Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

AC & heat Thermostate

slovichslovich Posts: 5Member
Hi, I have a gas fired hot water boiler with 3 zone valves and 4th for stand alone unit. I just replaced my old Lennox Heat- cool mercury thermostat. I replaced it with a Honeywell non programable round digital T8775C thermostat. Everything worked fine until the next day the ambient temp changed to 99 degrees on the display. I did find out I was supposed to install the two resisters on the first zone valve from power source point 1-2. After installing the resister assembly the display still reads 99 degrees. Will that change after it recharges ot is it toast. Thanks Richard

Comments

  • HVACNUTHVACNUT Posts: 2,272Member
    The T8775C is a COOLING only thermostat, so...
    If you want non programmable then I suggest the T6. It is a programmable thermostat but by going into installer setup it can be changed to non programmable and heat only.
    Silly rabbit.
  • slovichslovich Posts: 5Member
    Thanks for commenting , really I appreciate it. Actually its a T8775C 1005. It is heat on off and cool auto and fan only. By the way it still says 99 degrees after the resisters were installed on the first zone vale after 24 hours. Maybe its toast now from to much amperage when I first installed it without the resisters. It did show the correct ambient room temp and all controls worked fine. I'm told its a power stealing thermostat and gets trickle charge but without the resisters it gets slightly over and amp and wont charge back up. Please help. I need someone that has installed this thermostat on a gas system with zone valves. Anyone out there ever do this hook up ????
  • Jamie HallJamie Hall Posts: 10,548Member
    So far as I can make out tis is not a power stealing thermostat. It's a non-programmable, pretty plain vanilla replacement for the old mercury T87s -- which worked all the time, every time. I don't find any reference to resistors in the install manual.

    My betting is that somehow you got a power spike in there, and it fried. The only thing I would be interested in at this point is -- does it, in fact, turn the heat or cooling on and off when asked?
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • slovichslovich Posts: 5Member
    Gezz I don't like that mean face Icon. I wish I could change it. really know how I got it. Lol ,So Jamie. The Honeywell T8775C is a non programable replacement for the old Lenox mecury switch. Heat on off- cool- auto and fan only. This unit has a light up display window. Everything works fine heat and cool. Auto and fan only. The whole problem is its a voltage stealing thermosate as it says in the installation manual. They did realize that the capacitor would not recharge because of higher amps with zone valves. They engineered the resister assembly to keep the amps below 1 amp . Then it supposed to slowly have a trickle charge to keep correct ambient temp on window display. O ya, the fan just keeps running in cool because the room temp is stuck at 99 degrees. This is an add on . If you have the older manual it wont have it. The box has a decal saying new on it. Its the upgradded model so it will work in all kinds of systems now. If I had a oil unit without zone valves it would work fine I'm told. The problem is the zone valve pulling more amps than the unit was originally made for.
  • Jamie HallJamie Hall Posts: 10,548Member
    Fair enough. If the indicator says 99, and the fan keeps running despite what you do with the temperature setting lever, it's toast. Somewhere in its little solid state brain something fried.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • slovichslovich Posts: 5Member
    yes I agree, another problem now. I was taking the resister assembly off the zone valve today and I touched another terminal on the zone valve. She sparked. Now the boiler wont fire when the boiler water drops below the lower set point. I have a stand alone domestic hot water tank that heats the coil from the boiler water. If I go to main heater control box and pull the reset lever out it will fire but as soon as l let it drop it shuts down. The pilot light is lit. any subjections?
  • slovichslovich Posts: 5Member
    Jamie, what is the full description of the direct replacement for the old unit with mercury switch unit. I'll just put one of them in . Thanks.
  • Jamie HallJamie Hall Posts: 10,548Member
    I don't suppose... that you kept the old mercury unit... they last forever. However, the big box is your friend:
    https://www.homedepot.com/p/Honeywell-Round-Heat-Cool-Thermostat-CT87N/202219050
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • HVACNUTHVACNUT Posts: 2,272Member
    edited June 12
    > @slovich said:
    > If I go to main heater control box and pull the reset lever out it will fire but as soon as l let it drop it shuts down. The pilot light is lit. any subjections?

    Can you post a pic of the main heater control box and reset lever? Aquastat relay?
    If that's what it is you might have shorted the internal transformer.
    Also if there's an extra wire at the thermostat you might be able to use it as a common to 24v power the thermostat.
    I assume the other zone(s) dont work now?

    Edit: The T8775C does not offer a common terminal so that's out.
    Question- is A/C connected to the thermostat as well? Is the jumper removed from R and Rc, or is there just one (R) power source?
Sign In or Register to comment.

Welcome

It looks like you're new here. If you want to get involved, click one of these buttons!