My furnace fan won't run in the fan on position on the thermostat. I don't know the model number but my furnace is a Trane high efficiency with variable speed DC motor about five or six years old. It seems to be working OK in the auto position. It's keeping the house warm but I have an HRV and need to be able to keep the fan running while the HRV is. I suspect it's the thermostat but when I when I disconnect the red wire from it, turn the heat off, and put a circuit tester from the R terminal to the G terminal of the thermostat, it looks like the switch is working (i.e. The switch shows near zero resistance when switched to the fan on setting). Any suggestions would be greatly appreciated. I don't want to drop the cash on a new thermostat if its likely to be something wrong with the furnace.

Do you have a wire (most likely green) hooked between the g terminal on the stat and the g terminal on the furnace control board? If not, you need one for the fan on option of the stat to work.

Hi, Yes, the green wire is connected. The furnace has been running fine for years. The fan function not working just started a few days ago.

Okay. It could be the thermostat, or the fan relay, which is most likeky part of the control board, so if its bad, you would need a new board. One way to tell is to check for 24v between the g and c terminals on the control board. Of course the stat would have to be set to fan on, and the blower door switch would have to be held in so you have power to the furnace. If you have 24v between the g and c on the control board that means there is a call for fan to run. Next you would have to check power to the motor lead that is being used for fan on operation. Should have 120v there. Now, if you don't have 24v across g and c on the board, go up and jump r to g on the stat. If still nothing, you most likely have a bad stat wire. If when you jump r to g, the blower runs, a new stat it is. Let us know what you find. There are a few option here, and its never fun to just replace part only to find that they were not the problem. Good luck.

Thanks for your suggestions. I have finally tried all the tests you described above and it seems like it's the thermostat. I am still confused about why it appears to work when I disconnect it and test the conductivity across the fan switch. The resistance drops to zero across the red and green terminals when I switch the switch to fan. Am I missing something simple? Should the thermostat have some small resistance across those terminals when the fan switch is on? Is there any possibility that either the furnace or thermostat sends a digital signal through this circuit?

Thanks again

Thanks for your suggestions. I have finally tried all the tests you described above and it seems like it's the thermostat. I am still confused about why it appears to work when I disconnect it and test the conductivity across the fan switch. The resistance drops to zero across the red and green terminals when I switch the switch to fan. Am I missing something simple? Should the thermostat have some small resistance across those terminals when the fan switch is on? Is there any possibility that either the furnace or thermostat sends a digital signal through this circuit?

Thanks again

When you switch the fan to on, the r jumps to g and sends 24v to the furnace fan relay. If the stat isn't doing this, then you need a new stat. Did you jump r to g at the stat and the fan runs? If so, you need a new stat. They do go bad more often than you might think.

In heat mode The furnace control board will time on and off the blower. No need for a G call at the thermostat for heat. The question is how is your ERV controlled? The reason I ask is because a field installed control relay may have been added and breaking the blower circuit in heating. I don't know why it would but I have seen stranger things.

I have a brand new lp furnace and I am also having trouble with the "fan only" setting. I have tried 2 new thermostats and used different wiring for the "G" terminals and I still can't get just the fan only to run. The blower works fine when it's on the heat setting. I assume the heat/cool switch should be off when attempting to run just the fan. I also used a jumper wire at the stat and the blower kicked on. Same result at the furnace.

If you can jump r and g at the stat and the fan kicks on, then for whatever reason, the stat isn't doing its job when set to fan on. What kind of stat do you have? May have to look through the setup guide in the install instructions to make the fan on option works (basically program the stat to your equiptment) for your particular stat.

Had the same problem. Turned out it was the circuit board in my 2010 Trane xv95 2 stage DC gas furnace. Replaced the board under warranty and voilà! Everything works now.