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considering replacement for old millivolt thermostat

Have a single-pipe steam system and have been improving it, especially in the areas of venting and pressure, based on the excellent advice found on

Now I'm looking at the ancient Honeywell TS86A 1314 1 millivolt thermostat. The temperature calibration being off by five degrees is of no concern, but it does not appear to have an anticipator and despite much tuning and a Gorton #2 main vent the system continues to overshoot in a big way. Just two wire terminals, white and black.

I see three likely replacement candidates:

1) newer Honeywell TS86A with anticipator (off eBay)

2) Robertshaw 200-501 or -502 (from PexSupply)

3) the fancy White Rodgers 1F87-361 (Pex)

Am greatly interested in any thoughts, opinions, experience, etc. that anyone may have, including additional thermostat options.

The one constraint is that I am not willing to consider eliminating the excellent thermopile control or select a thermostat that requires external power. For ten days after Hurricane Sandy mine was the only house on the block with heat.

Measuring 0.49 volts at both the thermostat and pressuretrol (yes I'm planning to swap in a vaporstat eventually). I'm guessing that the long wire run and age of the thermopile accounts for the reduction from the nominal 0.75 volts--but it's working fine.


  • JStar
    JStar Member Posts: 2,752

    Just make sure the t-stat is battery powered and not power-stealing.
  • Tim McElwain
    Tim McElwain Member Posts: 4,481
    The TS86 has a fixed anticipator

    matched up to the self generating "Powerpile" gas valve you have.

    Anyone selling you a TS86 with an adjustable anticipator is ripping you off.

    The Robertshaw and White Rodgers thermostats you listed are compatible with powerpile systems.

    Any thermostat you install must be set up for use with millivolts if not you will have more problems not the least of which would be a high millivolt drop across the thermostat giving you no heat.
  • Binnacle
    Binnacle Member Posts: 126

    Thank you Tim for the information about the built-in anticipator!

    I spent some time carefully observing and timing the system and have come to realize that the 30-plus-year-old thermostat is perfectly fine and I'm keeping it as is.  Would replace it with a near identical TS86A from eBay if it ever came to that.

    Thermostat is breaking after about one degree in temperature rise, so fixing the overshoot will come from replacing the pressuretrol with a vaporstat and possibly adding a Gorton #2 to the contorted 20' riser off the end of the 40' main that feeds the t-stat radiator.  Have a #2 on the main-end already.

    It's nice that some very minor tweaking is all it takes to bring the 90-year-old steam system back to optimum operation.
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