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Comment by toomuchtodo | Hacker News Reader

If it’s the high limit temp switch, that can be triggered by extended furnace runtimes when the temperature is so cold outside that the furnace must run nonstop to keep up with thermal losses (polar vortex and whatnot). This will potentially manifest in smart thermostats as frequent reboots, as the limit switch cuts off power to the furnace (which cuts power to the thermostat) to prevent it from bursting into flames. I can’t speak to Nest thermostats, but ecobee will send you an alert.

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This is close, and I'd grant that limit trips are associated with longer run times. But in principle continuous furnace run time shouldn't cause a high limit trip. The limit is mainly to protect the heat exchanger from damage. The primary cause is usually low/reduced airflow, although bad sensors are also common.

A high limit trip will generally cycle the furnace off for a time period. Somewhere between 3-5 off cycles will result in lockout and the furnace will stay off. The control for this is more likely to reside in the furnace board vs an aftermarket ui.

I took great pains to setup my boiler to run nearly continuously by using its outdoor reset controls and an outside temp sensor (a simple 10K NTC) to adjust the flow temperature to match/ever so slightly exceed what’s needed to match the building heat loss.

This results in higher efficiency and better steady state comfort as the heat is on for always 18+ hours and often 22+ hours per day. It’s hard to say with confidence on the precision, but it seems like this is saving 8-10% in gas (in therms) vs last year when I had a constant flow temp (that I’d adjust manually every so often to try to match but where I’d have to leave it “too high” or risk a cold house on the coldest days).

The exhaust temps in the stack reach steady-state far before even a 30 minute run would be finished.

The high temperature limit switch should never be tripping, even when the furnace runs continuously.

This indicates you have a problem with temperature rise across the heat exchanger, probably from inadequate airflow. Your ducts may be sized too small/furnace too large, fan speed set too low, plugged filter, etc. You may be able to mitigate the problem by increasing fan speed, limiting heat by using stage 1 only, putting in less restrictive filter, add additional cold air return, etc.

Usually a failed limit switch means cycling and a bigger problem with airflow. However I have seen a high temperature limit switch fail without cycling - fell apart with age (40 year old furnace).

It was the high temp limit switch, and it was tripping at about 110 degrees instead of 180. It kept short cycling, and the house got down to 58F. Brrrrr

It took an extended run to get the house back up to temperature, but it had no problem with a correctly functioning switch.

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