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Thermostat Recommendation For Gas Fired Single Pipe Steam ?

SteamHeat
SteamHeat Member Posts: 151
Can anyone recommend a reliable thermostat for gas fired single pipe steam ?



I presently have a Honeywell RTH-6400D Thermostat wired into my Gas Fired Single Pipe Steam system. There are two problems.



One is that the temperature differential is only 1 degree and there is nothing in the install or user manual indicating that the differential can be adjusted.  The room temperature drops one degree, the boiler goes on and runs for 15 to twenty minutes because the first ten minutes are just to warm up the first radiator with boiling steam. One hour later it's on again.



I think a differential of 2 or 3 degrees would be more energy efficent because of the gas used to bring the water to a boil.



The second problem I have with the RTH6400D is that the inside temperature display rarely changes even when another accurate thermometer placed alongside the thermostat does by several degrees.



I have tried a second RTH6400D and it functions exactly the same, so it seems these observations reflect the design of the T'Stat not just a defective unit.



Can anyone recommend a reliable thermostat for gas fired single pipe steam ?



Thanks.

Comments

  • steam rated thermostat

    most people like the steam rated honeywell visionpro, set for 1 cph. an advantage of this model is the possibility of using a remote sensor which can be up to 100 ft. away. this enables the sensing to take place in the best possible sensing location, while the controlling "head" is placed in a more convenient  location, for changing settings etc. this feature was why i chose it.

    do a search here, and see what the general consensus is.--nbc
  • SteamHeat
    SteamHeat Member Posts: 151
    edited December 2009
    Differential ?

    Honeywell told me on the phone none of their current thermostats have more than a 1 degree differential. This sounds very odd to me.



    Does the Visionpro have this same 1 degree temperature differential ?



    Thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 15,411
    Short answer

    is yes.  The Visionpro does have a one degree (F) dead band.  Pretty much all of them do.



    The older mercury and bimetal had a similar deadband, and the really old all bimetal ones had no deadband at all.  Both of those had anticipators which heated the thermostat element up to fool it and set the run time.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamHeat
    SteamHeat Member Posts: 151
    Robertshaw Thermostats ?

    Thanks for the info.



    Someone told me that Robertshaw thermostats had adjustable differentials. I found their website and they do indeed have many models that have adjustable differentials (some as much as +/- 0.5 to +/- 3.0 degrees).



    Does anyone have any experience with this brand of thermostat either positive or negative ?



    Thanks.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,167
    I like the T87 heat only.

    make sure to trip the correct micro switches on the back for steam use. Customers with steam seem to prefer the simplicity of the round stats. They also make deep set backs harder to do as they are non-programmable
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
    https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating
  • john58
    john58 Member Posts: 7
    Differential setting

    The Maple Chase 9600 programmable t-stat has an adjustable diff range of 1 to 3 degrees. I have used many of these t-stats with good success, Would this t-stat work for you? John G
  • GBriggs
    GBriggs Member Posts: 15
    Steam Thermostat

    HI,

    Your stat should be set for 1CPH for steam service. This is done using the installer set up instructions. Most all Honeywell stats are set for Gas/Warm air 5 or 6 CPH out of the box as this is what heats most of the US. By changing the CPH you will have the best performance for your system.

    Now as for the inside display not changing Honeywell uses a temperature rounding algorithm that requires about one and three quarter degree change before the display will change. If you system is running correctly you might never see the display change.

    I hope this helps and you have a good stat too.
  • john58
    john58 Member Posts: 7
    Thermostats for steam CPH- why dif from hot air heat

    This is very educational. Why is CPH different for steam versus forced hot air heat?

    John G
  • Jamie Hall
    Jamie Hall Member Posts: 15,411
    Because...

    steam heat (and gravity hot water, which also should be 1 cycle per hour) has tremendous "thermal inertia".  That is, the radiation itself stores a lot of heat.  Takes time to warm it up, but it also takes time for it to cool off (radiators are often warm to the touch a half hour or more after the boiler shuts off).  Those warm radiators continue to heat the room air and you by radiation so long as they are warm.  Hot air, on the other hand, has almost no thermal inertia at all -- when the burner is firing and the blower is going, you're getting warm air blowing out the registers.  When the thermostat says stop, it does.  Right now.  10 minutes later and, if it didn't come back on, you'd start feeling a little chilly.



    There are hybrid applications.  For instance, if you have steam driving a unit heater or an air handler, the situation is more like hot air than "regular" steam -- and you may want to adjust the cycles per hour differently, to suit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Sly
    Sly Member Posts: 29
    CPH

    On the Honeywell Vision Pro once the temp is set how long does each cph run after to maintain the temp.  Is is fixed at 20 min or does it vary?



    thanks

    sil
  • Sly
    Sly Member Posts: 29
    CPH

    Sorry the visionpro is set to one for steam!!!! 



    sil
  • Jamie Hall
    Jamie Hall Member Posts: 15,411
    It varies

    The VisionPro will sense the space temperature and shut down at the top end of its dead band.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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