Help: Failing Secondary Heat Exchanger?

Old 02-24-12, 08:01 PM
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Help: Failing Secondary Heat Exchanger?

Recently, our high efficiency natural gas-fired furnace is not working properly. For instance, if I set the desired temperature to 68F, it could only go to 62F~64F. So we asked the technician to check it out. The furnace brand is shows as "Excel Comfort Systems" (Pro Series) with model #PSCMPE100U4 (Manufactured by Ducane). When we looked at the Furnace working, the symptom was that: The T-stat calls for heat and then the furnace starts. After several minutes (10 mins or so), a limit (seems due to overheating somewhere) is hit and the SmartValve shuts off the gas and the burner is turned off. After sometime, the furnace starts back on and then this process repeats. Basically, it could not produce enough heat.

So far, this is what happened:
(1) He replaced the control board which did not help.
(2) He put a new Smart Valve in but no use.
(3) He adjusted the gas pressure on the Smart Valve. Now the furnace can keep running without hitting the limit. However, still not enough heat was produced. It took several hours to raise the room temperature by a few degrees (7 hours from 60F to 65F).

Now the technician told me that the Secondary Heat Exchanger could be bad. I can either look for the original installer for warranty or replace the entire furnace. Is this warranty covered by the manufacturer or the installer? Any other possibility for its problem?

The furnace was installed on 2004.

Any help would be appreciated.
Old 02-25-12, 02:38 AM
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You have a good example of why guessing and supposing are unreliable methods to use in repairing equipment.

Does the main limit switch open? If so, the furnace is shutting off for a reason --- it's overheating, usually because the air flow through the furnace is impaired for some reason. The most common reason is a plugged filter that needs to be cleaned or replaced.
Old 02-25-12, 04:44 AM
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a secondary reason for it to shut off on high temp limit would be a dirty blower wheel or plugged evap coil if you have a/c. It is very unlikely that the secondary heat exchanger is failed and causing your problem, but it could be plugged externally and causing your problem. I would find a new tech. If he didn't show you where it was failed then he didnt do his job and is just guessing.
Old 02-25-12, 05:09 PM
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I agree with both the other guys. Your tech isn't doing a very good job of diagnosing the real problem.

To answer your question about warranty: The heat exchanger, if it is the actual problem (not likely), should be under warranty. Warranties are almost always for parts only. Labor & shipping would be at your expense.

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