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RTH221 honeywell thermostat install, hooked up blue wire from old thermo that hooked to C to o/b connection on new thermostat no work did I screw something up as old thermo will not work as well now. was not supposed to hook blue wire up I found out later on Internet. any easn fix, blown fuse??? etc. Thanks

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maybe a blown fuse if it has one... if not then you blew the transformer...
Check on the circuit board in the furnace or air handler for a fuse...
if there isn't one... then you will need to replace the 24v transformer... they blow very easily...


Posted on Aug 05, 2010


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This appears to be a thermostat installation that was made without regard to wire code. The Honeywell material I have suggests it may be necessary to check connections at the furnace end and work back to the thermostat. There may be compatibility problems between the number of wires in the old installation and terminal connections in the new thermostat. Honeywell may be able to help at 1-800-468-1502.

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First things first. What type of system? Heat only, boiler or furnace? Central cooling.
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Your new thermostat should have the same letters. Sometimes they mix up color wire with terminal letters...... example (yellow wire to w terminal)
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My wife bought a Hunter programmable thermostat to replace an ancient round Honeywell. The Honeywell is simple with the color coded wires simply going to the srews marked those colors. There are 5 wires,...

Normally the common wire is black, but installers can use anything, so you need to make sure that blue came off a "B" labeled screw. The common would normally be labeled "C". You need a simple multi-meter to check, although I have to believe that the instructions with the new thermo would have to cover the hookup and should show some extra connections if truly universal, as it would have to accommodate heat pumps and dual stage systems. With the old thermo mounted on the wall as it was originally with no wires connected, check for continuity between R and each of the other screws. Set thermo for cooling and turn temp down to 60 and see if you get continuity R to Y, set fan to on and see if you get continuity R to G, set thermo to heat and see if you get continuity R to W. If you just have a single wire that runs to the contactor in your condensing unit, and one wire for the heat, then the red is power from the transformer in the furnace, green is for the fan on all the time during cooling, yellow for the AC contactor, and white to bring on the heating cycle, IF ALL are single stage (only one setting for heat, cool, or fan that is either on/off). Check for continuity between R and B on each possible setting. If you get no continuity or greater resistance than on the other screws (when checking R to B), set meter to VAC and see if you get 24 volts between red and blue wires - the wires coming out of the wall and not the thermo screws. If so then you have a simple single stage for both heat and cool system. If you tape off the blue wire so that it can't contact anything in the new thermo then hook up the other wires as you state and see if everything works as it should. The blue wire in that case is a common that is not needed in your new thermo as it has its own internal battery power and does not need the common to power it from the transformer. Some systems actually use 2 transformers and the common does not run to the thermo as the other leg of each circuit is terminated elsewhere, but another wire does. If so, you will not get the readings above and you may need to do some rewiring - this is very unlikely.

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Go to
Under red welcome sign
Bottom red highlighted box literature/image search
Type in thermostat model number
Click on arrow to right
Then click on the PDF files owners manual / installation instructions
If you can’t find what you want, the bottom left and bottom of web
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Where to hook up wires?

First, take the old thermostat off the wall.

there should be different colored wires coming out of the wall still connected to the old thermo.

on the new thermo, there might be a wire color chart in the instructions or on the inside of the new thermo.

You should be able to line up the colors and install them that way. let me know if you have any other problems

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