Design, installation, capacity

QUESTION: My house is 3,000 square feet. All but a guest room and bathroom are on the main floor. When we set the air conditioning at 78 degrees on a hot day, it does not cool the house and will only cool to 82 degrees downstairs and will be 90-plus degrees upstairs. We have a dual-control Lennox SEER-rated 12.45 air conditioner but only one compressor. Would a second compressor help pull the hot air out? If that is not feasible, what would you suggest to at least cool the upstairs (other than a window unit)?
C. Ogden, via email
ANSWER: Vince McDonald of McDonald Plumbing, Heating and Air Conditioning in Sacramento believes that your system failure is due to one or more of the following:
The system needs repair;
The system was not properly designed;
The system was not properly installed; and/or
The system’s capacity is too small.
“My first suggestion is to have an air-conditioning contractor check to make sure the system is operating according to the manufacturer specifications,” he says. “A good contractor will run a series of tests, including one to monitor the system’s refrigerant charge. If the charge is too high or too low, it can have a large impact on its cooling capacity and efficiency.”
“The contractor should also make sure the filter and indoor components are clean, he adds. “This has to be the No.1 problem we find that affects the overall system performance. Also, homeowners should regularly change or clean their filters. Something as simple and easy to rectify as a clogged filter can cause your problem.”
“The contractor should also inspect your air duct system to make sure the ducts are free of leaks, are properly supported and that any exposed metal is insulated.” If you are still having problems after the checkup or they discover the system is clean, airtight and operating properly, McDonald suggests having the contractor inspect the system for proper design, installation and sizing.”
“Two-story homes are always a concern when designing and installing a HVAC system simply because hot air rises,” he says. “it sounds like you may have a zone system, with on thermostat upstairs and one down, to direct cooled air where needed according to the individual thermostats. Zone systems should definitely help even out temperatures, so have the contractor make sure your system was properly designed and installed.”
“Finally, the problem may be an issue of capacity, meaning there may be nothing wrong with your current equipment. The largest single residential air conditioning system is 5 tons. If you already have a 5-ton system, and you have taken steps to make your home more energy efficient (by adding more insulation, etc.), you will need to add another air conditioning system.”
“One option would be to take the upstairs ductwork off the current system and add another system that will serve the upstairs only,” McDonald says. “You mentioned that the upstairs consists of a guest bedroom and bathroom. If you use this area only occasionally when guests come over, this option would allow you to direct the full capacity of the existing system to service the downstairs only, and save energy by not cooling the upstairs when it is not being used.” he says.

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