Experts on Camera

Theresa Pistochini: Home heat pumps and indoor air quality

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Heat pumps can be used to both cool and heat homes—and the Inflation Reduction Act provides financial incentives for installing one. In tandem, ventilation and filtration systems can be used to improve indoor air quality and occupant health.

On Thursday, August 3, 2023, SciLine interviewed: Theresa Pistochini, the co-director of engineering of the Energy Efficiency Institute and Western Cooling Efficiency Center at the University of California, Davis. She discussed topics including: home heat pumps—what they are, and how they work; how efficient they are at cooling and heating homes; how switching to a heat pump can reduce a home’s global warming impact; how ventilation and filtration can be improved; and what else people should know when thinking about upgrading their heating, ventilation, and air conditioning (HVAC) systems.

Declared interests:

In Theresa’s position at UC Davis, she has received funding from the following entities in the prior five years: California Air Resources Board, Natural Resources Defense Council, National Philanthropic Trust, California Energy Commission, Trane Technologies, RMS Energy Consulting, Des Champs Technologies LLC, Daikin U.S. Corporation, Arbnco, and the Western Cooling Efficiency Center’s Affiliate Program for which members are listed at

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THERESA PISTOCHINI: Theresa Pistochini, engineering manager at the UC Davis Western Cooling Efficiency Center. And I study energy efficiency in buildings and also indoor air quality in buildings. And so that involves a lot of research in heating, ventilating, and air conditioning systems—or HVAC.


Interview with SciLine

What is a home heat pump, and how does it work?


THERESA PISTOCHINI: So, heat pumps are very similar to air conditioners. The technology has been around for decades. Let’s talk about an air conditioner first. So, what an air conditioner is doing is actually moving or pumping heat from inside your house to outside your house. Heat naturally wants to move from warmer places to colder places. And so what a heat pump does is it moves the heat essentially against the natural direction it wants to flow. And so we use a compressor and a refrigerant to absorb that heat inside and then pump it outside and reject it to the environment outside. And so that’s what an air conditioner is. And all the heat pump is is it contains a few extra components and a valve that for heating, takes the air conditioner and switches the direction that the refrigerant is flowing. And so now the compressor pumps the heat from outside your house to inside your house. And so we’re moving the heat that exists even in a very cold winter day. And we collect some of that and we pump it inside. And so basically a heat pump is an air conditioner running in reverse.

How efficient are heat pumps at heating homes?


THERESA PISTOCHINI: So, it’s difficult to talk about efficiency because it varies depending on the temperature that you’re pumping against. So, just like an air conditioner has to work harder to pump heat to air condition your house as it gets warmer outside, a heat pump has to work harder to pump heat when it gets colder outside. And so, the efficiency is quite variable. But let’s talk about a gas furnace for a minute. What that’s doing is it’s burning fuel at your home and it’s turning that fuel into heat. That process can never be more than 100% efficient. And generally speaking furnaces, depending on type, are around 80 to 96% efficient at capturing that heat from that burning process. So, heat pump now—we’re putting in some work, we’re putting in some energy to move heat. And so that process can be more than 100% efficient. Because we’re not generating heat, we’re just moving it from one place to another. And so heat pumps, depending on outdoor conditions and on the heat pump design, can be anywhere from 200 to 400% efficient if we want to think about it in those metrics. And because it’s difficult to know like what your exact efficiency will be in your climate zone where you live, one thing consumers can do is they can compare efficiency ratings between heat pumps. And it’ll be difficult to know exactly what that efficiency will translate to for your actual house but the relative efficiency will hold. So, if one is 10% more efficient than another you’ll still you know do better.

Are heat pumps more efficient than other air conditioners at cooling homes?


THERESA PISTOCHINI: They should be comparable. You can compare cooling efficiency ratings when you when you buy a heat pump.You can compare it to an air conditioner, but essentially they’re basically the same pieces of equipment. They just include valves that switch the direction the refrigerant goes. The thing that’s important to know is that air conditioning efficiency has improved over the last couple of decades. So, if you’re replacing an existing air conditioner with a brand new heat pump, you would expect a pretty big jump in air conditioner efficiency and a lot of savings on your cooling bills.

When a household switches to a heat pump, how can that affect its global warming impact?


