If you have chilly rooms, you may turn to the old adage that you can close some of the registers in your home to save energy and increase the air pumped into a room. Unfortunately, this is an HVAC urban myth in many ways, and can be a damaging one.
While closing a few registers does prevent air from getting to certain rooms and can raise the pressure in your system, it is not recommended to do this. The pressure that comes out of your AC or furnace is perfectly balanced for your home. Increasing the pressure is incredibly stressful on the system. It puts an increased pressure on your duct work that can lead to rips and it is hard on the internal parts of your HVAC. Furthermore, the air hitting the registers in a varying temperature room can produce condensation that can linger in the ducts and create mold.
Closing registers typically causes problems instead of solving them. Often times what people need is to not close the registers, but to call their HVAC service. The rooms furthest from your furnace or AC unit will be the most difficult to temperature control. This led to the creation of the myth.
In some cases, you may not have an appropriately sized system for your home if it is struggling to maintain the temperature in its furthest rooms. However, sometimes, you just need a better laid out system. These days, HVAC companies often recommend a system of multiple thermostats that control the temperature in zones of your home. It helps solve issues like poorly heated and cooled areas in the furthest areas from the home or a second story that is always the opposite of how you want it to be in terms of heating for the season. Doing a zoned system can actually be much more energy efficient and it is completely safe to do unlike closing the registers.