Two zone hydronic system with gas boiler issues

pacmanpasta

I just moved into a new place and am trying to figure out what is going on with the heating system. It is a finned baseboard system running off a Weil McClane CGi4 gas boiler. The system is separated into two zones with circulators controlled by a Taco SR-504 EXP switching relay. Two regular round mechanical thermostats are connected to the Taco. As I found the system I noticed the following: there was no connection between the Taco and the boiler, the two thermostat leads on the boiler were wire nutted together, and the Taco was set to priority off and in reset mode.

When either thermostat called for heat the relays did function and activate the circulators. The boiler is set to 180° but was always seeing a call for heat since it's leads were connected together and it will always stop firing at 140°. There is no other obvious temperature control in the system so I A: don't know why it's stopping at 140 and B: where the low temp limit is set.

To try and see if I could make some improvement I connected the Taco end switch terminals to the thermostat leads of the boiler. If I leave the Taco in reset mode as I found it the circulators will still activate but the end switch relay never activates. If I switch it into normal mode the end switch relay will activate and fire the boiler but it is constantly and the zone relays never turn on. In case it is important there is no relay present in the 4th zone socket. Also DHW is not involved at all - there is a separate electric hot water tank for that.

So...why is the boiler shutting down before it reaches its set temp and why is the Taco not operating as it should?

Thanks for any ideas!