Hold on. What’s a Heat Pump Water Heater Anyway?

By 

David Tuft

December 7, 2023

HPWH explained. How are they different? What are the advantages? An Interview with Harvest’s Pierre Delforge

A big reason Harvest was founded was to decarbonize the home’s biggest energy hogs: heating and hot water. We sat down with Pierre Delforge, a co-founder and Head of Product and Operations, to talk about heat pump water heaters. The conversation was so rich we broke it into three parts. In Part 1, we cover the basics. What defines heat pump water heaters, and why they’re key to the company’s groundbreaking smart thermal battery system. In Parts 2 & 3, we’ll discusses available incentives and  and  the future economics of heat pump water heaters and why they are instrumental in home decarbonization. Hope you enjoy!

David Tuft: Let’s start with basic definitions. What's the difference between heat pumps and heat pump water heaters?

Pierre Delforge: The terms that are best known are heat pumps for HVAC and hybrid water heaters for water heating. That's how the big guys market their heat pump water heaters. But heat pumps for space heating come in different types: the most common are air-to-air; they use refrigerant to circulate heat and cool from the outdoor unit into the home. But there are also air-to-water heat pumps for hydronic space heating–heating and cooling homes with hot and chilled water instead of refrigerant.

DT: So basically there are heat pumps that heat space and heat pumps water heaters for heating water, right?

PD: Heat pump water heaters are a special type of air-to-water heat pump designed to heat potable water.  Whereas typical air-to-water heat pumps heat non-potable water for [space] heating, not for domestic uses like showering, and dishwashing, right? So the difference is a heat pump water heater heats water for domestic uses. Whereas an air-to-water heat pump heats water for space heating and cooling purposes.

DT: And what's the kind of heat pump water heater where the heat pump sits on top of the tank?

PD: That's an integrated heat pump water heater. It means that the compressor is integrated with the tank into a single unit

DT: okay.

PD: Water heaters come in two main forms: integrated and split. Split systems are where the tank and the water heater are separate like with the SanCO2 unit that Harvest uses.

DT: Okay. Okay, I think that's pretty clear. While we're on the subject of heat pump water heaters, where do radiant floor heat, radiators and fan coils fit in? Can we talk about those three things?

PD: These are the three main ways of distributing heat from hot water (hydronic heat) — through fan coils, radiant floors, and radiators — which can be either hydronic baseboards or hydronic radiators.

DT: Do you want to talk a little bit about a fan coil?

PD: Yeah, a fan coil as the name says is a coil like a car radiator, you know, the radiator in front of the car's internal combustion engine that cools the engine down. You circulate hot water in it and the fan blows air across it and that air extracts heat from the water in the coil. So that's essentially a heat exchanger from water to air. It extracts the heat that's in the water and blows it into the ducts and the home.

DT: Before we get specifically to the Harvest thermal system. What are the advantages of heat pump water heaters? Compared to electric resistance a lot of people have tankless or instant hot water heaters. What do you see is the big transformation around heat pump water heaters?

PD: Well, the first one is efficiency. Heat pumps are 3 to 5 times more efficient than electrical resistance.

Pierre Delforge, Head of Product and Operations

The other thing is they draw much lower power from the grid. Because a heat pump water heater typically uses a low-power compressor to heat water, anywhere between 200 and 1,000 watts. Whereas an instantaneous water heater can be 12, 18, and even up to 36 kilowatts. That’s almost 200 times higher power than some heat pump water heaters. That's a huge difference for the grid and carbon pollution because those kilowatts are typically used in the mornings and evenings when there is little renewable energy available and the grid is dirtiest.

People use the most hot water in the mornings and in the evenings when electricity is supplied primarily by gas power plants, not by renewable energy. So tankless electric water heaters have very high power draws of relatively dirty electricity compared to a heat pump water heater, which is a much gentler draw. The heat pump water heater also has storage which allows it to be operated in a way that maximizes the use of clean energy. So it's a slow steady draw of clean energy compared to a very high-power, peaky draw of dirtier energy of tankless electric water heaters.

DT: Okay, so clearly efficiency and energy savings are huge with heat pumps.

PD: And the time energy is used is also huge. Not just for energy savings, but for carbon emissions -- for how much emissions your water heating is responsible for.

DT: Compared to gas there’s a huge emission benefit. Can you talk a little bit about that?

PD: Well gas is a fossil fuel that produces CO2 when burnt and also has a high climate impact — a high global warming potential  – when it leaks unburned, 86 times the impact of CO2. So gas has climate impacts in two ways: both in terms of the emissions when it's burned but also the emissions as it escapes along the way from extraction to distribution to use in household appliances.

DT: So let's take a look at the SanCO2 heat pump hot water heater that Harvest Thermal uses. Why did you choose that particular system?

PD: We chose it because it's one of the most efficient heat pumps on the market today with efficiencies of up to 550%. And because it produces high-temperature water, 150° F, in one pass. It's pretty unique. Most heat pumps increase the temperature of the water by 10 or 20 degrees every time the water runs through the heat pump and eventually, the water gets to the target temperature. But it has to run through the heat pump several times. Whereas the SanCO2 takes cold water and brings it to hot in one single pass.

That’s key for efficient thermal energy storage. You have access to high-temperature water as soon as it's produced from the heat pump without having to heat a whole tank which takes a lot longer and also has much higher thermal losses.

DT: That's super fascinating. Okay. I'm gonna go back to the comparison with air-to-air heat pumps. So we've established that we can store heat in hot water and then we distribute it through an air handler by blowing air over these warm coils. How does that compare with an air-to-air heat pump for space heating? What are the benefits of using a hot water heat pump and thermal battery for heating?

PD: Yeah, air-to-air heat pumps are not designed for storage. We can't store refrigerant. I mean technically you can, through heat exchangers, but why run refrigerant to a heat exchanger inside the home to heat water when you can do it in the heat pump itself without running a refrigerant line?

DT: You don't want R410 in your house.

PD: Exactly. You don't want that because it has a high global warming potential and can leak. And it’s not the most efficient way to do thermal energy storage.

DT: right

PD: Air-to-water heat pumps are great for thermal energy storage, particularly the SANCO2 and other heat pumps that can produce high-temperature water in a single pass through the heat pump. This is why we selected it as the heat engine for our smart thermal battery. Its efficiency is up to 550% depending on temperature conditions and it is ideal for thermal energy storage using hot water.

Heat pump water heaters efficient, cost-effective replacements for gas systems for all the reasons outlined above. But are they affordable? What kind of incentives are available? And are they the best investment for my money in the long-term? Stay tuned for Parts 2 & 3.

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