Gravity driven plate heat exchanger for DHW

[Dillsue]

30 minutes ago, JohnMo said:

Here are some proper layouts and flow diagrams - they feed into the secondary return port not the DHW out port.

 

http://heatweb.com/literature/Amazon_HXIN.pdf

Be nice to be able to add a dedicated new tapping but the jury's out on that just now

[JohnMo]

1 hour ago, Dillsue said:

Be nice to be able to add a dedicated new tapping but the jury's out on that just now

Would be good, but one chance to get it correct, then hello new cylinder

[Dillsue]

1 hour ago, JohnMo said:

Would be good, but one chance to get it correct, then hello new cylinder

I'm slowly coming round to the challenge just to get the HW entering in a better place. It's only 5 mins with a holesaw..........what could possibly go wrong:)

[JohnMo]

39 minutes ago, Dillsue said:

only 5 mins with a holesaw.

And an Essex flange

[John Carroll]

19 hours ago, Dillsue said:

I think/hope that there's more resistance through the PHE route than through the cylinder so most HW draw off will be through the cylinder. Via the PHE there's 2 x 22mm Tees, the pump and the PHE but via the cylinder there's nothing but the cylinder itself. I'll leave some space for a 2 port valve just in case things don't work out.

 

I looked at the Mixergy set up and as JohnMo says cooler water is drawn from the bottom and returned around a quarter of the way up. Would be nice to replicate that setup but I'm a bit wary of cutting a new hole in the existing cylinder and then feeding a new tank connector through the immersion boss and getting it into place and secured. Anyone successfully done that??

 

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.

[sharpener]

5 hours ago, John Carroll said:

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.

 

https://www.heat-exchangers.uk/online-calculator/

[marshian]

20 hours ago, Dillsue said:

I'm slowly coming round to the challenge just to get the HW entering in a better place. It's only 5 mins with a holesaw..........what could possibly go wrong:)

doesn’t seem too difficult 

 

 

[Dillsue]

I watched Rogers video earlier and it does look straight forward and not as uncommon as I thought it was. I think Roger mentions a max temp of 60 deg C which is too low for us as the solar thermal can take the cylinder well above that. I had a look on the manufacturers web site and it says up to 90 deg C so a bit of investigation needed.

 

Thanks for all the ideas and comments so far

Edited Tuesday at 16:28 by Dillsue

[sharpener]

1 hour ago, Dillsue said:

I watched Rogers video earlier and it does look straight forward and not as uncommon as I thought it was.

 

Looks plausible for a DIY job. Have a weighted hook handy to hang on the wire puller so it doesn't drop inside. 

I would have liked to see some kind of centreing device - maybe a step on the external washer - though if you cut the insulation to a close fit that would do it. 

 

Is your cyl a foam sprayed one or does it have a metal enclosure?

 

ATB with it, rather you than me!

 

 

Edited Tuesday at 17:36 by sharpener

[marshian]

56 minutes ago, sharpener said:

 

Looks plausible for a DIY job. Have a weighted hook handy to hang on the wire puller so it doesn't drop inside. 

I would have liked to see some kind of centreing device - maybe a step on the external washer - though if you cut the insulation to a close fit that would do it. 

 

Is your cyl a foam sprayed one or does it have a metal enclosure?

 

ATB with it, rather you than me!

 

 

 

I know for me the cyl is a spray foamed one from 2011/2012 and I'd have no issues with adding another hole because my intention is to replace the tank once I've proofed the concept of PHEX in my set up

 

The plus side of this conversation is I need to make sure any modern direct tank also will have a return point fairly high up the tank

 

 

[Dillsue]

11 hours ago, sharpener said:

Is your cyl a foam sprayed one or does it have a metal enclosure?

75mm of spray foam. Looks a bit of a stretch for gland pliers/pipe wrench so will need to find a box spanner or deep socket to tighten up the back nut

[John Carroll]

2 hours ago, Dillsue said:

75mm of spray foam. Looks a bit of a stretch for gland pliers/pipe wrench so will need to find a box spanner or deep socket to tighten up the back nut

 

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

Edited Wednesday at 07:15 by John Carroll

[John Carroll]

2 minutes ago, John Carroll said:

 

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

 

Double post.

Edited Wednesday at 07:14 by John Carroll

[Dillsue]

1 hour ago, John Carroll said:

 

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

ASHP pumps through the PHE primary side in the same way it would pump through a coil in the cylinder. On the secondary side a potable water pump will draw cool water from the bottom of the cylinder via the cold water inlet. The pump will pump this water through the secondary side of the PHE. Heated water leaving the PHE secondary will be injected back into the cylinder via the Essex flange fitted part way up the cylinder. I'm aiming to fit the Essex flange about a quarter of the way up the cylinder to avoid the coils inside the cylinder and any seams

[marshian]

9 minutes ago, Dillsue said:

ASHP pumps through the PHE primary side in the same way it would pump through a coil in the cylinder. On the secondary side a potable water pump will draw cool water from the bottom of the cylinder via the cold water inlet. The pump will pump this water through the secondary side of the PHE. Heated water leaving the PHE secondary will be injected back into the cylinder via the Essex flange fitted part way up the cylinder. I'm aiming to fit the Essex flange about a quarter of the way up the cylinder to avoid the coils inside the cylinder and any seams

I’m planning to put the Essex flange as close to the top section of the tank as is possible - In my view you want as little mixing at the top of the cylinder with cooler water already in the cylinder

[sharpener]

4 hours ago, Dillsue said:

75mm of spray foam. Looks a bit of a stretch for gland pliers/pipe wrench so will need to find a box spanner or deep socket to tighten up the back nut

 

I don't know what your HW scheduling is like but if you are heating it at off-peak times like 0400 and 1300 when you are not likely to be showering then I can't see it matters if you put the heated water from the HX back into the existing outlet at the top which will save a lot of b*ggering about. You will need the two-port valve though.

