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Help with Thermostat

Ed_5
Ed_5 Member Posts: 24
in THE MAIN WALL
I'm replacing an older mercury Honeywell thermostat with a digital non programable Honeywell CT500A for an air handler with AC only. On the old thermostat the W,Y,G,Rc are being used. Ther new stat has W,Y,G,R terminals only. New stat has a display and I can adjust settings, only the fan comes on when "Auto/On" switch is in the "on" position. AC unit and fan never come on together when calling for AC. Any thoughts? I called Honeywell and they said it was possible that it wasn't working because the CT500 is a "power robbing" thermostat. Is it possible that the CT500 does not support this wiring configuration? Thanks for any help. -Ed

Comments

  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    The CT500...

    is the retail equivalent of the Trade model T8400. I don't have access to the wiring diagrams for the retail products, but I will attempt to help using the T8400. The stat "steals" power to operate from the heating or cooling system. The terminals are as follows: R is power, W is heating relay, Y is cooling contactor and G is the fan relay. The system switch is heat-off-cool and the fan switch is on or auto. On your older stat, the Rc terminal was power for the cooling circuit, probably from the transformer for the system. I would suggest that you return the CT500 and see if they have a 5-wire model, the equivalent of the T8401. I don't know if such a stat is available at retail, if not, try your local hvac pro.
  • Ed_5
    Ed_5 Member Posts: 24
    Stat replacement

    Bill, how about the 8775c (Digital Round)? There is an "A" and "C" version. It looks like the "C" supports my wiring configuration. Thanks for the quick response. -Ed
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    The T8775C...

    has R,Rc,Y,W,G and also O&B. Just Ignore O&B, they're used for dampers or changeover on heat pumps. The other terminals are the same as I described. Rc is the cooling system power supply.
  • Ed_5
    Ed_5 Member Posts: 24
    Bill

    Do you think that there is a chance that I'll have a problem with the "power robbing" like the tech support person at Honeywell says? Thanks again. -Ed
  • Matt Undy
    Matt Undy Member Posts: 256


    Is the consumer version power stealing or does it require batteries to work? I had to buy from a supply house to get a thermostat that didn't run off of batteries(and cost more to operate and potentially would freeze the pipes should the batteries go dead while I wasn't home). If it isn't power stealing then it needs batteries to operate. Does it have a "W" terminal for a direct power supply? I was trying to find the manual but after about 10 minutes of not getting to thermostats at all on honeywell's site i gave up.

    Matt
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    The trade & retail models...

    may be different, but neither the CT500 or the T8775 use batteries.
  • Matt Undy
    Matt Undy Member Posts: 256


    Here is the manual for the ct500a:

    http://hbctechlit.honeywell.com/request.cfm?form=69-1525

    What do you see on the display when you turn the setpoint above the room temp? You could have broken a wire inside the insulation on the yellow wire. If possible try shorting the "g" "y" and "w" and see if the compressor and blower come on. Do it with the air handler disconnect off then turn the power on. Be careful to give the compressor a couple minutes to unload before starting it again if you are controling it manualy.

    Its set to cooling mode, right? The fan switch probably directly connects "r" and "g" so even if there isn't enough current to be stolen through the relays to operate the thermostat the blower would still work.

    Matt
  • Ed_5
    Ed_5 Member Posts: 24
    Matt

    When I connected the old mercury style stat back up it worked fine.
  • Glenn Harrison_2
    Glenn Harrison_2 Member Posts: 845
    Here's a thought

    If this is an Air Conditioning system only, what is "W" connected to? I would think that it is connected to nothing at the air handler, in which case it can't power steal in the cooling mode, so it won't work. If "W" is connected to something, please let us know what it is connected to. Since this is cooling only, you will need to get a battery operated t-stat, or one with a common, and connect a wire from common in the air handler to common on the t-stat.
  • Ed_5
    Ed_5 Member Posts: 24
    CT500

    After talking to a fourth person at Hwell teck support, I found out that I needed to connect the stat and wait 5 min. for the fan and compressor to turn on. I reconnected it last night, the display came on and the cooling indicator(snowflake) was flashing, after 5 minutes the indicator stopped flashing and the fan and compressor came on. Thanks for the feedback. -Ed
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Yes, that's true...

    I should have mentioned it, but I didn't. Sorry. That can be disabled for testing purposes, but the procedure is different for each stat. Refer to the instruction sheet for details, or go to http://hbctechlit.honeywell.com for a complete tech sheet. The purpose of the time delay is to protect the compressor.
  • Matt Undy
    Matt Undy Member Posts: 256
    Power Stealing

    I beleive power stealing means that the thermostat derives power without a connection to both sides of the transformer. How does this work in a 2 wire heat only system or a cool only system where there is no non-energized relay in the equipment durning a heat/cool call to run current for the thermostat through? Does it store power in a capacitor on the off cycle? Does it put some resistance in series with the equipment's realys to produce a voltage drop to derive its power from?

    Matt
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