Tank-in-tank TS

[dpmiller]

Never come across this before

https://www.greentherm.ie/product/greentherm-300-litre-combined-cylinder/

 

300l-sized mild steel TS but with an internal 80l  stainless vessel for DHW. Had a yarn with one of the fellas from Greentherm yesterday, querying recovery, losses etc but he didn't have much info.

 

How would the flow and recovery of this compare to a conventional TS with a coil? 80l stored at constant temp vs a DHW coil... What happens when you're drawing off? Sure there'll be 80l but (noting that CW is fed to the bottom of the DHW vessel)  can the stored body reheat the DHW in real time?

 

 

[SteamyTea]

It is really to do with minimizing mixing, the laws of thermodynamics stay the same.

[Jeremy Harris]

I think the re-heat time of these is reasonable, as the 80l internal tank has a fairly large surface area, and it's the surface area that tends to dominate the heating time, for a given temperature differential.  One advantage the tank-in-tank systems have is a higher potential flow rate.  The main disadvantage is that they tend to be physically larger than an equivalent size coil type thermal store.

[Nickfromwales]

A TS with a 22mm DHW coil gives LOTS of DHW flow, but I have gone to 28mm when combined with either an accumulator or a damn good cold mains. 
I have only ever fitted coil types so have no experience of TiT versions sorry. Can’t say I like the idea though as the heat transfer rate in the coil version is excellent, whereas a) the TiT version probably needs time to recover and b) it’s only 80L......