you mentioned that it sounds like I may have a vaulty aquastat on my indirect tank. The way it was connected to zone 6 on my Taco ZVC406 w/priority R & W is wired to 24v connections on the TPI aquastat and then the 3 & 4 terminals on the Taco zone controller have a jumper because I am using a pump on that zone. The two pump contacts on the TPI were not used. this arrangement caused the red light on the Taco zone 6 to stay on constantly but it would start the burner and run the pump that is connected to the the Taco controller whenever the tank called for heat. Recently I found the tank calling for heat but the burner never started. I was never sure that it was connected properly to begin with but it was doing what I thought it was supposed to be doing. I disconnected the wireing from zone 6 including the jumper. I connected a thermostat and zone valve that I had available. It worked as it should starting the pump and fireing the burner. I'm Not sure how to test the aquastat
Sorry, I thought you had a typical aquastat on the indirect.
R and W do not connect to the 24v terminals on the TPI, they connect to the pump/TT terminals. The TPI is constantly 24v powered so you need at least a 3 wire to use the control properly.
Let's say you have red, white and green wires. R,W and G for common. Connect the wires to the zone 6 thermostat terminals. Re install the jumper on 3 and 4.
At the TPI, connect Red to one terminal on pump/TT, then jumper that Red to the first terminal on 24v.
Connect the White to the other terminal on pump/TT. Connect Green to the other terminal on 24v.
On the bottom left of the zone board, I think that was correct.
Pump power comes into N.O. and out to pump on common.
Top left of zone board, XX boiler to TT on the boiler aquastat.
I'm not sure how you can actually test the TPI control, but jumping terminals at pump/TT should start the tank pump and start the burner if it's not already at limit.
Hope this helped.Let me know.