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Cycle Timing and Comfort

btc Member Posts: 43
This is a very interesting topic. I am having trouble with my single pipe steam system--both holding constant thermostat temperature and managing cycle times like Fred and others describe. If anyone has additional thoughts/ideas on the topic would appreciate hearing more about their situation and fixes. Thanks.


  • Drod
    Drod Member Posts: 59
    An earlier thread got me thinking about cycle timing

    I have a single pipe steam system that works very well, and is fairly efficient-thanks to all of you. I have a modern, programmable Honeywell thermostat that after much trial and error, I set on "Hold 69 degrees".
    The only complaint is from my wife. She says the house gets nice and warm just as she's leaving in the morning. It also get's nice and warm when she first gets home in the early afternoon, but it's not warm enough in the evening and during the day when she goes back home.
    I don't want to change the temp. setting. But, I was reading an earlier thread and now I'm wondering if the temp would even out if I chose a different cycle setting, i.e. so that it would cycle more frequently versus the steam system setting I've chosen (I think it's probably once or twice per hour).
    It's just so nice and warm when the boiler kicks on and the radiators heat up all the way...
    Any suggestions (besides the one I initally suggested to my wife about wearing a heavier sweater-I'm still paying for that one)?
  • R. Kalia
    R. Kalia Member Posts: 349
    pls clarify

    Even the steam setting is one cycle every hour, there shouldn't be any variations that last all morning or all evening. Especially since you don't have any recovery, you are using the same setting all the time.

    The Honeywell Chronotherms have a nasty feature in their firmware...once they get near the set point, they display the actual temperature as always being the set point, to make you think they are doing a perfect job of holding temperature. In fact there are always variations. Totaline thermostats display the atual temperature at all times.

    Have you measured the temperature variations, using a thermometer rather than a wife?
  • kk_2
    kk_2 Member Posts: 57
    Honeywell thermostats

    I was under the impression that they always round to the set point. I.e. if temp was 68.1 and set point was 69, they round up to 69. If temp was 69.9 it would round down to 69. If temp is 67.9 it should show 68 I would think.

    So you could have a 1.8 degree swing in temperature with the thermostat reporting no change.
  • R. Kalia
    R. Kalia Member Posts: 349
    more than that

    With a separate thermometer I have found that the difference can be 2-3 degrees (and it is not a difference in calibration because it can be 2-3 degrees either way). You can't trust the Honeywell display, so the OP needs a separate thermometer to check the exact temperature, and not go by how someone feels (especially since the OP apparently doesn't feel it).

    PS "rounding" 68.1 should give 68, and "rounding" 69.9 should give 70. So we agree that the Honeywell is deceptive.
  • Fred Harwood
    Fred Harwood Member Posts: 261

    My experience is that cycling a steam boiler once an hour results in temp overshoot, long cooling-off periods, and large temp differences in the house (mains, runouts, and rads should not go cold between cycles). Depending upon the system and house, a 15 to 20-minute cycle (on to on again) gives good results while not taxing the equipment too much. Even shorter cycles work well, but can wear out motors and relays sooner than a longer cycle. It all depends upon your preferences, the system, and the house.
  • Drod
    Drod Member Posts: 59
    Thanks, Everyone. Fred, your

    comment especially hit home. When the boiler starts to steam, I frequently hear sounds that indicate it's starting from cold, i.e. pipe expansion noise, etc. So, I suspect I must be overshooting the temp somewhat with a setting of only one cycle per hour.
  • Boiler Guy
    Boiler Guy Member Posts: 585

    I agree with Fred. THe more cycles the better. I have used Robertshaw 9701I computer stat complete with remote sensing to achieve a better average. This stat reads accurately to 1/10 of a degree. I set an old boiler up on high/low fire using 2 vaporstats to controll pressure @ 1.25 and 2.25 in conjunction with an Omron cycle timer to shut down 7.5 mins every 25 mins. The system works great. This building originally ran on/off @ 5.5lbs with EVERYONE complaining it was too hot. DUH! Not one temperature complaint since Dec 1 and it has been -1 to -51 here during that time. Thank you Dan for being such a TURNDOWN! (S)
  • Fred Harwood
    Fred Harwood Member Posts: 261
    cycle times

    For what it's worth, the standard T87F does the job spot on just by moving the anticipator to eliminate overshoot (usually lower amperage). Just don't cycle too often.
  • David Efflandt
    David Efflandt Member Posts: 152
    Thanks for improved comfort

    1-pipe oversize boiler running 1 cycle/hr on CT3500 rarely overshot, but felt a little cool between cycles and would pressure cycle (minimal pressuretrol setting) after about 20 minutes of run time.

    Last night I changed to 3 cycles/hr, so it runs often enough that the boiler steams quickly, but never gets up to 1 psi (no pressure cycling). I did have to adjust my vents to rebalance upstairs/downstairs (generally increased by varying amounts). Result is that the radiators are constantly warm and room temperature near thermostat seems to vary less than half a degree when around 0 degrees F outside. Hydronic comfort without the pumps. And I do not even have main vents (yet).
  • DaveF
    DaveF Member Posts: 7
    what about cost?

    Do all the extra cycles and pushing out air drive up fuel costs? That is my big concern. I don't care if it gets a little nippy between cycles.
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