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!


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




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




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 (https://www.saveonenergy.ca/Consumer/Programs/PeaksaverPlus.aspx?gclid=CPqi3Znxs7cCFe07MgodNVYA5 Q).
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.

It took me a while to get back to this subject.
My t-stat has a battery and it has Rh and Rc which are connected together.

At the t-stat
Black - RC
Red - RH
- RC and RH are connected together
Green - G
Orange - Y
White - W

At the furnace
Black and Red - R
White - W
Orange - Y
Green - G

It's a really hot day today so I'm hoping to somehow just get the fan going manually with the AC on.
BTW, thank you for all your help

I can't really say that.

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.


I should say that when I first tried to turn the AC this year, the fan stopped working and a technician replaced the transformer. This happened a couple of weeks ago.

I made sure the connections were tight and replaced G with R at the t-stat and replaced the G with R at the furnace as you suggested, I also removed the black wire from both sides as it seemed redundant

Turned AC on, fan on.
Fan is on, but AC is not on right away,
Two minutes later AC outside turns on and fan goes off.
I'm back at square one.

The transformer died probably because a wire is rubbing against ground someplace. Just a thought anyway.

Quote
1. Turned AC on, fan on.
2. fan is on, but AC is not on right away,
3. two minutes later AC outside turns on and fan goes off.
Unquote

Somehow, you may be talking about two different fans.
The inside fan (furnace/evaporator fan) and the outside fan (condenser fan)

If #1 Fan is the inside fan, that's OK.
#2 is OK
#3 A is OK, the fact that the outside unit turns on 2 minutes later.

The fact that it turns OFF is likely due to the AC unit kicking out because of "something"
Suggestions are
1. High pressure
2. Low pressure
3. Outside fan isn't running

The "2 minute" delay is usually built into the thermostat to prevent the compressor from coming on under load.

I can't quite figure out the pieces, because "fan" can mean multiple things. Outside or Inside.

The fact that the transformer died suggests another problem such as an intermittent short in the tstat wires heading to the outside unit.

Now, you could cut the AC power to the outside unit at the disconnect and watch/listen for the contactor to click after the 2 min wait. If the contactor stays in, you probably have an AC related issue. If it does not, then you may have a t-stat/wiring issue.

In most cases the outside AC contactor gets power from the furnace.



I'm back at square one.

The transformer died probably because a wire is rubbing against ground someplace. Just a thought anyway.
That is probably the case because I did notice the black wire that connects to the Rc or Rh was loose at the furnace



Somehow, you may be talking about two different fans.
The inside fan (furnace/evaporator fan) and the outside fan (condenser fan)


When referred to the fan, I've been referring to the Furnace fan circulating the air in the house. (inside fan)
When I referred to the AC turning on, it's the outside AC fan and compressor as I can hear them both.

I will try your other suggestion tomorrow.
Again thanks for your help.

This post got misplaced:

Do this also:

With the t-stat turned OFF which means
FAN --> Auto
Mode; Cool-OFF-Heat (turn to OFF)

At the furnace connect R to Y to G. Before you make the connections, turn the power to the furnace off.

If all goes to plan, the AC would turn on, but never turn off. The inside fan will run too.

---

Aside:

Something to consider, since you have the right number of wires: Make the Rc/Rh connection at the Stat and also bring Common to the thermostat.

Depending on the age of the furnace, Common may not be labeled, but if your tstat supports common, I would use it.

Common is simply the other wire of the 24 V secondary that isn't connected to R.

---

Do this also:

With the t-stat turned OFF which means
FAN --> Auto
Mode; Cool-OFF-Heat (turn to OFF)

At the furnace connect R to Y to G. Before you make the connections, turn the power to the furnace off.

If all goes to plan, the AC would turn on, but never turn off. The inside fan will run too.


Do I use another wire to connect R to Y to G? Or do I connect the R to the Y and the Y to the G and R remains unconnected?




Something to consider, since you have the right number of wires: Make the Rc/Rh connection at the Stat and also bring Common to the thermostat.

Depending on the age of the furnace, Common may not be labeled, but if your tstat supports common, I would use it.

Common is simply the other wire of the 24 V secondary that isn't connected to R.

---

So right now the R from the furnace is connected to Rc and Rh. The Black wire is available for use.
I do see an un labeled spot at the furnace which is probably the Common, and I can connect the black wire to it.

At the Stat, I don't believe I have the option for Common.
These are my options.
H2, H1, B, O, RC, RH, G, Y, W.

What would I be connecting the Black (Common) on my Stat?

OK, you don't have common on the stat. On the surface, you have a heat pump thermostat with humindity control. See: http://www.epatest.com/store/resources/images/misc/how-a-thermostat-operates.pdf

The giveaways are the B and O terminals. One of these goes to a reversing valve, that changes the system from heat to cool. B and O are complements of each other. Which one is used depends on whether the reversing valve is unpowered in heat or powered in heat.

H1 and H2 are humidistat contacts.

Rc and Rh we discussed.

G is the Fan

Y is the compressor. In a heat pump, the Y connection is used ALL of the time. It turns the outside unit on.

What confuses me "a little" is that I would normally expect W to be labeled W2. For aux heat.
Aux heat is usually a resistance heater in the furnace.

The stat could still have configuration options for conventional or heat pump. We need to nail what type of system you have? 1) Conventional or 2) Heat pump

With a heat pump, the outside unit comes on when your heating or cooling unless the outside temperature is too low. In a heat pump, there is a valve that reverses the pluming connections to the compressor.

