I use a one inch Danfoss esbe 4 way valve to control the direction of flow to/from my storage tank. This also requires a esbe motor if you want to do the switch over automatically. This method works very well. See here -> http://www.danfoss.com/North_America/Products/Categories/Literature/HE-HEC/ESBE-Mixing-Valves/ESBE-Motorized-Valves/ESBE-3-and-4-Way-Mixing-Valves-Type-MG/05a3144f-61d7-4d71-881c-70ef54edb0b6.html Another 4 way valve was once used to provide low temp boiler protection, but that has been removed. I now provide low temp boiler protection by controlling the flow in the Primary loop. If you have any difficulty finding a 4way valve contact me via private email. I have a used 1 1/4" and a few new 1" Danfoss esbe 4way valves.