As the Honeywell RTH6300B didnt require the C, and most installations use a 5-wire installation, is it possible that the wire was either cut off flush with the back of the thermostat or shoved back into the hole in the wall.
Typically the box will say "C wire required " Any 24v tstat will work, just make sure it doesn't say c required.
SOURCE: Intertherm to Honeywell thermostat
Typically if you have a yellow wire in the "y" terminal blue is used as common or "c". As far as your jumper from rc to rh, that will depend upon what type of heat you have. If all of your heat and cooling come from your packaged unit outside, yes you will need the jumper. If this does not work maybe you have a bad breaker or transfomer.
SOURCE: got a honeywell t8400c thermostat
R--- rc or rh y--yellow g---green w--white red is power yellow is cool g--fan white is heat
SOURCE: I'm installing a Honeywell RTH7600B
Every thing the same except take your blue wire and replace the yellow for it. So when you hook it to t-stat goes like this
Red to R
White to W
Green to G
Blue to Y
SOURCE: Replacing Round Honewell Mercury Thermostat
try to get one that is made for oil heat, the installation is easy only two wires, and the instructions make it easy.
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