There are many reasons why rooms don’t stay warm or cool enough even though they have similar sized ducts, and there are several easy ways to improve comfort.
If your heating, ventilation and air conditioning blower has an efficient variable-speed electronically commutated (EC) motor, switch the thermostat fan to continuous when problems arise. This keeps the air circulating to reduce room temperature differences. If your system has a less-efficient standard air conditioning blower motor, use this option sparingly, because it can use a lot of electricity.
Duct walls, especially if they are sheet metal, lose or gain heat as the air flows through them. To check for heat loss, hold a thermometer in the register outlet air flow in each room. If there is a 5 degree or more temperature difference, wrap insulation around as much of the duct as you can reach.
Another test: Hold your hand over room outlet registers to compare the air flow rates. Uneven temperatures are more likely the farther the room is from the blower and the longer the duct. You also can hang a thread from a stick and hold it near all the joints in the ducts to locate air leaks.
Seal these leaks with a duct joint sealing compound called mastic or use a HVAC foil-backed tape. Tip: despite its name, do not use the traditional gray duct tape, as it fails quickly.
Focus on the baffles
Check the baffles in the ducts near the heat pump or furnace to be sure the ones leading to the problem rooms are not partially closed and blocking air flow. Also, check to see that the room register baffles are fully opened and install deflectors to direct heated or cooled air out into the room. Be sure furniture is not blocking the air flow.
Try partially closing the duct baffles leading to the other rooms at least 45 degrees. This forces more heated or cooled air to the problem rooms. The settings will have to be changed to accommodate seasonal variances.
Boost air flow with a fan
Installing a duct booster fan, which fits into ducts near the furnace blower, can help get more air flow to problem rooms. Some have automatic sensors, and others have their own thermostat or can be connected to the main blower controls.
Register booster fans, which mount over the outlet register in a room, can also help. Plugging into standard wall outlets, they are easier to install than a duct booster fan and provide more control over the room temperature.
JAMES DULLEY is a nationally syndicated columnist who writes on energy efficiency and do-it-yourself energy topics.