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 Friday at 14:36 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.