Unfortunately, bigger is not necessarily better with a cooling system. An oversized system will experience short cycling periods, which means it will run just long enough to send cool air into the home, but not long enough to remove the humidity in the air.
Further, your new system is likely moving a lot more air than your 40-year-old system did, which would allow for better cooling, but poor humidity removal. An undersized unit will actually be more effective in your situation because it will run in longer cycles, which would still cool the space but work better at removing humidity and moisture.
There is also the possibility that the system was installed poorly, which would lead to duct-work leakage.
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Your best solution? Contact the company that installed this unit and demand they perform a Manual J calculation on your home to determine if the correct-sized equipment was installed. Have them also perform a thorough inspection of your entire system, including all duct work, to troubleshoot A/C problems and ensure everything is working properly and there are no air leaks in your crawl space or attic.
If you’re no longer comfortable working with that company, bring in another heating and cooling company, but do your research first and only hire an HVAC company with a good reputation in your area. Hopefully, you’ve kept records of all conversations you have with the original company, as well as copies of your receipt and warranty information. Those records will serve you well in the event you need to modify or replace any of the existing equipment.
Angie Hicks is founder of Angie’s List. Have a question for her team? Email firstname.lastname@example.org or tweet to #AskAngie. Follow her @Angie_Hicks.
Have you had problems with incorrectly sized HVAC systems? Tell us about it in the comments below.
Editor's note: This is an updated version of an article originally posted on Oct. 31, 2011.