New Trane XR-80 Furnace acting WONKY

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Old 01-03-17, 11:43 PM
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New Trane XR-80 Furnace acting WONKY

I just yesterday installed a new (old stock-the furnace is 4 years old but was never hooked up) Trane XR-80 100k BTU 80% efficient nat gas furnace in my shop.

I am using a Honeywell electronic thermostat utilizing only the R and W terminals on the thermostat and furnace (this should cause the blower fan to be in auto made, controlled by the furnace computer board if I understand right). I can hear it's internal relay clicking appropriately, so I think it's good.

The furnace appeared to work normally while I was checking it, raising the shop to 70 deg, and the burner, and then blower shutting off when the set-temperature was reached, but it appears to have a problem:

1) I ran the furnace a bit this morning, then turned it off and opened the doors for awhile to let moisture out. I closed the doors, and I turned the furnace on.

The furnace lit normally and ran for the time I was in there, and I left to have dinner (the shop was 36deg, and I set 68deg), expecting it to be warm when I returned..

At some later point, the flame apparently shut off (for no reason?) and the room was only 41 deg when I returned (thermostat was set well higher than the room temperature, so the burner should have been on), but the blower motor kept running,

I turned the mode switch on the thermometer from HEAT to OFF expecting the blower motor to stop (since the heat exchanger was stone cold and had been for a while I suspect), but it did not even after waiting 5 minutes.

I removed the front panel and the safety switch shut the blower down. I closed the switch and the furnace appeared reset. I then moved the thermostat mode switch from OFF to HEAT on the thermostat and the furnace lit and ran normally again.

The red DS-1 LED trouble code light blinks continuously 1 blink/sec with the furnace powered (but not running), and when heat is commanded, blinks about 2 blinks/sec.

I looked up the Trane codes and it said this is how it should blink if working properly, but obviously the blower motor should not (?) just keep running forever with the flame out for a considerable period.

When powered (115v to the unit, but not activated), the furnace emits a very quiet transformer hum (barely noticible).

As an aside, last night when leaving the shop I turned the gas supply off (until I observe it for a while, I am reluctant to have it running overnight unattended), but forgot to change the Thermostat mode switch to OFF, and when the shop cooled below the thermostat set point, I guess the furnace tried to light (but couldn't because there was no gas pressure). When I walked in this morning, the blower fan was STILL running.

Shouldn't the furnace sense that the heat exchanger is cold and turn the blower motor off? In the "old days" there was a "Fan Control Relay" that in "Auto" commanded the fan to run only when the heat exchanger was above a certain temperature (it was user adjustable).

There also was an adjustable "Limit Switch" that shut the gas off if the heat exchanger temperature exceeded a set limit.

I know modern furnaces have computer boards, but I would think they function about the same way?

So I guess my questions would be:

1) For what would my burner have shut off tonight (there is good gas pressure and plenty of airflow through the furnace, so I am sure the internal exchanger temp never rose above any "limit" sensor), and-
2) Why has my blower failed to shut off TWICE now (continuing to run for hours) after the flame went out while the unit was running (room temp still below set temperature) ?

Any suggestions welcome-
Bob
 
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Old 01-04-17, 12:12 AM
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Welcome to the forums.

The blower is time delayed from the control board.

It sounds like your furnace overheated. When you shut the power off and then checked the code it was normal because you reset it. You need to let it run and observe the LED without resetting power or opening the blower door.
 
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Old 01-04-17, 10:01 AM
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So if it senses an overheat, the fan blower will run until the motor bushings seize unless the power is interrupted by depowering the unit, or removing the cover?

In other words, there is no time after an overheat that the blower fan will ever stop on its own based on time or temperature? (Both times I mentioned, the blower had run for hours after shutdown, and the entire furnace was around 40 def F.)

The furnace has one 20X20X30 plenum boxes at each end. I don't have a filter in the intake now (It's installed in the photo below, but I slid it out after the first shut-down to see if that was restricting airflow through the unit), and the hot end has 400sq inches in louvers cut into it to discharge the air around the shop area. I picked the 400sqin number is it equals the area of the furnace discharge (20" X 20"), figuring it would provide a slight amount of back pressure.

