Gravity driven plate heat exchanger for DHW

[marshian]

26 minutes ago, Dillsue said:

More than happy for your input. You're wanting to do fundamentally the same as me so interesting to hear your take on things.

 

If your morning heating regime is based on cheap overnight eleccy remember that off peak tariffs are not always guaranteed be available....build in flexibility if you can

Thank you 

 

PS heating HW with gas boiler 😉

[John Carroll]

7 hours ago, sharpener said:

 

Yes but you will only have to heat up the replacement water for what you have drawn off, not the entire contents. So from the energy requirement pov it will be much the same, there will theoretically be a tiny saving bc the outside of the bottom of the tank will be at a lower temperature so the parasitic losses (something like 1 kWh/day) will be fractionally less. 

 

I doubt you will avoid disturbing the stratification in the tank when the pump is running, wherever its outlet is connected. My thermal store was configured with that in mind but it is one of the things that did not work; even with a baffle near the bottom to create a volumiser section the whole 300 litre contents get heated whether I want it or not, unfortunately there are not the valves to isolate it so I can modify the pipework without draining down.

 

 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?

[Dillsue]

10 hours ago, John Carroll said:

 

 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?

You could ask Mixergy if they considered or tested that set up? When they went down the PHE route it would have been fairly easy to test things with the PHE outlet feeding in nearer the top. In terms of manufacture its only going to cost a few extra £ to run the outlet up the side of the cylinder to the top, if it gave a performance advantage.

[JohnMo]

10 hours ago, John Carroll said:

PHEX outlet is connected say a 1/4 the way up

The Mixergy one isn't injecting a 1/4 of the way up, it may enter cylinder a 1/4 of the way but the pipe then drops to the bottom of the cylinder - it deliberately at the bottom, it takes water from bottom puts it back there warmed up. 

 

Mixergy don't use a PHE setup for boilers, PHE is designed specifically for heat pumps, to give best CoP.

 

[John Carroll]

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.

 

[John Carroll]

Just now, John Carroll said:

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, or does the flow into the PHE or whatever its called start rising as well?.

 

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.

 

 

[JohnMo]

50 minutes ago, John Carroll said:

it shows a pipe rising up through the cylinder

Believe there is another pump for that and it's only used for immersion use. It's mentioned in the thread I linked to 

.

54 minutes ago, John Carroll said:

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

Not sure you would get those conditions with a heat pump as the it runs a low dT typically 3 to 6 doing DHW. A heat pump will slowly ramp up temp to maintain a set dT. This way the CoP stays high. The heat pump will generally keep the same flow rate through out the DHW reheat.

 

A typical DHW heating cycle. The CoP is the falling blue line. The grey shaded area is the input electric which increases as CoP falls.

 

So secondary side flow rate would remain at a fixed rate. The difference in approach temperature would drive the primary temperature rise as it maintains dT by adding to flow temp 

 

[sharpener]

6 hours ago, JohnMo said:

A heat pump will slowly ramp up temp to maintain a set dT. This way the CoP stays high. The heat pump will generally keep the same flow rate through out the DHW reheat.

 

This is why returning from the HX direct to the hw outlet will temporarily result in lukewarm water at the taps. Of course it doesn't matter provided you do it in the small hours. And by turning over the entire contents you can guarantee it will all be hot.

 

I don't think there is a perfect answer which avoids this, achieves best CoP, maintains stratification and doesn't involve additional valves and pipework. It's all a compromise.

[marshian]

18 minutes ago, sharpener said:

 

This is why returning from the HX direct to the hw outlet will temporarily result in lukewarm water at the taps. Of course it doesn't matter provided you do it in the small hours. And by turning over the entire contents you can guarantee it will all be hot.

 

I don't think there is a perfect answer which avoids this, achieves best CoP, maintains stratification and doesn't involve additional valves and pipework. It's all a compromise.

I agree everything has compromises 

 

I could settle for

 

heating less water more frequently but efficiently

 

heating full tank more efficiently in a shorter time frame without resorting to 80 deg flow temps due to coil surface are of 0.25 m2 (best guess)

 

or something in between 

[JohnMo]

1 hour ago, marshian said:

I agree everything has compromises 

 

I could settle for

 

heating less water more frequently but efficiently

 

heating full tank more efficiently in a shorter time frame without resorting to 80 deg flow temps due to coil surface are of 0.25 m2 (best guess)

 

or something in between 

But adding in to the equation - the odd penny extra to reheat the cylinder, takes a long time to recover several hundred pounds spent.

