Dear Angie: I purchased a new air conditioner after my other one was vandalized. I was told by the company installing my new A/C unit that I would also need a new furnace because the two would not be compatible. My existing furnace was 40 years old and ran like a top. I changed the filter every month for the 25 years I have lived here.

After the installation of the new unit was completed, I arrived to my 900-square-foot home to find a 2.5-ton heat pump and a three-speed furnace. Immediately, I had condensation issues with water actually running down the windows and doors. I never had these issues with the old equipment, so I contacted the installer, who told me I need to call in a crawlspace expert. I went down in the crawlspace and it is completely dry.

I had another HVAC company come in and they said the unit is too big and not properly sized for the house. The indoor humidity in my home is high, making it miserable. If I turn down the temperature to pull the humidity out, then it’s cold and damp. I’m afraid if something isn’t done before prime cooling season I could end up with mold and mildew in my home. What do you think the issue is? – Deborah T., Indianapolis

Dear Deborah: This sounds like a classic case of being sold an oversized unit. The company that installed your new unit should first have done a heat loss/heat gain calculation (also known as a Manual J calculation) for your home before recommending a unit. A residential load calculation program factors in things like the size of the home, number and location of windows, and climate to determine the proper sized unit for your home.

Highly rated heating and cooling contractors I spoke with said a 900-square-foot home should only need between a one- and two-ton system.

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.

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 ensure everything is working properly and there are no air leaks in your crawlspace 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 one 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.

If you’re not getting the response you want from the original installer, consider going through the Angie’s List Complaint Resolution Service. We’ll work with the contractor on your behalf to help resolve your issue.

Angie Hicks is the founder of Angie’s List, a provider of reviews you can trust on contractors, doctors, dentists and other service professionals. More than 1 million consumers across the U.S. use Angie’s List to help make tough hiring decisions easier.