I was having an issue with my Honeywell Tstat RTH7400 --- when heat was turned on the fan would run in the auto position but would not run in the "On" position (regardless if it was heating or not it would not run in the "on" position). I called Honeywell and they could not figure out the problem. I got out the voltmeter and found the following:

Red wire carries 24 volts to green and yellow but not to white
Blue wire carries 24 volts to white but not to yellow or green
Green runs fan
White runs furnace
Yellow runs a/c

So I ran both Red and Blue into the R & Rc terminals with the jumper connected. This seems to have fixed the problem and everything runs the way it should.

Question: Is there any issue with running both 24v wires into the jumpered terminals? Would the system run properly if the jumper was removed?

Thanks is advance.

Here is a link to your t-stats instructions.

http://customer.honeywell.com/Techli.../69-1900EF.pdf

In some cases, life is complicated by having separate transformers for heating and cooling. In such cases, it is important to have both wires to a control connected to same transformer. If you properly match the phases, you can connect the transformers to each other. If your system works the way it is, it means either you have the red and blue wires both running to the same transformer, or the 2 transformers are in phase.