Lennox G20 - Damper stuck in open position

Problems related to residential installations.

Moderator: juster

Post Reply
LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Thu Dec 11, 2014 2:13 pm

Hello,

I am new to this forum. After reading several posts at the Residential forum to familiarize myself with Furnace issues, now, I am able to communicate about the damper issue with relative ease.

Our home runs on Lennox G20 (1996). The label says, Whisper Heat by LENNOX.
It is running very well.

After reading the post Lennox G20 Heat Issue, it becomes clear that the damper in our's is stuck open.

The damper flaps a little few times before Pilot lights.

I took off the damper door assembly to restore it to closed state -- as it would be before the Pilot call. Unsuccessful.
Hence I ask, how to restore it to closed state?

I will post a picture.
TiA!

LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

- Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Fri Dec 12, 2014 10:59 am

Here is a progress note.

Furnace is running, and home is heated.
Hoping to prevent interruption as the outside temp is close to freezing.

I read through the Operation Sequence 1 through 10.

The 5th step reads "CAB of the blower control energizes the damper motor (B17 on diagram) which opens the damper door. When door is in full open position, damper prove switch (S64 in diagram) closes".

With the damper door stuck open, what happens to the step #10? Are all three (a,b,c) cancelled?

A breakdown of step #10 of operation sequence is as -
a) when the heat demand is satisfied W1 of the thermostat de-energizes the gas valve and the damper spring closes the damper door.
b) as the damper door closes, the damper prove switch opens.
c) the blower runs for a designated period, which in our setup blower control time is set for 90 sec.

Is the damper door motor SYNCHRON pn 51H7401 needs replacing?
Any advice on where to buy?

Thank You!

p.s. I do not see option for loading photos?

LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

- Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Sat Dec 13, 2014 8:55 am

The furnace thermostat is set at 70º. It appears to be cycling fine with damper door stuck in open position. Any likely consequence of open damper door is not immediately visible -- a higher gas bill, limit switches at risk, or early failure of heat exchanger?

After reading the furnace's operation sequence few more times, I am isolating it to the failure of damper door motor -- SYNCHORN 51H7401.

The Lennox Plus store has it in stock but would not sell to a homeowner -- same response from other area supply store .
Now looking at purchasing online.

Thank you.

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- Lennox G20 - Damper stuck in open position

Post by Freon » Sat Dec 13, 2014 3:05 pm

If the damper door isn't 100% open the furnace ignition sequence stops. In your case with a hot air furnace, I'd set the damper to always open and forget it. The amount of heat you'd lose is negligible compared to the hassle. Now if it was a hot water boiler, then a damper begins to make sense.

Maybe the door motor needs a little oil. check to see if you can hear the motor try and work. Check that there's voltage getting to the motor. Otherwise look online or call a Lennox dealer for the part.

LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

- Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Sun Dec 14, 2014 12:13 pm

Would a fully and permanently opened damper door means the door prove switch stays closed -- spring pressing against it?
Is there a DIY for testing and then repairing the damper door motor?

For diligence, ordered one damper door motor from an eBay seller.

Many thanks Freon!

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- Lennox G20 - Damper stuck in open position

Post by Freon » Mon Dec 15, 2014 2:33 pm

You should have a manual lever to permanently open the damper door. Not knowing exactly how your damper is constructed, I can't say where the switch is or how it's closed. However all doors when open usually allow the furnace to ignite.

You can look at the furnace wiring diagram to see where and how the damper door switch works.

LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

- Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Wed Dec 17, 2014 4:30 pm

Thank you for the reply Freon.

Heat is uninterrupted and working.
It helps to know that the stuck-open damper door would not cause a furnace failure.

The new SYNCHORN motor replacement arrived in the mail today.
I will look closely at the damper door for how it works - motor, the door, spring, prove switch, etc.
My skills for testing supply of volt and reading wiring diagram is that of a beginner, but I do have a Sperry DM-6400 multimeter.

I am cautious.
I will search more, and read before testing the supply or motor -- tinker and repair.

Many thanks.

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- Lennox G20 - Damper stuck in open position

Post by Freon » Sat Dec 20, 2014 11:06 am

Be sure you turn off the power to the furnace. I would think the switch that proves the door is open would be some form of button switch but that's just a guess. Look for the switch control wires (not motor power wires) and trace them to the switch's location. You can test the switch once you determine its mode of operation. Set the multimeter to Continuity or Ohms. Disconnect the leads from the switch to where they join the furnace wiring... hopefully it's a simple wire nut connection. With the 2 wires from the switch now connected to the multimeter, press the switch several times and make sure you get a closed/open circuit indication with each press. That will establish that the switch is working as it should.

LennoxHVAC
Most Valued Contributor
Posts: 48
Joined: Thu Dec 11, 2014 1:34 pm

- Lennox G20 - Damper stuck in open position

Post by LennoxHVAC » Sun Dec 21, 2014 12:00 pm

Thank you for the reply Freon.

Turned the thermostat to OFF. Switched OFF power to HVAC.
Slowly working my way, disconnected supply socket to control wires. Unassembled the damper door function - from motor to prove switch.

The motor wire connection to supply socket was hour long. I did not know how to remove it. Little anxious, I decide to not tinker with multimeter, let go of all testing.

Re-assembled the damper door with new SYNCHRON motor. Re-checked my repair.
Switched furnace back ON, turned the thermostat back to HEAT.
I observe the operation sequence, and watch it cycle few times. It is perfect.

What is different...
- with damper door stuck - The blower/fan coming ON was random after flame. Often the blower would not come ON and flame will shut off itself. At times, temperature needle will drop to below 60º at thermostat before the blower will come ON. Longer furnace run, uneven between COLD and HOT inside the house.

- after restoring the damper door - It cycles like stated in the operation sequence. Frequent and short furnace cycle (4-min + 90Sec) - even home temperature. Girls are not wearing socks on bare floor.

Many thanks.

Post Reply