The placement of the Taco iSeries mixing valve is designed to temper the water going into the stove, however, that valve also limits the flow of water returning from the storage loop. I realized this in the middle of the night after completing the changes to the stove plumbing yesterday.
I reread the instructions for the iSeries three way valve and they show it installed in an isolated thermal loop that is feeding the heat/storage side of the system.
Looking at this diagram and one of the stove installation diagrams that uses a similar piping design, I redrew the plumbing with a similar thermal loop. The drawing duplicates the iSeries instructions and moves the mixing valve to the secondary storage side of the system.
Although this plumbing looks like the Taco install diagram above, it is NOT the storage side in this system that needs to be tempered, the water going to storage should be as hot as possible. Instead, the water going to the stove should be tempered with a mixture of warm and cooler water returning to the stove. The water in the return primary loop is mixed with water from the supply. So the mixing valve should stay where it is.
The solution that will provide cooler return water to the mixing valve and bypass the valve when returning from the secondary loop is to install another tee in the boiler primary loop. This design is similar to the mixing valve thermal balancing loop on the secondary piping in the Taco diagram but on the boiler side. It also closely spaces the return tee next to the supply tee in the secondary loop.
Now the fix is in place. I had to run around a bit finding a 3/4 inch brass tee, which apparently Lowes is dropping from its inventory, and to exchange the wrong Sharkbite tee that I picked by accident–it was probably in the wrong box and I didn’t notice. I hate having to spend construction time running around to various stores to get what I need. I added a shut off on the new loop too so supplies cost about $60. When I turned the water back on and it refilled the system, I only had one leak at a Sharkbite that was easily tightened. I also notice the air vent was closed! So once I opened that the air was able to fully escape from the pipes and the pump ran quieter too.
I’m anxious to have another fire so I can see if the aquastat turns on the secondary pump and the heated water gets to the storage area.