We were recently on a service call at a customer’s home because she received a letter in the mail from her energy company saying that she uses 67% more gas than other homes in her neighborhood. After checking for gas leaks as a possible culprit, we discovered that wasn’t the issue. She also had an older furnace, nearing 20 years old, but it was still 90% efficient after doing a complete furnace check-up.
So ultimately what was causing her high energy bills and gas usage? She was doing something many homeowners do to try to conserve energy: Closing furnace vents in unused rooms. Actually, she had 50% of her vents closed in an attempt to conserve energy!
Many people close the vents in rooms they don’t use and close the doors to those rooms. So this customer was definitely not alone in believing that closing vents will save money and cause the furnace to run less.
The Myth About Closing Vents
It logically makes sense: If you’re not using a spare bedroom, why not close the vents in it, save money, and redirect the heat to the other rooms? Most furnace registers (vents) even have a built-in lever that allows you to adjust the louvres behind the grille, change the direction of the air flow, or shut off the air flow altogether.
The Truth About Closing Vents
Closing vents and closing doors doesn’t prevent air movement, it actually interferes with it. The closed off room will be under pressure, and if it has a return air duct (like our customer did) it will pull in cold air from leaks around windows. The warm air trying to push up through the closed vent will create pressure as it backs up and it may leak out of ducts that are not sealed properly or force an opening in the duct work, leaking warm air into the floor cavity or back down into the basement.
Meanwhile your furnace will continue trying to push air to all the vents. Generally, newer high efficiency furnaces will detect the increased air pressure, step up the fan speed and attempt to push through that pressure (and lessen their efficiency). Older furnaces will continue running like normal but operating under this increased pressure will strain and wear the furnace motor. The pressure can damage the duct work or, worse still, if the pressure gets high enough it will damage the furnace.
Your furnace depends on good air flow to operate properly. If it does not have enough air being returned to it and then taking the heat away from it, the heat exchanger could crack and this would result in carbon monoxide leaking into your home, circulating through your vents. In addition to the huge health risk, your furnace would also need to be replaced.
To Close or Not to Close?
It’s not a good idea to close vents, but if you would like to shut off the heat to a room, especially one that is warmer than others, contact an HVAC technician you trust (like Gary from BOSS Services who helped our customer with her energy problem immensely!). They will tell you what can be done safely and what adjustments can be made to the operation of your furnace. A professional technician will ensure that your furnace is working well. Often, an HVAC technician can help you get your furnace operating more efficiently (and save you money) in ways that do not involve closing vents, which can increase gas usage or damage your furnace.
