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enter image description hereenter image description here....Question: On my gas furnace (DMO Industries MM series) electrical schematic diagram there are two places that have a symbol of a plus sign inside a circle.....what does it mean? (see wires 39 and 40 on photo going to symbol of plus sign inside circle on 2nd photo....see wire from GROUND on ingnition module going to same symbol on first photo ...Note: All grounds on diagram (furnace, blower, inducer motor grounds) are not denoted by this symbol but usual ground symbol)

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    Can you post a photo of the diagram? – kponz Nov 10 '17 at 6:07
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    Schematics don't show physical placement. They show electrical interconnections. To see physical placement you need the wiring diagram. Furnaces usually have both inside the fan cover. The symbol could mean many things and if there is no legend you will have to research it further. My wild guess is that it is a thermocouple. There should be at least one to detect flame and possibly others for high temperature limit, etc. Without seeing the schematic though it is just a guess. – ArchonOSX Nov 10 '17 at 9:39
  • ...Description: one of the 2 symbols has a wire from the ground burner wire terminal on the Honeywell Ignition module going to it,,,,the 2nd plus inside circle symbol appears beside the Blower Motor Relay with a wire from the main furnace ground and a wire from the 24v ground terminal on the ignition module both going to it......furnace uses Honeywell ignition module S8610U and Honeywell Gas Valve VR8204. – jackodylan Nov 11 '17 at 4:39
  • @jackodylan -- post the images to imgur and post a link to them in the comments, then we can edit them in – ThreePhaseEel Nov 11 '17 at 4:42
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They're ground screws

Those are screw-head symbols (stylized Philips heads) that represent where chassis ground wires are screwed to the furnace chassis/sheet metal. I cannot tell you why they're used for inside-the-control-box grounds though (vs. the use of the regular earth symbol for outside-the-control-box-grounds). Perhaps it has something to do with them being explicit wire connection points?

  • Thank you....But why would they use that symbol for those grounds and the regular ground symbol for the main furnace ground and the grounds for the inducer and blower motors? – jackodylan Nov 11 '17 at 4:54
  • and.....the screw head symbol beside the Blower Motor Relay....does that mean those two wires 39 and 40 must be grounded onto a screw that holds the BMR in the component box? – jackodylan Nov 11 '17 at 4:59
  • @jackodylan can you post a link to the full wiring diagram please? – ThreePhaseEel Nov 11 '17 at 16:37
  • see page 21 of the manual for the furnace at this link...inspectapedia.com/heat/… – jackodylan Nov 12 '17 at 18:25
  • My question is now is what the location of the two symbols of plus sign inside circle on the wiring diagram at the link means.....because if that symbol means to be grounded on the furnace frame it shouldnt matter then where on furnace frame those wires are grounded...right? But something tells me the location of those 2 symbols does mean something more than just to ground on furnace frame....? – jackodylan Nov 12 '17 at 18:29

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