Gravity driven plate heat exchanger for DHW

[Dillsue]

We have a relatively new open vented DHW cylinder with indirect coils for solar thermal, WBS and LPG boiler. Were adding a 7kw Therma V ASHP and retaining the cylinder as its a waste to swap it out and there's no BUS/MCS involved. I'm going to try the HP using the existing boiler coil at .44m2 with a back up plan of adding a PHEX if the small coil doesn't work or becomes costly. We're currently on Octopus Go so can run the DHW overnight in the cheap period so even with a poor COP it's unlikely going to break the bank, but I would like thing to be as efficient as possible so.....

 

The cylinder is 2 metres tall and I've got space to the side to fit a PHEX and tap into the CW feed at the bottom and HW outlet at the top with predominantly vertical pipework. What I'm pondering is if the circulation through the PHEX/DHW would work with gravity convection alone without its own circulation pump?? Anyone done it or know if its possible/impossible??

[JohnMo]

PHE work well due to the turbulent flow at the plates inside the exchanger. Using the change in density as the fluid heats may not/will not give the turbulent flow needed on the cylinder side especially at heat pump temperatures. So may be as good as useless.

 

I would add a DHW suitable pump, and pump from bottom connector through PHE and into top outlet connection.

 

The equivalent area of a PHE when compared to a 3m² cylinder coil is about 1 to 1.2m².

[Michael_S]

If there were the option of another of boss lower in the tank, I would pump back into the bottom of the tank using convection to mix the tank as this allows you to maintain separation of hotter water at the top of the tank.

[JohnMo]

1 hour ago, Michael_S said:

If there were the option of another of boss lower in the tank, I would pump back into the bottom of the tank using convection to mix the tank as this allows you to maintain separation of hotter water at the top of the tank.

Are you sure about that with an ASHP? You need to keep the return temp as low as possible for as long as possible to a get a decent CoP. You would kill CoP pretty quick. Can see an argument to push back in the centre the tank but not the bottom.

[sharpener]

I don't think you will get a useful amount of gravity flow through a HX.

 

But 7kW HP will probably be OK with existing coil (based on my experience with a 12kW in Noise Reduction mode). You will get some improvement - if needed - by circulating the (potable) water with a bronze pump, so buy that first and try it before shelling out on the HX. You will need to fit a NR valve as well otherwise there is a backflow path through the pump when it is off.

 

 

Edited August 15 by sharpener

[HughF]

I run a 9kW through a 1m2 coil - try it and see how you get on.

 

if you do end up using the plate, you won’t get any thermosyphon effect through it. you’ll need to pump the dhw side.

[Dillsue]

Thanks for all the info and comments. We'll suck it and see with existing coil for the next winter and fit a pumped PHE next year if we hit problems.

[Dillsue]

I tried using the existing .44m2 coil and it was hopeless so have resigned myself to fitting a PHE. There's likely a few ways I could trigger the DHW circulation pump but anyone know of proven ways?

[JohnMo]

If your 3 port valve has secondary contacts that close on valve moving to DHW position, use them to provide pump on off signal to pump. If it doesn't have secondary contacts, piggy back the power from the 3 port valve. 

[marshian]

9 minutes ago, Dillsue said:

I tried using the existing .44m2 coil and it was hopeless so have resigned myself to fitting a PHE. There's likely a few ways I could trigger the DHW circulation pump but anyone know of proven ways?

 Is this to avoid using a dedicated pump for the PHEX? 
 

I’m not sure I’d want a pump set up for DHWC to also be used for Tank circulation whilst heating using PHEX. I would have thought you would need to optimise the circ pump to work with the PHEX to maximise heat transfer?

 

you are adding to the circuit the PHEX has to work with increasing losses (even if the pipework is lagged you will get some) 

 

DHWC pumps are run at low flow rates and take from the top and return to the bottom in order to keep the tank stratified. 
 

I guess for a PHEX you could take from top and return to bottom but surely better to maximise differential temp at the PHEX by circulating from bottom and returning to the top?

[Dillsue]

The DHW circulation pump will draw from the cold inlet at the bottom of the cylinder, pump through the PHE then inject the heated water in at the top. That way it circulates cylinder contents through the PHE, pushing the hotter water in at the top. Primary side of the PHE is pumped by the ASHP

[marshian]

2 hours ago, Dillsue said:

The DHW circulation pump will draw from the cold inlet at the bottom of the cylinder, pump through the PHE then inject the heated water in at the top. That way it circulates cylinder contents through the PHE, pushing the hotter water in at the top. Primary side of the PHE is pumped by the ASHP

my apologies - I thought you were trying to use an existing HW circulation pump (the sort that stops you having to wait for HW at the taps) to also circulate the Tank water through the PHEX

[Dillsue]

3 hours ago, marshian said:

my apologies - I thought you were trying to use an existing HW circulation pump (the sort that stops you having to wait for HW at the taps) to also circulate the Tank water through the PHEX

No apologies needed. I was a bit confused by what you were saying but all is clear now.....for both of us

[marshian]

15 hours ago, Dillsue said:

No apologies needed. I was a bit confused by what you were saying but all is clear now.....for both of us

I’m looking at a similar set up for my HW (PHEX with my current tank which is a 2012 115 litre copper tank - spray foam lagged) Tanks with 3m2 coil area are £1300 - £1500 but I can get a PHEX kit for £450 and try it on the existing tank - if it works well I can get a high quality Direct HW tank (no coil) of a sensible size later as an upgrade for £600 - £700. Overall it would be saving against a Modern HW tank with 3m2 coil if it works well.

