I thought I would chime in with my drainback tank project. it is part of the ASHP that will replace my Viessmann boiler. First thing I did was to rip out the forced air from the house and replace it with floor heating in light weight cement and old cast iron rads where I couldn't put the floor heating. That was 5 years ago.
Now I am putting in a few more rads to drop the water temp needed to heat the place so that a Hp can be used. I would like to be able to heat with no more than 40C (104F).
Here is the pic of the tank I made out of PP and spray foam (don't do it this way, it was way too expensive)
Then, I got the innards from a German tank maker called Rotex. The tank was a display model with a big hole in it but the HX were intact. There is a HX for domestic hot water which takes the heat from the drainback tank as needed, one for the solar (at the bottom), one to go out to the house for heating and one for a backup boiler.
I will be putting the wall solar heat on to the solar HX, the boiler will stay put as a back up, the HP will heat the tank directly to keep the losses to a minimum and hopefully the DHW will work OK. I have only heard of these tanks before and never seen one in action so we will see.