With cold weather ahead, the experts at Efficiency Maine have some advice: Set your thermostat at a toasty 74 degrees or whatever makes you comfortable and don’t touch it again until daffodils are back in bloom.

That guidance, however, is only for homes with cold-climate heat pumps. They’re the hottest piece of home heating equipment in Maine today, but perhaps, the least understood.

This fall, that confusion is being amplified by mixed messages, one being sent by the state’s energy-efficiency agency and the other by the trade group representing many oil and propane dealers.

Most Mainers grow up turning down the thermostat when they go to work or bed. Many live in homes with multiple heating systems cobbled together. A cranking wood stove backstops the aging furnace on a bitter January night. The TV room space heater buffers May’s lingering chill.

But if you have one of the 30,000 high-efficiency heat pumps installed in Maine over the past four years, that Yankee frugality has to go out the (insulated) window.

“We have to have a whole culture shift in how we understand heating systems in Maine,” said Michael Stoddard, Efficiency Maine’s executive director.

Recent testing by the agency confirms that the way to save the most money with the latest generation of ductless heat pumps is to run them full time, all winter, at a steady temperature. That advice holds true, Stoddard says, even when the thermometer dips below zero.

This fall, Efficiency Maine is ramping up a consumer education campaign for heat pump owners that boils down to this: Set it and forget it.

But that hands-off approach runs counter to advice coming from the Maine Energy Marketers Association.

While some of the group’s members sell and install heat pumps, the association considers them to be best used as air conditioners in summer and heaters in spring and fall. In winter, it says, nothing beats a central heating system.

“Members that install cold-climate heat pumps recommend shutting them down in December and returning to them in early March,” says Jamie Py, the group’s president. “This allows the central system to operate when it is most efficient (cold temperatures) and the heat pump to operate when it’s most efficient (shoulder months).”

Heat pumps are confusing to most people because, unlike conventional systems, they move heat rather than generate it. That’s why they can achieve efficiencies well above 100 percent, even though they run on electricity.

Even when it’s very cold outdoors, heat pumps can extract the tiny amount of warmth that’s present in air, water or the ground and use a compressor and refrigerant to transfer it inside. In the summer, the transfer direction is reversed to remove heat from indoors, like a typical air conditioner.

Efficiency Maine has been promoting heat pumps to reduce fossil-fuel emissions linked to climate change, lower total energy bills, and reduce dependence on heating oil. A single unit costs around $3,500, and the state has encouraged the switch with rebates for the most-efficient models of $500 for the first unit and $250 for the second one.

Over the past five years, Efficiency Maine has paid out $19.2 million in heat pump incentives. It estimates the units will save Mainers $89 million in energy costs, over the full life of the equipment.

Roughly 5 percent of Maine homes now have heat pumps. That includes the Blaine House, where Gov. Paul LePage had a system installed in 2014. Many of the state’s 500,000 single-family homes are good candidates for conversion, Stoddard said, if they use oil or propane.

“They’re a technological breakthrough that has met a market need,” he said. “It came at a time when oil prices were high and people were looking for alternatives.”

COMPARING COSTS

Heat pumps have been around for decades in milder climates. But they didn’t work well in extreme cold, making them a poor choice for year-round comfort in places such as Maine. In recent years, though, manufacturers have made refinements.

Last winter, Efficiency Maine conducted its own tests to compare the costs and performance of the latest generation of heat pumps with an oil boiler. It presented the findings in August at its annual meeting, with an aim at debunking what it calls “myths” about heat pumps.

“We wanted to be really confident that when we tell people to prioritize their heat pump and leave it alone, we were right about that,” said Ian Burnes, who heads up strategic initiatives at Efficiency Maine.

A key finding: For equal units of heat, the modern, ductless heat pumps are cheaper to run than oil boilers, until the temperature gets well below zero. For that reason, Burnes said, the best way to use both systems together is to set the boiler’s thermostat at least 10 degrees cooler than the heat pump, so it only fires for backup.

Another data set shows that a home that maintains a constant temperature uses less electricity than a home where the heat pump is being turned up and down.

FRUSTRATED CUSTOMER

But despite these guidelines, not everyone has good experiences with their heat pumps. The tribulations of a homeowner in Saco underscore how details around use and installation matter with this sophisticated technology.

Mark Peterson had three heat pump units and a heat-pump water heater installed two years ago at his 40-year-old ranch-style house. The system cost $8,000 after the rebate, and Peterson said he was enticed by the state’s promotion and vow of savings.

That hasn’t happened. A failed part cost him $900 in repairs and routine maintenance uncovered mold, which required a deep cleaning. And though Peterson said he set the thermostats to constant temperatures, the increase in his electric bill canceled out savings on oil.

Peterson created a spreadsheet showing his power use is up 34 percent over two years, adding $1,091 to his electric bill. Oil savings over the period totaled $780. With the added cost of maintenance and repairs, he actually lost money. Peterson expressed frustration with Efficiency Maine and its advice.

“If you’re telling me it’s going to be better heating and I’m going to save money, that’s what should happen,” he said.

Stoddard said he wasn’t familiar with Peterson’s situation, but noted that some installers were still figuring out the optimum way to set up the systems. Peterson declined to provide the name of his installer.

Stoddard added that Efficiency Maine’s primary mission is to help Maine consumers lower their energy bills, regardless of which fuel they use. And he reiterated the agency’s guidelines.

“When consumers buy a heat pump,” he said, “we assume that they will leave their existing heating system in place and use it as backup and to heat other parts of the home. The advice we give Mainers who already have an oil or propane system is: If you buy a heat pump to save money, choose one of the highest efficiency, ‘cold climate’ models, set the heat pump thermostat a bit higher than the central system thermostat, and let it run all winter long.”

In late September, Peterson got a follow-up phone call from Efficiency Maine. A technician suggested ways to set the heat pump and oil burner for maximum savings. Peterson said he’ll try it.

“I will follow up with Efficiency Maine after a full winter’s experience to see how that works,” he said.

Proper installation of an appropriately sized heat pump, and correct use, are critical to customer satisfaction, according to Matt Scott, a co-owner of Dave’s World in Windham, one of the state’s largest installers. Scott said technicians need to make a point of explaining how to get the best performance from the units.

“It’s really just an education thing,” he said. “People have a fear of being sold something. But for us, it’s important to be educators, not just salesmen.”

WAYS TO USE LESS OIL

Maine’s pivot to heat pumps coincides with a move away from oil heat. Roughly eight out of 10 Maine homes were warmed with oil before 2000, the highest share in the country. Today, it’s closer to 60 percent.

Beyond heat pumps, other alternatives are competing for market share.

Highly efficient boilers that run on propane or natural gas, small enough to hang on a wall, are a choice for many builders. Direct-vent gas units that warm portions of homes, such as those made by Rinnai, have become a popular strategy. A similar, sealed-combustion unit that burns the low-sulfur heating oil now being used in the Northeast, the Toyotomi Laser, is another emerging alternative to central heat.

Sales of these systems suggest that the familiar sound of a fossil-fuel burner kicking in, followed by a surge of heat, will prevail in Maine homes for many winters to come. That suggests that the penetration of heat pumps, and Efficiency Maine’s efforts to re-educate homeowners, may have their limitations.

Scott was asked if he knows some homeowners who just prefer to use their oil-fired central heating systems in the dead of winter. One immediately came to mind.

“My father,” he said. “He’s an old Mainer and it’s his habit. It’s something that works for him.”

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