My heat works good. The central air doesn't. No matter how high I set the thermostat, once the air kicks on, it stays on and doesn't shut off for sometimes up to 9 hours because the cooling doesn't affect the thermostat enough to shut it off. Originally, I thought the vent slots were too small so I took the registers out and just left the open holes in the floor and the A/C would go on and off like it's suppose to. This year, I bought registers that had larger openings and installed them but the last couple days, we've had the same issue. A/C won't shut off. The house is a single floor ranch with a finished basement. Someone told me all the registers should be open for air flow but I'm ready to shut the registers for downstairs so more air is available for upstairs. Thanks for any help.

Since cold air is heavier,the basement will be cooler. Try closing them down to see what effect it has but keep the returns open. Plenty of attic insulation? Good attic venting? Clean filters?