Nest Learning Thermostat. Boiler turning off and on a lot before reaching target temp

gmase001

I’ll start by saying I’m the farthest thing from an expert on my heating system.  I had a regular thermostat which I replaced with a Nest learning thermostat.  I have a two wire system, and it doesn’t seem I need a C wire to maintain 3.7-3.8 volts on the Nest.

Today was the first cool day since installing the thermostat so we turned the heat up about five degrees.  Boiler kicked on as expected.  It ran for 10-15 min then turned off.  Temp had not been reached and Nest showed it was still calling for heat.  Boiler turned on a few minutes later. Ran for a bit and turned off again.  I thought the Nest might be turning off early assuming the boiler would provide sufficient heat from the radiators to reach target temp.  So I raised the temp over ten degrees.  Same behavior.

With my old thermostat the system stays running until target temp is reached.  Any ideas on why this is happening?  I don’t want to damage my boiler.

pecmsg

NEST is the absolute last t-stat I’d install. 

Any other is better. 

pedmec

boiler could be cycling off/on from boiler aquastat. check boiler water temperature against setting of aquastat. make sure water is being circulated thru heating circuit

gmase001

pedmec said:

boiler could be cycling off/on from boiler aquastat. check boiler water temperature against setting of aquastat. make sure water is being circulated thru heating circuit

Would I need to increase the aquastat temp to keep the heat on?  It’s set for 180 degrees and the boiler turned off when it hit that.  At least I think it’s set for that.  Here’s what it showed.  It jumped to 181 for a second after it turned off but went back to 180.

Jamie Hall

If the system worked with the old thermostat, and it doesn't work right now -- the problem isn't in the system. It's in the Nest, assuming that the ONLY thing you did was disconnect the old thermostat (two wires?) and connect the Nest.

Beyond that I'm not going to go. There are too many ways for a Nest to mess up a perfectly good heating system to even try to diagnose it remotely.

unclejohn

Are the rads or baseboards getting hot?

gmase001

unclejohn said:

Are the rads or baseboards getting hot?

Yes,  the heat is working but the boiler doesn’t usually turn off until the target temp on the thermostat is reached.  In this case it’ll run for ten or 15 minutes and turn off before reaching target temp.  It’ll turn back on after a bit.

ratio

It kinda sounds like the aquastat is stopping the burn because the water is up to temp (180°) before the stat is satisfied. Which is kinda normal if the boiler can heat the water faster than the radiators/baseboard/whatever can cool it (=heat the house). I can't see how the Nest can be causing that—but you say that it wasn't happening before.

Are the boiler cycles with the Nest longer than with the old stat?

unclejohn

AS bad as the nest is its not as bad if it has a "C" wire.

gmase001

ratio said:

It kinda sounds like the aquastat is stopping the burn because the water is up to temp (180°) before the stat is satisfied. Which is kinda normal if the boiler can heat the water faster than the radiators/baseboard/whatever can cool it (=heat the house). I can't see how the Nest can be causing that—but you say that it wasn't happening before.

Are the boiler cycles with the Nest longer than with the old stat?

Maybe I’m just thinking about it wrong.  I always thought the boiler kept running until the temp on the thermostat was reached but maybe not.  Maybe it’s just that in winter it takes longer to heat the water to the aquastat temp, so the boiler runs longer, making me think it was only running until target thermostat temp was reached.  But really it was running longer because it takes longer to satisfy the aquastat temp in winter making for a longer cycle.

Jamie Hall

gmase001 said:

ratio said:

It kinda sounds like the aquastat is stopping the burn because the water is up to temp (180°) before the stat is satisfied. Which is kinda normal if the boiler can heat the water faster than the radiators/baseboard/whatever can cool it (=heat the house). I can't see how the Nest can be causing that—but you say that it wasn't happening before.

Are the boiler cycles with the Nest longer than with the old stat?

Maybe I’m just thinking about it wrong.  I always thought the boiler kept running until the temp on the thermostat was reached but maybe not.  Maybe it’s just that in winter it takes longer to heat the water to the aquastat temp, so the boiler runs longer, making me think it was only running until target thermostat temp was reached.  But really it was running longer because it takes longer to satisfy the aquastat temp in winter making for a longer cycle.


Very very likely you've hit it

bburd

When the burner stops before the thermostat is satisfied, does the circulator pump continue running? If so, the system is operating normally.

MikeAmann

I know nothing about the NEST, but let me just throw this out there.....
Could this have anything to do with the anticipator settings?

Hot_water_fan

Pretty interesting study here - an easy rule of thumb is 1% fuel savings for every degree lower the thermostat is set, for periods of 8+ hours. Also, savings are higher when setbacks occur at night when it’s colder outside. Knowing this, why not just return the nest and get a simple programmable thermostat? Easy enough to program a nighttime setback so you get the benefit without the extra cost. 

https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_condensing_boilers.pdf