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Is my thermostat defective?

bob mac
bob mac Member Posts: 2
I am a new homeowner trying to understand thermostat operation. I have a forced hot water with baseboards gas heating system and a digital programmable thermostat. My question is: When I set the thermostat to 78, I can hear the furnace turn on; but it then will shut itself off before my thermostat reaches 78. Shouldn't the furnace keep running until the thermostat reads 78? or am I just not understanding how the system works. I had the thermostat set for 78 all day yesterday but the highest readout only reached 72. Thanks in advance.


  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Your Anticipator setting is probably off

    or the cycles per hour setting. If you have a typical mercury switch thermostat like the old round Honeywell, there is a setting under the ring marked with .1,.2 ,.3, etc. It sounds like this setting is too low. The proper setting depends on the amperage draw of the 24 volt controls on the thermostat circuit, the type of system you have, the size of the boiler versus the load, and how much system capacity you have versus the heat loss of your home.

    If you are willing to experiment a little, set the anticipator to the highest setting (usually about 1.2) and see if the heat stays even (no significatn variation in temp from boiler on to boiler off). If there is a uncomfortable swing in temperature, turn in down a little. Do this until the temperature stays even. Then check to make sure the biler is not short cycling. Most hot water heating systems work best and most efficiently when the heat is cycled on and off about 1 to 3 times per hour.
    If this doesn't work, take a look at the find a professional section for a contractor that knows hot water heat.

  • bob mac
    bob mac Member Posts: 2

    My thermostat is digital and programmable. Am i correct in assuming that the furnace should continue burning until my desired temperature is reached? Could someone explain to me what should happen inside my heating system when I set my thermostat to a certain temperature? Thank you.
  • Chuckles_2
    Chuckles_2 Member Posts: 52
    not enough info

    Your boiler may go on and off a bit, but it should keep running most of the time until your house hits the set temperature. If you set 78 but don't pass 72, something is probably wrong---unless you have outdoor reset so your water is (intentionally) too cold to take the house temp any higher.

    In any case it is not reasonable for anyone to do long-distance diagnosis with the limited information provided. All I can say is that it's not the anticipator (digital thermostats don't have anticipators, and even if they did, this would not cause the problem of setting 78 but getting 72).

    Newbies tend to assume that if there is a heat problem, it is the thermostat. That's because the thermostat is the "face" of the system, but the system is much more than the thermostat. Most likely the problem is elsewhere.
  • Ted_9
    Ted_9 Member Posts: 1,718

    Its also not correct to think that the boiler has to be firing in order to get heat. the boiler stops firing when it hits the set boiler water temp(say 180 degrees in you case?). So the boiler wmay stop firing, but the circulator will keep running suppling you heat.

  • Frank_17
    Frank_17 Member Posts: 107

    the t-stat does not control the firing of the boiler, it controls the circulator, other controls start and stop the boiler. some diditals do have anticipater settings but not all.this setting shuts down the circ, early anticipating that the latent heat from the bb will heat to the set point temp. on digitals it's in degrees not amps. usually 1-2 degrees. if its not even coming to temp, sounds like another problem, not enpough bb, air in system,setting problem on boiler water temp,......
  • Mitch_4
    Mitch_4 Member Posts: 955
    Look at

    The installation instructions. Many digitals that have problems in my area have different switch/screw settings for a hydronics system. The Honeywell magic stats for example require setting the stat using screws on the back.

    It may be as simple as that.
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    If it's a Honeywell Chronotherm 4 or later model...

    it should be set to 3 cycles per hour for the type of system you have. If it's a Chronotherm 3, look on the back,and set the screws according to the instructions on the back of the stat. Your type of system will continue to give off heat after the boiler shuts down, so the boiler may well shut down before the stat reaches the set point. If you don't have the instruction manual that came with your stat, just find the model number (T8600, T8000 etc) and go to http://hbctechlit.honeywell.com, and select and print out the owner's manual.
  • Thad English
    Thad English Member Posts: 152
    Hey Boiler Pro & BillW@honeywell

