Ecobee blower issue?

dmachek

I have an 02 Lennox gas furnace 2 stage. I recently installed a ecobee tstat. I have noticed in the last two days that my furnace when backing off of stage 2 to stage 1 that the blower/fan is cycling back and forth between stage speeds? Then finally going to the lower speed and shutting down. Is this a control board issue or is my tstat not working right? Had a tech out this weekend and after an hour of him looking around he seems to be puzzled and now wants to replace the board. Looking for insight. Could the tstat cause this? Or is it the board or something else.

ratio

That's kind of a strange problem if it just started happening when you changed the stat. I'm inclined to blame the stat just because it's (as far as we know) the only thing that changed. Any chance you can put a multimeter on the stat terminals in the furnace & see of the Y2 call is coming in and out when the fan changes gear, that would confirm the stat as the cause. Does the furnace & stat have multiple fan stages (G/G1, G2) wired as well as cooling (Y/Y1, Y2)?. The other thing that comes to mind is if the fan speed is controlled by temperature, it (the temperature) might be bouncing around enough to change the speed of the fan. But that doesn't explain why it started with the new stat.

What is the timing like? quickly, bounce in & out of high for 10-30 seconds over the course of a minute or two, or is it a few minutes low, less than a minute high, few minutes on low again; or vice versa? Describe a typical cycle, with a reasonable guess at the timing.

dmachek

The wires connected to tstat is rc, c, g, y1, w. The tstat is set so the HVAC controls fan speed. Tstat is set to 67deg and when house temp falls below the temp it calls for heat the furnace does its normal start up. Blower starts In stage 1 low speed then after a few minutes the blower goes to stage 2 blower is going just fine. But once the temp hits the program temp the furnace starts to shut down blower then goes back and forth with speed. It does that for about 30-40 seconds then you hear blower going slower for another 2-3 mins then shuts down completely.

ratio

Huh. IDK why I was thinking cooling, sorry.

If you don't have a W2 wire landed, you're not controlling it as a two stage furnace. Was W2 landed with the previous stat? Some 2 stage furnaces can be configured to operate on a single-stage stat by switching to high fire via a timer on the control board.

dmachek

No the w2 want connected to the old tstat

ratio

It could be the control board, but without testing things I'm guessing. It could be the motor itself. Is a relay clicking when the motor is changing speeds?

Does that furnace have a diagnostic LED on the control board somewhere? They might provide some insight.

When you hooked up the new stat, were there any spare wires? I think you'd gain better control, if not efficiency, by letting the thermostat control the second stage. What's the model number of the furnace?

John Mills_5

If W2 isn't being used and the timer in the board is controlling staging, you won't be backing off 2nd stage until the thermostat is satisfied and then the furnace will shut off, not drop to low fire.

SuperJ

The first thing I would do is try disabling the dipswitch that allows the furnace to control 2nd stage. It's usually just a timer. The Ecobee can be smarter about it and base it on space temperature error as well as time. The ecobee has several staging options to tune things nicely.

Once you've set to tstat control of W2, Run the furnace, it should just stay on stage 1 forever (or until the heat call is satisfied.
I would then reconfigure your Ecobee to use temporarily use the y1 (you will have to move it to a different terminal at the stat and furnace) wire for second stage heat (w2).

Unlikely, but if the furnace has some sort more advanced internal staging algorithm (possibly based on RAT) maybe you have some return air that is short cycled supply or something leading to erratic RAT readings causing it to stage between 1 and 2.

If the furnace is under warranty it would make sense to check out the control board completely, if you're only using W1 at this point I doubt it's the Ecobee's fault.

Another thing to check would be to remove the G wire. The furnace should run the fan on a heat call even without a G wire. Maybe the Ecobee is doing something with the fan during a heat call? But then again the furnace should basically ignore what's happening on the G wire during a heat call.

There are usually some dipswitch settings to control fan ramping behavior at the start and end of call. Some units even have a false (fan only) second stage where all they do it move more air but don't produce any more heating/cooling.