These new double-duty heat pumps can warm both air and water

Heat pumps are a more efficient way to heat homes. Startup Harvest Thermal says its tech can turn them into thermal batteries too.
By Jeff St. John

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An installer with AirCare Heating and Cooling installs a Harvest Pod in a Bay Area home (Harvest Thermal)

It’s a lot easier to store heat in water than it is to store heat in air. Why not make a household heating system that can ease strain on the electric grid by taking advantage of that fact?

Longtime Silicon Valley energy industry executive Jane Melia asked herself that question in 2017 when she was looking for a more eco-friendly replacement for her home’s aging gas furnace. She ended up buying a more efficient electric heat pump to replace it, but wondered, What about hot water? I need another heat pump for that.”

Then, Melia reasoned, why not combine water and space heating in a single unit? That’s the genesis of Harvest Thermal, the startup she co-founded in 2019, and the Harvest Pod, a device that’s now being tested in homes in California’s Bay Area.

The pod connects to a standard heat-pump water heater on one end and to an air handler on the other (that’s the part of a forced-air heating system with fans that push air through a home’s air ducts). Hot water is piped through coils in the air handler, warming the air that’s being blown throughout the house.

This isn’t a new idea, Melia emphasized. Similar hydronic” air-handler systems are made by a number of major water-heater manufacturers and are becoming a feature of high-efficiency heating systems in large buildings and homes alike.

A diagram of Harvest Thermal's heat pump water-heater-to-air heating system
(Harvest Thermal)

Where Harvest Thermal differentiates itself, Melia said, is in its software and controls to optimize that hot-water-to-hot-air cycle to help shift a home’s demand for electricity in ways that are valuable for balancing the power grid.

By overheating water when electricity is plentiful on the grid, then using that stored heat to reduce electricity demand when the grid is under stress, we’re shifting 65 percent of [a home’s] load to the lower-cost time of the day,” she said, referring to periods when electricity costs less.

That makes for a combined air and water heating system that can reduce carbon emissions not just by switching from fossil gas to electricity but also by reducing its demand for electricity at times when the grid tends to rely on the most carbon-intensive sources of generation.

Shifting electricity demand to midday periods when California’s grid is flooded with solar power also helps soak up more of that sometimes-curtailed zero-carbon electricity, as this graphic indicates.

A diagram showing how Harvest Thermal's heat pump system can shift household heating electricity demand
(Harvest Thermal)

That’s how we’re reducing emissions by 90 percent compared to gas, which is good, but also by about 50 percent compared to heat pumps for air and water heating,” she said. And unlike systems that require homeowners to turn down their thermostats to reduce electricity use, the heating is stored up in the [water] tank, so you don’t deprive people of heating when you have to conserve electricity.”

These are the features that have prompted Peninsula Clean Energy, the community choice aggregator serving the Bay Area’s San Mateo County, to put $250,000 into a pilot test of the Harvest Pods in a select set of homes. They…want to electrify homes and [are] concerned about the cost of electrifying homes — but what about the cost to the grid?” Melia explained.

A home heating system to meet California’s grid needs

Electric water heaters have been the target of utility demand-response and load-shifting programs for decades, largely in colder climates and in more rural areas. The National Rural Electric Cooperative Association estimates that U.S. rural co-ops save about 500 megawatts per year in such water-heater programs across 35 states.

But the vast majority of those programs are built around electric resistance water heaters, which use electricity to directly heat metal coils to heat water. That’s a very energy-intensive way to heat water compared to heat pumps, which work like air conditioners in reverse, using condensing liquids to move temperatures from outdoors to indoors.

Heat-pump water heaters can also be controlled to reduce electricity demand. But their inherently greater energy efficiency means they can’t offer as much load-reduction capacity as resistance water heaters can during times of grid stress. Simply put, because heat pumps use less electricity in a more stable and constant pattern, they offer less of a peak to reduce.

Meanwhile, the push to fight climate change by replacing fossil-gas-fueled heaters with heat pumps is likely to drive up grid demand overall as people switch from gas to electric. At a grand scale, that could mean creating new grid peaks to manage. At a local scale, that could mean adding more electricity demand to a home than its electrical panel or grid connections can handle, forcing expensive and time-consuming upgrades.

It’s not easy doing a conversion from methane gas equipment to electric technologies,” said Rafael Reyes, Peninsula Clean Energy’s director of energy programs. Like many of California’s community choice aggregators, PCE has set aggressive decarbonization goals, aiming for a 100 percent renewable electricity mix by 2025 and a carbon-free community by 2035.

We’re looking at residential decarbonization as one of our principal focus areas,” he said. But upgrading a home to run all-electric heating and cooking — and provide power to an electric vehicle charger — can often require adding multiple 240-volt circuits to power those systems. If those new loads exceed the capacity of a home’s grid connection, that creates not only costs…for the owner but also enormous costs…for the distribution grid.”

