heat anticipator


  #1  
Old 09-30-10, 09:22 AM
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heat anticipator

Hi, I've been adjusting the heat anticipator on my son's furnace and I got to thinking about how it works. My understanding is that a small heating element
heats the thermostat mechanism slightly so it turns the furnace off a little early
so it doesn't "overshoot". The adjustment on the antcipator is a variable resistor
that changes the current to the heating element. On a heating only system there is 24v across the 2 thermostat wires. But when the thermostat switches the heat on, it completes the circuit and the 24v potential between the wires is gone. Where does the current for the heat anticipator come from? I'm sure I'm missing something basic. Can anyone explain it?

Thanks, Steve
 
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Old 10-01-10, 06:09 AM
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A voltage reading of 24v tells you that there is indeed power but there's also an open in the circuit because you are measuring the potential difference of each side of the break. When the t-stat calls for heat and closes the switch, the meter reading will say 0 because that is now the potential difference between the two sides.
 
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Old 10-01-10, 08:51 AM
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Right- so where does the current come from (to heat the anticipator) when the potential is zero?
Steve
 
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Old 10-01-10, 08:54 AM
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Current is flowing thru the resistor & can be measured with an amp meter. If you had a sensitive enough voltmeter you probably could also read a very small voltage drop.
 
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Old 10-01-10, 09:34 AM
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The current just comes from the low voltage circuit of your heating system. The anticipator is wired in series with the gas valve which is why the amp draw of the valve is used for setting the anticipator.
 
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Old 10-01-10, 09:51 AM
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Thanks, I just examined the thermostat carefully and realized that the antcipator is wired in series with the actual "switch". DUH I don't know what I was thinking!
Thanks, Steve
 
 

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