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Thermostat adjustment

Discussion in 'Home Power and Microgeneration' started by Don Lee, Jan 30, 2004.

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  1. Don Lee

    Don Lee Guest

    My thermostat at home is the type with a mercury-in-glass contact type.
    Besides the temperature setting, there is also another adjustment with
    numbers ranging from .18-.9. Can anyone tell me what this second adjustment
    is for? The house is using in floor water radiant heat system. TIA.
  2. Most likely setpoint range. A perfect single thermostat would power on the
    heat at a temperature, and then almost immediately flip it off as the air
    warmed. This isn't healthy for burn chambers, motors, etc. Allowing a couple
    of degree slop between power on and power off provides a reasonable time for
    the system to operate.
  3. Don Lee

    Don Lee Guest

    So, in my case with in floor water radiant heating where the temperature
    does not change quickly, I should set this to a tight range or low number?
  4. Steve Young

    Steve Young Guest

    For the life of me I can't now remember the technical name of it (over-run
    compensator, anticipator?). What it is is a small single wire rheostat which is
    actually a small heater in the thermostat. It has markings on it that
    approximate the current required for the gas valve. You set it for the label
    current marked on the particular valve. It's purpose is to keep a furnace from
    overrunning and over heating the space. What is does, is adjust for the delay of
    the new heat making its way to the thermostat. Correctly set, it will reduce the
    temperature swings of the space, though will cause a furnace to cycle more often
    for shorter runs.
  5. It's an "anticipator", that shuts the thermostat off before the air
    reaches the setpoint, as the air temperature will continue to rise for
    a period of time after the thermostat shuts off. The length of this
    period of time varies depending on the furnace type, house
    construction, and a number of other factors, so it's adjustable.
  6. daestrom

    daestrom Guest

    It's call the 'heating anticpator' adjustment.

    When a heating system kicks on, it puts a lot of heat into the baseboard or
    the forced air heat-exchanger. So when the thermostat senses that room
    temperature is where you set it, and shuts off the heater, heat continues to
    be delivered. In forced-air units, the blower runs for a short time *after*
    the burner shuts off. For baseboard units, the heating element in the
    baseboard is still warm after the flow/electricity is shut off.

    So the room would tend to warm *above* the thermostat setpoint after the
    thermostat shuts off the heat.

    To help minimize this, the 'anticipator' is used. Usually a small nichrome
    wire resistor in series with the circuit, it heats up the thermostat
    slightly *above* the room temperature (about 1F warmer). So the thermostat
    reaches the setpoint *before* the room actually warms to the setpoint. It
    shuts off the heater and the stored heat in the baseboard continues to warm
    the room the rest of the way up to the desired temperature (about 1 more
    degree F).

    The numbers are the current flow expected through the thermostat when
    calling for heat. This could be the gas-valve current, or zone valve
    current for a hot-water baseboard system. If you're really fussy about the
    exact room temperature, you can calibrate this so the room temperature
    'coasts' up to the desired reading after the heater shuts off. Most folks
    don't notice a half a degree or degree overshoot and don't worry about it.

  7. daestrom

    daestrom Guest

    No, it's not a 'span adjustment'. It is a heat anticipator. See my other
    post for details.

  8. m Ransley

    m Ransley Guest

    Setting the anticipator is a matter of comfort and cycles per hr . You
    dont want to get cold between cycles having a 3-4 degree spread and
    cylcling every 1-4 hrs. But you really dont want it so tight it cycles
    5 -7 times an hr. More cycling means more wear and tear on equipment .
    Each system and house are unique . Adjust for a balance of the 2
    getting comfort and proper cycle rate.
  9. Caleb Hess

    Caleb Hess Guest

    Several posters have described the function of a heat anticipator. This
    reminds me of a Heathkit gadget I had ~20 years ago. It was a pre-microprocessor
    auto setback thermostat. Used a mechanical timer to switch the heat
    anticipator on and off, thus shifting the thermostat's effective switch
    point relative to room temperature.
  10. Don Lee

    Don Lee Guest

    Thanks for the input. I take it that I will have to play with each
    thermostat in each zone, and that the higher the current (number) the
    earlier the thermostat shuts down the heater, and in my case the radiant
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