Is shutting off heating vents in an unused room a good idea to increase my energy efficiency? I just had a customer ask me this very reasonable question. She lives alone and wants to save on her forced air furnace heating bills by closing the baseboard supply vents in unused rooms as well as shutting the doors. While on the surface, this seems to make sense, it actually doesn’t do much, if any, good and could even do harm. So short answer is that I do not recommend this. Read on to find out why.
Typically in a home heated by forced air, each room has at least one supply air heating vent that warm air comes out of. Some rooms also have return air vents that move the air back into the furnace. Both the supply ducts and return air are essential for proper functioning of your furnace, and require knowledge and training to properly design and install. The forced air heating system depends on air pressure differentials to function effectively. The side of the blower in your furnace that pushes out the warm air into the supply ducts and out the heating vents is the higher pressure side. This creates lower pressure on the side of the blower that pulls in the returned air. Ever wonder why you occasionally see bedroom doors with a big gap at the bottom of the door, or even ventilation grills above the door? These help keep the return air flowing out of the bedroom when the doors are closed. While I prefer to see return air vents in every bedroom, many builders don’t want the extra costs that incurs.
So, air flow through a furnace depends on the air handler pushing out warm air through the supply ducts and out the heating vents and pulling it back in through cold (really just cooler) air returns. If you shut off a heating vent, you have just increased the pressure in the supply side ducting because you eliminated a path for the air to exit the ducting. What this does to your furnace depends on what kind of blower you have. If you have an adjustable speed blower (ECM), it will speed up because it must work harder to push warmed air out due to the increased air pressure in the supply ducts. Or, if you have a single speed blower (PSC), the blower is forced to slow down because it is pushing air out against the increased pressure. Perhaps you got a little more warm air into the rooms you use, but your furnace worked harder, which may have offset any cost savings. A side note here is that a furnace that is oversized compared to the ducts may be working hard to push too much air through the ducts and out the heating vents. (Think about how easy it is to blow through a paper towel tube versus how much harder you have to work to blow air through a straw.) This can wear out the fan motor prematurely.
Furthermore, because shutting off heating vents increases the pressure in the supply ducting, air tends to get pushed out through leaks in the duct, perhaps through unsealed joints or cracks. That wasted heat defeats the purpose of trying to be more energy efficient.
A more serious consequence could result from shutting off heating vents. If your blower slows down, less air moves through the system, and thus less air comes back into the furnace. Your heat exchanger relies on a certain flow of cooler air over it to prevent it from overheating – it loses heat as it warms the air. Without the proper air flow, your heat exchanger may get too hot, then crack and release potentially fatal amounts of carbon monoxide into the household air. (Note that this is a good reason to keep your air filters clean – clogged air filters reduce the air flow through the furnace. )
So rather than shutting off heating vents in unused rooms, my advice is to save by keeping your furnace running efficiently through annual tune ups and clean filters, setting the heat a little lower during the day, and several degrees lower at night. A heating/cooling system is arguably the most complicated part of your house, and crucial to your comfort and enjoyment of your home. KJ Thomas Mechanical has the training and experience to help you with your heating and cooling needs. Call us at 303-435-8141 or schedule online.