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The irriating questions i just dont know!!! but i should!

tangled in wires
tangled in wires Member Posts: 23
in THE MAIN WALL
Why on "newer" furnaces do the blowers no longer come on due to temperature.  Instead they come on with a timer. they use a  60- 90 seconds delay or are told when to come on by an integrated fan control.  Why is this?  I know it must be due to the higher resistances in the new heat exchangers. My question is does the fan no longer operate based off temperature??

Comments

  • Ironman
    Ironman Member Posts: 7,441
    Design

    All of the newer furnaces are fired in a counterflow fashion: the flow of combustion is counter to the airflow. Ther burners are at the top and air flow goes up. If you attempted to use an electromechanical fan control inserted midway in the air stream, the upper portion of the heat exchanger would over-heat before the control responded.



    Carrier has used electronic boards since the 70's to control the fan.



    **** Smith was the engineer for Carrier who basically invented the first 90% furnace. After that, York hired him to do one for them. He spec'd the old style electromechanical fan control with an internal warp switch wired parallel to the gas valve to time on the fan. Some knuckle head in purchasing ordered the standard fan control and the first generation of those furnaces came through that way. The all had early heat exchanger failures.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tinman
    Tinman Member Posts: 2,808
    Timed on, timed off...

    has been the rule for quite awhile now on the forced air end of things. I think it's less troublesome. Multiple fan speeds as always to dial in temperature rise and dip switches that gives flexibility on the "fan off" settings.
    Steve Minnich
  • unclejohn
    unclejohn Member Posts: 1,833
    It's all

    About the money. less wiring in the furnace.
  • archibald tuttle
    archibald tuttle Member Posts: 1,094
    NFN but i got rid of the plenum temp switch before modern furnaces

    cause i couldn't rub two nickles together and my house had a cracked heat exchanger, but you would only get that sooty thing when the house fan wasn't running.  when the house fan was running, the air pressure in the plenum was plenty to exceed that in the fire chamber so no exhaust leakage.



    not something i'd recommend for clients but getting rid of the temp. sense bought be 4 years on that furnace . . . .



    brian

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