John Carroll

If you can move that as far up to the cylinder top as possible and run on the electric immersion and drain down as you say maybe 20/30L to get the stat to cut in, then wait until the stat cuts out, and repeat this test say one or ideally two more times then that will provide the info I'm after hopefully.

Again, thanks, but a horizontally mounted heating element heats the cylinder in a completely different fashion to a top mounted vertical one.

All I am interested in is the HW cylinder outlet temperature while using a top mounted vertical electric immersio I'm only interested in the HW (top) displayed temperature when using the (IF) you have a top mounted vertical immersion, this also incorporates its own thermostat, "the bottom one where the boiler tank thermostat is", this is only used when you use the heating coil to heat the cylinder.

That's interesting, sometime maybe when using the immersion only, you might switch it on until it cuts out, note the displayed temperature, run off ~ 15L of HW to get it to cut in again, then again note the displayed temperature when it cuts out again.

Dom't know how far away your W/machine is from any sink but I have my Dish washer drain hose connected as shown below, the hose can be seen next to that yellow container on the right, the other waste discharge nozzle is blanked.

Photo of typical top mounted single element immersion below. My Cylinder is only 14 years old, the original, 39 years, which I removed and installed a twin coil (Solar+oil fired) in 2011. I just removed the original immersion but the element had no limescale whatsoever so I desided to re install it in the present cylinder where it sits today.

Does anyone have any views /experience on/of the above re unstable HW outlet temperatures even with a perfectly operating thermostat which is the same length as the heating element. My views. With this top mounted immersion you will only heat the full immersion length of the cylinder from a fully cold cylinder. 20/25% of the HW must then be drawn off before the stat will cut back in, for example, with a stat setpoint of 60C and assuming 50C cut in, (hysteresis of 10C), and a cold mains of 15C then ~ 22% of the HW must be drawn off to enable the stat to cut in. If 22% of the HW is drawn off, just enough to get the the stat to start calling at 50C then the top 78% will/can still be at 60C and the remaining 22% at 15C then the actual final HW temp will be far? higher than 60C because the vertical element will heat the cylinder without any destratification and you might end up with 78% at 70C and the remaining 22% at 25C on stat cut out. If the above cycle is then repeated can it lead to still higher temperatures even though the stat is still only sensing the average of 50C/60C cutin/cutout temperatures?, if not, then easy enough to take a few tests over a few days and set the stat appropriately to give 60C. I have a 53 year old dual element top mounted immersion but only use the shorter element which heats 30L, it doesn't have the rod type stat but has a surface mounted one on top of the immersion so I can't carry out any tests, almost the full 30L has to be used before the stat cuts back in but it does give a very stable HW temperature of 65C.

Two oil fired boiler oil valve solenoid valves, One type, like mine has two windings, a pull in winding of 1.4ohm resistance which is supplied with 24VDC for a second or so, power, (24squared/1.4), 411W, the other, the hold in winding of 1400ohms is supplied continuously with 5VDC, power, (5squared/1400), 0.0178W. The other type has one winding of 100ohms resistance which is supplied with 30VDC for a second or two, power, (30squared/100), 9W, 3VDC is then supplied continuously to the same winding, power, (3squared/100), 0.09W. It seems strange that less that one fifth of the hold in power, 0.0178W vs 0.09W is required for the two winding coil, the second coil requires 0.09/9, or 1% of the pull in power but even if 9W was sufficient pull in power for the two winding coil then the hold in power is still only, 0.0178/9, less than 0.2% of the required assumed 9W, seems extraordinarily low.

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.