Problem with home AC

Problems related to residential installations.

Problem with home AC

Postby craigm » Mon Jul 11, 2011 2:56 pm

I have a 2.5 story house with two HVAC units. The unit for the second and "third" floor is a a Carrier 38CKC030330.

The second and third floors always seem hot. If I have the thermostat set at 76 for those floors, the temp will get up to around 80 degrees for most of the day while the air conditioner is running. Typically you can't get the temp to 76 or below until the evening hours (I live in North Carolina so the outside temp gets up there). It will typically run almost constantly from 10-11am until 8-9pm with short breaks throughout the day (15-30 minutes).

I had someone come look at that AC unit a year or so ago and he said that the compressor wasn't keeping a constant pressure. He tried removing all the refrigerant and putting a "negative pressure" on the system for a certain amount of time, then refilled the system with refrigerant. After he did that, the system still wouldn't keep a constant pressure and he said that it looked like the compressor would need to be replaced, but due to the age of the compressor and building code changes I couldn't replace just the compressor and would have to replace the entire system. The estimated replacement cost for just this one system was about $7,500.

Does that sound like it could be true? Is there anyway that I could buy a replacement compressor and just pay someone to install it?
Is there anything else I can try to make my AC unit run more efficiently.
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- Problem with home AC

Postby nomadpeo » Tue Jul 12, 2011 9:26 pm

firstly, building code has nothing to do with it. the only industry mandate for residential equipment at this time was issued by the epa prohibiting manufacturers to sell new equipment after the end of 2009 containing r22. it is not necessary to make up non-existing rules to sell you something you need unless he thinks you are not going to confirm the information he's giving you. also, the teminology and test procedures don't add up. i agree with the duke. get a second opinion.

a couple of points. stratification of warm air will naturally cause the top two floors to be warmer, particularly the third floor. you mentioned there are intervals, sometimes up to 30 minutes after the thermostat is satisfied set at 76. this actually sounds like normal operation. of course, if you set the thermostat lower, it probably would not cycle off during that hot period. the issues are probably more likely to be related to duct and system design and heat gain. it's difficult to deal with the rising warm air, especially in an open architectural design. i would have someone qualified to come over and assess air delivery and load calculation to see if there are any modifications that can be made to better serve the top two floors with the existing system. they should also be able to advise regarding correctable heat gain issues and determine if your system is possibly undersized. a company capable of properly assessing should also have a qualified tech to analyze the refrigerant system and compressor functionality.
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