[marshian]

Just now, JohnMo said:

But adding in to the equation - the odd penny extra to reheat the cylinder, takes a long time to recover several hundred pounds spent.

 

Yes wise words - same would apply to replacing the boiler when the previous one worked (but was 15 years old), HW tank (currently 14 years old), circulation pump (was 20 years old) and many other items - none of them will recover the costs in terms of savings (well except maybe the pump because the old grunfoss pump was a watt hungry little bugger.

 

However from my own perspective I'm getting close to retirement so currently trying to mimimise extraordinary household expenses in retirement by choosing wisely whilst I have the disposable income to facilitate the purchases.

 

Of course I can never eliminate all potential expensive surprises in retirement but if I get the main ones done I'll feel a lot happier with a far lower income stream

[JohnMo]

3 minutes ago, marshian said:

 

However from my own perspective I'm getting close to retirement so currently trying to mimimise extraordinary household expenses in retirement

I'm in the same place. I have all sorts of expensive stuff that comes up on eBay for pennies in the pound sat in the plant room.

[marshian]

1 minute ago, JohnMo said:

I'm in the same place. I have all sorts of expensive stuff that comes up on eBay for pennies in the pound sat in the plant room.

 

I'm not adverse to picking up things second hand but at a good price, However the chances of an ASHP compatible indirect HW tank fitted with a 3m2 surface area coil and a volume capacity of 125 to 150 litres appearing on Ebay, gumtree or FB Market place would have some serious odds against it happening

[Dillsue]

48 minutes ago, marshian said:

 

I'm not adverse to picking up things second hand but at a good price, However the chances of an ASHP compatible indirect HW tank fitted with a 3m2 surface area coil and a volume capacity of 125 to 150 litres appearing on Ebay, gumtree or FB Market place would have some serious odds against it happening

60 ebay results on a search for "heat pump cylinder". Start searching and collecting well in advance of starting a project and most things come up at the right price:)

[marshian]

4 minutes ago, Dillsue said:

60 ebay results on a search for "heat pump cylinder". Start searching and collecting well in advance of starting a project and most things come up at the right price:)

 

Oh I'm sure they will work - just not much better than what I have which is definitely not a heat pump compatible cyl

[John Carroll]

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

[marshian]

7 minutes ago, John Carroll said:

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

 

Then we are going to need to work out which is better PHEX first and then HW Cyl Coil before return to the boiler or viki verki?

[JohnMo]

9 minutes ago, John Carroll said:

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

The trouble with approach is the two two exchangers can start to work against each other. If you have the PHE as a primary HE, the heat source water leaving the PHE has had all the heat removed, so then can act as a coolant in the secondary coil. Would take plenty of calculations and compromises.

[JamesPa]

11 hours ago, John Carroll said:

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

 

 

 

10 hours ago, JohnMo said:

The trouble with approach is the two two exchangers can start to work against each other.

Its also a little pointless because the coil isnt adding much extra heat transfer!  KISS!

[marshian]

I've actually found an alternative 117 litre cyl (copper and vented like current system) But it has a 2m2 surface area coil 

 

https://pirateheatingsupplies.com/product/ultra-high-recovery-117-litre-vented-hot-water-cylinder-900-x-450/

 

Only downside is the heat loss is a bit steep compared to modern encased cylinders

[John Carroll]

2 hours ago, JamesPa said:

 

 

 

Its also a little pointless because the coil isnt adding much extra heat transfer!  KISS!

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.

Edited Friday at 10:34 by John Carroll

[JohnMo]

1 hour ago, John Carroll said:

2 port valve

A fail closed valve with secondary contacts. Valve opens on call for heat, once open triggers pump to start. Keeps it simple, charge when not likely to be used, as whole tank will be stirred up. No cold water draw by accident, no new holes, ok efficiency wisey

[John Carroll]

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.

 

[JohnMo]

43 minutes ago, John Carroll said:

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.

 

First thing to remember is this setup is heat pump specific. So is basically playing with the coldest water for as long as possible. As long as the water goes back into the cylinder without too much churn the cold should stay at the bottom. So say the bottom of cylinder is at room temp (20 Deg), the heat pump would start flowing at approx 25 (dT5) then slowly ramp up temp as the return temp increases. So slowly heat over 40 mins + until bottom of cylinder gets to same temp as above the thermocline. If this isn't hot enough, it will just continue heating from the bottom - similar to a bottom immersion.

[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!.