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New Honeywell RTH9580 Jumper Loop Concern

Squidstro Member Posts: 54
Hello all. I just installed a new Honeywell RTH9580 to replace my old TH4110D1007. I have a Sentry S-120 EDP gas fired boiler for heat with Honeywell V8043E1012 zone valves. Then I have a central air conditioner with a York MC48C3XC1A evaporator coil.

On my old Honeywell TH4110D1007 thermostat, I had R and W going to my heat and Rc, Y and G going to my AC. No common wire. The thermostat was powered by two AA batteries.

I had 18/3 wire going to my heat with the spare wire being unused since this house was built in 1980. So I put the spare wire to use as a new common wire for this new Honeywell RTH9580 thermostat.

Here is how I wired the new thermostat:

Their instructions say to remove the jumper loop if you have both R and Rc wires which I do. However, the thermostat will not power on unless I add that jumper loop to that current wiring setup.

I have the heat successfully working with that setup but I have not tried to turn on the AC just yet. I am afraid to fire up the AC because of that jumper loop. I contacted Honeywell and they are asking me to swap my R and Rc wires and remove the jumper loop to see what happens. They position that as a "we'd like to try something". I understand what they're trying to do, but I don't like their trial and error attitude. I also believe if the R and Rc wires are swapped, then my heat and AC calls would be backwards.

So before I humor Honeywell, put your electrical engineering hats on and send me your thoughts and comments about my situation please.

I could always leave my wiring setup and just try to fire up the AC regardless. But I'm just worried with that jumper loop.

Thanks in advance people!


  • Squidstro
    Squidstro Member Posts: 54
    Honeywell confirmed what a bunch of users on the internet stated. The Honeywell thermostats look for common power on the cooling circuit, not the heating circuit. They designed it this way because most boilers don't have a true common built in. So I understand, and I also understand why the jumper loop makes the thermostat have power.

    But now I'm at the question of the calls for heat and AC vs the powering of the thermostat. Those obviously have to be segregated electrical circuits. Three to be exact. One for the call for heat, one for the call for cooling, and one for full time powering of the thermostat. I want to know if I can leave the jumper loop in and leave my current setup, and have the calls for heat and AC also still work. Remember, my call for heat is working just fine. I have not tried the call for AC yet. But if that jumper is powering the thermostat via the Rc terminal and the common terminal, it hasn't created a call for AC (which I think it would if it was a problem). It also doesn't create a call for heat until the thermostat requests it. So it may be working as desired. It's just my concern of the circuitry. I don't want to create a short situation. I'm asking Honeywell to validate it. But knowing litigious companies, I'm sure they won't support it and ask that I get a common wire from my AC.

    If you guys are wondering why I don't just get a common wire from my AC, that air handler is up in my attic crawlspace in a super hard area to work. I'd have to get up there and figure out where the thermostat wire access panel is to set a C wire live. I'd rather not do that unless it's a last resort.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,038
    You answered your own question. Either get the common from the AC, or put a separate transformer for the thermostat all together.
    But either way, remove the jumper so you don't cause any damage.

    There was an error rendering this rich post.

  • Squidstro
    Squidstro Member Posts: 54
    Thanks @STEVEusaPA. Honeywell did say "There should be no jumper because there are two separate transformers. Leaving your jumper in will cause damage to your system."

    So I got up in my attic and found the common terminal of my air handler and setup a common wire from there. I got a good coating of sweat from being up there in such a tight space. What a pain crawlspace attics are.

    I'm now wired like this:

    I successfully tested a call for AC, a call for heat, and my thermostat powers up! Jumper loop removed, of course.

    Sorry for wasting everyone's time. Hopefully this thread can be informational to some.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,038
    No one's time was wasted. You provided good information for anyone who comes across the post.

    There was an error rendering this rich post.

    mattmia2Erin Holohan Haskell