    I am trying to wring the maximum efficiency out of my heating system I am wondering if there are any adjustments that I can make to the settings on the thermostat to make the system achieve longer burn times. The system is a 2-pipe air-vent steam system. The boiler is a peerless, with insulated steam mains, 2 big main vents, all controls are working properly, and the thermostat in question is the 'old round honeywell.' The setting on the side of the thermostat that boilerpro mentioned in a post above is at the maximum, 1.2. Currently the system cycles on 4 minutes, off for 26, and the temp varies minimally between cycles. The temp usually hits just below wherever I have it set, e.g. if it's set for 67, the room temp will hit just below 67 and then drop to 66 when the boiler will fire again. The short 4 minute burn time is not allowing two rads at the end of the run to get any steam. The house overall is comfortable, but the two rooms that have those rads are cooler. Is there anything that I can do to the 'anticipator' setting to get the burn time to be longer and hopefully lengthen the off time, too?
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Setting back steam systems is controversial...

    since they have such a pronounced flywheel effect. Right now, you could do yourself a favor by replacing the T87 with either an electronic round version, digital, but not programmable, +- 1 degree accurate, mercury-free T8775A1009 (heat only) or a T8775C1005(heat and cool) or you could treat yourself to the new FocusPro 5000, part number TH5110D1006 (standard screen) or the TH5110D1022(large screen) our new digital, non-programmable. Set either one up for 1 cycle per hour. Either one will provide tighter temperature control than the mechanical T87 ever will.
  • Thad English
    Thad English Member Posts: 152
    Don't want to setback


    I don't want to setback, but I do want to lenghten the cycle time. I'd rather not replace the thermostat right now so how do I setup the T87 for 1 cycle per hour? Currently we're at about 2 cycles per hour. (4mins on/26mins off)
  • Norbert_2
    Norbert_2 Member Posts: 6
    Stats- Vision Pro or Focus Pro as a replacement?

    I also have a one pipe steam, just replaced the boiler(did the dirty removal myself but let a pro put the new one in), new wet return and main vent,as well as Hoffmann 1A adjustables on the rads.
    My Honeywell stat is a TM850A1C1, Chronotherm (circ 1952?)still works, a classic! but the wife would like me to "modernize" with a digital setback. Is the Vision Pro suitable for steam,or must I use the Focus Pro?
    Should I hold onto my classic?
  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Bill, your Chronotherms work great for steam setback!

    I have a church that comes up from a 55F setback temperature up to 70F for the past 4 or 5 winters and it never overshoots like it used to with the t-87 manually setback.
    If I understand the internal logic correctly, I suspect it works so well becasue the thermostat limits the speed of recovery, so the system is still cycled during recovery to maintain a recovery rate of about 4F per hour (or something like that). In this way, the rads don't get so hot as to overheat the space when the setup temp is reached. I believe your competition just lets the system run full out until the setting is reached causing overshoot.

  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    You probably have a very high amp draw on the circuit

    I believe you would have to install an isolation relay with a amp draw of .6 or lower and then set the anticipator for twice this amp draw.

  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Thanks for the interest...

    The Vision Pro is our full-featured touch-screen programmable, with all the extra goodies. As I mentioned in a previous post, setting back steam systems is a controversial subject, and depending on how your home loses heat, it may not be adviseable. In that case the Focus Pro, which is digital, but not porgrammable, will work just fine. Both will work on steam, just set to 1 cycle per hour, since they come from the factory set to 6 cycles per hour.
  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Thanks, Boilerpro!

    All the Chronotherms from the early Chronotherm 3's, Chronotherm 4's and now the VisionPros feature Adaptive Intelligent Recovery, a program that insures that the set point temperature will be reached at the time the "wake" or "return" or whatever period starts. It remembers the previous 3 days temps, and gradually ramps up the system until the setpoint is reached. It's entirely automatic, and needs no input from the homeowner. The old mechanical timers just took the systems out of setback at a certain time, and the home could be hot or cold, until the system caught up. I'm glad the stats are working well for you.
  • Thad English
    Thad English Member Posts: 152
    I think I get it......


    So if my anticipator right now is at the highest setting of 1.2 then I have to make sure that the amp draw is below .6, right? Then teh system should run at about 1 cycle per hour? or is what I am asking just not possible with the T87?
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