Luxaire cycles burners on high limit (set at 150F)


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Old 02-14-14, 12:00 PM
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Luxaire cycles burners on high limit (set at 150F)

Hi

I have a Luxaire upflow 90+ furnace from 1987. The problem is that the furnace cycles the burners on the high limit. It has a Honeywell fan-limit switch (L4064Y-1433). The control module is a Honeywell 50E47-70.

This Honeywell fan-limit switch has a built-in timed heater element, so that the main blower turns on after a time period (as opposed to the Fan-On setting). The main blower usually goes on in about a minute, at around 100F.

In any event, the three settings on the fan-limit switch are Fan-Off at 85F, Fan-On at 125F, Limit at 150F (Limit is set right against the Limit-Stop at 150F).

Even though the Limit is set at 150F, it appears to turn the burners off at about 130F (as measured from inside the plenum about 4 feet away from the switch and as seen on the face of dial of the fan-limit switch).

Here is the basic sequence:
1. call for heat
2. inducer starts
3. ignitor glows for 30 seconds or so
4. gas valve opens
5. burners ignite
6. about a minute after the burners ignite, main blower starts up
7. burners run for about 2-3 minutes until about 130F and then gas valve closes and burners shut off
-- cycle repeat --
a. ignitor immediately starts glowing again after burners shut off
b. gas valve opens again
c. burners run again for about 2-3 minutes until about 130F and then gas valves shuts off
-- cycle repeat --

These last three steps (a) to (c) keep happening until the t-stat has been satisfied. During these three steps, the inducer and main blower keep running.

Some questions.
Is the 150F limit setting/stop too low? Should it be higher? If set higher, will the cycles just be longer (but still cycling)?

Once the main blower comes on, should the fan limit dial start rotating to a lower temperature? It seems it should.

Thanks.
 
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Old 02-14-14, 12:34 PM
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You must have a very similar Luxaire to mine. My hi limit is also maxed out at 150f.

You are restricting the air flow thru your furnace. Possibly a problem with the blower fan, blocked return register(s), too many closed down supply registers or generally too small return ducting. A dirty A/C coil can also cause issues.
 
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Old 02-14-14, 12:46 PM
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Here's a bit more data.

My unit may be oversized (100K btu for a 1600 sq. ft. house in Wisconsin), but all the supply vents and returns are open and unobstructed. I seem to get sufficient flow through the vents.

Note, I also ran the furnace with the lower door off (door to main blower) to give it more return air and it did not seem to slow down the rise/heating of the fan-limit switch -- still burners off at 130F. Nothing seemed to changed.

1. Should this unit be able to run properly with the high limit set at 150F? In other words, when the main blower comes on, what should the fan-limit switch do (continue to rise, stabilize at that point, or lower??)

2. Does the 130F reading, mean that the limit setting (at 150F) is off by 20 degrees, or is that just the way it is?

3. To clean the A/C coil, I have to go into the plenum area above the furnace, right? I assume once I open it, I just have to vacuum things out or is it more complicated?

4. Could this also be a dirty/clogged secondary heat exchanger (which is located above the main blower)?

Thanks again, folks, much appreciated.
 
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Old 02-14-14, 01:24 PM
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My fan/limit control runs the blower fan for heat as well as the hi limit for the burners. My dial rises rapidly until the blower starts and then it creeps up very slowly towards the hi limit point. It doesn't usually hit the hi limit unless someone has a register or two closed too much.
Leaving my blower door open has a noticeable effect on it which tells me that the my return ductwork is undersized.

Those fan/limit switches are usually fairly accurate so either yours has a problem or you're measuring it too far away and it actually sees a higher temperature.
 
 

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