I am trying to hook up a Honeywell RTH 7400 Tstat to the existing Rheem HVAC in my house. Currently using Rheem thermostat that came with the house (non-programmeable, using mercury tubes). Heating is provided by a heat pump as well as Natural gas furnace for aux heating (besides the air conditioner for summer). This is the original system installed in the house when it was built 4 years ago. See details on current Rheem connections below. The proposed new connections using Honeywell RTH 7400 are also below with a link to the installation instructions.

The problem is the Blue wire in my current Rheem system. As noted in other postings as well, Rheem systems have separate O and B wires.

Common Thermostat Questions (https://www.askmehelpdesk.com/heating-air-conditioning/common-thermostat-questions-40771.html) (https://www.askmehelpdesk.com/heating-air-conditioning/common-thermostat-questions-40771.html)
B=reversing valve (heat pump) energizes on heat (mostly just Rheem/Ruud/Weatherking)
O=reversing valve (heat pump) energizes on cool (Most all heat pumps)

In the Honeywell Tstat the O and B terminals are not separate. So what happens to the extra Blue wire? I doubt it can be left unconnected? Any help is very much appreciated. If the answer is that this Tstat cannot work with this HVAC system then that's OK too. I will return it and find one that does work. Any recommendations? Thanks for the help.

Below are the details on current Rheem connections and the new Honeywell RTH 7400 Tstat.

Current connections of Rheem Tstat:

Wire ----> Connected to -> T-stat Terminal
--------------------------------
Green ---> Connected to -> G
Red -----> Connected to -> R
White ---> Connected to -> W2 and E
Yellow --> Connected to -> Y
Orange --> Connected to -> O
Black ---> Connected to -> X
Blue ----> Connected to -> B

Proposed connections using Honewell RTH 7400 Tstat

Wire ----> Connected to -> T-stat Terminal
--------------------------------
Green ---> Connected to -> G
Red -----> Connected to -> R (jumper to Rc)
White ---> Connected to -> Aux (jumper to E)
Yellow --> Connected to -> Y
Orange --> Connected to -> O/B
Black ---> Connected to -> C
Blue ----> Connected to -> ??

Honeywell RTH 7400 Installation Instructions (http://customer.honeywell.com/Techlit/pdf/69-0000s/69-1900ES.pdf) (http://customer.honeywell.com/Techlit/pdf/69-0000s/69-1900ES.pdf)
Pages 10 & 12 for wiring reference. Pages 18 & 19 for System Setup of options 170, 180 and 190.

Rheem uses blue not orange, go into the installer set up and change it to reversing valve energizes on heat. Function 0190 set to 1. Also jumper Rc and Rw.

Thanks for the reply NorthernHeat.
Are you saying that the Blue wire should be connected to O/B on the Honeywell Tstat and then leave the Orange wire disconnected? If that's the case then I can see now why function 190 should be set to 1.

Ok the next question may seem dumb... and that's because I don't understand the terminology and I am simply reversing the heat/cool concepts :)

Does that also mean that when summer arrives and I need to change the Tstat to "cool", I will need to change the wiring at that time and connect the Orange wire to O/B? And also switch function 190 to 0?

Thanks again for the help.