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T-stat for 1-pipe steam sysyem

I believe that the T87F is the best simple thermostat for one-pipe steam, and that it is grossly under-appreciated. For example, using the anticipator, the HO can abolish overshoot and establish a two or three cph pattern that virtually eliminates cold rooms (with proper mains and rad venting). Using the T87 and a chart temperature recorder I have equalized whole houses to within one degree (provided the radiation was equal to the task). Once properly set, the cph pattern holds for all outside temps. It's an uncanny device.


  • Bill_80
    Bill_80 Member Posts: 1

    Can someone recommend the right t-stat for a 1-pipe steam system. Right now i have a white-rodgers model that goes 1 degree above set point. I also have the Honeywell favorite round t-stat. Would i be better off using the honeywell,and not have it automatically setback the temp. I usually have the t-stat set for 67 when i am home ,and have it go down to 64 at night when i go to bed. Looking for some direction to go in .thanks
  • wsdave
    wsdave Member Posts: 97

    I've been using the Honeywell VisionPro 8000 Programmable T-stat on a one pipe steam system with no problems.

  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Honeywell stats...

    Any VisionPro programmable, or FocusPro non-programmable will work, as will the new T87N&K mercury free Round stats, as long as you don't have a millivolt ignition system on your boiler. Just remember to set them for 1 cycle per hour for your steam system. I agree that a deep setback is not practical, 5 degrees is about the best.
  • Steve_126
    Steve_126 Member Posts: 19
    Honeywell T-stat

    I have an old school Honeywell mercury t-stat that appears to be causing short cycling. If the temp is way below the setting I don't have any issues as the boiler will run until the stat is satisfied. But once it reaches the setting it seems to cycle on an off every 10 minutes or so. Is this due to the sensitivity of the mercury? I checked and it is not cutting out on pressure or anything else. Wouls you recommend a replacement to a digital round t-stat?
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    The old style T87

    could be calibrated by using a very small wrench to adjust the bi-metal. Also, your heat anticipator could be set wrong, check the gas valve or oil primary for the amp draw, then set the anticipator accordingly. If you would like to update, the new mercury free T87N or K will fit in the same spot and work just like the old T87 did, but you will need to set the dip switches inside for 1 cycle per hour for your steam system. If you would like a digital, non programmable, try the TH5110, or any of our Vision Pro programmables.
  • Steve_126
    Steve_126 Member Posts: 19

    I don't believe this is an old T87. It is a rectangular box that you manually move a slide to adjust the temp setting. It also has red and blue pins to enable you to program a timer. I am certain it has mercury and is not bi-metal, so I don't think I can make any adjustments. Not sure if this one has an anticipator as it seems pretty old. What replacement would you recommend to solve the short cycling problem? Is it the T87 digital round or the TH5110? Also what does the DIP switch do? Does it limit the boiler from making one cycle in any given hour? Will that impact the boiler if another stat is calling (indirect). Thanks.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Sounds like a Chronotherm

    You should be able to find the anticipator, which is a rheostat with an arrow and the word "longer". Slide it in the direction of "longer" and see if the short cycling decreases.

    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Chronotherm

    Sounds like an old mechanical Chronotherm. It's similar internally to a mechanical T87, except for the timer. Both use mercury switches mounted on a bimetallic coil. They've also got anticipators, and you should probably check the setting before condemning it unless you've already made up your mind about a new thermostat.

    Is this what you've got? The anticipator is shown on page 22.


    The recalibration that Bill describes is shown on page 25, but it doesn't sound to me like you need to do this.
  • Steve_126
    Steve_126 Member Posts: 19
    Nice work!

    That is exactly what I have. Where did you dig that out from? Any idea at to what the appropriate anticiaptor setting should be on a one pipe steam system. Where would I look to find this info as I don't have an ammeter. Might as well try this out before replacing.
  • Piece of cake

    Your description sounded just like an original Chronotherm, so I moseyed on over to Honeywell's site and searched their literature for "chronotherm."

    An ammeter's really the best way to do this, and they're pretty inexpensive. But if you know exactly what the thermostat is connected to, then you can check that device's nameplate to figure out how much current will be pulled through the thermostat. This depends on your system. The thermostat might be directly connected to an oil primary, or a gas valve, or a relay, or a zone valve. (Well, it probably wouldn't be a zone valve on a steamer.)

    In a simple heating-only steam system, you'd usually be looking at the oil primary or the gas valve.

  • chuck_6
    chuck_6 Member Posts: 107


    I don't mean to through a monkey wrench into all of the Honeywell mentions, but we have a Lux t-stat for our 1-pipe steam system and it works great. Like you we turn it down overnight and have never had a problem.

  • GusHerb
    GusHerb Member Posts: 91

  • V8toilet
    V8toilet Member Posts: 71

    The problem I have always had with my steam system was overshoot especially with the old round mercury T-87 thermostat. The anticipator in the T-87 can only compensate for 1.5 degrees each way. The T-87 is obsolete compared to what’s available today. You need to find a computerized thermostat that is capable of learning how your system runs. You need one that learns both run time and start time so that it can compensate for your systems overshoot and if you have a programmable one it can also learn how long it takes for the system to heat the house from the time it calls for it.

    I like the Honeywell RTH7400. This thermostat has a computer that will learn when to shut off your system so that it won't overshoot by more than 1 degree. If you balance all your radiators in conjunction with your thermostat you can get the whole house to be within 1-2 degrees of each other, which for an 80-100 year old steam system than ran on a coal boiler is awesome. This thermostat will cost you some money (about $80) but it will pay for itself in fuel savings. Mine has paid for itself multiple times now.
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