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Thread: HSI's

  1. #1
    Join Date
    Jun 2005
    Posts
    1
    Hey guys Just a quick question, are there any hot surface ignition systems that are intermittent ignitions ( on for the complete burn for residental furnaces), would really apprciate a reply. My HVAC teacher says no but had a boss that says yes, just trying to clarify the situation.

    Thanks for any input

  2. #2
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    Generally speaking...
    the heating cycle of a Hot Surface Ignition cycle is like this.....


    Stat calls for heat
    Inducer comes on and runs for 30-45 seconds (Makes sure no gas fumes are in heating section before trying to light)
    Voltage is applied to hot surface ignitor
    (Some units apply voltage for about 10-15 seconds.. then shuts off.... then kicks back on again for about 15 seconds)
    Voltage is applied to main gas valve. Gas valve opens,
    gas is sent to burners, the gas hitting the HSI ignites.
    On the oposite end of the burners from the HSI is a flame sensor.
    Seems like 3-5 seconds after the sensor proves flame, the HSI goes back out. (I've never seen an HSI left on for the entire run cycle.)
    45-60 seconds passes and the main blower comes on.
    After the stat is satisfied.
    The gas valve shuts down
    60-180 seconds the blower shuts down.

    IF the flame sensor loses the flame signal during a run cycle, it will react as if the stat was satisfied but once the blower finally shuts down, it will start the run cycle over again.

    Some HSI's 'appear' to running during a run cycle because the flame is heating them up. But they do not have voltage applied.


    THere are some HSI systems that use a HSI to light a pilot instead of the main burner too.
    (Honeywell makes one called Smartvalve)
    It operates the same way, but instead of lighting the main burner... it lights a pilot. The pilot assembly has the
    hot surface ignitor and flame sensor built into one assembly. Once the flame sensor detects flame... the gas valve opens the main valve and lights the main burners using the pilot flame.

  3. #3
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    Be sure to fill out your profile and
    put in there that you are an HVAC Student.


  4. #4
    Join Date
    Mar 2003
    Location
    North St Paul MN
    Posts
    858
    I'm wondering if your boss is thinking that the HSI is on during the heat cycle as a flame sensor. Some older units did this (Rheem)
    I'd rather have a bottle in front of me, than a frontal lobotomy.

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