If anyone knows HVAC I'd love some help.

Viessmann oil boiler, hot water baseboard radiators. Also connected to a corkscrew heat exchanger for water cycling to swimming pool.

Swapped out an old Honeywell round for a Nest thermostat Tuesday night. Just two wires, red and white. Connected them to Rh and W. Nest came on, seemed happy. Heat came on, all seemed fine until I went to sleep. Around 10am I noticed that although the heat was set to 60 and Nest thought heat was on, the actual temp was 52 and not rising. Went home and checked and the boiler was not running. The "need reset" light wasn't flashing but I tried reset button anyways. Boiler didn't turn on. Turned off at breaker box, back on, boiler didn't come on. Tried manually turning on the pool cycler for a few minutes, boiler didn't come on. On the boiler itself turned the switch off then on - boiler immediately came on.

Things seemed to work all day yesterday until I went to sleep. First thing this morning, heat was 60, temp dropped from 54 to 52. Boiler not on. Reset button, nothing. Off/on, boiler came on.

It's possible that the new thermostat has nothing to do with it - I'm not usually in that building in the morning. Yesterday I waited a good hour or so to see heat come back but maybe I needed to be more patient?

If anyone knows this stuff and has an idea of what might be wrong I'd love to hear it. We've had the oil company out for other things more times than I'd like to think about recently...

Sorry this will probably not be that much help, but while you investigate would it make sense to put the old thermostat back on, set it and check it for a day to see if the boiler comes on and the heat kicks in to the set temp?

Yup, that would probably be interesting, but I'm worried about getting it connected properly again - that mercury bubble seems awfully fragile / imprecise...

You could also pick up an inexpensive wall tstat that doesn't have the Mercury switch and give that a try.

My Nest downstairs let the inside temp fall to 62 and was not kicking on to heat the house. I went into the settings and did a reset and it started working again. Also, check your schedule on your Nest and make sure it's not learning and making it's own decisions.

Did you figure it out?

Not entirely but it has something to do with the aquastat on the hot water tank... When the house ends up cold if I turn that aquastat up... Even momentarily, then all goes back to normal for a while...

Might be a problem with the Nest itself. Mine has an issue where, randomly, it'll say that there's no power connected to it, so I have to disconnect the wire that runs the A/C.

But, your system is totally different than mine. Too bad getting ahold of Nest isn't super easy, since they're based in the UK.

I think the Oil company fixed it yesterday. They seemed to think the boiler internally had some special connection with the aqua stat. Still need to find out what they actually did.

Apparently this was all about logic configuration. I didn't try to understand all the details. Seems to be working though.