I installed the RTH 230B thermostat, Theheat only works when set to the cool mode, I only have gas furnace(no ac unit). My old wires are Red, white, green and blue. What wires go where?

Check thermostat wires maybe to long and touching in the back of the thermostat

I can give you a start, For heat only, R terminal on furnace to Red wire to R or RH terminal on thermostat. W terminal to White wire to W terminal on thermostat. G terminal to Green wire to G terminal on thermostat. This will get you heat but not air conditioning.

Check for a bad thermostat and wire by jumping the red and white wire together at the thermostat. If the unit fires, you have a bad thermostat. If it does not, jump the R and W terminals at the furnace (or where ever the thermostat wires are connected to). If the unit fires you have a bad wire. If it still does not fire, you have an issue with the heating system not the thermostat or its wires.

Typically the four thermostat wires are:

R = power (or common)
G= FAN
W = Heat
Y = Cool

Assuming that you have a normal electric or gas furnace and air conditioning, if the heat really is coming on, then just switch the position of the the Yellow and White wires.

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

disconect the dark blue on the circuit board of the furnace. if there is no central air the c terminal was powering the old t-stat

This is typical wiring for a 4 wire thermostat

R = Red One Side of 24 volt Transformer
G = Green Blower relay
Y = Yellow Cool Condensing unit
W = White Heat Furnace

Check to see if there is a short between the green wire and the red wire. Green is the fan control wire for AC. Sounds like some wires may be touching. How did the old t-stat get fried?

new unit and old thermostat/wire? thermostat could be bad or bad thermostat wires. try putting in new of anything old. do this wiring practice with the terminals being the letters.

r- red (24 volts)
w- white ( heat)
y- yellow (cooling)
g- green (fan)

put the jumper back in RC and RH and that should do it.