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I have a strange problem.

I have an IPC/Tempstar T9MPD075F12A3 condensing furnace, roughly 12 years old. It uses a Honeywell SV9541M "SMART VALVE".

EDIT: Just found the service manual, checking it out now: Service Manual

EDIT2: Was not much help.

The furnace is short cycling. The inducer comes on, the ignitor comes on, the flame comes on, the main blower comes on and stays on for a couple of minutes.

Then everything shuts down, blower first, then inducer for about a second, and then the startup cycle happens again. This goes on indefinitely.

There are NO trouble codes. I verified that the pressure switch is working both on and off, the limit switches are NOT tripping, the fire rod sensor/ignitor combo is OK (swapped it with the downstairs furnace that is working fine).

The interlock switch is functioning normally.

The odd part is that if I remove the bottom panel where the main blower is, and defeat the interlock the furnace runs fine without short-cycling. So it seems like the extra intake air solves the problem. There are no obstructions in the intake plenum, I even removed the filter and still get the same symptoms.

If it was a matter of not enough air causing the exchangers to overheat, wouldn't the limit switches trip? And wouldn't the valve blink a trouble code? If I manually pull the spade lug off of the limit switches the furnace shuts down and blinks a trouble code. Same with the pressure switch.

What could cause short cycling with no trouble codes, that resolves itself by opening the lower panel to let more air into the main blower?

EDIT: As a test, I've bypassed all the limit/pressure switches on the wiring diagram one at a time, and the problem still occurred every time. I thought maybe the control board mounted by the main blower might have a thermal problem that was alleviated by opening the bottom panel (then intake air flows across the board) but I put some fiberglass insulation over the board and could not get the problem to occur as long as the lower panel was open.

  • What sorts of static pressures are you seeing in your duct system? This easily could be a problem with excess static causing the blower to short-cycle on the plenum thermostat control without the high limit ever getting involved.... – ThreePhaseEel Nov 7 '18 at 1:38
  • @ThreePhaseEel Thanks- I don't have a way to measure the static pressure, but I don't see a plenum thermostat on the wiring diagram. Also this furnace has been working for 12 years and just started having an issue. I looked inside the intake and there's no restriction from the intake to the furnace, and the blower sounds normal. That's about as far as I've gotten on the blower side of the system. – John D Nov 7 '18 at 1:56
  • @ThreePhaseEel What I meant by "I don't see a plenum thermostat" was that the only plenum thermostat is in series with the other two limit switches, and causes a trouble code when it opens. I also shorted it out temporarily and the problem still occurs, no trouble code. – John D Nov 7 '18 at 3:50
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Do you have a single return grill near the center of the house or do you have numerous return grills in many rooms? If it is a single return grill, check to see if it has a filter that needs to be changed. Also it is possible that the blower fan's vanes have accumulated enough dirt and dust to reduce their ability to move enough air through the duct system but would allow the furnace to operate properly when the blower door is removed.

  • Single return, but removed the filter and looked into the duct. I could see the blower and there were no obstructions. I ran it without the filter and the problem still occurred. I switched the motor winding taps to run the blower at higher speed as a test, and as long as the lower panel is in place the problem still occurs. I'm back to thinking it's a thermal issue on the control board and the extra airflow across the board when the panel is open mitigates the problem. I may do a better test of that when I get time. – John D Nov 7 '18 at 16:02
  • I did have a highly rated HVAC company out to look at it and they were unable to diagnose the problem. Their solution was to install a new furnace, but they are booked through the end of the year so they can't do it until next year sometime. Great. – John D Nov 7 '18 at 16:02
  • The problem could also be something stuck in the fan blades (paper, piece of filter, accumulation of dirt on the blades, etc). Replacing the furnace after 12 years seems a bit of what we call "over kill", but if that is your solution and choice, go for it> – d.george Nov 8 '18 at 10:39
  • I've examined the fan blades with an inspection mirror, they look fine, nothing seems to be stuck and they're not excessively dirty. The fan sounds normal. I don't plan to replace the furnace if I can fix it, but the HVAC company wasn't interested in spending the necessary time to diagnose it since it's not an obvious problem. They played with it for an hour and said they couldn't figure it out so their solution was just to install a new furnace, that's not my planned course of action until I give up. – John D Nov 8 '18 at 15:52
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This turned out to be a thermal problem on the control board in the main blower compartment. By opening the lower panel, the extra airflow cooled the board and allowed the furnace to run. My experiment with putting insulation on the board with the cabinet open didn't sufficiently insulate the board to expose the problem.

Swapping the control board with our other identical furnace solved the problem in the original furnace and moved it to the other one, so I ordered a new control board.

There's no schematic available for the control board so it's not worth trying to troubleshoot any further than this.

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