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question? Aquastats, What does the Differential dial do

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Robert_4
Robert_4 Member Posts: 1
in THE MAIN WALL
looking for a non technical explaination for the differential dial on aquastats. what does it do and why would i use different settings?

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  • Unknown
    Unknown
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    Differential

    allows you to make adjustments to boiler water temperature functions such as domestic hot water, pump control etc.

    All switches used on boilers have a built in 10 degree diff.

    For example a SPDT control such as Honeywell L6006

    R-B provide high limit switching, normally closed opens on a temperatur rise. The built in 10 degrees is subtracted from the set point.

    EXAMPLE: Set at 200 degrees when boiler reaches 200 degrees the switch will open it will remake at 190 degrees.

    ADJUSTING THE DIFFERENTIAL

    R-B break and R-W make temperature will be the set point temperature plus the DIFFERENCE between the differential setting and 10 degrees.

    EXAMPLE: Set point at 140 degrees

    Differential 25

    On a temperature rise, the switch will break at 155 on a temperature fall it will remake at 130.

    This was determined by taking the diff 25 - 10 = 15 added to 140 = 155. Then the built in 10 subtracted from 140 = 130.

    Another important setting is that the low limit must be 20 degrees or greater seperation from hi-limit.

    EXAMPLE: Hi limit 200 degrees low limit can be 180 or 170 or 160.

    Hope this helps.
  • J.C.A.
    J.C.A. Member Posts: 349
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    good explanation Timmie !

    I've found by trial and error that the reaction time also becomes a factor where a tankless coil is involved , and will set the "differential" a bit lower if possible,than I would if the unit is a heat only application (say 10-12° as opposed to 20° re:8124 or equivalent)Capilary type controls take time to respond , and beating them to the punch comes with too much experience. The heating can take its time , but folks want their hot water on demand. Non adjustable(builtin) "differentials" tend to vary quite a bit according to boiler gauges. Some will be close , and some will be REAL slow to react .

    Chris (lets put in an indirect) A.
  • Terry
    Terry Member Posts: 186
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    side note

    ever wonder why standard water heater gas valves have a builtin/ non consistant differential of 30-55 Degress?

    Often wondered bout this after seeing water heater "morning sickness".... When a wtr htr is so well insulated along with that big differential. It may not fire until the tank has cooled right down.. which is typically the point that the HO wakes up & wants a hot shower!

    Terry
  • Unknown
    Unknown
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    Most water heaters

    use a rod and tube principal to sense water temperature change. They then activate a snap disc to open and close the valve seat. Per a factory bulletins from several water heater companies:

    If there is a difference greater than 10 degrees with a 3 to 6 gpm draw then the control needs calibration, check dip tube, remove and clean sensing element (rod and tube) or replace the existing water heater control. Greater than 6 gpm may cause an appreciable drop in temperature. It is recommended that tank size be refigured in cases of high usuage or large family size. This all has to do with recovery capacity which is related to BTU input as compared to gallons of water and the weight of a gallon of water (roughtly 8 lbs per gallon) Rule it takes one BTU to raise one pound of water one degree (F). Increase BTU with gallons constant greater recovery.
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