Weil-McLain EG/H: thermostat and pressuretrol shut the damper, but not the spill switch.

These always confused me as well and I am an electrician. Looking at the ladder diagram

Basically 4 wires to the damper:

1. Constant hot 24v from transformer "R" Black wire to #1 on the damper
2. 24 volt common from transformer "C" Green wire to #4 on damper
3. #2 on damper yellow wire to 24v on ignition control (starts burner)
4. yellow at damper. Starts at "R" red on transformer through limit and operating controls to damper #5

I have looked at the internal damper wiring. The way I think it works is:

thermostat and controls call for heat, this pulls in a relay (internal in the damper) to open the damper but it just operates the relay. The damper power comes from the "constant hot wire". When the call for heat ends or pressure control or LWCO opens the damper loses its "signal" and the internal relay drops out and the damper is driven closed also by the constant hot.

As for the damper position. The limits and thermostat circuit have to be closed for the damper to open. Once the damper opens the damper motor is stopped by the internal switches. Once the damper is open if a limit or stat opens the damper will drive closed.

But if something was wired into the "constant hot" and that opened the damper will stop wherever it is.

I don't see a spill switch on the diagram. If it was wired into the constant hot and it opens the damper can't move.

The confusing part is the hot from the thermostat and limits just operates the internal damper relay. The damper power comes from the constant hot.

Hope this helps. Maybe @EdTheHeaterMan or someone else who knows this better than I do will comment.

To me an open damper just means the damper is open, with no guarantee of anything else.

Developing skills with reading the diagrams and use of a multimeter and tracing out the wires or the circuit path (with wiring diagrams or not) may be better for the troubleshooting process in the long run.

Assessing the state of the limits and controls should not take long and may be a quicker path to an accurate diagnosis than assuming an open damper means the same thing in every case.

@109A_5

In @JohnNY diagram there is no spill switch shown

In the first diagram you posted if the spill switch opens the damper will drive closed

In the second diagram you posted the spill switch opening will stop the burner and the damper will stay open if the controls are calling for heat.

So here are two other variations. The first example a tripped Blocked Vent Switch will close the damper. A tripped Flame Rollout Switch would result in the damper staying open.

In this case a tripped Blocked Vent Switch (SSW) or a tripped Flame Rollout Switch (RSW) would result in the damper staying open.

As far as your original Query "For troubleshooting purposes, I'm trying to explain to my guys that on a no-heat call the damper being open is the first indication there is a call for heat, but it will remain open if the spill switch is activated yet will close if the pressure is satisfied. The included wiring diagram does not support my claim. All the safeties and limits are before the end switch of the damper. Or am I reading this wrong? What goes on inside the damper has always been at the very limits of my electrical comprehension. Power to open, power to close? Whyyyyyy? Can't the damper just spring close like a zone valve?"

I might suggest that the Roll Out Fuse and the Spill Switch are not specifically noted in the Schematic Wiring Diagram or the Ladder Diagram. Look closely at the Roll Out Switch actual wiring on the appliance to see if that was added by the installer to break the circuit from the #5 on the damper plug to the 24V on the ignition control.

I'm not sure those two schematics are for the same damper, but in any case that synchronous motor isn't likely to be reversable. It'll drive open & closed going the same direction each time.

I'm not sure why they did it that way because now they have to be clever to make it work—and clever is rarely simple to understand or diagnose.

Synchronous motors like that will run in either direction. Often if needed there is a mechanical jamming mechanism to make them run (start) in only one direction (like an old clock) or if needed reverse and start in the other direction.

With @EdTheHeaterMan damper example it may be a switch with segments like a wafer switch or segmented slip rings.

With @EBEBRATT-Ed damper example the micro-switches use a cam assembly that rotates with the damper plate to monitor its position.

With the upper original schematic the motor control switch that changes state by the movement of the motor and indexed to the damper plate can be simplified and drawn like this in the lower redrawn schematic. Although the switch in the simplified drawing does not show the 90 degree damper plate indexing.

@109A_5 I saw some flaw in the wiring logic so I fixed it. That first old diagram I copied was never proofed to see if it works… it was just a concept drawing from taking apart some old damper that is no longer made. It took a long minute to find it but I knew it was on an old computer somewhere

@EBEBRATT-Ed said: @EdTheHeaterMan your internal diagram is giving me a migraine. Not understanding that one.

Take each circuit one at a time if you want to understand it.

Everything in Red is powered and the blue is the return path to the transformer. The purple Is the path from the rotary switch arm to the hot side of the damper motor.

The switch arm in the closed position has no power until there is a call for heat

That call for heat causes three things to happen (See Orange wires).  The Closed contacts get power to start the motor.  A relay coil causes the power to the Open section to the rotary switch to lose power and the motor then reaches the “Always Hot” portion of the rotary switch so it continues to operate until it is completely open. 

When the motor reaches the full open position the motor stops since there is no power to the Open contacts on the rotary switch. Also the Cam Switch (End switch) closes sending power to the burner circuit (Green wire).

The burner operates safely with the damper open. Eventually the call for heat or a limit stops the burner.

The orange call for heat looses power and the green to the burner looses power but the thermal switch may keep the damper open for a time in order to clear out the byproducts of combustion from the boiler or furnace.

Eventually the thermal switch will close and power the open portion of the rotary switch.  The motor will start and travel to the always hot portion of the rotary switch and finally end at the closed portion of the rotary switch then stop.  The cam switch will open as soon as the damper motor starts to close the damper. 

@EdTheHeaterMan My eyes aren't so good but the first diagram you posted looked like the relay contacts were NO instead of NC. I guess thats what threw me off.

OK @EdTheHeaterMan just conceptual, looks like it would work. Yeah the Flue Temp Switch is in a non-helpful place.

I think you would find the circuit board version is pretty much the same as the version that @EBEBRATT-Ed posted, just built different but electrically very similar.

They probably were NO on the first diagram. There were 2 other mistakes too. I fixed them in the final diagrams.

In the 1980s and 90s there was this toilet paper (janitorial supplies) salesman that would try to sell anything to make a buck and he tried to cash in on the automatic vent damper craze They were called Energy Stack Vent, and he needed someone that understood how to fix the mistakes his installers were making. I probably put in over 100 of them and fixed 200 more.

He would buy the vent damper from some manufacturer for about $60. and pay an installer $85. to install it. He was selling them for $600.00 and telling the customer that it would pay for itself in 2 to 3 years. The vent damper company went out of business because they made too many people sick with CO. But the guy made a small fortune for about 3 to 5 years selling hundreds of them. That is the piece of junk I took apart to get that diagram.  

I like the Honeywell D896 paperwork you were able to find.  Thanks for that.