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Reading Electrical Schematics - Vent Damper

Reading and understanding electrical schematics and ladder diagrams is not one of my strong points and I'd like to improve my troubleshooting abilities here, with your help, by asking some specific questions.

I often get "no heat" calls and find vent dampers no longer working, interrupting the safety chain and preventing the boiler from firing. I consider vent dampers non-essential and troublesome just like the car in front of me stopped at a green light while they are talking on their cell phone.

Referencing the schematic below, I see the vent damper receiving continuous 24 volts from "Y" (neutral) and "R" (hot) wires. When there is a call for heat and the master relay closes, 24 volts passes through closed aquastats and powers the vent damper "O" wire which somehow opens the vent damper and also closes a switch to wire terminal "P" which sends 24 volts to the power vent (if any), the remainder of the safety chain and eventually to the ignition control module to start the ignition process. So, to bypass the vent damper, all I have to do is make sure the vent damper is open and jumper the "O" and "P" wires, correct?

8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab


    EBEBRATT-Ed Member Posts: 15,622
    Yes. And with a defective damper I would remove R & Y from the damper as well.
  • ratio
    ratio Member Posts: 3,651
    Correct, a jumper from O to P will bypass the vent damper. It's my understanding, however, that bypass dampers have manual override switches, to physically hold it in the open position, which will keep the safely switch closed. It would be better to use that, if available.
    HVACNUT Member Posts: 5,863
    Years ago (maybe they still do, I dont know), Honeywell or the boiler manufacturer provided a Male molex jumper if the damper wasnt being installed. Just plug it into the aquastat.
  • Zman
    Zman Member Posts: 7,573
    The above advise is spot on.
    The way I visualize this, R and Y are the hot and common for the motor load. When R is energized, the damper motor opens the damper. O and P are connected to a switch which proves that the damper has in fact opened. It is just a switch in series with the flame rollout and others. In order for the boiler to fire, all of the safeties need to close.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • gerry gill
    gerry gill Member Posts: 3,078
    Yes sir. And somehow make sure the damper blade can never close again.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • pecmsg
    pecmsg Member Posts: 4,883
    Sorry I disagree...Never bypass a Safety. That's what a volt meter is for. Wiring, screwing, welding the damper open does not replace the function of the Safety!
  • mattmia2
    mattmia2 Member Posts: 9,747
    All the controls being black boxes does not make it easier to read. It isn't organized so well either.

    So 24 vac between o and y causes the motor to open the damper, then that trips an end switch that closes between o and p when the damper is open. This would be a lot easier to understand if they showed the damper instead of a disconnected plug.
    HVACNUT Member Posts: 5,863
    edited June 2020
    > @pecmsg said:
    > Sorry I disagree...Never bypass a Safety. That's what a volt meter is for. Wiring, screwing, welding the damper open does not replace the function of the Safety!

    Right, if the damper is not being used, then remove it completely. Cut a piece of 26 ga. to fill the gap or go to the next joint. 3 screws per joint.