John Carroll

Have you reduced the static pressure?

All sealed system boilers have PGs, cant recall reading of any weeps/leaks. But I would install a PG just before that PRV above with a isolation valve, the PRV PG port can allways be plugged after any commissioning/adjustment. I do find it extraordinary though that PGs are not installed as standard on all UVCs.

You can then use that outside tap screw on PG (just leave it slightly leaking) instead of the PRV PG to set up the PRV (static) pressure to the 2.5bar or so.

You can allways stick a plug in that PG port once you have taken any required readings. Without the upstream pressure though you don't really know where the pressure drop is occuring, you could either install a PG (with isolating valve) just upstream of the PRV or tee in the outside tap upstream of the PRV, you can then test the pressure with the screw on pressure gauge. What is the ~ distance from the main ("street") stopcock to the PRV and the pipe type and diameter(s)?.

Even though downstream doesn't show the dynamic mains pressure you should get a fairly accurate reading by allowing a very small leakage from where the PG is screwed onto the tap, this will allow the NRV to reopen when the pressure drops?. Does the UVC noise only happen after a reheat when a hot tap is opened or does it also happen at other times?. If you just open a hot tap very slightly then turn the PRV setting screw slowly anticlockwise until the pressure falls from ~ 4 bar to say 2.5bar ish, then the only flow in/out of the EV will occur during a reheat (in) and opening (out) a hot tap, you can allways reset it to 3.5/4.0 bar when test complete.

Is the outside tap teed off before (upstream of) the PRV or after it (downstream)??. What pressure reading do you get from that screw on pressure gauge when the PRV pressure gauge is showing 2.0bar or whatever at a high UVC flowrate?. The Caleffi PRV should have its model labelled somewhere on it, like a 535H or similar, also the size should be stamped on it, ie, DN20 or such.

What is the make/model/size of this PRV?. You said the dynamic pressure falls to ~ 2.0bar, can't remember the flowrate, can you take the PRV upstream pressure anywhere like from a outside tap which will give the pressure loss in the piping up to the PRV. All PRVs fall off in pressure with increasing flowrate even with a constant upstream pressure, but nowhere near the 2.0bar you are getting but you should try and establish that upstream pressure if not allready done so.

Suggest setting it to say 2.2/2.3bar with a very small flowrate, this means that with no flow the downstream pressure should still be ~ 2.5bar (static) but should still ensure that the PRV goes fully open at higher flowrates. When set, and with no flowrate, either hot or cold, keep an eye on it eo ensure it doesn't rise above ~ 2.5 bar over a short period and doesn't reach the full static pressure of 4.0bar or whatever which would mean that its not drop tight. What pressure is it reading now with no demand immediately after water shut off and say 15/20 minutes later still with no demand?.

If you ensure the stopcock PRV is set to a max of say 2.0/2.5bar then (and assuming the EV precharge is still set to 2.8/3.0bar) then the only possible movement of water discharge from the EV is after a reheat, at all other times, opening/closing a hot tap should not cause the EV to trigger anything?

The balanced cold should really be taken off downstream of the UVC valve set but it is what it is. You said the UVC is set non adjustable at 3.0bar, if the stopcock PRV is set to 3.0bar and assuming the dynamic pressure falls to 2.0bar then, assuming, EV precharge set to 2.8bar and no heating on, 0.9L will flow in/out of the EV on opening/closing a hot tap. After a full reheat, 3.4L will flow into the EV to give a final pressure of 3.99bar, a total of 4.3L will flow back out of the EV when a hot tap is opened and 0.9L will flow back in on closing the hot tap. If the stopcock PRV is set to 2.0bar then no water will flow in/out of the EV with no heating on as the pressure downstream of the stopcock PRV is now almost constant. After a full reheat, 3.4L of water will flow into the EV to give a final pressure of 3.99bar. So, the only movement of water in/out of the EV is a very slow one during heat up and a fairly rapid one on opening a hot tap when the pressure falls from 3.99bar to 2.8bar. I'm assuming that the stopcock PRV then also feeds to the UVC PRV inlet??.

Are there two PRVs between the stopcock and the UVC cold inlet?, one, with the pressure gauge, the other, at the UVC cold inlet?. Also, I think you said somewhere that Megaflo offered you a new UVC?

The closer the evaporating and condensing temperatures are, the less compressor power required, a very quick look at 410A refrigerant tables shows, purely based on the table pressures, ~ a pressure ratio of 3.148 in compressing from -5C to 35C and 3.727 in compressing from -10C to 35C so ~ 18/19% more power required maybe?

One might think that, assuming a modulating as against a on/off ASHP, that the pump should be automatically controlled to give maybe a fixed dP of say 5C, the flowrates above of max/min of 540/1205 LPH would then give a ASHP output of 3.14/7.0 kW.

You shouldn't have to drain down the UVC, just shut the mains supply to it, open a hot tap and any cold tap off the bananced cold, the water should then stop fairly quickly as there is/should be a check valve in the combination valve set.

Friction loss tables would indicate that the dP through that pipework should be well < than 0.25bar at 11.6LPM, (one table shows a loss of < 0.1bar), add the elevation loss of 0.15bar gives a total (max) loss of 0.4bar, a 1M dP should give a flow of 18.34LPM and still leave ~ 1.8bar after the PRV.