[Dillsue]

50 minutes ago, marshian said:

I’m planning to put the Essex flange as close to the top section of the tank as is possible - In my view you want as little mixing at the top of the cylinder with cooler water already in the cylinder

?? If you don't want mixing at the top wouldn't you put the flange lower down? Mixergy have their return point around a quarter of the way up so I'm following their lead.......they've likely done extensive testing so I'm trusting that their positioning is optimal??

[Dillsue]

59 minutes ago, sharpener said:

 

I don't know what your HW scheduling is like but if you are heating it at off-peak times like 0400 and 1300 when you are not likely to be showering then I can't see it matters if you put the heated water from the HX back into the existing outlet at the top which will save a lot of b*ggering about. You will need the two-port valve though.

Our cylinder is 2 metres tall with a mass of pipework above and around the top so access isn't the best. Nearer the base is largely clear so less buggering about to put the return where I beleive/hope it will perform better. The flanges aren't cheap but not needing a 2 port valve for top return will pretty much offset the cost of the flange

Edited Wednesday at 09:46 by Dillsue

[John Carroll]

12 minutes ago, Dillsue said:

Our cylinder is 2 metres tall with a mass of pipework above and around the top so access isn't the best. Nearer the base is largely clear so less buggering about to put the return where I beleive/hope it will perform better. The flanges aren't cheap but needing a 2 port valve for top return will pretty much offset the cost of the flange

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.

[JohnMo]

1 hour ago, marshian said:

I’m planning to put the Essex flange as close to the top section of the tank as is possible - In my view you want as little mixing at the top of the cylinder with cooler water already in the cylinder

If you look at the mixergy discussion I linked to earlier. The water is taken from the bottom of cylinder the injection point is about 1/3 up the cylinder and then piped down to the bottom again.

 

Reason this is good for a heat pump it is always working in the coldest part of the tank, holding back flow temp rise for better CoP. Hot water will float to top naturally. Would also be good for a boiler as it would hold back on increasing flow temp as long as possible.

 

The main issues injecting high is

1. It destroys stratification instantly as you get a cold slug of water hitting the top of cylinder as cycle starts

2. You have to heat top down, so no hot water available until 45 Deg hot water comes out of PHE.

 

[Dillsue]

2 hours ago, John Carroll said:

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.

 

2 hours ago, Dillsue said:

Mixergy have their return point around a quarter of the way up so I'm following their lead.......they've likely done extensive testing so I'm trusting that their positioning is optimal??

I'm sure they have more knowledge and experience than me

[sharpener]

2 hours ago, John Carroll said:

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.

 

I don't think that will work. The NRV will do nothing there: when the pump is off it will still allow water straight from the cw inlet to the hw outlet bypassing the tank completely. [In my tank where the pump serves a different purpose (stirring the tank while hw is being heated by the coil) the flow is the other way up, and works fine. ]

 

If you do not care that there is lukewarm water at the outlet at the start of the run then that will work for the OP too, though it will be less efficient as the HX isn't fed with the coldest possible water. But I think that benefit may be illusory as the tank will get mixed very quickly unless the flow through the HX is heavily restricted.

[marshian]

I don’t really want to piggy back this thread with my plans but it’s a good discussion with lots of input from knowledgable BH members so hopefully the OP won’t mind too much?

 

Firstly we never heat water in a period where we could potentially be used (always early morning before we are awake) So no concerns for getting cool tank contents before the HW tank heating process is complete.

 

Secondly - if using a PHEX process there might be a potential to heat less water than a full tank (currently with coil in tank process we have to heat whole 115 litre to desired target temp (currently 52 deg C once a day)

 

if circulation through PHEX is returned to top 1/4 of the tank and PHEX process is effective at generating water at a temp of say 50 Deg C to the tank without upsetting stratification we might be able to heat say 50 L of water to target temp rather than whole tank

 

if we feed return from PHEX lower then we will have to heat more of the tank to target than we potentially need each time we re-heat

[sharpener]

8 minutes ago, marshian said:

if we feed return from PHEX lower then we will have to heat more of the tank to target than we potentially need each time we re-heat

 

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.

[Dillsue]

1 hour ago, marshian said:

I don’t really want to piggy back this thread with my plans but it’s a good discussion with lots of input from knowledgable BH members so hopefully the OP won’t mind too much?

 

Firstly we never heat water in a period where we could potentially be used (always early morning before we are awake) So no concerns for getting cool tank contents before the HW tank heating process is complete.

 

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