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Question re: Signaturestat

It's been a while and I hope all are well...

I wonder if anyone may have the answer handy... when a Lennox Signaturestat (or the White Rodgers equivalent) calls for heat (W), does it also turn on the fan? (i.e. "G") I'd like to think that "G" would be left alone but this may be a stumbling block in my quest to run the radiant heating and the AC system off the same thermostats... and if the Signaturestat is not suitable for a boiler/AC System, which alternative would you recommend?


  • Wayne_16
    Wayne_16 Member Posts: 130
    Seperate Rw from Rc

    Does this thermostat have a jumper wire from Rw (heat) to Rc (cool)?  Remove the jumper wire, wire the radient heat to Rw & W, and the fan coil/ac to Rc & Y, G.   Now you will be using the thermostat to control a heating unit and the same thermostat to operate a seperate a/c blower equipment.  Set the programable portion of the thermostat to control radient floors.  If I can find my copy of the installation manual, I will post any additional features I find.

    Thanks, Minnesota Wayne
  • geno54
    geno54 Member Posts: 43

    Looking at the manual on the Signature stat there isn't any independant Heat/Cool  (Rh/Rc) only R. So you would have to use an isolation relay for the heat because you don't want to mix up transformers between the two systems. I have done that many times. And no it should not turn on the fan if it is the conventional stat, the furnace would do that. There are A few models of the Sig Stat. Which model do you have?
  • Constantin
    Constantin Member Posts: 3,782

    I looked through the manual, saw no mention of radiant heating  as an option - they only have gas and Hp modes in those 'stats, I think.  Not sure there is a separate Rc and Rh either, but have that covered (I think - see further answer below). Thanks for the info!
  • Constantin
    Constantin Member Posts: 3,782
    edited February 2010

    Isolation relay - check!

    Also time to start shaking your head. I'm an engineer. I can't help it...

    Specifically, you can read the gory details over at oil tech talk ([url=;action=display;num=1254779568;start=13#13];action=display;num=1254779568;start=13#13)

    These homebrew boards will isolate the heating portion from the cooling side. The power draw should be quite manageable - that relay draw is 400mW, IIRC. Single zone, no standby loss. Dirt simple circuit and 1-Wire monitoring. They'll be mounted inside an Al enclosure, 0.25A fast-acting fuse on the 120VAC circuit, 1.25A polyfuse on the 24VAC side for the zone actuators. Haven't tested the fuses in anger but expect it to work...

    As for the Signaturestat models... hah... I have been accumulating Signaturestats via ebay for a while. I have greenies and bluees with a mix of capabilities (i.e. some can do more stages than others).

    All signaturestats seem to work and with my recent upgrade from a Harmony II to a Harmony III zoning system, humidification is more targeted and works better. Pretty interesting to see the massive change in construction from the Harmony II to the Harmony III, i.e. transitioning from mostly through-hole components on the PCB to surface-mount stuff. Had to undo a bunch of kludging re: the humidifier wiring, the old control method included a single horse-hair actuator, etc.

    And yes, I will include a circuit diagram, extra fuses, a description, with every box in the system. You guys get to scratch your heads enough as it is! :-D
  • geno54
    geno54 Member Posts: 43

    Slow down a bit. No need for that relay. that woulld be a bit overkill for what you need to do.  A simple "peanut" or General purpose relay is sufficient.

    The early models of the signature stats had some internal overheating problems when in cooling mode. When the stat called it would generate some heat inside the stat and drive the sensor up keeping the system on and over cooling. I believe that has been corrected though.

    "Also time to start shaking your head. I'm an engineer. I can't help


    Thats OK I understand your dilemma. I've  worked with many Engineers and Harmony systems. :):)
  • Constantin
    Constantin Member Posts: 3,782
    edited February 2010
    Looks big, doesn't it?

    The relay is about 1" long, the green light is a 5mm LED sticking up. So it's not that big... The big hulking Triad-labeled thingie in the back is the transformer for the zone actuators. Instead of using a zone manager from the likes of TACO, I'm using the relay to turn on the 120VAC supply to the transformer, which in turn allows the 24VAC zone valves to open up. So, it is a peanut of a relay, rated at just 3A but passing a mere 0.25A at full load (6 zone valves), but I figure a generous safety factory will enhance its lifetime to the point where someone else will have to worry about it.

    What's nice about this setup is its simplicity and zero standby. When the Tstat is not calling for heat, there is no power to the transformer, less passive heating of the house, etc. Since the AC system will always have a transformer running, I'll simply be piggy-backing off of that.

    As RS485-based systems evolve (i.e. climatetalk, etc.) upgrades may make sense. However, since Lennox appears to have chosen a closed-system approach at the present time (like Carrier), my guess is that it'll be the next revision of the AC system proper before we get to enjoy the many benefits of serial communication, closed-loop control, etc. 24VAC-based command and control does have some benefits though, one of which is that even I can diagnose issues sometimes.

    A picture of the circuit is attached. The 1-wire stuff can be safely ignored, it's just for the whole-house monitoring system. Power from the Tstat comes into the Tstat part of the board, is rectified, then turns on one LED (status) and the relay. Once the relay closes, the transformer is turned on, starts pushing power down to the 24VAC side of things (via a polyfuse). A LED comes on down there, a opto-coupler keeps the 1-wire system abreast of things also. The transformer is IIRC a 40VA unit, more than enough for up to 6 zone actuators that draw maybe 6W on startup and 2W while running.

    As for the overcooling, I wager that dehumidification settings may have something to do with that also. IIRC, the out-of-box allowable overcooling is 2*F, quite noticeable.
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