Harvest Thermal’s architecture requires only one 240-volt circuit for the heat-pump water heater of the customer’s choice, plus a 120-volt circuit to run the Harvest Pod, Reyes said. That means lower costs to install compared to two systems and lower…costs [for the utility] because you [need] fewer service upgrades.”

Beyond these upfront cost considerations, Harvest Thermal’s load-shifting capability allows you to use energy when the cost is lower,” he said. California utilities and community choice aggregators like PCE are required to implement variable time-of-use” rates that charge less for electricity when it’s plentiful on the grid and more when it’s in short supply, which in California means during late afternoon and evening hours in summer and early fall when solar power fades away while air conditioners are still running hard.

These grid peaks are driven by air-conditioner demand rather than heating demand, but they can be exacerbated by standard heat pumps that are designed to keep using electricity at steady rates to maintain desired indoor temperatures. That means the devices will keep drawing power during peaks if they don’t have some form of digital controls to alter that behavior.

Adapting home heating for an all-electric future

Even water heaters that can store heat and then shut off may not be well suited to the load-shifting task, Reyes said. Most heat-pump water heaters and hydronic heating systems aren’t designed to be turned on and off frequently, and may not be equipped with the temperature gauges or software smarts that can track how those on-and-off commands affect their core function of providing ample hot water.

Melia highlighted this distinction between the Harvest Thermal technology and those products on offer from other companies. Retrofit options such as heat-exchanger systems that use hot water to heat air don’t have knowledge of what’s happening inside the tank — and what’s happening inside the tank is really important,” she said. That’s because it allows a system to know when it’s tapping too much heat, leaving the water in the tank too cold to meet a home’s hot water needs.

The Harvest Pod, by comparison, is continuously measuring usage, it’s monitoring flow, it’s monitoring temperature.” That allows the system to fine-tune its operations to prevent the cold-water problem, she said. But it also allows the system to use weather forecasts to preplan its heating cycles, learn from a household’s energy-use patterns to adjust operations, and detect changes in heating efficiency that might indicate failing equipment, to name some examples of what its digital intelligence can enable, she said.

Harvest Thermal's Harvest Pod installed next to a heat pump water heater tank
Harvest Thermal says its hardware and software can turn heat pump water heaters into load-shifting home thermal storage systems. (Harvest Thermal)

Ray Tam was one of the first Peninsula Clean Energy customers to participate in the Harvest Thermal pilot project. So far, he has had no issues with running out of hot water, even during bath times for his two sons, ages 2 and 4, during the same evening hours the heat pump isn’t running.

Nor did the installation require any upgrades to his Daly City, California home’s electrical panel or utility service, Tam said in an interview. He and his wife have been thinking about alternatives to fossil gas appliances for some time and are considering an induction stove as their next step, he said.

As an associate with Pyatok, an Oakland, California–based architecture firm specializing in multifamily housing, he also has a professional interest in learning about the technology and understanding the data it generates, he added.

The idea that it uses machine learning to improve over time is really exciting,” he said. The traditional heating system…just [turns] on and off.”

All of this intelligence comes at some additional cost compared to a standard heating system. But Melia said that the cost of the Harvest Pod, along with the plumbing to carry water from it to the air handler, can be made up for by the reduced costs of not having to install a second heat pump to heat air as well as water in a home.

The fact that Harvest Thermal’s system requires replacing both air and water heating with one system does present a barrier to fast adoption, Reyes said. Normally homeowners say, I want to do the water heater today, and I’ll get to the furnace when it’s time for it to go,’” he said. This is basically a whole-home upgrade, and that will require more programmatic support for people to get there.”

There’s also the question of how well hydronic systems work in colder climates where winter heating loads make up a significant portion of electricity demand. For the time being, Melia said that Harvest Thermal is very…focused on the Bay Area where we’re rolling out, making sure we have trained contractors in place. We have a long waitlist of hundreds of people who want to electrify their homes.”

Harvest Thermal has received funding from a combination of federal and state grants and a $1.9 million seed funding round in December that was led by Astia Angels and included VertueLab, Energy Labs and the Band of Angels. That investment will allow the company to expand its production capacity and improve its hardware and software.

Melia would like to see Harvest Thermal expand to handle even a small portion of the roughly 500,000 home heating retrofits that take place in California every year. There’s a long way to go on that front, but state incentives and utility load-shifting imperatives might help push the market, she said.

Wouldn’t it be nice if, in five years’ time, when your furnace breaks down, the contractor has a heat pump on the back of the truck?”

Jeff St. John is director of news and special projects at Canary Media. He covers innovative grid technologies, rooftop solar and batteries, clean hydrogen, EV charging and more.