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A Bit of Confusing Heat Pump Terminology

Oh, the wonderful world of heating and air conditioning and its confusing terminology.  I’ve been doing building science for nearly two decades now, and I still have to ask around sometimes to find out what what a particular term means.  Here’s the one that got me going on this today:

Air-to-water heat pump.  You probably know that a heat pump is a clever device that can move heat from a warm place to a hot place, as unlikely as that sounds.  And for its second trick, it can also move heat from a cold place to a warm place.  It’s actually the same trick, involving two heat reservoirs, one known as the source and the other as the sink.

Once you have that basic knowledge, you can look at the term “air-to-water heat pump” and deduce that the air and the water must be connected to the source and the sink.  Right?  But which is which?  And does it change with the seasons, as a heat pump switches between cooling and heating modes?

As I’ve seen this term used over the years, I’ve come to see that it’s mostly used in ways that imply that the first part, air in this case, refers to the outdoor heat exchange, and the second part refers to the indoor heat exchange.  So, an air-to-water heat pump would be what’s also called an air-source heat pump with hydronic distribution.

In plain English, that means the outdoor unit uses the outdoor air as a sink for dumping heat from indoors when it’s cooling the house and as a source of heat to warm the house in winter.  The indoor unit collects or distributes heat with water running through pipes connected to radiant panels or some kind of radiator, like the one shown in the photo above.

I’ve occasionally tried to find a source (heh heh heh!) that could verify that as the correct usage, always unsuccessfully.  Recently, I asked three of my go-to HVAC buddies (John Semmelhack, Mike MacFarland, & Kristof Irwin) about this term, and they all confirmed that my assumption is correct.  MacFarland even sent me a paragraph from a book called Troubleshooting & Servicing Heat Pump Systems by Richard Jazwin, which states, “The first word in the description identifies the physical location of the outdoor coil.”

From that, you can figure out exactly what kind of system would be called air-to-air (the most common configuration), water-to-air, water-to-water, or even a water-to-wine heat pump (mostly used in churches).  We know the formula, so now it’s easy to decipher any combination.

Of course, if water is the first term, we could make things even more confusing by trying to describe what kind of system uses water for the outdoor heat exchange.  Is it a geothermal heat pump?  A ground-source heat pump?  A water-source heat pump?  Geoexchange?  Earth-coupled?  Oh, the fun we could have!

But this is New Year’s Eve, and even in a pandemic, I’m going to save up some fun for this evening.  Happy New Year, everyone!


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

This Post Has 13 Comments

  1. Allson I am inspired by the “Cat on the Radiator”, which is possibly a cat on a convector. The temperature and construction of the heat delivery device determines whether the predominant heat transfer method is convection or radiation. It seems to me that most “radiators” are actually convectors (unless you make the mistake of putting your hand on it. Then it turns into a conductor |:(

    1. John, yes, you’re right, and I thought about bringing that up in this article. But the cat looked so peaceful, I didn’t want to disturb it. ;~)

  2. In the UK where everyone heats with hot water, almost every heat pump sold is air-to-water. If you order an ASHP that is what they will send you.

    1. Lloyd, your mention of ASHP reminds me that I didn’t really explain that the source in an air-source heat pump is the outdoor air, even though it acts as the sink in summer.

  3. Happy New Year, Allison! I’m “pumped” for 2021. For a good overview of terminology and technology, see the ASHRAE Handbook – 2020 HVAC Systems and Equipment volume, Chapter 9 Applied Heat Pump and Heat Recovery Systems.

    1. Bill, I don’t know why I didn’t look there because I have a copy on my desk. Happy New Year to you, too!

  4. “cat on radiator”…only it really ain’t a radiator…its primary mode of heat transfer is natural convection – air moving up along fins, so it is a “convector”

    Same applies to automotive “radiators” – in fact most heat transfer is via forced convection (vehicle forward motion or fan), not radiation.

    Heat transfer components that get hot enough to glow (1600*F +) such as electric or gas fired infrared heaters are properly called radiators, but the cat would be long gone…

    1. Yes indeed, Curt! As you point out, if that “radiator” really did transfer heat to the room mainly via radiation, it would also be a fryer.

  5. “A household built with proper HVAC appliances is a blessing for the people living in it as it makes life much more comfortable and convenient for them.

    But if someone is looking forward to installing HVAC products, it sometimes becomes confusing when it comes to its terminology, which was rightly mentioned in the article. Air to a water heat pump is a new terminology you get to know about, and it’s like increasing your HVAC vocabulary.
    HAHAHA, Thanks, mate, for this one!!”

  6. Glad another finds annoyance in recent GSPH lexicon shifts.
    Clearly, however, I was duly tickled by the one, the church-HP–“water-to-wine!”
    …Hmm, wonder if that applies to the opossum-shack?

  7. When it comes to heat pumps that can heat or cool a house by exchanging heat with the ground, I claim that they should be called “ground-coupled” heat pumps. “Ground source” is wrong because that only applies in heating mode. “Geothermal” is even worse. In a true geothermal system, you should be using the heat from the ground directly for heating without a heat pump. This can be done anywhere if you drill deep enough.

  8. The water to wine heat pump is of very limited manufacturing, I’ve only heard of a single unit being made which only functioned on one recorded occasion ;).

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