Nest inconsistently starts Boiler Gas Valve

[Peter_M]

I recently replaced my old Honeywell 2-wire thermostat in my single family home. I ran new wire through the wall, connected my new Nest to our 2004 Yorker gas boiler (L8148E Aquastat). I connected the Nest's 'R' to the boiler-aquastat's 'Tv', and 'W' to 'T'. I connected the Nest's 'C' to the aquastat 'Z'.

This setup worked great for a month. Now that there are negative temps outdoors in my area (<50F in basement where boiler is) the gas valve is acting up occasionally. Sometimes, when the nest calls for heat in the middle of the night, the boiler's pump starts, the flue-vent opens, but the gas valve does not open. To get the flame started, I have to turn the Nest down all the way, turn the gas valve off and on, and turn the Nest up again. This usuals works but is getting less reliable. It helps if I poke at the wires on the hydrostat during the process. 

I have an Hvac person coming on Tuesday, but I'm not sure they will know enough about nest. I am worried I have to get a different thermostat, or a new boiler with a real 'C' port on the hydrostat.

Any help or insight greatly appreciated / will save marriage, thanks!

@CoolingWizard  I thought maybe you'd have some insight?

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[CoolingWizard · Accepted solution]

@Peter_M , your thermostat is wired correctly to the Honeywell L8148E.  Your system has a 24VAC gas valve and the only thing I would check is the power transformer output under load. 

AC Cooling Wizard

[dkdbsi]

Related experience with L8148E Aquastat. How many amps can pass through the various Nest thermostats. The boiler L8148E Aquastat has a load of .5 amps on the 24 volt line. With the white Nest the relay in the L8148E Aquastat clicks on and off frequently. Everything checks out with the L8148E Aquastat, voltage values on 24 and 120v are good. When I wire nut the red and white wire at the wall the boiler works fine. The capacitor on the circ pump failed, probably from the excessive cycling.

[dkdbsi]

Related experience with L8148E Aquastat. How many amps can pass through the various Nest thermostats. The boiler L8148E Aquastat has a load of .5 amps on the 24 volt line. With the white Nest the relay in the L8148E Aquastat clicks on and off frequently. Everything checks out with the L8148E Aquastat, voltage values on 24 and 120v are good. When I wire nut the red and white wire at the wall the boiler works fine. The capacitor on the circ pump failed, probably from the excessive cycling.