John Carroll

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.

Just added a few more calcs here assuming water at a very minimum of 5C, so if you increase the EV capacity by 80L to give a total of 102L, the final pressure after a full reheat from 5C to 80C will still be < 3.5bar at 3.31bar. (prepressure of 3.0bar) EV CalcsJK Rev1.xlsx

The attached may be of some help re Accumulator. For example assuming a 200L accumulator and a required UVC pressure of 3bar, then if 4.0bar mains is available to recharge the accumulator you end up with a net vol of 40L. If you install a pumped accumulator and assuming you can pressurize it to 6.0bar then the net vol is 86L Accumulator CalcsJK Rev0.xlsx

Not quite that prolific, I've used just a bucket of rain water with a teaspoon of that car wax for the past 60 years and just hose it off for 5 minutes or so, people often congratulate me for polishing my car, can any one think of a more wasteful obsession?

52 times turned on/annum & 52 turned off/annum. they are Chinese V&G valves but do feel fine and "stiffish" in operation which is indicative of little or no wear?, when I installed them I left the outlet disconnected and they were both drop tight. I had another one somewhere which was permanently closed (3.6bar mains) and this started dripping after ~ 5 years even though it was probably ony opened about a total of 4 or 5 times during this period for a few hours.

Its passing, gland fine, water is perfectly soft. I also have a few gate valves, ~ 20 years old around the place which don't pass a single drop of water.

I had a inside 1/2 ins (+NRV) isolating gate valve on a supply to a outside hose used to wash the car, after ~ 45 years I decided to splash out and installed a 1/2 ins (reduced bore) ball valve, this valve is either just fully open or fully shut say once/week. After around 4 years this valve started leaking slightly so I just replaced it with another ball valve but, this valve is again leaking slightly after 3.5 years. My mains pressure is a very constant 3.4/3.6bar. Do all ball valves behave like this?

I'll just leave it at that. 😀

I doubt if theres any requirement for balancing, the positive head pumps don't have any connecting pipe I think. I wouldn't have any big worries with a shower where the HW is say 60C and the cold 10C, you need 9LPM at 60C mixing with 6LPM at 10C to give 15LPM at 40C, the hot might be reduced to say 50/55C by dilution but no problem for a thermostatic shower, however if say running the HW cylinder at even 50C where you the require 11.25LPM of hot mixing with 3.75LPM of cold to give 15LPM at 40C, this might well result in significant dilution to then cause shower temperature control. People with ASHPs and vented HW cylinders probably often run with a cylinder temperature of 45C or even lower, this might result in a HW temp as low as 35C, not too pleasant for either showering or normal kitchen use. Anyhow, all it requires is for someone with one of the hundreds and hundreds of these pumps to run a few tests at different flow rates and temps, that will soon put all this to bed, hopefully not in the way I fear it might.