John Carroll

A 37kW combi will give a flowrate of 15.2LPM at @dT of 35C, you said in your first post that the pressure was a bit low, by which I take it you mean flowrate, this may have nothing to do with the boiler output but the actual flowrate itself is too low, in which case the boiler will just modulate down in output to match the demand. If two could shower at the same time at a flowrate of 7.6LPM each, would this be acceptable? the Navien will give 11.0LPM each if 2 showers in service and theoretically 7.3LPM if 3 showers in service, practically speaking I would think its unrealistic to expect 3 showers in service together so maybe go for a well known boiler make of ~ 40/50kW rather than the Navien which might only have a limited pool of experienced installers for servicing etc.

The 54kW Navien, will give a flowrate of just over 22LPM at @dT of 35C, in other words a showering temperature of 40C from a winter very cold mains of 5C, or 42C from 7C, so will supply 2 showers at a flowrate of 11LPM, acceptable? or 7.3LPM if 3 showers in service. What is the existing make/model/output of your existing Combi?

I think @Indy has lost interest anyway.

Surprised to hear that, here, oil fired boilers are banned from 2022 and gas fired boilers from 2025, in new builds.

The Grundfos UPM seems a very popular choice of circulator for UFH, wonder why since one would think that any pump with constant pressure mode/settings would do the job. Can you remotely see if the system pressure is normal?. Assume the Willis is installed correctly, with the immersion head on the bottom with the inlet on the side and the outlet straight out the top, otherwise air can build up if installed incorrectly, may not be a huge problem with the sealed pumped system.

Just did a few calcs on this elusive unlimited HW. Assumed, 300L cylinder heated from 7C to 60C by using a PHEX combined with a 20kW boiler with a PHEX secondary flowrate of 5.41LPM discgarging into the top of the HW cylinder at 60C and absorbing the full 20kW boiler output as soon as there is a HW demand, my calcs show that the 300L contents will be exhausted after 22.6 minutes at a HW demand of 30LPM at 40C, say 2 showers which will require 18.68LPM@60C mixed with 11.32LPM@7C to give 30LPM@40C, this gives a total vol of 678L at 40C vs 482L if the cylinder alone was not augmented with the boiler firing at full output, the other advantage with the PHEX (maybe) is that once the cylinder is exhausted then the boiler's 20kW will still provide 8.7LPM@40C continuously but will only feed one shower then at that reduced flowrate, enough for some if happy with that flowrate. I don't know how this compares with a conventional coil and again assumes the boiler fires up immediately on HW drawoff, will the total time still be 22.6 minutes before exhaustion and how long is the waiting period to achieve a comparable flowrate at 40C as the PHEX?.

Perfectly Nick, but find it difficult to see why, forgot to mention that its set to 30C, I'd love to look at it but can't get the daughter to remove the cupboard full of stuff but I've no doubt its installed as shown. If there was/is a blank before the T to the left of the flowmeters then it might make a bit of sense of if there was nothing on the bottom manifold apart from the hot flows?. See attachment.

Son in Law sent me on these photos ~ 10 years ago when the Ivar UFH system was being installed in a extension, its worked perfectly since with a oil fired boiler either on its own or with the existing Rads. I only came across them recently and just drew a very rough schematic of what I think it looks like, the Pump is pumping downwards as shown and the TMV capillary is inserted in the lower manifold. It just doesn't look right to me if I have the direction of flows correct in the schematic, @JohnMo might comment since I believe he has this system or similar.

Yes, I can obviously see that the hot cannot be returned into the cylinder top because of the ~ 5C dT (ASHP) it would only destratify and cool down the cylinder but thought it might have been returned higher up to give cold inlet water longer, a ASHP producing say 7kW at a dT of 5C means a secondary circ flowrate of 20LPM, so if the hot is returned say 50L UP from the cold, the 50L will stll be heated by 5C in only 2.5minutes (theoretically) so probably no advantage in doing so and MIxergy have no doubt tested various configurations, makes more sense, at last!.

Can someone, maybe @JohnMo, explain how this actually works?. It shows the hot outlet discharging adjacent to the "cold" inlet where one would think that the cold would actually be quite warm because of its close proximity to the hot, also how does the hot rise up (if it does) through the cold without heating it, one would think that the cold "1/2" would rise uniformly in temperature, then when it reaches the same temp as the upper 1/2, that the whole cylinder should then rise further in temperature unless its setpoint has been reached.

