What's the Name of The Outdoor Part of a Heat Pump?

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The outdoor unit of an air conditioner or heat pump

I was on a tour of passive house projects in Seattle a couple of days ago when someone asked me, "What do you call the outdoor part of a heat pump? The condenser? The compressor?" For someone not immersed in the field of HVAC (heating, ventilating, and air conditioning), that can be confusing. So let's take a look at the basic terminology of air conditioning and heat pump systems.

The best way to know the correct terminology is to understand the refrigeration cycle. It's the physics behind air conditioners, heat pumps, dehumidifiers, and yes, refrigerators. I wrote a couple of articles about it a few years ago, and that's a good place to go if you want to understand how these devices move heat from one place to another. I wrote the first article at a very basic level, without the technical terms. The second one went into a bit more detail, naming the four main components involved in the refrigeration cycle:

  • Metering device
  • Evaporator coil
  • Compressor
  • Condensing coil

If you don't want to understand the refrigeration cycle, those components give us everything we need to make sense of the name of that metal noisemaker that sits outdoors when you have an air-source air conditioner or heat pump.

Air conditioners

The photo above is what you see sitting outdoors. You can't look at it and tell immediately whether it's an air conditioner or heat pump, but it's not hard to figure out. (Yes, I've written about that, too.) But if it's an air conditioner, that outdoor box contains the compressor and condensing coil. Most often people call the outdoor unit of an AC the condenser or the condensing unit. Its job is to dump heat from the refrigerant into the outdoor air, which is quite a job on a hot day.

Heat pumps

One thing heat pumps have that air conditioners don't is a reversing valve. That doohickey does exactly what its name implies. It reverses the flow of the refrigerant, with the result that the functions of the indoor and outdoor coils reverse. In summer, a heat pump is just an air conditioner. The outdoor unit has the compressor and the condensing coil. In winter, though, it has the compressor and the evaporator coil.

So if you see the outdoor unit of a heat pump and call it the condenser or condensing unit, you're wrong. Well, at least you're wrong when it's in heating mode. So what should we call it? The best name is what I just called it: the outdoor unit. That's exactly what it is and it covers both summer and winter.

Package units

Since we're going down this path, we ought not to ignore this other type of system. A package unit — whether it's an air conditioner or heat pump or a gas pack (AC plus gas furnace) — has all four of the components I listed above in the outdoor unit. The units I described above are part of split system air conditioners or heat pumps, with both an indoor and an outdoor part. Package units aren't split. Everything that's part of the heating and cooling sits in that one package outdoors. The best name for this one is...package unit!

Two special cases

In the passive house where I got asked this question, the piece of equipment in question was the Sanden heat pump water heater. It's a split system, too, but not in the same way as air conditioners and heat pumps used for space heating and cooling. The indoor unit is a tank for hot water. The outdoor unit is a package unit heat pump. If all you saw was the outdoor unit, you might think it's a mini-split heat pump because that's what it looks like (photo below).

Sanden heat pump water heater outdoor unit

Finally, there's a dehumidifier model that's a split system. The Ultra-Aire SD12, unlike other dehumidifiers, dumps its waste heat outdoors. (Disclosure: Therma-Stor, which makes the SD12, is an advertiser here.) The outdoor unit of a split-system dehumidifier is always a condensing unit because that's where you're dumping the heat. So you could call it a condenser.

Or you could just stick with outdoor unit. In fact, in all of these systems, you can call the part that sits outdoors the outdoor unit. Pretty simple, eh?

 

Related Articles

The Magic of Cold, Part 2 - Intermediate Air Conditioning Principles

3 Ways to Tell if That Contraption Is a Heat Pump or an AC

An Easy Way to Save Money — Let Your Air Conditioner Breathe!

 

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Comments

Does a heat pump water heater require an outdoor unit? I recall seeing a heat pump water heater with the only "unit" visible on top of the water heater storage tank. This was in Maine, if that helps to answer the question.

I am considering using one located in my basement not too far from my wood stove. I believe such a system would be a lot easier to install than installing a water heating system on the wood stove, which BTW would only provide hot water during the cold seasons.

I also question the lifetime of a heat pump system water heater.
I have had very negative performance from standard electrically heated water with quality systems lasting less than 5 years due to galvanic action between heater components and well water. Switching to propane, the systems have lasted well over 10 years.

