I need to replace an old Thermostat that just has 3 wires coloured Red, Blue and Yellow. The Honeywell THR830TUK Thermostat I have bought shows 5 wires on the diagram. I'm not sure which wire goes into which Terminal. Can you please advise? Regards Tim

Hi,

Here is a list of each common terminal and what their commonly used for. Alsolisted is what colors are commonly used. NEVER ASSUME these are what youhave. Verify all your wires by following each to each piece of equipment. Thiswill help you to be able to match your equipment with the terminal.

Here is the rest of the tip that I wrote...

Thermostat Wiring and Terminal Designations
heatman101

The old thermostat has a second stage heat which is W2. The new one doesn't have second stage heat.
The wiring your describing on the old thermostat is telling me you have a heat pump with second stage heat and the new thermostat is for air conditioning only. You will need to get a thermostat for a heat pump.

It depends on the type of thermostat you have.
Do you have a 4 wire or 5 wire T'stat?
The red jumper wire was for a 5 wire t'stat with 1 transformer, the red jumper actually should have been from R to 1.
So you need to trace the wires back to the furnace and make sure they go to the locations identified below.
if you have a 4 wire t'stat then you don't need that jumper, if you have a 5 wire t'stat then you do need that jumper but it should go from Rh to Rc.
4 Wire Termostat
R = Red One Side of 24 volt Transformer
G = Green Blower
Y = Yellow Cool
W = White Heat
5 Wire Thermostat
with one transformer for both units
Rc One Side of 24 volt Transformer
Rh Jumper Rc to Rh
G = Green Blower
Y = Yellow Cool
W = White Heat
t'stat?

check wiring to furnace check wire hook ups on board match letters and colors sometimes installer use what ever wiring they chose on letters

RC AND RH CONNECT BOTH WITH RED
Y TO THE YELLOW
G TO THE GREEN
W TO THE WHITE

If you have a 5 wire thermostat then you have a two transformer system a cooling transformer and a heating transformer.
The cooling transformer wires to Rc - Y - G,
and the heating transformer wires to R - W the R terminal heating would be the #4 terminal from the old stat.

Hi,

All you need to do is look at the wiring diagram on the back of the old room stat, you will have the following:

(normally wired with these colours)

OLD COLOUR CODING

Live (sometimes called common) - Red
Neutral - Blue
Switch Live - Yellow

NEW COLOUR CODING

Live - Brown
Neutral - Grey
Switch Live - Black

All you need to do is wire up the new stat with this in mind. (please note: if the wires are not colour coded like this then do not proceed and get an electrician)

Any problems or questions let me know

I hope this helps

Steve

What is the blue hooked up to now?

rh and rc should have a jumper between them.connect red to either.y=yellow g=green w=white and do not use the blue.your new stat will run on batteries.

Normally, you should have 5 color wires: W-White, Y-Yellow, G-Green, R-Red and C-Blue.

The Blue wire is a common from the transformer on older systems, which is probably hooked to RC terminal.

Try this: Remove the blue wire and jump the RC to RH with the red wire and you will be fine. Blue wire should not be needed with the new thermostat.

This should also be stated in the instruction manual.
Hope this helps.