Advice on Replacing Furnace Control Board


Old 12-13-16, 08:12 AM
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Advice on Replacing Furnace Control Board

We are about to occupy a house that has been unoccupied for an unknown period of time. The furnace is a
Luxaire model # pcuhbld12n100c furnace serial # EKHM791522

It doesn't show any indication it's getting power, and it appears the hot line wire going to the Control board has made a burnt mark at its attachment point. Name:  burntCB-20161212_200947.jpg
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When I press in the hot wire, it began to make noise but quickly disconnected. I didn't push it again.
I'll remove the board, and I'll try to solder the failure point, but I don't expect success. I'll looking at a new board.
When I look up the model number I get
OEM Part - Manufacturer #S1-33103010000
as the replacement conrol board.
But these replacement boards don't look like the one in the furnace. See:Name:  FurContBoard20161212_200902.jpg
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Is this just that the new boards look different? Can I safely buy the ones listed as #S1-33103010000 replacements?
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Old 12-13-16, 11:23 AM
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Usually replacement boards have the same layout for the connections. Sometimes a pin or two moved or in rare cases an adapter harness comes with it. See what the part number is on the board itself as maybe it is not original.

Here is the generic replacement which you can see if the connections are the same.
Old 12-13-16, 11:59 AM
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Replacement boards can be a nuisance. Your furnace lists two different boards and neither looks exactly like your old one.

As mentioned.... we need the full number of your current board. My guess is that may not be the original board.
Old 12-15-16, 10:25 PM
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We already have the replacement control board. Actually, someone else ordered it using the numbers I provided.

New Board 50A56-956
existing Board 50A55-843

The first 4 digits are encouraging.

I'm looking it up now.

The existing 50A55-843 is (second row in 1st table)
Universal Integrated Ignition & Fan Control Boards
The New 50A56-956 (seventh row in 2nd table) is described in the next table as a
OEM Specific/ Direct Replacements

The last replacement number is the right cell of the New one lists S133103010000, which is a board number listed for the pcuhbld12n100c model furnace.

I'll have to do some reading before I attempt the replacement. I have coffee, and It's warm where I'm at now.
The New comes with a metal mounting plate.
I'm a bit concerned about the cause of failure. As you can see from the earlier pics, where the brown power wire connects to the board (on the left), it's burnt.
I'm concerned that when I install the new board, the same thing may happen.  50A55-843-Existing-Control-Board-Furnace-20161214_141422.jpg
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Last edited by Nehmo Sergheyev; 12-16-16 at 12:56 AM.
Old 12-15-16, 11:28 PM
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One other peculiarity I should point out. On the pic of the whole compartment, on the bottom of the control board plate, you can see the base of a disconnected capacitor. On the floor, is one hanging from the wires with electrical tape on its connectors.
I don't like the position of the replacement capacitor. I'm worried water may have got to it.
Old 12-16-16, 04:38 AM
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New 50A56-956
Existing 50A55-843

Page 4 of these PDFs show the pinouts of the boards. They are pretty much the same. The New allows for a 2-speed Cooling attachments for the Circulator Blower, and the New has a Y1 wire on the thermostat.

For my specific job, I'll first test the capacitor on the floor to see if that is damaged. I'm concerned because of its position.
Then I'll remove the Existing board > physically install the New > transfer the wires from the Existing to New.
Old 12-18-16, 09:07 AM
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Job done

I successfully replaced the control board in the furnace, and the furnace now functions.
Looking at the old board now that it's out, I realize it would have been a major job to repair. The burn-out part to too large.
The existing board had a jumper between the Heat and the Cool terminal (actually, the terminals for the circulation fan), and the New board has Heat plus High and Low Cool. I had to choose one of the Cool terminals for the wire previously attached to Cool, so, I picked High Cool.

When everything was hooked up, I rigged the door switch closed and started the furnace. It started up but then turned off.
The 4-flashes indicated an open limit switch. These limit switches are simply attached in series and none can be open. Since I didn't hear the circulation fan turn on, I reasoned the limit switch labeled 180 F, was opening because the fan didn't cool the chamber. Sure enough, the circulation fan was stuck simply from being still for so long. Moving it by hand loosened it.
The next time I started the furnace, the circulation fan started as it should, and everything worked.

It turned out the furnace only has one air outlet. (I hadn't noticed this before.) And even though the furnace blew continuously, it was so cold outside, it never caught up to the thermostat setting.
I still have to fix major holes in the vent work that release heat to the basement. The furnace had been installed in 1999. I don't know what was there before that, but the house is so old, there's what remains of a coal chute going into the basement.

This happened just at the right time too. It got bitter cold outside, and frozen sleet made driving impossible. When we tried, we couldn't get enough traction to climb up the hill to get out. We were lucky to be able to negotiate the car back into the garage.

My wife, dog, and I stayed in the house for a few hours. We did have coffee and a coffee maker. Eventually, we just walked out, leaving the car there. Name:  50A55-843 White-Rodgers Control Board, burnt out-20161217_065028.jpg
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Old 12-23-16, 11:23 AM
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Name:  Burnt-board-back-furnace20161218_172451.jpg
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Size:  37.5 KBI've already replaced the board, and things seem to be functioning normally.
But what caused this burn-out in the first place? It looks like a serious short, but how could that have happened spontaneously?
The left of the pic is toward up, and the burn hole is where the hot line (from the door switch) connects. The nearest other connection is to the transformer.
The furnace was installed in 1999, and I don't know when it failed, but it must have worked for several years.
The smoke from the failure was so bad it left smoke stains at every exit grille.

Last edited by Nehmo Sergheyev; 12-23-16 at 11:39 AM.

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