I recently started using one of these and it makes a big difference for bringing cool air into a room.
This runs while the central air is not running and has a thermostat to switch off when the desired temperature is reached.
The minimum for the opening it must go into is 24.5 cm x 9.6 cm. This is the size of the enclosure of the fans which go into the end of ducting in the wall or floor.
The actual size of the face of the unit is 29.2 cm x 14.0 cm.
I recommend this unit as it will save on energy cost since this pulls air from other rooms into a problem area, instead of having the cooling\heat running to the whole house to cool a particular area.
There are different sizes and shapes depending on your application.
Supplier link - https://acinfinity.com/hvac-home-ventilation/register-booster-fans/airtap-t4-quiet-register-booster-fan-system-white-for-4-x-10-registers/
This can be used together with a thermostat sensor and a smart plug to get even better control of different rooms in a home.
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