Jon Gorey-Global Correspondent

By now, you may have heard of heat pumps. But do you know how these mysterious machines actually work? Or do they make sense to your home?

Meg Howard, the project director of the Massachusetts Clean Energy Center, said that heat pumps are hyped for a reason: They are the key to electrifying buildings and can greatly reduce greenhouse gas emissions across the state.

Howard said that because of their incredible efficiency and the composition of our power grid, heat pumps already produce less emissions than oil, natural gas, or propane heating systems. With every solar array and wind farm coming online, the grid becomes greener, Howard said, “Heat pumps will only become cleaner for the entire life cycle you own them, so they really put us on the path to eliminating emissions. The road comes from the building.''

Howard said that environmental benefits alone have motivated many homeowners. However, there are many other good reasons to invest in heat pumps. "The biggest factor driving many people is to increase the chance of cooling," she said, especially for old houses with radiators or skirting boards for heating.

Because, despite its name, the heat pump is essentially an air conditioner.

"In short, a heat pump is a reverse-working air conditioner," said Richard Trethewey, a long-term heating and plumbing expert on the home improvement show "This Old House." "When you have an air conditioning system, the refrigerant absorbs the heat in the air and discharges it outside. The heat pump just reverses the process. It finds the heat outside and delivers it to the building."

Early heat pumps won a bad reputation because they could not keep up in severe cold weather. But in the past 20 years, Trethewey said that Asian manufacturers have made tremendous progress in cold-climate heat pump technology. Now, as long as there is heat energy in the air outside—and there will always be some, until we reach absolute zero (minus 273.15 degrees Celsius/minus 459.67 degrees Fahrenheit)—the variable-frequency heat pump can draw heat energy into its refrigerant, pressurize it, and Move it indoors. Modern cold climate heat pumps have been proven to operate effectively even at temperatures below zero.

The main reason heat pumps are so efficient is that they do not generate heat, but simply move heat. According to data from the US Department of Energy, although the most efficient gas-fired boiler produces almost as much heat energy as it consumes, measured in BTU, the BTU that an air source heat pump can provide is three times the amount it uses.

Different types of heat pumps extract and transport these BTUs in different ways. Air source heat pumps extract heat from ambient air and are the most common-you may have seen heat pumps installed outside office buildings or neighboring houses. Ground source or geothermal heat pumps are more efficient, but also higher upfront costs: they need to drill a deep well to make it more suitable for new construction projects.

Depending on your heating infrastructure, the way the heat pump delivers warm or cold air to your home will vary. In households that use forced hot air, the central heat pump can simply replace or increase the furnace by connecting to the existing piping system.

At the same time, homes with radiators or baseboards for heating can use ductless mini split heat pumps. "It's like you cut the window air conditioner in half with a chainsaw," Tretway said, putting the noisy condenser part outside and the quiet blower part inside, and using a pair of refrigerant lines to cut the window air conditioner in half. Connect the two. An external condenser unit can provide heating and cooling for a single indoor head or multiple consoles with their own controls, which can create different zones throughout the home.

Trethewey says that some newer systems can even operate in different modes at the same time: for example, cooling a sunny south-facing room and transferring excess heat to the colder, north-facing side of the house.

According to MassCEC data, in 2018 and 2019, the average cost of a single-head residential heat pump installed in Suffolk County, including Boston, was $6,474. However, since then, the installation price will almost certainly rise, and the cost of the entire home system will be higher. In the 2020 MassCEC pilot program, the median cost of installing a whole-house heat pump system is $18,400-new buildings and bowel repairs are cheaper, and existing buildings are more expensive to renovate.

Some of these costs can be compensated by lowering energy costs. "Based on current prices, houses that use oil, propane, and resistance heating will generally see their costs remain the same — or generally decrease — if they switch to heat pumps based on current prices,” Howard said. She added that at current prices, this is not the case for natural gas customers-although the US Department of Energy expects natural gas heating costs to rise by 30% this winter. For homeowners with solar panels, heat pumps are also a wise investment because they use free or discounted electricity.

Bill Stack, an energy efficiency expert at Eversource, said installation costs are an obstacle for many homeowners-the company works with State Grid and other utilities to fund and operate the Mass Save program to encourage improvements Family efficiency. But through Mass Save’s hot loan, Stack said, “People can borrow up to $25,000 at a 0% interest rate to pay for the installation.”

