[JKPRODUCTIONS]

I am switching from Honeywell CT87a (subbase Q539A4034) to Honeywell RTH7400 seems simple enough but I am bad when it comes to this sort of stuff. Anyway. I have Red, White, Green, Blue wiring coming out of the wall and have a electric air/heat system.

Currently wired to 7400 as
R-R
W-Y
B-W
G-G

I would have thought W-W and B-Y but those don't work either.

I looked at the old manual and the new manual and have tried variations of the connection devices and still cannot even turn anything on.

Someone please help.

[hvac1000]

What kind of system is this on. Heat pump? Gas furnace? Got to know before wiring discussion.

[NorthernHeat]

https://www.askmehelpdesk.com/heatin...tml#post268211
Need to also check the furnace and see what terminals are connected to what color wires.

[JackT]

Does the new Honeywell thermostat have a "C" terminal? Many programmable thermostats require 24 volts or a battery to maintain voltage to the thermostat. I can't recall what the 7400 requires. If it has a "C" terminal you will most likely need a five wire thermostat cable.

[JKPRODUCTIONS]

I am using an electric heat system (both a/c and heat are electric)

I went to look at the transformer in the electric furnace system (not sure if that is what it is called). The wires are labeled as follows:
W1-White
G-G
R-R
B-C

I currently have wires in thermostat as follows:

W1-W
G-G
R-R
B-Y

Please let me know if you have any ideas.
THANKS!

[JackT]

There should be two wires going out to your outside air conditioning condensing unit from your air handler. One wire should be connected to "C" off the transformer and the other wire should be connected to the wire that goes up to your thermostat to the Y terminal. You should also have a red wire that comes off your transformer and connects to R on your thermostat. You also need a wire from the G terminal on your thermostat and connected to G on your air handler. You'll need to read the instructions to see if you need a wire to run from the C terminal on your air handler up to the C terminal on your thermostat. Did you install the battery when you tried the new thermostat? What does the new thermostat show or not do? Good luck

[hvac1000]

i am using an electric heat system (both a/c and heat are electric)

I went to look at the transformer in the electric furnace system (not sure if that is what it is called). The wires are labled as follows:
W1-White
G-G
R-R
B-C

i currently have wires in thermostat as follows:

W1-W
G-G
R-R
B-Y

please let me know if you have any ideas.
THANKS!

Here is your install manual

http://customer.honeywell.com/techli.../69-1900ES.pdf

Looking at the manual I believe you will need a 5 wire setup as was mentioned by another poster. You will need a C wire for it to operate properly.

The only thing you can try is putting the B wire on the C terminal on the new thermostat. But I still think you need a 5th wire to activate the A/C unit outside. NOTE: leave the R and RH jumper in place. Changing to a new style thermostat is not easy sometimes. Good luck.

[JKPRODUCTIONS]

Here is the pic of the actual a/c heat unit.


LEFT SIDE
On the left side red wire is hooked up going into box 1 @bottom
On top blue and green are going into box 2 @ top


RIGHT SIde
On right side wiring is coming out of box and tied to wiring going to actual tstat
Also at top white wire is going in box #3


Here is a pic of wiring on tstat as is now


After I spoke with both homedepot and Lowe's they both informed me that it should be a universal tstat and should some how hook up. My guess is one of two things.
I am not getting power to transformer:
I am not getting wiring correct at tstat:
I am continuing to try variations of different wiring combos but no luck.
THANKS for any help.

[hvac1000]

Wow what a rats nest. I have seen better wiring in a mobile home. Good luck.

[JackT]

What's under the wire nut? If it's a spare or unused wire, connect it to the C terminal at both the thermostat end and in the air handler. It also appears that you have a total of 15 kw of heaters making believe you have just a air conditioning unit and not a heat pump. If this is the case you don't need a wire to the B/O terminal. Remove it and use that wire to connect the C terminals.

[T-Top]

Take a wire and jump from R to G the blower should run, jump from R to Y next the out side A/C should run. If those parts work when jumped. The stat is wired wrong. You do need the common at the stat to make it work. With the pic you gave its hard to see witch is common and witch is W- heat.