C4 Thermostat turning on HVAC fan for about 5-10 seconds when temp is okay

[calbear]

Can't seem to figure out a causation or solution for this; would appreciate any insight or ideas.

 

I have two identical HVAC systems in the house, each tied to a C4 Thermostat.  Everything works great except every now and then (a pattern I can't discern) I get the following:

 

* While HVAC is off

* And temperature is already at the set point (or within the delta)

* Fan turns on for about 3-7 seconds, then shuts off

 

This happens on either HVAC system, making me think this is a software issue, rather than hardware.

 

Anyone else experience this or have any recommendations?

 

Thanks!

Dan

[Cyknight]

It could also be something the HVAC does all by itslef actually - some systems turn the fan on if not run for an extended time to circulate. Though it seems rather short for that.

Easy way to find it it's that is to disconnect the G wire from the Tstat - if it doesn't happen then you know its C4 or not.

 

Hard to give detailed insights not knowing: if it's got a Common (OC/TS connection) wire, if it has powerstealing enabled or not and what the toggle switches at the back of the thermostat for type of furnace are set to.

[calbear]

Thanks Cyknight.   I'll run the experiment and double-check all the settings/toggles.  Challenge is the frequency of this happening is somewhat random, can be many hours or longer between episodes; hard to pin down any correlations from that (and wife might not like the heat being off for a long time while it's freezing out)...  

 

The furnaces themselves are about 30 years old (original equipment in the house) so they're not "sophisticated" per se... I don't even know if they have any circuitry to auto-on anything...  

 

But if nobody else has ever experienced this with C4 Thermostats, then perhaps it is just some gremlin I need to hunt down...