Tell if fan is running and why?

Posted on
Sun Nov 12, 2017 10:34 am
dduff617 offline
Posts: 407
Joined: Jul 05, 2006

Tell if fan is running and why?

I'd like to know if my HVAC system's fan is on for any reason. Ideally, I'd also like to discern the reason(s) the fan is running. I can already infer when there is a call for heat or AC (via isheating and iscoooling) and perhaps when it is running with the timer function (via fan_timer_timeout, maybe). I'm not sure if it is possible to tell the difference between when fan is running due to a manual user request ("stay on for 15 minutes") vs. an automatic/scheduled request ("run for 15 minutes each hour between 10am and 5pm").

Anyone know if this possible the Nest API and if so via Nest Home?

Posted on
Wed Jul 11, 2018 2:33 pm
dduff617 offline
Posts: 407
Joined: Jul 05, 2006

Re: Tell if fan is running and why?

So I've done some empirical analysis of my system and have some findings. I'll share in hopes others may also find it useful.

The state fan_timer_active will be true when the fan is running for any reason. This translates to:
the system is heating, OR
when the system is cooling, OR
when the system is executing a Nest scheduled fan run OR
or when you are doing a manual timed fan run.

The state heat_or_cool is "Heating" when system is heating, "Cooling" when system is cooling and "Saving Energy" when system is neither currently heating nor cooling.
States isHeating and isCooling also work as you would expect with values "No" and "Yes" when the system is curerntly calling for heat or cool respectively.

the system mode setting hvac_mode is "heat", "cool", "heat- cool", or "off".

fan_timer_duration sounds potentially useful, but afaik is always zero.
fan_timer_timeout contains the absolute time of a scheduled shutoff when fan is running due to a manual timed fan run. it is jan 1, 1970 when there is no such event active.

Btw, the IOM (?) field fanIsOn on my unit is always false. I'm not sure I exactly grok the concepts here and how inheritance is supposed to work for plugin classes, but that may be a bug or an oversight in the Nest Home plugin design. It seems like it would be straightforward to make Nest thermostats comply with the Indigo standard api here.

So with combination of states fan_timer_active, heat_or_cool, and fan_timer_timeout and some logic, I can answer my original question - that is, figure out whether fan is currently running and if so, why.

Posted on
Thu Jul 12, 2018 1:18 pm
Colorado4Wheeler offline
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Posts: 2723
Joined: Jul 20, 2009
Location: Colorado

Re: Tell if fan is running and why?

dduff617 wrote:
Btw, the IOM (?) field fanIsOn on my unit is always fals

Yes, I discovered that a while back as well when I wanted to know if my fan was on or off, I've had it on a back burner to fork the Nest plugin and fix that as I'm sure it was just an oversight by Chameleon when he wrote the plugin.

The Nest, in general, has a lot of odd settings to try to figure out if it's running or not and in what mode. Sounds like you did some sleuthing and got it figured out before I could write up an answer for you as I've already been down that path myself.

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