Extra heat not required, the PHEX can be sized to suite, it would now not require a extra hole in the cylinder, which was the intention. The PHEX alone is the best option IMO if its outlet is just returned to the cylinder HW outlet (no hole cutting) but because of short circuiting might require a 2 port valve, I thought a NRV with a fairly strong spring might suffice but depends on the dP through the cylinder when HW is drawn off? If a PHEX is installed with the return low down then its suggested I think that a 2 port valve is not required but needs that hole.

Would it be feasible to use a PHEX plumbed in series with the existing 0.44m2 coil?

So how does the Mixergy manage to get HW at the top?, it shows a pipe rising up through the cylinder, if it just deposited the water into the bottom again then the cylinder should just heat up uniformly, the same as a botom mounted short coil, Just a general question re ASHP & PHEX, if you had a secondary flow/return of say 10C/45C, then 8kW output will require a secondary flowrate of 3.28LPM, the primary flow&return will be somewhat higher maybe by 3C to 5C, this will result in a certainn COP, if the primary flow temp rises to say 15C then what happens if both flowrates stay the same?, does the COP go up or down, the output will probably change but that's not my question really.

If the PHEX outlet is connected say a 1/4 the way up and close(er) to the cold PHEX inlet, IMO the PHE inlet will increase in temperature quicker due to faster mixing, downward convection, whatever, than having it mounted as high as possible, does this imply that a ASHP COP will benefit from this then, the lower inlet?

I can't see how injecting the HW lower down than at the cylinder HW outlet is a advantage, if injected at the HW outlet you will get HW very rapidly since the heating is from the top down, in either case a spring loaded NRV ( no need for a 2 port valve?) should stop any cold water short circuiting through the PHEX when HW is drawn off and diluting it. Injecting at the HW outlet should also give the best solar coil output v/s injecting it a 1/4 way up the cylinder, it should't be too difficult to run a length of 3/4" pipe from the PHEX to the Cyl HW outlet, especially since its a pumped system and a few bends here and there won't interfere with the circulation.

Double post.

If you don't mind, can you just explain yout intended PHEX install? with its DHW & Cylinder inlet and outlet positions.

If a PHE is being considered with a ASHP then its very important that its sized correctly to get the minimum approach temperature between the ASHP flow temp and the DHW outlet temperature, so one needs to know the ASHP/PHE power, the ASHP flow temperature and flowrate and the required DHW temperature, the correctly sized PHE can then be estimated and/or the above numbers be given to the PHE manufacturers who will give a very accurate calculated size.

Yes, The bottom attachment shows the gist of it. The combination valve set. below, has, from the right, cold mains inlet, then balanced cold out after the PRV, then the EV connection and out of the top of the expansion relief valve, the discharge to waste, but there is a NRV incorporated in (after) the PRV to stop any expansion back to the mains as suggested by @Ajm

A extra 28/30L of EVs to give a total of 50L should satisfy your requirements but with precharge/filling pressures of 2.5bar/2.7bar with both teed into the cold feed if possible, the 22L looks as if its perched on top of the UVC so receiving hot expanded water which further increases final pressure, you may well find that the dynamic pressure at even reasonable flowrates is never higher than 2.5bar anyway, a total of 50L EVs with the above settings will then meet your "worst" conditions of 7C/80C.

Have you installed the extra EV yet?.

I finally got it to work without adding or subtracting from the D values after reading the above but didn't fill in all the values, John Only Expansion Calc using Steam Tables.xlsx

Can you have a look at the attachment sometime please. Why do you add +1 to both C7 & C8 in the Indirect function? I'm a 60 year experienced "steam man" so I use the Spirax Sarco Sub Saturated Steam Tables and the relative water densities. My calc results are practically the same as yours but if I don't add that +1 to both C7 & C8 then it doesn't compute. I have just inserted a few values in column I. I modified your calc slightly, it will work with either the +1 added on or not added to C7& & C8, gives slightly different values. TEMPORARY Expansion System pressure calculator.xlsx

Nice one Jack, where you just input the start/end temperatures to do the calcs.