I have an odd problem that I can't seem to find an answer for.

I attempted to turn on the air conditioning tonight since it was hot and humid. The AC unit outside turned on okay and the fan started to spin. I expected the furnace blower fan to start shortly thereafter but it never kicked in even if I set the fan mode to be "on." What's odd, though, is that if I shift the thermostat selector to Off (or even Heat---but without the heating relay engaged) the fan can turn on (again, in the "on" position).

I've removed the thermostat and checked with a multimeter that the G and RH/RC pins get shorted when the fan is switched to "on" and that the Y and RH/RC pins are shorted when the cooling relay kicks in so I'm suspected there's something wrong with the furnace circuit somewhere?

Any ideas what might be wrong?

Thanks!

In the AC mode, the thermostat usually controls the fan.

As a test. Cut power to the AC unit (outside disconnect or breaker)

Short Rh/Rc to G and Y with the tstat in cool mode. Turn on power and see if the fan and AC turn on.

This bypasses the delay which should occur with modern thermostats, so make sure the AC is off for about 5 minutes before switching it back on again.

Another test to try is to measure the voltage ACROSS Rc/Rh and G and Rc/Rh and Y when the fan is supposed to be on. Voltage should be near zero.

And a reminder that usually Rc is connected to G when there is a call for fan in AC or fan mode.

I disconnected the breaker, and shorted rh/rc to G and Y as you've recommended and it results in the same symptoms.

The fan will run for a second or two and then turn off.

Thanks!

Quote:

---

Originally Posted by KeepItSimpleStupid

In the AC mode, the thermostat usually controls the fan.

As a test. Cut power to the AC unit (outside disconnect or breaker)

Short Rh/Rc to G and Y with the tstat in cool mode. Turn on power and see if the fan and AC turn on.

This bypasses the delay which should occur with modern thermostats, so make sure the AC is off for about 5 minutes before switching it back on again.

Another test to try is to measure the voltage ACROSS Rc/Rh and G and Rc/Rh and Y when the fan is supposed to be on. Voltage should be near zero.

And a reminder that usually Rc is connected to G when there is a call for fan in AC or fan mode.

---

Are we having problems with the definition of fan?

Blower - inside blower also known as a fan
Compressor - outside
Condenser fan - blows air over outside unit.

The outside unit (fan and compressor) usuallly comes on together. If the low or high pressure switch were activated, then the outside unit would turn off.

What complicaes the behavior of the inside blower (fan), is that in heat mode, the furnace controls the fan. There can be a delay mode or a temperature mode where the furnace ignites and a short time later, the blower runs. When shutting down, the furnace shuts down and sometime later the blower shuts down. The time delays can be done with temperature.

The heat and cool mode select whether the furnace controls the fan or the tstat.

Correct... the fan that turns off is the one in the furnace (the blower fan). The condenser fan on the outside unit turns on and, as far as I can tell, so does the compressor.

I tripped the breaker for the AC to cut power from it for about 30 minutes and then fired it back up again. First, I had the thermostat set on Off with Fan On. The fan runs fine. As soon as I switch to Cool and set the target temperature below the ambient, the thermostat relay clicks and then about 2 seconds later, the blower fan of the furnace shuts off.

It's odd.

Thanks,
Charlton

Quote:

---

Originally Posted by KeepItSimpleStupid

Are we having problems with the definition of fan?

Blower - inside blower also known as a fan
Compressor - outside
Condenser fan - blows air over outside unit.

The outside unit (fan and compressor) usuallly comes on together. If the low or high pressure switch were activated, then the outside unit would turn off.

What complicaes the behavior of the inside blower (fan), is that in heat mode, the furnace controls the fan. There can be a delay mode or a temperature mode where the furnace ignites and a short time later, the blower runs. When shutting down, the furnace shuts down and sometime later the blower shuts down. The time delays can be done with temperature.

The heat and cool mode select whether or not the furnace controls the fan or the tstat.

---

Now you have me thinking again. If you waited for 3-5 minutes, will the AC/Fan start running again?

Is it possible that G and Y are reversed? Reason why I might be saying that is that there is a delay on break safety that is usually performed by the tstat. This prevents the compressor from starting under high head pressures until a few minutes have elapsed.

It would kind of, I think, do as you suggest. Cool set point satisfied, the inside blower would go off and will not start again for about 5 minutes instead of the outside unit going off and it being unable to restart until about 5 minutes have passed.

The delay on break timer is supposed to work like:

Tstat powered up like from a power fail, outside unit is not allowed on until about 5 minutes have passed.

Once a call for cool is ended, the compressor stays off for 5 minutes or so before it's allowed to start.

What if the G and Y were reversed? It won't make much difference in heat mode because the thermostat controls the fan.

05/24/2010 SAME PROBLEM HERE!
We just came across the Same problem!

A serviceman replaced our main board last winter because the heat quit.
We wonder if he didn't connect the A/C correctly (if it's possible).
The A/C worked last year, (before the board). Nothing else has changed.

I plan on buying a new thermostat today, to see if that fixes it.

Could you please let us know how you solved your fan problem?
Blower fan turns off in AC mode, but works fine otherwise!

-jim

Quote:

---

Originally Posted by larrivee

I have an odd problem that I can't seem to find an answer for.

