I recently installed a 2nd generation NEST thermostat. I'm not home very often, but I was paying particularly close attention to the furnace after installing this thermostat and I noticed that the fan would not shut off.

Thought it was a faulty NEST so I put the old honeywell back on and same problem.

The furnace is set to a 60 second delay so the fan should shut off 60 seconds after temp (heat) is reached.

I opened up the furnace and i found a relay (p283-0294) that has terminals 4 and 6 jumped from "line" (110v). when the coil at 1 (24v heat) and 3 (neutral) is energized (heat is on) then 5 (red, low fan), 6 circuit is completed. When heat is off 4 and 2 (black, high fan) circuit is completed.

I went in and removed the wire from terminal 5 and now it behaves as I would expect.

Why is this relay in place? Why would anyone want the fan to be on all the time like that when there is no heating nor cooling and the fan is set to auto?

This system does have a white-rodgers electronic controller. The only connection to this board from the relay is the heat connection on terminal 1. The connection for 4/6 is pigtailed off of the line connection to the board. The neutral is pigtailed to neutral.

I feel like i just saved myself a bunch of money by not having the furnace fan running all the time but did I create a different problem?

furnace is a york gas upflow model P3URC14N09501C. i inherited it when we moved in. The only service I have not done on it myself is had the main fan wheel blade assembly replaced when it somehow got warped a couple of years ago.

no error codes / lights on via board diagnostic led.

Edit: should the red wire i pulled off of relay's terminal 5 be going to the board controller instead of directly to the fan? seems logical to me that the board would decide when to run that fan speed instead of all the time.

  • Was the fan always on, or just after the nest installation? – Tester101 Mar 19 '15 at 20:37
  • not certain. i certainly cannot prove it either way. uninstalling the NEST didn't help though. And yes fan setting was "auto" on the thermostate – gillyspy Mar 20 '15 at 0:06
  • If the fan runs when there's no thermostat connected, there's either a short on the thermostat wiring (R to G), or the furnace is wired to always run the fan. If I get a chance, I'll try to look up the schematic for your furnace. – Tester101 Mar 20 '15 at 0:29
  • Turns out York is stingy with their schematics. Any chance you could post a photo of the diagram? You should be able to find it either in the owner's manual, or on the inside of the unit (usually inside the access panel). – Tester101 Mar 20 '15 at 0:42
  • i'll see what i can find. If there was a short from R to G then unplugging the low speed terminal would not have prevented the fan from coming on correct? Note: I still have separate fan-only control if /when desired -- it's just that after my change when heat is off the fan is also off – gillyspy Mar 20 '15 at 4:50

There is often a "fan" switch on the thermostat, which can be set either to "auto" or to "on" - the "on" setting runs the fan all the time, which can be helpful with distributing heat (or cold) in houses that otherwise tend to be very cold downstairs and very hot upstairs due to natural convection currents, especially if the furnace/AC does not run for quite a while.

It can also be helpful in running more air through the filters. It does use more electricity running the fan - how much depends on the fan.

  • you didn't really answer the question -- more of a comment i would think. the fan setting was on "auto". Also removing the thermostat completely and the fan was still on. Also running the fan test on the NEST and it never shuts off I use vent covers in the winter on the upper floors to force more hot air through the vents on the lower floors. The NEST also has it's own fan schedule feature separate from heating/cooling which allows me to run the fan when and how much i want. – gillyspy Mar 20 '15 at 0:03
  • And I quote: "Why would I want the fan to be on all the time like that when there is no heating nor cooling?" – Ecnerwal Mar 20 '15 at 1:10
  • for anyone else that reads each sentence in a bubble... the question should then be -- "why would anyone want the fan to be wired like that (and thus forced on) no matter how the system is configured?". better? – gillyspy Mar 20 '15 at 4:46

Put nest in last December. Yesterday the fan turned on and stayed on. FIXED. Read the support article on Common wire. I had installed it with the 4 wires from existing thermostat but since we live in AZ we haven't run the system much in April. Turns out I needed the common wire to keep the Nest battery charged. Luckily the cable had unused wires. Connect one end at the ac C terminal and other end to the Nest. Easy fix.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.