I am trying to change from a white rodgers 1F76-357 thermostat to a Honeywell 7 day programmable thermostat (RTH7500 series). The wires that I have don't match up with the installation directions. Here is what I have:
2 wires coming from the wall, one goes to the B post and the other goes to a screw on the box labeled A (there is no designation for an A wire in the honeywell guide.)
The other wires I have are a yellow wire that is connected to the Y spot on the old thermostat but it then is connected to a rivet on the back of the old thermostat. The other wire is connected on one end to the RC post and on the other to the RH post. Any help?

You only have heat? No ac? And just to wires correct? Red to rh and white to w

What about the other wires that were connected in there? Do I need to connect the RC and RH ones? We do only have heat.

I did find this document that mentions A & B wires. http://www.michaelshell.org/files/robertshaw_400_subbase.pdf

I really don't know what "A" is for. Connect Rc to Rh and connect to your old R. Without Rc, the fan switch may not work.

The other question I have is if there is a way to tell if I have a high voltage or low voltage system without using a voltmeter?