THERESA PISTOCHINI: So, when we’re burning natural gas or some sort of fossil fuel at our house, we’re creating emissions on site at our homes. We’re creating mainly carbon dioxide, and we can’t burn all the fuel. So for example some natural gas is escaping to the atmosphere, and that has global warming potential. So, the benefit of switching to a heat pump is now we’re using electricity—there are no on site emissions. Now the discussion becomes what did we do to create that electricity, and what was the global warming impact associated with that. And that’s changing rapidly over time as the way we generate electricity is evolving with the increase of renewables on the grid. And the other tricky thing is that when you’re buying heat pump you’re making—or a furnace—you’re making a decision for the next—you’re probably gonna have that piece of equipment for at least 15 years, probably longer. And the electricity grid is going to evolve a lot in the next 15 years. So, based on an analysis we did here at at WCEC, in almost all geographic locations based on current forecasts for electricity generation, buying a heat pump today will reduce global warming impact, basically everywhere, except maybe in the very coldest climates. And those improvements will only increase with time.

Are there other reasons to switch to a heat pump?


THERESA PISTOCHINI: Natural gas furnaces are fairly clean burning, and those emissions are generated outside. But there is some evidence at least that we do get some methane leakage inside our homes from natural gas appliances. And so there are expected to be some indoor air quality benefits from switching to all electric appliances, so that there’s essentially no combustion on site at your home. You know, the other thing is that this is a bit of a—electrification is a societal movement, right? And so by purchasing electric appliances, essentially society is sending a message that this is where we’re going, right? And the utilities need to be prepared, and we need to get more renewables on the grid. And so it’s sort of one person at a time, right, making that decision to stop burning fossil fuels.

Why should people care about indoor air quality, and how can they improve it?


THERESA PISTOCHINI: People spend on the order of like 85 to 90% of their time indoors. And so we have to be concerned about what we’re breathing. And there’s increasing evidence that, in particular, the more particles we breathe—the more particulate matter we breathe—that that has impacts on our respiratory health and our lifespan. And so there’s essentially an opportunity to create a safer environment in our home. So, things that people can do are to think about how to reduce—first of all, how can we reduce indoor sources of air pollution? So, things that create indoor sources of air pollution are: cooking, particularly without appropriate ventilation, like an exhaust hood, or open windows; burning things or burning candles, any type of scented products, essentially—if you can smell a scent, that’s—those scents are created by chemicals, right?—and so, thinking about, you know, your cleaning products, burning incense, you know—none of these things have a good impact on indoor air quality; and then certainly not smoking, obviously, avoid smoking in general for your health, but smoking indoors in particular, for thinking about secondhand smoke. And then the other thing we can think about is what do we do when the outdoor air quality is poor, right? If we, what do we then? And so in that case, we want to tend to isolate ourselves from the outdoors by closing windows and weather proofing, and then trying to clean our indoor environment through filtration. And so portable air cleaners are really great at this because portable air cleaners can remove particles that infiltrate from outdoors, and they can also remove particles we generate indoors, like from cooking. And so using portable air cleaners is really helpful. And these can be purchased online or also people building Corsi-Rosenthal boxes, CR boxes, which are like a type of do it yourself air cleaner—are pretty popular. But all of these things can reduce that particle count indoors and those are those are part of—any particle you capture in a filter is a particle that doesn’t deposit inside your lungs. Right? So, that’s a good thing.

How do we know when the air quality is so poor that we should change our behavior?


THERESA PISTOCHINI: When the air quality index start to get above about 150, into that unhealthy range, that’s when I start to change my behavior. You know—not exercise outdoors, definitely run my air filters indoors. And some people are also just more—there’s also there’s these potential long-term impacts to our health in terms of what we breathe, and then there’s short-term impacts. Some people are just way more affected by these short term—like asthma attacks, and trouble breathing and respiratory irritation—than others. And so, you know, people are probably going to make their own determinations, just kind of also based on how they feel.

What else should people know when thinking about upgrading their heating, ventilation, and air conditioning (HVAC) system?


THERESA PISTOCHINI: I think one thing that’s really important is that most of us, for obvious reasons, right—it’s very expensive to replace a heating and cooling system. And so, we generally don’t do it until we’re forced, right? And that’s usually when something fails. Generally, that happens when your system is sort of pushed to the brink of its limits. So, it’s generally really cold or really hot when that happens. And then you really have your back up against the wall because you have very little time to make a decision. And whoever is going to replace your system only has limited things available to them in stock. And they’re going to be charging a premium price because everybody else is calling when it’s 110 degrees outside or 25 degrees outside. And so my suggestion is if your system is like more than 15 years old, definitely if it’s more than 20 years old, start thinking maybe during like a non-busy time, like spring or fall, about getting some bids to replace your system and see what options are available to you, so you’re ahead of the curve and not having to make a rush decision.