I attempted to turn on the air conditioning tonight since it was hot and humid. The AC unit outside turned on okay and the fan started to spin. I expected the furnace blower fan to start shortly thereafter but it never kicked in even if I set the fan mode to be "on." What's odd, though, is that if I shift the thermostat selector to Off (or even Heat---but without the heating relay engaged) the fan can turn on (again, in the "on" position).

I've removed the thermostat and checked with a multimeter that the G and RH/RC pins get shorted when the fan is switched to "on" and that the Y and RH/RC pins are shorted when the cooling relay kicks in so I'm suspected there's something wrong with the furnace circuit somewhere?

Any ideas what might be wrong?

Thanks!

---

Jim:

Does the fan work when you put the fan to ON in the thermostat?

Is this a heat pump or a gas, oil or electric furnace?

Yes, the fan works when we put the fan to ON in the thermostat.
... As long as the mode switch is Not set to COOL.

The fan (inside blower) does not work wihen the A/C is switched on.

If the A/C is switched OFF, the fan runs fine with no problems.
The Fan works fine when in the Heat mode, ON or AUTO.

(1a) With the mode switch turned to OFF (Not in Heat or Cool mode),
We turned the Fan switch to ON. The inside fan started, no problem.

(1b) When we turned the mode switch to Cool, the condenser fan & compressor outside started,
But then the inside blower fan stopped (even though it WAS running in step 1a).

(2) When we put the Fan switch to AUTO, and the mode switch to COOL ,
The outside condenser fan starts and the compressor starts.
However the inside blower fan never started.
We let the condenser unit run for a long time, the lines sweated and frosted a bit,
You could hear the comppressor running, the condenser fan was running strong.
The insdie Fan would not start (Fan switch ON or AUTO).

Basically, the inside blower Fan will not work when in Cool mode regardless if the Fan switch is ON or AUTO.

The inside Fan works fine in Heat mode, ON or AUTO.
And the inside Fan works fine manually while the Heat/Cool mode is OFF.

I believe the furnace and A/C are Trane. (No relics, great shape. Up to date).

SO:
We replaced the thermostat yesterday, that didn't solve the problem.
Still have the same problem.
Inside Blower Fan will not run when in A/C mode.

I'm having the same issues to the letter I sure hope someone can figure this out I'm going to bookmark this page I sure hope a crackerjack is reading these post lol

Has this question been answered? We are having the same problem.

Tried everything new thermostat and new contactor everything works fine until we set fan to auto and turn on ac

I know this is an old thread, but was anyone able to find a solution to this? We're having the same problem. It would be fine if I could figure out how to manually turn the fan on until we can have someone take a look at it. But I can't figure out how to do that. Some of the older units had a fan/limit switch that had a button to manually turn on the blower and override the thermostat settings, but our current unit doesn't seem to have this. Can anyone help?

I am having the same problem... any solutions? Also, my oil furnace and tstat are new. Please help.

I'm having same problem and my furnace and tstat are new... please help.

I just had the same problem and found my issue and now it is fixed. We had a new circuit board installed and when the technical person replaced the board, they hooked up the m1 connection from the old board to the eac on the new board. On our new board it needed to be added to the park slot on the control board, now it is working fine!

If you do not have an air cleaner on your furnace, simply pull the eac connection and apply it to the park slot!

These are very close on the old board and if I did not have a puicture I would have never found it.

I'm having the same issue as all of you. Has this solution worked for everyone? I will have to research first what your saying exactly :) and then I'll post my results.

In AC mode, the thermostat controls the FAN. In heating, the Furnace controls the fan.

Heating may allow the thermostat to override the FAN to ON.

Thanks Kiss, so is your suggestion to just replace the thermostat? I'm getting my thermostat replaced by my power company /PeaksaverPlus.
The thing is that I need everything to be in working condition for them to replace it.

I can't really say that.

In a typical non-heat-pump, single stage t-stat, there are 4 to 5 wires.
R, W, Y, G; usually Red, White, Yellow and Green

Connect R to W and the heat cones on. The furnace will actually will turn on and off the fan based on a delay or plenum temperature.

Connect R to Y and the AC outdoor unit comes on.

Connect R to G and the Fan in the house comes on.

C or Common is used by newer thermostats so that the thermostat can receive power all of the time. This wasn't necessary with a mechanical thermostat. Almost all newer stats will use the C terminal unless there is a battery in the stat. There are ways a contractor can make 4 wires do the work of 4 without running a new wire.

Now, I hate that your link doesn't tell you what they do. I suspect.
1. They put a radio xmitter/receiver on the AC unit outside, so they can turn it off.
2. Put a relay/xmitter/reciever on the water heater so they can turn it off.
3. Integrate both to the ZIgbee xmitter that would be in your electric meter.

Your tests could be to connect R to G at the thermostat and then R to G at the furnace and see if the fan comes on. A loose or broken wire at a connection is a usual cause. So is rubbing by vibration.

A few other comments: New stats may have Rc and Rh instead of R. Rc & Rh must be connected together at the stat if your furnace just has R.

The thermostat uses 24 VAC. R & C MUST NEVER be connected together. This is the 24 VAC source. This will either blow the transformer or a small fuse on the furnace board.