Should I block my basement air returns to better cool my top floor?

Question

My house has three floors (basement, main floor, and upstairs) and the top floor is much warmer than the basement, especially in the summer. I have an air conditioner attached to my furnace, and it works well, but it mostly cools the basement and the main floor, while my upstairs floor is still too warm.

My brother suggested using magnetic vent covers to cover my basement air returns. His logic was that this would force the system to suck more warm air from the top floor air returns. It made sense to me at first, but now I'm thinking that I'd like that cool air in the basement to be sucked into the basement air return and distributed to the top floors.

Can somebody tell me the most efficient air return/vent setup for cooling my top floor in the summer?

Answer 1

Heating and air conditioner units have a fan that pushes air and it has a maximum ventilation capacity. The air return “allows” the air conditioner to draw air back into the system and is called “make-up air”.

That is to say, the fan can only “push” so much air, whether you’re heating or cooling and your unit will draw the same amount of air back into the system.

If you are “cooling” the basement too much and not cooling the upstairs enough, then you want to distribute more air to the warm areas. This is called “balancing the system”.

Without re-sizing all the ducts, you could cover the “supply” vents in the basement, which will force more air to the other areas. (You may be able to just partially close the downstairs vents, if you have adjustable vents.) By redistributing that air to other areas, those areas my get too cold, so you’ll need to “balance” the system by adjusting those vent covers.

During the cooling season, we like return air vents where we can bring cool air into the system and re-cool it to the desired temperature. (Likewise with the return air for the heating season, we like to have warm air returned to the system so we can re-heat it.)

I doubt if you have 2 return vents, I’d just cover some of the supply vents in the basement until it’s comfortable.

BTW, by covering some vents, you are forcing more air into other ducts. If those ducts are too small to “handle” the additional volume, you’ll hear a slight “whistle” sound. If so, you’ll just have to re-adjust how many vents you cover in the basement.

Answer 2

A mistake so many people make when having a single system and many levels is running fan in auto. The circulating fan needs to run constantly to keep the cool air moving to the hottest points in the house (upper floors). In addition to closing supply in basement if you run circulating fan constantly, you will have more uniform temperature. It will never be perfect, for that you would need zone cooling, but it should improve temperature in upper floor greatly.

Answer 3

Multiple floor homes are usually hotter on the upper floors because warm air is lighter and rises and cool air is heavier and falls. I would not close the returns on the lowest level, just close the supplies on that level. If you have a multi-speed fan on your furnace then a higher speed is usually selected while in the cooling mode since the cold air is heavier and needs more power to get it to the upper floors. The duct work sizing and layout may also be a contributing factor on which I can not comment since I am not there to physically inspect it. A better explanation of the supply and return registers on all floors would be helpful. For example, where are the supply and return registers in relation to the rooms and where on the walls are they located? Also, what is the temperature drop across the cooling coil? If these problem can not be solved by you, and it is really a problem you want to address, I would try to find a reputable HVAC company and ask them for an assessment andpossible solutions. Hope this helps.

Answer 4

I close all but two supply vents on the lower level of my home in the summer months, and also have a second temperature sensor ran up to the second floor to use during that time. It works well and prevents the lower level from becoming a frozen iceland. HVAC experts will tell you that this can be hard on your system, but I have been doing this for over 20 years with no issues.

Another trick you can try is to use supply register vents that are powered with fans. You can leave them on all the time to pull the lower cooler air upwards.