unvented cylinders don't have a pressure gauge.
The precharge pressure should be set to the dynamic pressure or slightly lower, 0.2/0.3 bar is recommended but I would set it to 0.5bar lower to provide more of a buffer to stop the diaphragm stopping hard up against the EV vessel water end.

Your UV/EV volumes of 250/25L with precharge/dynamic pressures of 3.0/3.0bar will give a final pressure of 3.77bar if the whole cylinder is reheated.
precharge/dynamic pressures of 2.7/3.0bar gives final pressure of 3.85bar and 2.5/3.0bar gives a final pressure of 3.91bar which shouldn't cause the slightest problem in falling to 3.0 bar.
You can read most of the advice given in the above posts, one other thing you could try is before retiring for the night is to just crack open a HW tap to give a tiny flow sufficient to prevent any pressure rise during the next (overnight?) heating time or if you know the heating time is say 30 minutes then open the HW tap to give a flow rate of say 150ml/min ( a mere trickle) as the expanded volume is only 4L after a full cylinder reheat.
Then, just before the heating session ends open the hot tap fully and see what happens.

You might post a close up photo&details of the expansion valve showing especially where the EV outlet is installed, also the expansion valve setting, it's stamped on the end.


Edit: https://www.diynot.com/diy/threads/...-dynamic-flow-pressure-to-ev-pressure.614395/

Installing that pressure gauge would be my priority.

 

[Matchlessman]

Many thanks again for your helpful advice

 

[Matchlessman]

I followed your advice this morning and found that by cracking open the hot tap during the final stage of the heating period the EV only made a very small "click" noise. However, the EV still made quite a loud rattling/thud noise opening a hot tap shortly after heating period had finished.

Does this indicate that the EV cannot control a rapid discharge of hot water? Would perhaps a PVR set at 3 Bar installed in the hot water outlet help control the pressure surge from the EV?

The inlet PRV is an integral part of the combination valve assembly that is manufactured by Reliance specifically for Gledhill - I've attached a close-up of the valve showing the EV take-off directly after the PRV.

 

[Madrab]

The EV pre-charge needs to be adjusted to suit to ensure that there is a min disparity between the EV pressure once the cylinder is fully heated and the static pressure the cylinder sits at. That's a job for your qualified unvented specialist.

Would perhaps a PVR set at 3 Bar installed in the hot water outlet help control the pressure surge from the EV?

Nope, an unvented system isn't designed to work that way.

How long has the system been installed? Do EV membranes get tired as they get older, they must do, so that may be a contributing factor.

 

[Johntheo5]

I followed your advice this morning and found that by cracking open the hot tap during the final stage of the heating period the EV only made a very small "click" noise. However, the EV still made quite a loud rattling/thud noise opening a hot tap shortly after heating period had finished.

Does this indicate that the EV cannot control a rapid discharge of hot water? Would perhaps a PVR set at 3 Bar installed in the hot water outlet help control the pressure surge from the EV?

The inlet PRV is an integral part of the combination valve assembly that is manufactured by Reliance specifically for Gledhill - I've attached a close-up of the valve showing the EV take-off directly after the PRV.

As stated above, you don't really know what the dynamic pressure is when you open the hot tap fully, with the trickle of water the pressure(S) will be (more than likely) 3bar so the EV diaphragm will be hard up against the water end so even if the dynamic pressure falls to say 1.5bar then it should have no effect on the EV diaphragm, the problem is you dont really know what the static pressure is or if the PRV is passing, IMO, it should be drop tight to maintain 3.0 bar so again pressure gauge required, you asked if attaching it to the PRV balanced take off (not used) would be of any use, It would be of some use only as it will show the pressure after the PRV and possibly the cylinder/EV pressure with no draw off or no heating on but will show the correct pressures (dynamic) once hot water is being drawn off.


@Madrab will advise but these problems do seem to be associated particularly with Gledhill cylinders.