Adding to new setup need help

Joebif

Here is a drawing of my setup with the existing propane heater on the right, except for the mixing valve and ball valve B and everything on the left of the variable speed circulator that is using a DELTA T that is not there yet. Everything works great and was tested from the past 2 days of colder weather.



I want to add the 500 gallon open storage tank for the wood fired boiler and future solar panels. In doing that I added a ball valve B and ball valve C and a mixing valve to my existing system and need help on a problem I see.



If I want to force the return of my radiant system to go through the storage tank heat exchanger I close ball valve A and open ball valves B & C.  The mixing valve then will not have a source for the cold side to mix with and need help on where that should come from. When ball valve A is open and B & C are closed then the water will just pass through the mixing valve and not be used.



I was thinking that I could always keep ball valve A open but was wondering if my return would for sure still go to the storage tank and get used or not.



I could have the cold side of the mixing valve get cold at a T underneath the mixing valve between the ball valve from the cold from the source and put on a backflow preventer so if I wanted to put in glycol I could but was not sure if that was correct or not.  Or is there another way?



I am learning a lot here.

Thanks