Welcome to the Energy Efficiency Myths series from Direct Energy! As many myths arise from incomplete knowledge, they can create seemingly possible answers that many people accept as fact. Each month, we will examine common misunderstandings about energy efficiency — whether it’s in your home or about the energy industry — and deliver real facts behind the myth (and how they might be costing you money).
Closing Vents or Doors Saves Energy
Wrong. The idea may sound right but you need to remember that HVAC systems are closed loops, blowing air into the same rooms they pull air out from. Consequently, two factors are in play when you close a vent in a room.
The first thing is that when you close off a vent, the air gets backed up down the entire length of that particular duct branch. Where this duct branch splits off from the main duct work trunk or plenum, this back pressure forces more air to flow out other open registers, usually at a slightly higher speed. All that’s happening here is that you are diverting more heated or cooled air to rooms with open vents. Your HVAC is still using the same amount of energy to heat or cool the air to the temperature setting in the room where your thermostat is.
Ah, but doesn’t that mean all the air is being diverted to where my thermostat is? Won’t the room heat up or cool down quicker AND save me money?
Not really. Remember that backed up air in the branch duct? All the energy used to heat or cool and move that air is really being wasted creating back pressure to divert air flow away from that duct. The energy efficiency here, if any at all, is negligible.
Plus, the more vents you close, the more air backs up into the ductwork and causes back pressure to build deeper into your home’s HVAC system. Research in 2003 at EOL Berkeley National Laboratory showed that closing vents actually degrades the efficiency. Ideally, HVAC systems are most efficient when they are balanced The more resistance or back pressure you add to the system by closing vents, the less efficient the system will be. In fact, result in the Berkeley National Laboratory results showed that when 60% of a home’s vents are closed (eg. 6 out of 10 vents), the blower’s ability to move air is reduced by up to 20%. The more vents that get closed, the less air for the blower to move, especially if your home is well sealed. The lack of air flow can cause air conditioning heat exchangers to ice-over, causing damage to the compressor, or while heating hit their thermal limit and go into a safety shut down. If they blow out 1000 cubic feet per minute (cfm) then they should pull in 1000 cfm. In either case, you’ll need to call in a repairman for some expensive maintenance.
The second thing that happens when you close a vent in a room is that the room’s return vent is still sucking in air. Without that balanced pressure, the return duct will more likely pull in more air through leaks in the exterior walls, including windows, doors, chimney, and water heater or furnace flues. Instead of using less energy, you wind up using more.
Cranking the Thermostat Heats/Cools Your Home Faster and Saves Energy
Blatantly wrong. Some may think the thermostat works like a volume knob that you can turn up to 11 to heat or cool your home, but that may not be the case. HVACs system can not vary their heating/cooling energy output so setting it higher for heat or lower for cooling than you need will actually waste energy and cost you more.
Why does it take so long for my house to heat up or cool down?
Your home has a thermal mass or the ability to absorb and store heat energy over time. Lots of heat can be stored in bricks, concrete, and tile while less dense materials such as wood don’t store as much. How fast your home can lose that heat also depends on its thermal mass. Heat from inside (during the winter) or from the outside (during the summer) is stored to a degree in your home’s thermal mass. Insulation and air sealing resist the transfer of heat from the inside to outside in winter, outside to inside in summer. Landscaping, including shade trees, can have a significant effect on how much heat your home can absorb/lose.
How long it takes to heat or cool your home depends on the weather, the amount of heat your home has stored, how well air-sealed and insulated your home is, and the age and efficiency of your HVAC system.
Vinyl Duct Tape Is Fine for Sealing Ductwork
Somewhat true. If you prep the surface of the duct so that it is clean and dry, the vinyl tape will seal around the duct joint quite nicely…BUT— odds are the tape will fall apart in 1 to 5 years.
Lab experiments have shown that the vinyl stuff’s adhesive does dry out on average with in one year— less if the duct is subjected to consistent heating for extended period of time…such as on a heating duct. In other applications, the tape tends to contract and wrinkle with the outer vinyl skin separating from the adhesive mesh.
The right stuff to use is either aluminum HVAC tape that’s labeled in accordance with UL 181A or 181B. This tape usually comes with a removable paper backing and it takes a little practice to put on neatly. There are also oriented polypropylene tapes that has a higher temperature range that is approved for or sealing flex duct to metal duct. But for metal ductwork, mastic provides the best method to completely seal both the connections between pieces of duct work and the seams on the individual metal pieces, especially boots and elbow joints. With a consistency similar to tarry-peanut butter, it seems easier to spoon it out of the contain with a putty knife and then smooth it over the individual pieces you want to seal with a paint brush. Fortunately, water-based mastic is easy to clean up. Dry time varies between manufacturers, with pressure-test seals ready between 48 and 72 hours.
Do you know of any Energy Efficiency Myths you’d like us to dispel? Share with us in the comments!