I have a 2-zone hydronic system running off an ancient Bryant boiler. Power is supplied by a 24V transformer, and each red wire passes through a Taco zone valve before heading off to the appropriate thermostat. There is no C wire currently connected.
For the past year, I've used a Nest thermostat for the 2nd-floor heating control. I simply replaced the old battery-powered digital thermostat with a 3rd-gen Nest, hooked up the Red and White wires, and was on my way.
I recently purchased a Nest E (the cheaper one) for the first floor and hooked it up the same way. It refuses to work. It repeatedly goes into a "delay" countdown for a few minutes after attempting to heat, then restarts the countdown a few seconds after arriving at zero.
The Nest on the 2nd floor continues working fine. The Nest downstairs just won't switch on. It's not giving me any indication as to why it's doing a 3-minute delay. I left it go overnight after installing it one evening and came down to a 5-degree temp drop and repeated countdown.
I tried hooking up a C wire for the downstairs thermostat (by attaching to C on the Nest plate and attaching the other end to the common terminal on the transformer) and it didn't help.
I swapped in the old digital thermostat and the heat came right on again.