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Damper motor troubleshooting, Part I

Feature Story

By George Lanthier


A lot of us do work on what I call commercial burners. Those burners are, by my definition, anything that fires lo-hi-lo, also called two-stage, and that normally means a firing rate between 6.00 gallons per hour and 35.00 gph. All those burners that fire single-stage below 6.00 gph are nothing but big residential burners to me, while those that modulate and do multi-fuels are industrial.

Some who do troubleshooting around these burners are, in fact, terrified of them and that’s too bad since they are just larger versions of residential burners. I’m going after some of the scary parts on the servicing of these burners and hopefully I’ll give you some of my tricks along the way.

First things first. Most of the problems on these burners seem to come from the damper motor. Although damper motors are going through an evolution right now, the venerable, or is it infamous, Honeywell damper motor still rules the roost in the field. One of the things that you absolutely have to have, in my opinion, is a test light like the one shown in Figure 1. These are purchased at places like Radio Shack and can be made up with the clips for less than $4.

In Figure 2 we see the internal workings of the Honeywell damper motor and its wiring diagram in Figure 3. This motor is not only used for warm-air zoning, but it is also an integral part of the commercial burner.

The motor’s operation is very straightforward. When power is applied to the power terminals, the motor starts and continues to turn until the arm connected to it has traveled about 90 degrees. We say ‘power terminals” because the motor is available in both a 120 VAC model and a 24 VAC model. The Honeywell M436 motor has power terminals marked ‘L1-L2″ and the M836 has terminals marked ‘T-T.” Both motors are equipped with an auxiliary switch, or end-switch, that has terminals marked ‘R-W-B.” Terminals ‘R-W” are normally closed and terminals ‘R-B” are normally open.

Normally found on warm-air systems, the M836 would be powered by an additional transformer wired in parallel with the zone thermostat. The ‘R-B” terminals would then be connected to the system operating control.

Probably the most difficult thing about these motors is setting the switch for operation on commercial burners. In Figure 4, the motor is shown mounted on a Carlin burner. In Figure 5 we show you the inside of the motor and the location for adjusting the ‘make point” of the internal switch. In Figure 6, you see the location for adjusting the switch enlarged so that you can see the adjusting cam.

To make it easy to adjust the cam, you need a way to tell when the high-fire oil valve actually makes. The easy way is to use the small neon light shown in Figure 1 to create a ‘power-on indicator.” Place the test light across the incoming neutral or common terminal ‘L2″ and the powered terminal ‘B” of the switch. Remember, the switch makes R-B on the auxiliary switch during the power stroke and so this is also the terminal that your high-fire oil valve will be wired to.

The power stroke is when the motor shaft moves in the direction of the ‘open” arrow on the outside of the case. Power the motor so it runs to the open position. Note the point of the motor stroke at which the switch operates and your indicator light comes on. If the switch operates correctly for the application, proceed to check out the installation.

Never tamper with or adjust the linkage of any burner unless there are specific directions within the manufacturer’s literature for how to do it. This is the No. 1 way people screw up Carlin, Beckett and many other commercial burners. If it isn’t broken, you can’t fix it, but you can sure screw it up.

Keep in mind that the internal switch of Honeywell’s M436/M836 can be adjusted to operate at any point between 5 and 70 degrees of the motor stroke. It has a one- to two-degree non-adjustable differential. If the internal auxiliary switch of the M436/M836 must be adjusted, follow this procedure:

Remove the motor cover.
Install your power on indicator (test light).
Run through one cycle and determine if the switch is closing too soon or too late.
Shut down the motor.
Insert a screwdriver in the slot in the switch cam (white plastic) located near the center of the motor. Each slot in the cam equals approximately 20 degrees of motor rotation.
If the switch takes too long to open, adjust the switch to operate nearer the open (maximum rotation) motor position, move the cam one notch in the direction of the ‘close” arrow on the outside of the motor case.
If the switch is opening too soon, adjust the switch to operate nearer the closed motor position, move the cam one notch in the direction of the ‘open” arrow on the outside of the motor case.
Remove the screwdriver and operate the motor again. Check the point at which the switch makes and breaks.
Continue to adjust the cam in one-notch movements until correct operation is obtained.
Remove the power-on indicator and replace the cover.
With the cover in place, operate the motor through its complete open-close stroke. Check for proper operation, and make sure that the linkage does not bind and that the motor travels smoothly through its fully open and closed positions.

If the motor does not operate properly, check for proper voltage or mechanical binding in the linkage or damper. If the linkage sticks or binds on any surface, simply clean it with an abrasive such as grit-cloth or steel wool and then wipe it down with WD-40 or an equivalent. Since commercial burners operate in some nasty environments lots of things, including moisture or chemicals in the air, can mess up the linkage surface. A final motor checkout should prove that:

The motor operates the load.
The motor responds properly to the controller.
There is no binding of the linkage or motor.

Another thing that comes up is just when should that switch and the high-fire oil valve come on? Well, this is one place where a little smoke is allowed. With today’s burners you should be setting them for ‘true zero smoke;” a trace of smoke just doesn’t cut it any more and the heat exchangers show it.

The trick here is to achieve a smooth and quiet transition between low-fire to high-fire and back to low-fire on demand. We’ll talk about controls in the next article, but how long does transition take? In my experience, about five to 10 seconds and any smoke you make will be gone before you can find it. If the valve makes too late you will get a rough transition and may even shake the building a bit. Allow the motor to make the oil valve before high-fire air is reached and clean up the air setting on the damper. If you have zero smoke at high-fire and low-fire, who cares what it is in between? So, next month we’ll look at the controls that make these motors work and get rid of some more scary stuff.


See ya.


George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax (781) 641-7099 and his e-mail is FiredragonEnt@comcast.net



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