Tank-in-tank for Air Source Heat Pump

[CurvedHalo]

I've been reading about air-source-heat-pumps, and that a new cylinder would need to be fitted which has an element with a larger surface area (I guess as the water is coming at a much lower temp in the ASHP vs a conventional boiler). I also have read about the use of 'tank-in-tank' cylinders, where domestic water is stored at a high temperature, but much lower volume, and then central heating (UFH) water is stored a larger volumes but much lower-temperature. 

Does anyone have any experience of these? Are they beneficial? Does this mean the flow/return to the UFH mixer is now independent to the DHW? 

 

Finally, hypothetically, in a system using solar panels, would it be better to use the generated electricity to heat the water in the cylinder directly using an eddi? or is it better to store this, and use the ASHP to heat the water from stored electricity I have generated?

 

Thanks!

[JohnMo]

2 hours ago, CurvedHalo said:

'tank-in-tank' cylinders

Think they would be more suited to something other than an ASHP.  Keep it simple

 

UVC, with a large coil for good heat transfer at lower temps, so about 3m2 coil or bigger. If you only have UFH, you really don't need a mixer/pump, let the ASP circulator do the UFH directly. Then you just need a diverter valve to direct the flow to UFH or cylinder.

 

2 hours ago, CurvedHalo said:

hypothetically, in a system using solar panels, would it be better to use the generated electricity to heat the water in the cylinder directly using an eddi? or is it better to store this, and use the ASHP to heat the water from stored electricity I have generated?

Lots of variables

Do you get paid for export?

What do you do when the battery is full?

 

I had a diverter, but had two fail, so skipped the idea of paid for diverter. I use home assistant and do two things. All I needed was one Shelly relay (second operating point for ASHP switch) and one SONOFF High Power Smart Switch Power Meter Switch (POWR3 for immersion), so about £70 all in, and does a way better job when you have a battery.

 

ASHP - have a second operating point set up, so when battery is 97% full and generating xkW, the ASHP is started - generally get a CoP of 4 or above, doing this. Second if the battery get to 98% I power a relay to divert to cylinder immersion.  Leave both to run until battery drops to 95%. Then stop both processes two hours before sunset, so battery can get to 100% or close to that.

[SteamyTea]

4 hours ago, CurvedHalo said:

I also have read about the use of 'tank-in-tank' cylinders, where domestic water is stored at a high temperature, but much lower volume, and then central heating (UFH) water is stored a larger volumes but much lower-temperature

DHW and space heating are different things, at different temperatures, and at different times. It is not normal to combine them. A thermal store is the exception, but even those are really just large, high temperature, water cylinders.

The Tank in Tank idea is sold on two points, neither of which are very valid. The first point is that it reduces 'stratification' in a cylinder, the second is that there is a store of hotter water surrounded by cooler water.

Stratification does not really happen in a cylinder, it is generally caused by different salinities, not temperatures. What you do get, in any cylinder is a temperature gradient with hot at the top  cooler at the base.

The second point, a cylinder, surrounded by another, lower temperature body of water, is just going to loose temper to the larger, lower temperature, surrounding water, rapidly. That is just thermodynamic.

[CurvedHalo]

@SteamyTea sounds like you're not an advocate of tank in tank. So separating the two tanks for you is a better solution then?

[Nickfromwales]

19 minutes ago, CurvedHalo said:

@SteamyTea sounds like you're not an advocate of tank in tank. So separating the two tanks for you is a better solution then?

Yes, separate the two tanks to the two different disciplines; 1x UVC and 1x buffer, if you need a buffer that is?

 

Have you eliminated the possibility that you could drive the UFH directly, without a buffer?

 

With a HP I am a 100% advocate of installing a much bigger cylinder and storing at the lowest temp possible, and this will help massively with reducing the chances of freezing up the unit, routinely, over winter, when you go to heat it up overnight on cheap rate (as the solar will have next to now output then).