Time to Replace Furnance/AC?

Old 09-02-02, 08:44 PM
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Time to Replace Furnance/AC?

My house was built in 1989. When I purchased it, it had the original gas furnance and cooling system. The furnace is a TRANE XE70. Though I live in Southern California, I use the air conditioning in frequently. Last summer, I used it regularly and it would cool the house
completely within an hour and a half.

This summer I used it once a month, evenings only, a couple of days. Everything cooled quickly and well same as before. Then, about a month ago, I went to turn it on and it sounded like the fan in the furnance had broken. The sound it makes is more than a hum...it almost sounds like the fan is having trouble turning or is something. Its noisy. I left it on for about an hour but the house didn't get cool and it appeared there was no air coming out of any of the vents.

So, I called a maintenance company. Their tech came out and cleaned the whole system. He tested the air and showed me it coming out of one of the downstairs vents (dummy me, I didn't have him check upstairs). He told me the system was old and cheap. That it would only work at about 80-90% of capacity and would be noisy. Otherwise, he didn't see any problem with it. He told me if I run it infrequently to run it 15 minutes a month to keep it oiled. The sound did not change with his work.

Two weeks later and this weekend was a scorcher. I turned on the A/C. The odd sound was still happening. I left the house for a couple of hours and came back to find that only the lower half of the house was cool -- at that, very little. The upstairs had hardly changed temp and I could barely detect any air flow.

I want to call the technician back but I don't have a clue what to tell him to look for. Can you provide me with some suggestions as to what might be the problem? The technician said the system is old. What is the life-span of this model? I don't really have money to throw around. The technician told me if I replace the furnance I should replace the A/C unit as well. Of course, I don't have too but is it advisable? He implied they should always be bought as a "unit" or they don't work well.

Thanks in advance for any help.
Old 09-03-02, 09:18 AM
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If the air filter is clean, then it sounds to me that your cooling coil is dirty and clogged. The evaporator coil is located usually above your furnace, there are 2 pipes that go to it, one will be insulated and the other is not. Usually PVC pipe is used for drainage. By your description, it doesn't sound like the service person cleaned the coil.

When a coil is dirty like that, what happens is the condensation doesn't drip off the coil and freezes on the coil. Once that happens, no air can get pass the coil. The result is, you don't get any air coming out of your vents and the air blows back towards the fan, which will eventually damage the fan. That's the noise you are probably hearing.

In my opinion, all you have to do is have the coil cleaned and have the system checked to determine if the air filter is doing its job correctly, so the coil doesn't get dirty. Because you are an infrequent user of the A/C, you probably don't need a new unit.
Old 09-03-02, 09:35 AM
lynn comstock
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It worked OK last year. The design is not the issue, something is wrong.

1) The blower could have lost a little clip that the factory attaches to a blade to balance the wheel. The wheel could be warped or bent. Spin it and look for wobble as it spins on the shaft. Unevenly weighted dirt in the wheel will cause the same imbalance.
2) I believe that the blower wheel slipping on the metal disk that connects it to the hub and motor may cause the noise. This can reduce airflow and generate considerable noise. With the power off, hold the hub and try to move the wheel rim in either direction.
3) The setscrew that holds the wheel to the shaft may be loose and allowing the wheel to slip with the same results as 2). Rocking the wheel rim sharply back and forth may show the slip for 2) or 3).
4) If you need to go further, FEEL each and every blade for movement at the crimps? A visual inspection is not enough.
5) Do not wear loose clothing around moving parts, and be CAUTIOUS of electrical hazards also. With that in mind and with the blower on, CAREFULLY press the HOUSING with your hand to see if the noise changes and feel for vibrations. Also press on other parts of the air handler to see if the noise changes. Pressing on the non-shaft end of the motor may also affect the noise. If the vibration and noise do NOT change, the wheel itself is the most likely source of the rattling noise,
6) If the problem is 2) or 3) I would expect the noise to increase with the fan access panel open.
7) Most systems need cleaning, but the problem developed too suddenly for the underlying problem to be dirt. The noise that started about the same time is a significant clue also.

Last edited by lynn comstock; 09-03-02 at 09:59 AM.
Old 09-03-02, 04:39 PM
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the most common

cause of what you describe is the motor bearings overheating, and either seizing or slowing the motor. both would cause diminished airflow hence loss of load and frozen coil. as lynn mentioned, the blower wheel may be loose from the shaft, and the noise either coming from the wheel scraping on the shaft or the wheel hitting housing.

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