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Old Thermostat Differential Question

CKNJ Member Posts: 57
During the lockdown, I had been reading old (pre-war) and early post war materials, mainly Honeywell. I began to notice that for coal fired and some oil fired systems the thermostat temperature differential (swing) was really varied. Example is the T-42A controlling a coal feed was 2 degrees (non adjustable), while their Acratherm line (with a heater) was recommended to be set for 1 degree (but adjustable).

And this got me thinking. With for example a one pipe system, is it better to have say a 1 degree differential or a 2 degree differential? Was there any particular reason they would be so varied? Really appreciate any answers.


  • Steamhead
    Steamhead Member Posts: 16,821
    Much of this was dictated by how long it took for the system to respond. Coal boilers responded slowly so longer cycles were called for. Oil and gas were faster in those old boilers but not as fast as with newer boilers.

    Another of the biggest factors is how well the air leaves the system. Today, we install large main vents so the steam will distribute quickly. This shortens the time needed to heat the building.

    What kind of boiler do you have? Are your mains well vented? Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • CKNJ
    CKNJ Member Posts: 57
    edited June 2021
    Thanks Steamhead. My single pipe system operates very well (well vented mains and while 2 story single zone (built around 1900), I have balanced the temp difference between 1st and 2nd floor to within 1 degree of thermostat setting). Have read all of Dan's books too along with other period materials.

    I was just looking for some education on what the thought process was at the time. I have my differential at 1 degree, but when i saw some of these old books ranging between 1 and 2 degree differentials, i couldn't find anything to explain why. I was just curious if they thought 2 degree differential would provide any cost savings over 1 degree? Or was there something else.
  • mattmia2
    mattmia2 Member Posts: 9,622
    Probably more like 2 degrees was close enough and still far better than having to shovel coal and adjust dampers
  • Big Ed_4
    Big Ed_4 Member Posts: 2,780
    I have been taught 4 degrees setting was the ideal thermostat differential setting back in the days we could adjust them ...
    I have enough experience to know , that I dont know it all