I understand the principle behind the 120v overtemp protection relay. However, I need to find out how to tie it into my existing system without adding zone valves/circulators. My current heating system uses 24v zone valves. Thermostat calls for heat, opens the zone valve and powers the circulator through an 8148 Aquastat Controller. Without adding a dedicated 120v zone valve for the overtemp relay, I would like to install a 24v step down transformer and tie the feed into an existing zone valve. In retrospect, making the wood boiler act as a thermostat. However, won't the feed from the existing thermostat back-feed through the newly added 120v/24v overtemp relay when the thermostat calls for heat, and vice-versa? Econoburn mentions that the overtemp relay is a rare occurance. And the chances are that both call at the same time is even more so. I've gotten several mentions of a rib relay but I'm unfamiliar with these and specifically which one would suite my application. Any suggestions?