John Carroll

IMHO, there is only one logical explaination for the huge dT between the target/flow displayed temperature and the measured flow pipe temperature (and confirmed by the attending WB engineer) and that is, the displayed temperature is not tbeing taken from the flow pipe sensor but probably from the boiler HEX. It doesn't matter what kind of throttling or flowrates, as long as the boiler is firing continuously then because there is probably well less than 0.5L of water between where the boiler flow sensor is attached and the manual measuring point which means that even with a flowrate as low as 5LPM should only "take" this vol of water 5 or 6 minutes to reach the manual measuring point once it does then there should be little or no difference since the flowtemperature is then very constant for relatively long periods.

A ABV will only work properly IMO when using fixed speed (constant curve) mode where (on a centrifugal pump), the head increases as the flow rate decreases, some suggest that it will work with the pump in CP (constant pressure) mode but then also requires a fixed restriction (a valve?) installed in series with it, I really can't see this working properly. If the ABV is installed as far away from the pump/boiler as possible then this should possibly help as the pipe loss decreases with decreasing flow which means increased head at the ABV and vica versa, the constant pressure is only maintained at the pump.

Found this in the attachment but seems strange that this subject unit does not have its own pump?

Just wonder if there is a actual interlock via a flowswitch, etc, it just says the unit can stop, page 23, I think they can trip if the flow/return dT is too low, some alarm is surely flagged if the unit refuses to start. Could be something else stopping this unit from starting. There should really be a ABV between the flow&return anyway.

Agree, also, the Wilo Yonos Pico is the RR in pump control because it's head (and output) in CP mode can be incrementally changed in 0.1M steps to give almost infinite control, assuming of course that it can achieve the ASHP minimum requirement of 17.0LPM. @SimonD, where did you get this dP of 0.5M at 17LPM from and is 17LPM written in stone or can it be changed?, if it can, and could be reduced to say 14.5LPM to get the ASHP away, even as a temporary fix just now before any plumbing changes, then maybe worth looking at.

The (my) last suggestion is to just fully shut/open the TMV, the shuttle may be partially jammed/seized, it will always shut off due to the hydraulic pressure exerted by the controlling pin but only spring force opens it.

If it means, which I'm sure it does, fully open to hot, then can't see that being the problem although I would still remove the setting knob and have a look. What model mixing valve is it, might be on the label. The 4/40W shows it is a 6M pump, you should still be able to see (but maybe it doesn't display this any more now) the actual power displayed. There's not much else you can check out except see if you can find the ASHP pressure loss, SimonD says "Your heat pump at the required 17l/min has a pressure drop of 0.5m head" so that definitely will not be the source of any restriction, the dP at a flowrate of 22.5LPM would still only be 0.9M. Its a bit strange as you now have 2 powerful pumps running in series at 5M head each and still only getting 15LPM?

Rough Schematic below of how I imagine the system is now configured, a UFH circ flowrate of 15.0LPM is only possible now vs 22.5LPM with the original UPS pump circulating from a "heat store"??, despite the Wilo running at a 5M head when pumping water through the the ASHP, it would appear that the restriction is caused at the ASHP side since both pumps are/should now be in series. It would be useful if the ASHP heat exchanger pressure loss is known. On the other hand, the mixer is now calling for full hot water so it might be possible to make a adjustment by removing rhe setting knob and increasing this port's opening as it may be restricting the flow, when mixing, there is only ~ 25% hot mixing with 75% cold

Simon said "The loops are pretty much fully open, in fact one completely undid to much merriment!. Pumps on max." so not much more scope there for improvement?. 90M of 16mm/12mm ID pipe has only a loss of 0.61M and the UPS 2 when using sun amp was circulating a total of 22.5LPM, at this flowrate a UPS 2 will run with a 4.3M head, At the present set up it will be running at 5.3M at a total flowrate of 15LPM (1.5LPM/loop), similar to the Wilo if (a 6M), I would check that the Wilo is orientated correctly, ie pumping in the "right" direction, by looking at the arrow on the pump body, also as requested earlier, check the power in watts which will identify whether a 4M or 6M pump.

Your Wilo pump should also display the power W (watts), not sure if it alternately switches from flow, m3/hr to power, W but there is certainly some method of having a look quite easily, I have a 6M Wilo (Yonos) Pico that only displays the power W but quite easy to read off the flowrate from the pump curves. If its the 4M model then the power at 0.9ms/hr is only ~ 17W, if its the 6M model, then the power at 0.9m3/hr will be around 40W + a quite hefty pump head of ~ 5M. (speed 3, C3) What is the UFH manifold pump model and setting? What are the flow indicators showing in LPM?. What dia are the Loop pipes and roughy the loop lengths?

Might find this simple Spreadsheet handy for EV sizing calcs etc, or just for interest Expansion MASTER Vessel Calculation extract rev0.xlsx

Can see this now more clearly on my Laptop, the relief valve that is lifting is the expansion relief valve, while not a safety issue this should not be rated the same (6bar) as the TPRV which is stated on the cylinder data as being 6.0bar (& 95C), just ensure ASAP that the TPRV rating IS stamped 6.0bar as labelled on the cylinder. (Your Caleffi combination valve does appear to be plumbed correctly, as you described) Normally a UVC with a 6bar TPRV would/should have a 4.5bar expansion relief valve and if the precharge/filling pressures are 2.8bar/3.0bar then the final pressure when UVC is heated to 60C will be 4.0bar or 4.24bar with precharge/filling pressures of 3.0/3.2bar. You can see from the screenshot that the expansion relief valve setting is 4.5bar.

Havn't read all the posts but a 22L EV should be OK for a 248L UV cylinder. Assuming a (now) cold mains temp of 10C and a final HW temp of 60C and a full cylinder reheat each time, then...... precharge/filling pressures of 3.0/3.448bar = final pressure of 4.63bar. No problem. precharge/filling pressures of 3.0/4.25bar = final pressure of 6.0bar. Assumes PRV is passing and rises to 4.25bar precharge/filling pressures of 1.31/3.448bar = final pressure of 6.0bar. Assumes EV air end pressure (precharge) has fallen to 1.31bar. (50psi = 3.448bar) If the TPRV is set to 6.0bar, then the expansion relief valve should be set lower and should have lifted first, what is this set to??, it should be stamped on the end of it (red). Is the pipe from the EV connected in at the "bottom" of this expansion relief valve??. Also what is the Caleffi PRV model, it may not be drop tight. The Caleffi 535 is drop tight The Caleffi 533 is not drop tight

The devil is in the detail. They obviously don't make them like they used to. No pesky rod thermostat in mine, still working perfectly after exactly 53 years. Sink/Bath c/o switch conveniently on top of the Immersion.