The fan speed is selected "yellow-tap" or the lowest speed for heating only.

This is a picture of the installation-
I just have the power and fuel temporarily hooked up for testing of course)

There are louvers on the far right end of the plenum, and also on the bottom of the plenum, The sheet metal plenum is about 110 deg F when running (I have not measured the discharge air temp).

At what temp would the furnace sense an overheat?

None of the Trane codes I have talk about an overheat shutdown- would you know how many flashes that would be?

imagehosting

Thank-you,
Bob
 
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Old 01-04-17, 10:08 AM
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Is that a down draft furnace? It looks like the discharge air is on the left where the filter is located
 
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Old 01-04-17, 10:18 AM
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It is indeed a downdraft furnace, and the intake is on the left, with discharge on the right. The heating contractor who bent up my plenums recommended this orientation (left-to-right air flow).

In other words, the furnace is lying on it's "left side," with "down" as the furnace sees life, is on the right.

The discharge air is on the right side right past the heat exchanger.

Bob
 
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Old 01-04-17, 10:31 AM
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I would bump the blower speed up one step.

The code you'd get would be for open limit(s).
The furnace stays in "fan only" to let you know it has been tripped and to cool it down.
 
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Old 01-04-17, 10:37 AM
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Four flashes would indicate over heating.
 
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Old 01-04-17, 10:53 AM
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I will try the next higher blower speed, then. I was afraid of causing too much back pressure if I did that at first. It blows a lot of air now, but more would actually be better to circulate around the shop.

Should I cut more louvers into my discharge plenum to allow a higher airflow (more CFM past heat exchanger = lower exchanger temperature), or does it look OK?

OK, so "Open Limit" (4 flashes) doesn't mean the limit circuit is open (from a bad limit sensor or wiring fault), it means the sensor has tripped (opened up) due to sensing it's high limit temperature.

The overheat overrides the authority of the thermostat (temp switch AND mode switch) so once the high limit has been tripped, the blower fan NEVER stops until manually reset by 115v power removal?

Do I have it right now?

What would be the highest discharge temperature at the discharge plenum I should accept? I would say the far end of the metal plenum is now at 110 degF or so, which probably represents a 50 deg temp rise from ambient intake air temp.
 
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Old 01-04-17, 01:27 PM
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UPDATE-

I went out this morning, expecting to do a blower speed change. The shop was 40 deg F.

I figured I'd warm the shop a bit before doing the wire swap to the higher motor tap.

I hooked up 115vac noting 1 flash/second on the light. I then switched the thermostat to "HEAT (from OFF), expecting the usual sequence of draft inducer motor, igniter, ignition, and finally the blower cutting in, but instead I got a series of relays clicking in the furnace, with ONLY the blower motor starting a few seconds later. (no hot igniter, so flame, no long delay for the blower to start)

I climbed up the ladder to the furnace and could see the draft inducer motor was winding down (in the relay clicks it got powered briefly and had started, but the board evidently removed power immediately).

The red light was flashing code 9 blinks, or "Check Igniter." I didn't think a brand new igniter with maybe 10 starts on it would malfunction, so I unplugged it and checked it with an ohmmeter, and it showed GOOD at about 10 ohms cold.

Just in case, I re-plugged all of the block connectors on the IFC circuit board (after-all, the new furnace had sat unopened and unused for 3-4 years in the box), and no change.

Obviously I reset everything and tried it again-same deal.

BTW, I ASSUME that unpowering the furnace circuit re-sets the furnace computer board just like opening the front panel and allowing the safety switch to re-set it?

What the heck? It was working (running/heating) last night when I put it to bed by switching the thermostat to off, and after everything shut off, de-powering it's 115v circuit.

Ideas?
This is getting old fast.

(I checked furnace power for proper polarity and grounding-all good BTW)
 
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