Mass Save also offers discounts for the purchase of heat pump equipment, ranging from US$250 to US$1,250 per ton of heating capacity. (Customers in municipal lighting districts are generally not eligible for Mass Save rewards-but some communities, including Hull, Marblehead, and Peabody, offer rebates through MuniHELPS.org.) Mass Save’s heat pump rebates are currently beneficial to homeowners who use oil and propane for heating Or resistance, but natural gas customers are expected to get more discounts in 2022.

In October, public utility and state officials are finalizing a new three-year Mass Save plan, which will determine the rebate structure and financial incentives from 2022 to 2024. "The rebate level for heat pumps will be significantly improved. This is our recommendation for the next round," Stark said. "It's still under negotiation, but we know that it is certain that the emphasis on heat pumps and reward dollars and rebate dollars will increase significantly."

Before installing a heat pump or mini split system, make sure your house is properly insulated and airtight. "If your house is leaking like a sieve, it doesn't make sense to upgrade to this kind of high-efficiency equipment," Stark said. Mass Save will pay 75% or more of the cost of most insulation and weathering work.

Like any home renovation, it is best to complete it with a larger renovation project so that you can use open walls to hide refrigerant lines and electrical wiring, and even install a new plumbing system. But it is very simple and can be installed as a one-time project in almost any building mini-split. "In a renovation, you may be forced to place high-side wall units on the exterior wall and then lay pipelines outside the building to make them look like downpipes in the gutter," Tretherway said. “It’s not visually appealing to me, but sometimes it’s the only way you don’t take the house apart during renovation.”

Lynnfield's homeowner Alex Jovanovic and his wife Nada bought their oil-heated Colonial in 1982, so he has installed several heat pumps. Jovanovic said that this was initially an experiment when they were looking for an affordable and good solution to heat the children's basement playroom and chose Mitsubishi Hyper Heat mini split air conditioners. "In the first year, I ran it on the coldest night to see if it would still get hot? Will it heat up?"

Indeed, when the family converted part of their home into a guest suite for Jovanovic's in-laws, they installed a second mini-split system to heat the space. "In the coldest days, we still use petroleum heating, but in fact it is more of an emergency backup," Jovanovic said.

Finally, when the central air conditioner in the house broke down, they decided to replace it with a 60,000-BTU Bosch central heat pump. The outdoor condenser is connected to the distributor in the attic, which connects to the existing piping system to provide central cooling and heat. "It actually works well," Jovanovic said, although he later realized that the air conditioning ducts might be too small to provide heating. Therefore, when the outdoor temperature is below 35 degrees, the Ecobee smart thermostat will switch to the relatively new Buderus oil-fired boiler. "Last winter, we cut oil usage by half by using heat pumps. This is my top priority-I want to get rid of oil completely."

Howard said that, as proven by MassCEC's recent pilot program, it is now more feasible to retrofit houses to be heated entirely by heat pumps. Some participants left the old fossil fuel heating system in place just in case-but later decided that the old equipment was no longer needed. 95% of people are satisfied with the heating comfort of their entire home system. "Even a few years ago, people had a lot of concerns about whole-house heat pumps in retrofit situations," Howard said. "The important lesson for us from the pilot is that yes, these can be done. Even in old houses, you can retrofit them with heat pumps and eliminate fossil fuels from home heating."

In general, Jovanovic is "very satisfied" with the heat pump, but he offered a warning. The largest outdoor condenser will make a loud sound, just like a central air conditioner is running at full power in summer. Their old air conditioner is right next to the garage, but in order to shorten the hose connection and improve efficiency, they installed a 5-ton heat pump in a different location-under the bedroom window. "We can definitely hear it, especially at night," he said.

Trethewey says that apart from cleaning the filters regularly, the heat pump does not require extensive maintenance. "The most difficult part is to install them correctly initially, ensuring that the installer puts in the right amount of refrigerant, the correct size device. But once they are in and running, they are very bulletproof," he said. As with central air conditioning, it is worthwhile to check the old system regularly.

Contrary to the frugal instinct, experts say that turning down the thermostat of the heat pump at night is not worth it. "The problem with heat pumps is that if they are in cruise control, they will be happier," Howard said. "When they try to quickly heat the space from a big setback, the efficiency will decrease a bit."'Just choose a temperature that feels comfortable, She said, then set it and forget it. If you like a cool sleep temperature, set the heat pump temperature to gradually drop at night and slowly warm up in the morning.

"There are not many shortcomings I can think of," Trethewey said. "It's like everything except prepaying more money. The interesting thing is that it is."

Jon Gorey blogs about houses on HouseandHammer.com. Send comments to [email protected]. Follow him on Twitter @jongorey. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp. Follow us on Facebook, LinkedIn, Instagram and Twitter @globehomes.