SO, there is a CHANCE that the thermostat can do conventional or heat pump by configuration.

==

So, let's ignore the R to Y and R to G for now although I will try to describe what I meant. Leave the wires as they are connected at the furnace and have the thermostat off. It doesn't matter if you use two wires or one, just as long as R,G and Y are connected together and you do the connections with the furnace disconnect off.

If this is a heat pump, you may end up with heat or cooling. Usually heat. The idea is if a reversing valve fails, you want the system to be stuck in heat, not cool mode, but that is dependent on the heat pump operation.

So,
1. is outside unit a heat pump?
2. Is the thermostat the correct type or can it be configured?

So I bypassed the stat and ran the tests you suggested.

I can get the inside fan to go on, the heater to go on, I can get the AC outside fan and compressor to go on, But when I connect the AC with the instide Fan, the fan just does not want to turn on..

This in my opinion eliminates the possibility that the stat is at fault. I took a picture of the motherboard and it looks burned out.

I am now looking for a "ST9160B 1050" motherboard, but all the parts I've seen online don't match the one that I have. Any suggestions on where I can purchase the correct one? If I were to purchase a "ST9160B 1050" with a green board and a different layout, would it still work? Are there different versions?

You did good.

I tend to agree with about 95% certainty. We have not eliminated a short within the t-stat wiring or the wiring to the AC unit.

I am trying to think of other scenereos that may cause your issue. Fresh batteries in the stat? I would not expect this to be the case, but if the AC system had it's own transformer and it was connected like you have it, it MAY be possible for the 24 VAC to drop to zero. Two transformers in parallel.

We never did determine if the stat was a heat pump/regular stat. The furnace definitely looks conventional.

So, I would suggest to remove and label all of the wiring going to the thermostat at the furnace (that will leave two wires going to the AC outside) and repeat the R to Y and R to G test.

Another odd ball suggestion is if there are any EXTRA wires in the AC unit, change briefly to another two running outside. Really doubt a problem exists here.

I'm trying to 100% eliminate everything else. I don't know if you own a multimeter?

Aside:

Note the automotive type fuse in the upper left hand corner. This is the fuse to the thermostat 24 VAC secondary.

Note the yellow an the Blue wire. Blue "probably" connects to one end of the fuse and then to R. Verify!

That would make the Yellow fat wire, the Common terminal on the stat. For future reference a Piggyback spade connector would be a way to add a common terminal should you decide to replace the stat.

The furnace boards DO NOT have to look the same. Some actually come with various harnesses to allow them to work with other models. This is just one example listing: Honeywell Fan Control Board ST9160B1050 ST9160B 1050 | eBay (http://www.ebay.com/itm/Honeywell-Fan-Control-Board-ST9160B1050-ST9160B-1050-/370452834737)

TIP:
If you do not have a hole house surge suppressor, I would consider investing in a wired in suppressor just for the furnace. Most furnaces have nothing in there for protection against spikes and RFI.

If you can't afford that, I would add a TVS on the 24 V secondary side. For about $5.00.

I HAD to add a $80.00 RFI filter on a carrier furnace because it was interfering with my power line automation controls.

I do live in an area where power problems are very rare including outages. The two important things, the computer network and the phone/answering machine and a PERS alarm are on a UPS. One repeater on my network is not. That power supply got knocked out at the same time a neighbor's oven controller was knocked out.

The importance of a commercial product is not the cost, but the warranty, Some have a $50,000 to $100,000 insurance policy on connected equipment and offer a lifetime warranty.

I am not 100% convinced. Maybe 95%, but not 100. Why, one side of the 24 VAC is usually grounded.

One commercial AC guy actually ordered a ceiling heat pump because he thought it was bad. When all of the effort to install the HP including overtime, it still would not work.
He asked me for some help. In about 10 minutes, working together, I said you have a bad t-stat wire and left him. Turned out the wire had shorted at a metal stud during a remodel.

In another instance, the installer didn't pay attention to the way the outside unit was wired. Two wires were paralleled and he paralleled the wrong ones. To make matters even worse for trouble shooting, the installed spliced the wires about 2 feet from the AC unit and then used different colors. The splice was crappy too without a drip leg. Using the wrong colors was definitely bad. It was a really tough problem to find.

So here is my solution to my furnace problem.
I ordered a universal replacement Control Board (ST912ou1011) from HVAC Parts Outlet which is meant to replace my OEM board.
I connected it according to the diagrams but it would not work, the blower fan would turn on even if the stat is unplugged. When I would turn the AC on from the stat, it would not turn on. I had my brother come in and test all the connections and follow the diagram to make sure I'm not missing anything and I wasn't.
On the diagram for the board, it showed that M1 and M2 were not being used and that I should have a cable for Data and Cont.
When I saw that the forecast for this week was going to be in the high 30s and low 40s I decided to call a professional. I called the same company that replaced my transformer over a month ago and the same person showed up.
When he came in, he spent about 30 minutes diagnosing the connections I've made, then proceeded to tell me that these universal boards don't always work and that I need an OEM board.
It took him over an hour to go pick up the new board which to my surprise was an identical board, same universal model. He told me that this is the only one as a replacement. When he plugged it in the same way as I had it, everything worked fine.

I'm happy that I finally have the AC working but I'm quite upset that I had to spend $340 on the technician when I could have resolved my own issue if the board just worked. I also have my doubts that I'll be getting my money back on the defective board.