DHWC's in series

[trialuser]

I have a setup whereby a new extension will be heated by ASHP / UFH, approx 100sqM footprint.

The existing house is heated by a new condensing oil boiler and radiators (old stone cottage, no insulation to speak of (yet)

The extension will be effectivelt sealed from the old part.

Hot water to both parts is from the oil boiler and a 300L cylinder.

The ASHP was an secondhand ebay buy (3 months use) and came with a preplumbed 250L cylinder. (Ecodan 8.5)

I am proposing to link the DHW cylinders in series and heat the first cylinder to 40C (ecodan min hot water temp) to hopefully get a decentish COP, and feed the outlet into the inlet of the second cylinder and heat it from 40 to 60 ish with the oil boiler.

All pressure reducing valves, temp and presure relief valves and expansion vessels will be fitted.

Is this OK?

[trialuser]

Pretty please, anyone, @Nickfromwales

Edited March 10, 2022 by trialuser

[Nickfromwales]

15 minutes ago, trialuser said:

Pretty please, anyone, @Nickfromwales

Chill Winston.......your time will be coming soon.

 

Then I will gladly kick the shit out of your idea!  

 

Oh, and never beg ok  

[JohnMo]

Don't see any issue.

 

Cold water feed to HP cylinder, then the outlet fed to oil cylinder.  So long as both have the cylinder protection on (relief valves etc), should be no issues.

 

Why heat to 60, a lower temp should be fine. Even maintaining the second cylinder at 40 or just above would be fine, as you will have 550 l of water available.

[Iceverge]

Watching with interest here as I was contemplating doing the same with an all in one ESHP. The payback is questionable at the moment but if we could heat ~600l of water to the low-mid 40's by putting it in series/parallel with the existing UVC, the bounce in COP might make it a runner.

 

 

[trialuser]

Thanks All, although I'm not sure I understand Nick, but I seem to spend half my life begging 😁

My reason for doing this is I have a highly insulated extension with ufh which is bigger than the original cottage, the ashp hasn't got a hope of heating the old stone cottage, and the ecodan cylinder was essentially a freebie with the pump. It would be a hassle not to use it as it has the ftc5 and the valves, thermistors and circulatorfor the ufh bolted on the side of it, by the time that was stripped off the cylinder would be worthless. My hope is that pre heating the water by ashp would lower the dhw cost slighty and give a bit of a buffer and backup.

[PeterW]

Domino tanks are easy to set up as you push cold into the last and treat it as a single volume.  Heating it is as simple as using zone valves on the return from the coils to divert back along the run dependent on volume required. 
 

For simplicity sake, a double pole switch and a manual 3 way valve to direct flow and switch between the thermostats is all you need. It’s not like you will be using complex controls to decide how much water you want to store as it will be planned. 
 

Designed a couple of these using multiple 250 litre tanks and works very well. 

[Iceverge]

How about mounting the Oil heated cylinder above the ASHP but plumbing in parallel.

 

Would this not allow the upper cylinder to be heated by convection to 40 deg but prevent the lower one being heated by oil. 

[trialuser]

That's beyond my understanding. I wasn't aware of any convection flow. My understanding is that cold mains water enters the first cllinder at the bottom and displaces hot water at the top of the second cylinder?

I don't have the room to physically mount one cylinder above the other.

Thanks again.

[tonyshouse]

Commercially and large domestic I have seen de-stratification brass pumps use to mix all the water in both cylinders to maximise the volume of HW available.