I have the same problem as :
https://www.askmehelpdesk.com/heatin...tat-14250.html

i.e..
The thermostat has the following terminal designations: (RH, RC, G, Y, W)
I have the following colored wires: Red, Blue, Green, Yellow, White.

I tried R --> Rh, B--> Rc and other 3 colors match. It didn't work. Fan doesn't come in Auto mode and thus heating not occurring. I can hear the heating relay clicking when the temperature dial is rotated.

The following answer to the above Question (in the above link) is unclear to me.
[If after you remove the blue wire, the fan still doesn't work, check the limit switch that keeps it from coming on until the furnace heats up.]

Remove Blue wire ? From where ?
Limit Switch ? Which limit switch ?

Any other suggestions?

The post you referenced says he jumped Rh and Rc and call it R Your post indicates you didn't do that.

Hi, Thanks for answering.

I tried the following:
Red --> Rh, Blue --> Rc
Red --> Rc, Blue --> Rh
Red --> Rh, Jumper to Rc, Blue (taped)
Red --> Rh, Jumper to Rc, Blue Rc -

...

G --> G
W --> W
Y --> Y

But any combination for Red-Blue-Jumper didn't work for me as expected. In each case different scenario, but A/C, Heater, Automatic Fan do not work as expected.
Earlier with the old thermostat, it worked.

The red to RC and RH, blue taped should have worked. It is possible you have blown the fuse or burnt out the transformer. There should be a small blade type fuse on the circuit board the wires connect to at the furnace. IF you need more details, check the sticky at the top of the forum.

Hi Labman,

I don't think the fuse is blown off because, I can get A/C , Heat and Fan to work separately (AC/Fan or Heat/Fan)

In your previous post u mentioned about "Limit Switch". What is that?

What I found is, if Blue is taped, nothing works ( No Fan in Auto or Fan On modes ).
But if I connect B to Rc, R to Rh
Fan is ON
-- auto - A/C,
-- Fan ON
But Heating Part NOT working.

Now I'm in the office and I'll give u all the details, (ie. For each connection, what works & what doesn't), once I get back home.

But before anything else, I'll check one more time (not sure whether I overlooked) your suggestion.

Thank you sir for your valuable time.

I don't understand this at all:

I don't think the fuse is blown off because, I can get A/C , Heat and Fan to work separately (AC/Fan or Heat/Fan)

A few things to be aware of:

It may be easy to pop the fuse for the outdoor unit unless the AC has an anti-short cycle timer in it, but the stat usually implements that. An anti-short-cycle timer means that it may take up to 5 minues before the AC outside to come on after re-applying power. The indoor fan will come on right away.

R to G - makes the fan continuously run
R to W -Call for cool, fan runs too
R to Y - Call for heat, fan runs too

Put the old stat in and check again, or wire the above temporarily. Check downstairs in the furnace and see where they go. Hopefully someone didn't change the colors along the way.

There is something funny going on here. It could be wires switched at the furnace. I think your next step is to go to the sticky at the top of the forum and read through thermostat wire colors and the sequence of operations of a gas furnace. The furnace fan uses a different relay, wires, and motor winding as the A/C fan. Same fan and motor, but a completely different electrical circuit. I am not sure why I mentioned the limit switch. Very few modern furnaces use them, relying on a timer on the board to let the furnace heat up before the fan starts. Make sure all the wires are connected correctly at both ends. Then go through the sequence of operations and see where the fan fails.

I just noticed the wire colors are wrong in the above post, W is heat, and Y is the A/C compressor. The fan only runs red to green or the timer or limit switch.

Hi Labman,

What is "sticky at the top of the forum" ? I couldn't check the inside wiring.

Here is the full list of what I did & what I got.
I assume current wiring scenario is correct, since it was connected to the old thermostat with exactly same color scheme and it did worked. I replaced it as it started malfunctioning in its own (not because of wiring).

G --> G
W --> W
Y --> Y

Scenario 1:
R --> Rh, Rc & Blue Taped
A/C :
Works / Fan comes in Auto mode, Fan ON when put to ON

Heating:
Can hear the relay clicking sound when the temperature dial is rotated to higher side,
Fan not getting ON in Auto mode.
Fan can be put ON (manually)
A burning smell comes from the air outlets, when Fan is put in Heating mode.

Scenario2:
R --> Rc & Blue Taped
Cooling operations OK
Heating No Clicking sound, No Auto Fan, No Heat

Scenario3:
R --> Rc
B --> Rh
Cooling OK
Heating Not Working (No Click, No Auto Fan, No Heat)

Scenario4:
R --> Rh
B --> Rc
Cooling NOT OK
Heating : (Click, No Auto Fan, No Fan ON)

Scenario1, which is your suggestion is almost OK, except for 2 things.
Those are:
In heating, the Fan doesn't come up Automatically
In heating, a burning smell ( sort of high heating of a metal) comes out.

Please advice me.

For reasons I don't understand, sometimes installers use separate transformers for heating and cooling. In which case the heating transformer needs to to be connected to RH and the cooling one to RC. With only one transformer, power needs to be connected to both RH and RC.

The thermostat switches RH to W when it calls for heat. It switches RC to Y and G when it calls for cooling. It switches RG to G for fan on.

Once the thermostat switches power to W, the controls in the furnace switch power to the relay coil for the low speed motor winding. If the fan doesn't come on shortly after the gas, as I said, you have a problem that circuit. If it smells hot, it could be the motor, or the furnace overheating before the high limit kicks off the gas.

The sticky is at https://www.askmehelpdesk.com/heatin...ons-58313.html
It sticks to the top of the H&C forum.

Hi Labman,

I read that info about color codes. I opened, whatever possible wiring chambers, but I couldn't see, much of the things matching the details given. There is no way for me to find, which wire controls cooler, fan, heating etc.

So let me know, if there is a way to determine the “should-be-color” of the wires.
e.g.
Red --> 24V AC or DC ? And with it is respect to which wire?
When Red Wire is connected to Yellow --> A/C should start?
When Red Wire is connected to Green --> Fan should start?
Also let me know what should not be done, to avoid a blowing off the equipment.