Harman XXV Wont Start due to Differential Switch??

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New Member
Jan 17, 2023
I have an older 25th anniversary XXV stove, going on 12 years old.
After a Chimney cleaning a few months ago the stove would not start. I gathered it was from the ash and such so I cleaned all the piping it it start up fine. Recently though my stove has been having a problem that unless I bypass the Differential Switch (Pressure switch) my stove wont start.

So I went and did the following
Combustion motor was failing so I replaced that with a pellethead replacement motor, it sounds much better now.
Differential Switch, mine was old and I test it by applying negative pressure to the tube and I could hit it "trip" and connect
Door gasket
Hopper Gasket
Tube from hopper to Differential Switch, though the ID of the one I got online looks a tad smaller
Cleaned all exhaust pipes

The Differential Switch seems easy to manually trip so it looks like it would connect the circuit to start ignition
I can feel air coming out of the exhaust pipe when I pulled the stove away from the wall (xxv stove pipe adapter)
I have not replaced the Hopper Muffler, looked ok and I could get air through it when I pulled it
Amazon product ASIN B07CVJR529
I am a bit on a loss why the Differential Switch wont still connect?
As a follow up in case other people have the issue. It looks like it was a variety of issue in my case leading to the Differential Switch not sensing the proper draft. I replaced the hopper gasket, replaced the Differential Switch and also made sure the fines cleanout area removable gasket/metal cleanout was on tighter. After that was all done the Differential Switch is working and the stove will start.
The micro switch in the differential vacuum switch almost never fails. The bladder in the vacuum differential switch gets old and brittle from the heat and leaks and will not hold enough air to keep the switch closed. When this happens the switch should be replaced.
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