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Honeywell T651 A & B questions

JoeBoulder Member Posts: 1

I couldn’t find much about this thermostat on here and what’s discussions I did were closed, so sorry if I missed the right discussion and poorly created a new one here. 

I live in an older than me originally built in 60s condo in Boulder, CO. I’m betting this Honeywell T651 A&B is original, but tell me if I’m wrong. It controls the temp fan on/off of my unit with the central boiler and chiller feeding water. The thermistor it uses to switch modes between between seasons has failed. They can’t find a replacement. Thus I wondering what to do. For now I just circumvent the thermistor but then I’ll have to rewire it every seasonal change. I was thinking I could put in a switch instead, to manually switch the system with the seasons. But also its ability to sense and hold a temperature is off. I pretty much just have to use it as an on/off switch. And manually change the fan speed at a switch on the unit. 

So I wondering how I can make this better. I’d love to switch to something much more current and maybe even like WiFi controllable but I can’t find anything like it on the market today with line level voltage 3 wire 120v. 

I’ll take anything better you can suggest or even directions on how to “tune” the temperature sensor I’ll call it so it just not an on / off switch. 

I’m in your hands. 


  • 109A_5
    109A_5 Member Posts: 1,344
    edited June 2022
    @JoeBoulder, I'm thinking the electrical contacts are worn or pitted to the point of no or poor operation (Age). My guess it it works with bi-metal spring and there is actually no thermistor like found in newer electronic units. It looks like there is a 30,000 Ohm, 5%, 1 Watt Resistor (the brown cylinder with the Orange, Black, Orange, Gold bands) which probably acts as a Heat Anticipator. With incorrect wiring you may loose the Heat Anticipator functionality. Be careful that unit is made for 120 Volts AC and those are THHN type wires, not thermostat type wires. You need more information to go beyond the 'like' replacement.

    There is a 'like' replacement and a few digital versions. Could not find a WIFI type.


    Edit: Thanks ED, not sure what I was thinking about with the 24 VAC comments (now removed).
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,590
    This thermostat does have a modern day replacement. The wires in you picture look as though they are 14 gauge 3 conductor with a ground. I assume there is a Summer/Winter switch at the other end of that wire on somewhere on the fan unit. Here is what your current thermostat has:there is a newer version of that thermostat which is a direct replacement T651A2028
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics