Thermostat adjustment

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.
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Don
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That is the anticipator which controls temp swing, how even your thermosat will keep the temp. For a round honywell to the left gives more swing , more even temp to the right.
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Heat anticipator. It is an adjustment that controls how fast the system cycles. At a higher setting, the furnace will run for longer periods of time per cycle.
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