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Thermostats and CPH for Steam

Synbio
Synbio Member Posts: 14
I've read many times that Nest are bad for steam boilers- well it's what I have and I'm trying to determine if it's bad as people think it is. The main difference I can tell with other thermostats is that there is no way to set the dead-band hysteresis (stuck at 1F) and cycles per hour (CPH). I have a relay w/ 24V transformer so current drain is not an issue.

What I've seen on the forums is people use Honeywell with 1 or 2 CPH for their steam systems. Looking at my history, my Nest is doing on average 0.5 CPH (it runs every 2 hours). Sometimes up to 0.7 CPH or so, but never gets to 1 CPH. From my understanding, when you set the CPH on a thermostat like Honeywell, that is the max CPH allowed, so it isn't allowed to run more frequently than that. So, would I not benefit from this at all?

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,999
    Nest was designed for the mass market. That being the majority of applications would be on a Gas furnace with air conditioning. All relays (Heat, Cool and Fan relays) are powered by the same control (24VAC) transformer. Gas warm air systems are quite comfortable when there are 5 to 8 CPH. Steam systems require longer run times, therefore the installer must take that into consideration. When the installer is a DIY homeowner, that does not always happen.

    Also, the wiring of a residential steam boiler places the thermostat R connection on some connection point that has at least one operating control that will break the R terminal from the transformer to the R terminal on the thermostat. That operating control may be the Pressure control or the Low Water Cut Off (LWCO). These two controls may open the transformer to thermostat circuit more than once per call for heat. This open circuit causes the thermostat to produce an error code. Usually "No Power to the R terminal". To solve this Nest recommends an isolation relay. (you stated you have a relay)

    The cycles per hour selection on the Nest is the least frequent question on this forum. That us usually solved in the thermostat's Installer Setup menu.

    As far as your steamer operating once every 2 hours. that will depend on the heat loss of your building, the amount of radiation in the building, the outdoor temperature, and the boiler size. I wonder what the outdoor temperature was on the day(s) you were testing the system?
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
    Synbio
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,170
    @Synbio

    Bottom line is is the building staying comfortable?. If the boiler is short cycling that is not good too many cycles is hard on the equipment and may overshoot on temperature. If the building cools off between cycles then you don't have enough cycles
    Synbio
  • Synbio
    Synbio Member Posts: 14

    @Synbio

    Bottom line is is the building staying comfortable?. If the boiler is short cycling that is not good too many cycles is hard on the equipment and may overshoot on temperature. If the building cools off between cycles then you don't have enough cycles

    I would say overall it is comfortable. Only downside I can think of is that the pipes get cold in between cycles and have early cycle noises. I know there's some other issues I need to address like the wood hole the pipes are going through in a few places is too small so there is ticking.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,080
    What do you think about programing the nest to radiant , one of the few choices they give you . I am thinking for steam. trying to set it for the longest cycle .
    I have enough experience to know , that I dont know it all
  • Mad Dog_2
    Mad Dog_2 Member Posts: 3,942
    I still prefer my Honeywell T-87 Round (mercury-era) .  Nothing worked better on one pipe steam. I hoard themand purchase them at garage and church sales when I see them. Mad Dog
  • Synbio
    Synbio Member Posts: 14
    Big Ed_4 said:

    What do you think about programing the nest to radiant , one of the few choices they give you . I am thinking for steam. trying to set it for the longest cycle .

    I have mine set to Radiant. It's VERY important you do because it enables a feature called True Radiant. After a day or two, it learns how long your radiators take to heat up so that it doesn't overshoot. You can also use it for anticipation but my system heats so fast I disable that feature.
  • Synbio
    Synbio Member Posts: 14


    As far as your steamer operating once every 2 hours. that will depend on the heat loss of your building, the amount of radiation in the building, the outdoor temperature, and the boiler size. I wonder what the outdoor temperature was on the day(s) you were testing the system?

    We just had a record breaking low yesterday down to -10F before the wind chill. The boiler ran for 11 cycles in a 24 hr period. So it's still at 0.5 cycles per hour! The total run time reported by the Nest was 5.5 hrs. So the cycles average 30 min each. That doesn't include the "short cycling" within a cycle due to the vaporstat. The Nest reports it as the boiler is on even though it shut off waiting for the pressure to decrease, so the true run time is a little less.
  • Kickstand55
    Kickstand55 Member Posts: 84
    I had a customer that installed a Nest thermostat on a gas fired steam boiler. It was rapidly short cycling beating the gas valve to smithereens.
    For the sake of safety, I replaced the gas valve and installed an R845A relay and solved the problem, until months later the t'stat crashed. So, they got a T87F for keeps. No complaints yet.
    Mad Dog_2
  • Waher
    Waher Member Posts: 100
    edited February 6
    Ecobee does a better job with Steam than Nest. (this is with the default minimum 5 minute run-time setting) [Ecobee can adjust the minimum run time from 0 minutes to 30 minutes]
    Brutally cold subzero New England temperatures

    vs. more typical 30f temperatures

    The only short cycling is for the low water cut-off self test every 15 minutes that the thermostat is smart enough to figure out.
    Mad Dog_2Synbio
  • Mad Dog_2
    Mad Dog_2 Member Posts: 3,942
    I like it..mad 🐕 Dog