The old thermostat uses only W1 and RH for heating only. After replacing it with Nest learning thermostat connecting only W1 and RH, it's acting weird.
I measured the voltage between the W1 and RH, before install nest, the voltage is stable around 25v/26v. However, after installation, the voltage became unstable, the number is consistently changing and sometimes went down around zero, which will trigger the boiler unexpected (the current temp is even higher than the setting temp).
Is it the expected behavior (i.e., unstable voltage) for nest learning thermostat if it only connects W1 and RH? If so, is it fixable? Is it because it uses these two wires for power, causing unstable voltage, and adding a c-wire will fix this?
