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fenwal 05-21 HSI ignition flame sense

Del Member Posts: 52
Tim-Was on a call on one of these boilers, ended up being a bad pressure switch as the boiler got hot, but at first I thought it was a flame sense issue.  The way the flame sensor is hooked into the wiring with a T tap connector threw me a bit and I'm wondering how this particular control senses the flame since the flame sensor is tied into the neutral of the ignitor leads.  Does the control send voltage back through the neutral somehow, or does the ignitor itself do anything in the process?  I know there are some that sense through the ignitor. thanks again.

<a href="http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/products/discontinued/discontinued-boilers/he-boiler/hevhe3hsisupp.pdf">http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/products/discontinued/discontinued-boilers/he-boiler/hevhe3hsisupp.pdf</a>


  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    Yes once igniter warm up time is complete

    the rectification system will be energized as the neutral leg of the igniter has zero potential it will allow the sensor to complete the circuit for the correct microamp signal back to the Fenwal control. Then the gas valve is powered if the signal is in the microamp range of the 05-21 Fenwal control. It is a little tricky as that neutral leg does double duty both to complete the circuit for the igniter and then the superimposed voltage signal for the rectification process.
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