 

I’m using gas boiler but running low flow temps for CH but because of the small coil in the current HW tank meant for higher flow temps when doing a 45 min tank heating cycle with a flow temp of 64 Deg C I’m only in condensing mode for the first 10 mins

 

I’d like to heat the tank to 50 Deg C and be in condensing mode the whole cycle so 55 to maybe 58 Deg flow temp with a better heat transfer from PHEX keeping the return tenp below 54 Deg C 

[sharpener]

23 hours ago, Dillsue said:

The DHW circulation pump will draw from the cold inlet at the bottom of the cylinder, pump through the PHE then inject the heated water in at the top. That way it circulates cylinder contents through the PHE, pushing the hotter water in at the top. Primary side of the PHE is pumped by the ASHP

 

Yes, if you run the secondary pump off the HW valve supply then it will not de-stratify when you are not heating the water. It will in addition need an NRV so you do not get back flow from cold inlet straight to the hot taps when the pump is not running. For best heat transfer make sure you plumb the HX in the counter-flow configuration.

[Dillsue]

6 hours ago, sharpener said:

 

Yes, if you run the secondary pump off the HW valve supply then it will not de-stratify when you are not heating the water. It will in addition need an NRV so you do not get back flow from cold inlet straight to the hot taps when the pump is not running. For best heat transfer make sure you plumb the HX in the counter-flow configuration.

Thanks for the tip about counter flow....not sure I would have considered that.

 

Not sure I follow where you're suggesting an NRV is needed? I'm going to pump from cold inlet, though PHE and into the HW outlet. I cant fit an NRV to block that flow when the pump isn't running as it will block the flow when the pump is running???

[JohnMo]

9 hours ago, marshian said:

Tanks with 3m2 coil area are £1300 - £1500 but I can get a PHEX kit for £450

Look on eBay, they can be way cheaper.

[marshian]

43 minutes ago, JohnMo said:

Look on eBay, they can be way cheaper.

I have been looking but smol tanks with 3m2 coils don’t come up often

[sharpener]

1 hour ago, Dillsue said:

Not sure I follow where you're suggesting an NRV is needed? I'm going to pump from cold inlet, though PHE and into the HW outlet. I cant fit an NRV to block that flow when the pump isn't running as it will block the flow when the pump is running???

 

I don't think you can do that bc of the parasitic path from cold inlet to hot outlet when the pump is not running. If you do it the other way then the NRV will stop that. Or use a 2-port valve not an NRV.

 

However my application is slightly different, to stir the tank while I am heating it bc of the small coil, so I am not sure what is usual. Worth looking to see what Mixergy do.

[JohnMo]

Discussion on plate loaded cylinder here

https://community.openenergymonitor.org/t/anyone-else-with-a-monitored-heat-pump-and-a-mixergy-cylinder/23825/69?page=2

 

It shows a mixergy cylinder layout, it draws cold water from bottom of cylinder and deposits hot back at bottom of cylinder also.  So I assume the hot water at top cylinder isn't disturbed.

[Dillsue]

13 hours ago, sharpener said:

 

I don't think you can do that bc of the parasitic path from cold inlet to hot outlet when the pump is not running. If you do it the other way then the NRV will stop that. Or use a 2-port valve not an NRV.

 

Worth looking to see what Mixergy do.

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??

[JohnMo]

If you are pulling from the bottom and injecting higher up, you will be pumping up hill. Depending on pipe runs there may be a thermosyphon formed as hot water cools, this will travel downhill, so you will need check valve downstream of pump.

[sharpener]

4 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.

 

You may be right. You might though want to restrict the flow through the HX so it slowly pushes the thermocline down rather than mixing the entire tank when the pump starts, that way the inlet to the HX will get cold water for as long as poss. You could achieve this with either a flow setter (expensive for what they are) or a simple gate valve. This would also reduce possible flow through the path you don't want when the pump is off, but any flow at all will dilute the hot water supply with a proportion of cold. 

 

Like you I would not want to try adding an extra connection to an existing tank! I have tried various heroic repairs and mods in the past, not all of which have worked.

 

[JohnMo]

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

Edited Monday at 16:31 by JohnMo

[Dillsue]

28 minutes ago, sharpener said:

Like you I would not want to try adding an extra connection to an existing tank! I have tried various heroic repairs and mods in the past, not all of which have worked.

 

Waiting to hear back from the cylinder manufacturer on viability of doing this. I'm sure it'll be a negative response but interested to know how they would add one in the factory..... I think they are soldered which is a bit of a none starter with spray foam insulation