Hi, I have a Lennox Furnace, when you switch on the thermostat to heat and adjust the temp. accordingly, the burner does not ignite. Pilot flame is working, fan works. But no flame on the burner.
Here is how the thermostat is wired (Is it right? )
RC- blue wire,
G- black wire
Y- red wire
W- white wire

UPDATE... Thanks people for your input... the thermostat did not have a jumper cable for `rc-rh` put one on and `presto` done deal
It`s all good... thanks again...

These wires are not connected as normally it may be some one changed the thermostat and did not connect them properly, the "w" terminal is for heat , the "y" terminal is for cooling, the "g" terminal is for the indoor fan and the red is the wire from the transformer that connects to the "rc/rh" The rc is for cooling and the rh is for heating, if you have one transformer then you put a jumper wire between the rc and rh and connect the red wire to either one.
The lettered terminals correspond to the color of wires. Y is yellow but in your case some one has substituted a different color wire, go to your furnace and look to see what color wire from your thermostat goes to the outside unit .Good luck, Mike

You should have R, G, Y, and W at the furnace too. Although it is easier to understand if the red wire is on R and the yellow on Y, the important thing is to have the same wire connected to R at the furnace connected to RH and RC at the thermostat.

If the cooling worked with the thermostat wired this way than the heat should also. Turn the power going to the furnace off and then back on this will reset the control board. Turn the T-stat back to heat and see what it will do.( Did you change out any parts or thermostat?)