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Old 01-04-2016, 02:47 PM   #1
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Carrier furnace with 33-limit faults


We installed the Carrier 59MN7 furnace along with an a/c unit last summer. The a/c modulates the fan speed up and down as it cools, and we found that it was nice and quiet as it blew cooled air at the lowest fan speed. However, ever since we began running the furnace for the first season, we find that rather than heating at the lowest blower speed, the blower runs at full speed on every heat cycle. Then, the gas switches off and the fan returns to the normal "low" speed we have set for continuous fan operation.

I discovered the error code 33 showing on the thermostat, which means that due to repeated "limit faults", i.e. overheating, the system locks itself on high blower only, which exactly explains what we're experiencing. A power cycle will reset the fault condition, but I've found that it always returns within a few hours.

After copious study of the various Carrier docs, I've replaced the filter, checked to be sure registers are all open, set staging to Furnace, set the parameter Efficiency rather than Comfort. The only result so far is that now, it takes longer for the 33 error to appear; formerly, we could run for maybe 1/2 hour or so, but today, it's been up for around 4 hours so far, cycling on and off in low mode, yet I know eventually it will pop up the error again as it did sometime last night. It sure seems odd that it is able to run like this for hours and hours, yet at some point it always will fail again. Is it likely we just have a bad limit switch that is always on the ragged edge of popping?

I haven't heard from my dealer yet so I was hoping to hear any tips for anything else I might be able to do to fix this.

Hoping for news...

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Old 01-04-2016, 02:56 PM   #2
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Quote:
Carrier 59MN7
very complex expensive furnace, the dealer should come back and fix it.

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Old 01-04-2016, 03:28 PM   #3
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That is a modulating furnace and you need the dealer to find out why it is overheating. May not be modulating properly. May have a diagnostic history built into the circuit board. DO NOT reset the power or it may erase the codes and when the tech gets there he won't have them to work with or a proper batch.

Get him there soon as overheating is hard on the heat exchanger.
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