Could use some help on controlling flow to different zones on a solar slab heat.  One of the challenges with solar heating is that of the 6 or 7 hours that is collected the first 4 to 5 does most of the work and achieves most of the temp most of the time.  The last 2 hrs. the system chugs along trying to capture a few degrees.  With the slab already warm and the sun going down there is a bunch of waste.

30 years ago I used to have my wife do the washes in the afternoon to extend the collection time by injecting cold water into the bottom of the tank.  Mrs. Flathead won't put up with my energy saving antics anymore but the principal can be the same for slab heat by sending the fading afternoon heat to cooler perimeter zones.  Question is how to do that?

I believe that most of the conventional heating is done on setpoint.  Over here in the solar ocean we're using differential.  We have priority controllers but the priority is setpoint.  Best we've been able to come up with is two differential controllers.  First triggers the system.  2nd runs the 2nd pump for the cooler loop.  Only thing we've been able to come up with is to run the second controller on a timer (say 2 o'clock).  It comes on and cools the loop and shuts the first controller down.  This is a bit primitive but we are limited with our knowledge and logic and what's available.  We talked to Watts at ASES and they are very interested in this if we develop it.  I could use something better than a True Value timer in this layout.  Remember conditions change all the time in solar.  Comments and constructive criticism welcome.