Nest Thermostat + Fast-Stat Compatibility

July 15, 2020 9:57 AM

I've been using an original Nest thermostat for almost a decade with no problems. Lately, the fan will sometimes not run when I activate it via the Nest app (or the thermostat itself).

Often the fan makes whining or clicking noises when I try to run it, or just does nothing at all. Of course, every time I call the technician it starts working again. The last technician said it is happening because we have a Fast-Stat device that adds a common "C" wire, and can be flakey when used with smart thermostats. He said we should upgrade to a "5-wire".

Is he correct? The first technician we called didn't mention this at all, and since the fan was working when he visited, he couldn't diagnose the issue. I don't know why this would only recently become a problem.

Note that the fan always works if the A/C is running.

posted by krunk to Home & Garden (3 answers total)

It looks like the "Fast-Stat" is a device that permits 4 physical wires between the thermostat and the furnace to carry 5 signals: specifically two power wires, and three signal wires: fan, AC, and heat. It would consist of two parts, one behind the thermostat and one in the furnace.

The technician may be recommending getting rid of the Fast-Stat device and replacing it with 5 wires, which would work if the Fast-Stat device is the problem. And the Fast-Stat device _could_ be the problem, but it's not guaranteed.

If I were trying to diagnose it, I would remove the Fast-Stat, and wire up the Nest with 4 wires leaving out heat, and test. If that works, the problem is the Fast-Stat. If that doesn't, the problem is either your Nest or the furnace mainboard.

If the fan doesn't start properly in a 4-wire configuration, the next step would be replacing the Nest. I've heard of reliability problems in the electronic switches that the Nest base uses to trigger the furnace, so from what I have seen I would be most inclined to believe that the Nest itself is the problem given your symptoms.
posted by doomsey at 10:42 AM on July 15, 2020

Response by poster: Thanks, I'll get some wire and test it out. I don't know why they would use a Fast-Stat, considering that the thermostat is about 10 feet above the furnace and wouldn't require complex re-wiring when they put in the furnace.

Outside of the fan inconsistency I haven't had any issues with the Nest.
posted by krunk at 11:18 AM on July 15, 2020

Many furnaces don't allow the thermostat to control the fan independently - they have the fan logic built into their board.

So - I tried and failed for some time to control our fan with the Nest - but it would only work under certain conditions.

Wiring your thermostat yourself isn't terribly hard (everything is well labelled), however, I have ruined one control board that cost me $200 to get replaced by someone who actually knew what they were doing.
posted by bbqturtle at 11:31 AM on July 15, 2020