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

Thermostat question

JohnW2 Member Posts: 21
We've lived in a house with a one pipe steam system for 10 years. We previously had one of the old round Honeywell mercury thermostats, and our system ran very well. It did a great job of keeping nice even heat in the radiators. Recently, the system started to go haywire. It would cycle on every five minutes, run for a minute, and then shut off. The guy who looked at it said that the heat anticipator in the thermostat was shot. He thought that a new gas valve we put on at the end of last year had caused the problem because it was 24v. However, I still have the old gas valve that we took off, and it was also 24v., so I don't think that was what caused it. Anyway, he installed a digital nonprogrammable thermostat that has no setting or adjustment for steam. It kicks on when it is one degree below the set point and shuts off when it is one degree above it. The problem is that it takes approximately 45 minutes for it to raise the house two full degrees, but by then the radiators are so hot that the temperature will rise an additional two degrees above the setpoint. So now the system will not kick on again until the house drops four degrees, which takes a couple of hours. So we're getting long cycles once every several hours, and the radiators are always going from too hot to too cold instead of maintaining even heat. We're also getting annoying banging sounds from the radiators that we never had before; we used to have very quiet radiators with our old thermostat. I suspect that the problem is from the expansion and contraction caused by going from too cold to too hot. Is it possible to still get thermostats with heat anticipators? I'm gong to go crazy if I have to live with the system this way. Thanks for any suggestions.


  • Si_zim
    Si_zim Member Posts: 40


    I believe most modern digital thermostats which include features named like "smart response" have logic which learns the reactiveness of your system and try to plan for that... which is better than a simple anticipator in theory.

    Otherwise I think Honeywell's newer (post-Hg) Round tstats like T87 still have anticipators.
  • Jamie Hall
    Jamie Hall Member Posts: 17,122
    By far the simplest thing to do

    is to get a replacement Honeywell T87N round.  It doesn't actually have an anticipator; rather, it has a setting for 1 cycle per hour which is usually correct for steam.

    Cheap.  Reasonably accurate.  Reliable.  Amazon has them, if your local big box doesn't.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JohnW2
    JohnW2 Member Posts: 21

    Thanks. When you have a CPH thermostat, does it force it to run the number of cycles that you set if for or is that just a maximum number? For instance, last night mine cycled on once every three hours and I had the freeze/burn issue. Would one of these thermostats force it to run exactly once per hour?
  • Mark N
    Mark N Member Posts: 1,090

    What thermostat was installed on your system? Did you check the installation manual to make sure it is set up correctly? The CPH setting on a Honeywell replaces the anticipator. If is set to 1 it won't force the boiler to run once an hour. It will run when when the room temp drops to set point minus the differential and then turn off so as not to over shoot the set point.
  • JohnW2
    JohnW2 Member Posts: 21
    Robertshaw RS 2110

    The Thermostat is a Robertshaw RS 2110. He left the box here, and I couldn't find anything on the box or online that says that this thermostat should be used with a steam system. In terms of setting it up, there's really not much to do. It's working the way he said it would: it kicks on at one degree below the set point and off at one degree over. So if I set it for 67, it kicks on at 66 and off at 68, but then the radiators are so hot that it will get up to 70 or 71. Then it has to drop four or five degrees before it will kick on again, and at that point the pipes are cold again. Thus the burn/freeze cycle. There's nothing on it that you can set so that it will shut off before it reaches the set point. Thanks for your response to my CPH question. I guess I'm confused as to what the term means. Does setting it at one CPH mean that that it will run no more than one cycle per hour?
  • Mark N
    Mark N Member Posts: 1,090
    edited January 2014
    Diff setting

    According to the manual for that thermostat on the factory default setting on that thermostat is 2. You could try adjusting it to 1 and see how it works out. Push the up and down arrows for 3 seconds. Diff should be displayed use the up and down arrows to adjust.
  • JohnW2
    JohnW2 Member Posts: 21
    It's at 1

    Thanks, I checked and it's already set for 1. I feel like I need something that's adjustable to .5.
  • Si_zim
    Si_zim Member Posts: 40
    Maybe get a replacement

    Maybe try to get a simple Honeywell 6350 from a big box retailer (or Amazon for just over 40bucks - but delivery will probably get snarled by Hercules).

    For that price it should be worth it to sort out your issue - and if it doesnt do what you need you can return it
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,510
    The first thing to do:

    Short out the thermostat wires to imitate a constant call for heat, and then see if the short-cycling disappears.

    If it does, then of course the problem is in the thermostat. If not, then check the venting, and pressuretrol.--NBC