This article appeared in the Nov/Dec 2020 issue of This Old House Magazine. Click here to learn how to subscribe.

Myth #1: Closing forced-air registers in unused rooms saves on heating and cooling costs

In fact, the opposite is true. Shutting down the flow of heated or cooled air to a room increases pressure (known as “back pressure”) in a forced-air system, so the blower has to work harder, using more electricity.

A better move: Ask your HVAC pro to create more zones by adding motorized dampers to your ductwork. Or install smart registers, such as Ecovent. These battery-powered devices automatically open or close as needed to maintain a set room temperature, while monitoring back pressure and communicating with one another wirelessly to balance airflow. Ecovent says a typical system costs about $1,200, delivers average energy savings of 25 percent, and could pay for itself in about three years.

Myth #2: Replacing old windows is a cost-effective way to save energy

Installing new, insulated windows is a sure way to reduce the heat loss and air infiltration typical of older, single-pane units and boost comfort and convenience. But their high initial cost means you can wait as long as 40 years to break even, based on energy savings for whole-house replacement.

For about a quarter of that cost, you can add Energy Star–certified storm windows. When fitted with heat-reflecting low-e glass, these can lower heating and cooling bills by about 10 percent and pay for themselves in five to seven years, according to Energy Star.

Interior storms control air leakage better than exterior storms because they fit tightly into the window opening. With either type of storm, updating the weatherstripping on the old windows is an inexpensive way to improve overall performance even more.

Myth #3: A tankless water heater is the most cost-effective way to generate hot water

It depends. Tankless heaters do save energy by eliminating standby heat losses—the energy lost while keeping water hot in tank-type heaters. In a household that uses less than 42 gallons of hot water per day, tankless units are 24 to 34 percent more efficient, saving an average of $100 per year, or more. But in a busy household, standby losses are less of a factor, and increased efficiency falls to 8 to 14 percent. In that case, it could take 12 to 15 years to recoup the $2,800 cost of installing a gas-fired tankless water heater. (Fortunately, these heaters last 20 years or more.)

For about the same price, you can buy an Energy Star tank-type water heater with a condensing gas burner or a hybrid electric heat pump. While tankless heaters don’t run out of hot water, simultaneous or high-volume use may be limited by the size of your gas supply line. Tank-style heaters don’t have that limitation, and, in the case of condensing-gas heaters, bounce back so quickly that running out of hot water isn’t an issue. When heavily used, these gas heaters achieve payback in about eight years. In warm climates with high electricity rates, the payback on heat-pump heaters is just four years.

Myth #4: An electric space heater is a cost-saving way to warm a room

At best, it’s a temporary, occasional solution. Regular use can be pricey: about $3.20 per 16-hour day, based on the average cost of electricity in the U.S. The more energy-efficient—and safer—cures for chronically cold rooms include extending forced-air ductwork to them.

Or, if you have a boiler, you can run pipes to a wall-mounted hot-water radiator, which, unlike a space heater, produces comfortable radiant heat.

A third option: Add a wall-mounted gas- or propane-fired furnace that vents to the outside. It costs about 85 cents per day to run either a 3,000-Btu hot-water panel radiator or a natural-gas wall furnace with the same output.

Myth #5: If you want to save on energy, go solar

There are lots of reasons to go solar: Rooftop photovoltaic cells produce clean energy, reduce carbon emissions, and can lower your electric bills (although pushback by utilities may limit savings in some areas).

But in terms of bang for your buck, a better investment is to properly insulate the big gaps in a house: the attic floor, floors above a crawl space, and the rim joist that sits on the foundation. Doing that saves an average of 15 percent on heating and cooling at a cost of only $1.50 to $3.50 per square foot. Unlike solar panels, these improvements don’t wear out in 30 years, and they do their job 24/7, whether the sun is shining or not.

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