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

Honeywell T775R2035 ?

james patrickjames patrick Member Posts: 70
Anyone used one?

Looks like a way to control a buffer tank temp. Based on outdoor reset?


  • wogpa67wogpa67 Member Posts: 238
    I haven't used the Honeywell.
    The Tekmar 256 would do it, and it's cheaper and very reliable.
  • james patrickjames patrick Member Posts: 70
    I want the buffer tank to be the boilers only load and its temperature maintained at minimum required.

    System will draw heat out of the buffer tank as thermostats require heat.

    When the buffer tank temperature drops below where ODR needs it to be some device must issue a "heat call" to boiler TT terminals. Thermostat won't work.

    As temperature in buffer tank is variable, it can't use a standard aquastat.

  • james patrickjames patrick Member Posts: 70
    A Tekmar 256 needs an "input heat demand"
  • SWEISWEI Member Posts: 7,356
    Just jumper those terminals and it will maintain the ODR setpoint temp.
  • wogpa67wogpa67 Member Posts: 238
    edited May 2015
    Is the system zoned? If so the zone control would give it the heat demand. If not a RIB could be used to initiate the demand.

    I always try new to me controls at my house first. I want to know the up and downsides of them before getting the late night phone calls.

    I just read the brochure on it. I will try one out at some point , has a lot of nice features.
  • james patrickjames patrick Member Posts: 70
    A boiler demand is required in order for the 256 to provide heat to the heating system. A boiler demand is generated by connecting terminal T(7) to terminal C-(9) through a switching device. Once the switching device closes, the Boiler Demand pointer is displayed in the LCD. The 256 calculates a BOIL TARGET supply temperature based on the outdoor air temperature and settings.

    When the 256 determines that boiler operation is required, the Boiler contact terminals (5 and 6) close. While the Boiler contact is closed, the burner segment in the LCD is displayed.

    SWEI wouldn't that act the same as jumpering the boiler TT & run the boiler over & over needlessly?
  • james patrickjames patrick Member Posts: 70
    RIB ?
  • wogpa67wogpa67 Member Posts: 238
    It's an enclosed relay.

    I am very familiar with all of Tekmar's controls and how they work.

    Is this a zoned system? Does it have a zone valve or circulator control?
  • SWEISWEI Member Posts: 7,356
    Won't the 256 fire the boiler as needed to maintain the calculated temp at the sensor? If you open the TT terminals, it will let the tank temp drift down until they are closed -- or did I miss something?

    Tack SR501-OR-4 might also work, and costs even less.

    RIB = Relay In a Box.
  • james patrickjames patrick Member Posts: 70
    Zone valves
  • wogpa67wogpa67 Member Posts: 238
    So your end switch from zone valves would initiate the heat demand to the whatever ODR control you use.
  • james patrickjames patrick Member Posts: 70
    What would control buffer tank differential ?
    How often would boiler fire ?
  • wogpa67wogpa67 Member Posts: 238
    Any ODR control you put on it would control the tank temp.
    The boiler would only fire when low tank temp minus differential was met.
    The 256 which I use on Geo buffer tanks is a good control. User friendly and all the parameters are setup by the installer.
  • SWEISWEI Member Posts: 7,356
    This is a very standard ODR control setup -- and yes, your end switches or zone control can make the call. Diagram in the top right portion of this shows the wiring clearly.
  • james patrickjames patrick Member Posts: 70
    Thanks, guys

    As I want the buffer tank temperature controlled separate from any system "heat call" it looks like a Tekmar 256 with T7 jumperd to C (boiler demand) should work fine.

    The sensor wired to terminal T1 to C would be placed in the buffer tank aquastat well instead of on the boiler output. Boiler would fire when buffer tank cooled to 1/2 of the programmed differential below the outdoor reset controlled temp.
  • james patrickjames patrick Member Posts: 70
    I hope my post above configuration will work?

    The Honeywell T775R2035 would also do it, but costs more.
  • wogpa67wogpa67 Member Posts: 238
    As Spock would say what is your logic?
    Are you wanting to keep this tank at a setpoint all year?
  • james patrickjames patrick Member Posts: 70
    Buffer tank needs to be warm to hot about 9-10 months per year, zone 7 Crested Butte Colorado
  • james patrickjames patrick Member Posts: 70
    Thinking of using the IFTTT app on Apple IPhone along with WeMo internet controlled switch.

    Something like:
    If outdoor temp > 65*F for 3 days in a row turn off Tekmar 256 power.
  • SWEISWEI Member Posts: 7,356
    The 256 and the SR501-OR-4 both have WWSD. The taco is fixed at 70°F, the 256 is adjustable.

    Shoulder seasons are tricky, especially with higher mass buildings and tight reset curves. With DDC, we sometimes average ODT across the past 24-48 hours and then look at the indoor temp to make the call. Lacking that, a manual switch can sometimes prove to be the best answer.
  • james patrickjames patrick Member Posts: 70
    Tight reset curve?
  • james patrickjames patrick Member Posts: 70
    DDC = Direct Digital Controls
  • SWEISWEI Member Posts: 7,356
    Tight reset curve means one which, when paired with a reasonably well-balanced balanced system will produce stable indoor temps despite not having a thermostat.

Leave a Comment

BoldItalicStrikethroughOrdered listUnordered list
Align leftAlign centerAlign rightToggle HTML viewToggle full pageToggle lights
Drop image/file


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