Allison
Bailes

Thomas, the Sanden is the only HPWH I know of that has the heat pump located somewhere other than the top of the tank. The benefit for you folks in cold climates is that it doesn't steal heat from your home to heat the water — and thus doesn't cool your home. It's much more efficient, too, and works down to about -20° F, I believe. 

The Sanden is pretty new and I don't know how long it will last but heat pump technology has been around a long time and works pretty well these days. It's not the same process as electric resistance heating and I don't know what their components are like in terms of not suffering the kind of problems you've had with well water.

Water heating heat pumps extract heat from the air at the evaporator coil then exchange the heat energy plus some heat of compression through the refrigeration cycle to the water at the condensing coil. The integrated type you mentioned probably uses a condensing heat exchanger inside the tank rather than pump water out of the tank, through the condensing coil and back to the tank.

The idea behind most household-sized heat pump water heaters is to use otherwise wasted heat from the house to heat the water. As they extract heat from the air they cool and dehumidify it of course so you need to decide if that's the right thing to do in the space you're considering. A crawlspace in most climate zones is a great place for them, but split systems (heat pump separate from tank) are less common these days so installation might not be easy.

Their are some challenges with water heating heat pumps that modern technology has helped with. Max temp. Slow recovery. High pumping ratios (related to Max temp). Back in the day we had two refrigerant choices readily available that would operate in the high temp range we needed; R-12 and R-22. R-12 was easier on equipment but less efficient than R-22. Today we have much better refrigerants including Carbon Dioxide. I haven't studied their characteristics but hopefully at least one will be a better choice than the old ones. The pumping ratio for most AC systems is 3 or 4:1, and these systems last for decades if properly maintained. WHHP's with the old refrigerants had a pumping ratio of 7:1 and more. That's tough on a compressor, especially when trying to heat water to 130-140°F.

Slow recovery is really a lifestyle issue as much as a technology challenge. We Americans expect hot water NOW! .....and plenty of it. We tend to not like waiting for tankless water heaters and since a WHHP takes much longer to recover, we either need larger tanks or we use at least one heating element in the tank, which sort of defeats the purpose.

To summarize: Water Heating Heat Pumps are more efficient than electric resistance. They're slower. They provide cooling and dehumidification, and they generally operate down to about 45°F ambient air temperature. Allison can explain how the defrost cycle works and WHHPs generally don't have a reversing valve so they can't be operated at temps where the evaporator coil will frost over.

Your issue with electrolysis might be solved with a powered anode. Many plumbers won't know about these, but you can probably find a factory rep to guide you. If you don't use a powered anode, you can replace the anode rod in the tank every couple of years and extend the life of the tank for decades. Be sure to flush the tank at the same time. I shut off the power to the tank and wash my car with water from the bottom of the tank. That usually flushes out the mineral deposits and I get warm water. Be sure to turn the power/gas back on or you'll either get a cold shower or a family member yelling at you.

Hope this helps.
Mac

PS: If anyone finds an "Oregon Water Heater" heat pump, working or not, I would love to have it. We built about 10,000 of them in 1980 - 1983 and I failed to keep one for myself.

The Sanden units are pricey but since they use CO2 refrigerant, they produce high output at much lower ambients (down to -15F). Recovery rate and efficiency are significantly higher at a given ambient temperature than a conventional HPWH. And since the tank is SS, it should last a very long time (I can't speak to the mechanicals).

As Max said, conventional HPWH's are only good down to about 45F, which is why the evaporator must be indoors or in an enclosed garage or crawl.

@Thomas, since you have a basement with woodstove, a conventional HPWH should work well. As Max said, recovery is slower than a standard electric heater (and much slower than propane heater). For this reason, all HPWH's (except Sanden) come with a supplemental electric element. Household size and lifestyle determines the supplemental ratio.

For example, a 2-person household may be able to avoid supplemental heat by splitting showers between morning & evening. In that case, it's best to disable the supplemental element to avoid unnecessary use. But don't forget to enable when you have guests! Note that HPWH energy ratings aren't particularly useful since the supplemental ratio has an huge impact on actual efficiency, and automatic controls are optimized for the DoE test procedure, which may or may not reflect consumption patterns for a given household.

All electric water heaters come with a passive anode rod that must be replaced when it's "spent." The replacement interval varies greatly with water chemistry and HW use. For example, at one year, my standard heater's anode had almost no degradation, and after five years it was greater than 50% intact. If your tank lasted less than 5 years, I'm guessing you failed to inspect and replace the anode rod as required. It's an easy DIY project if you have the right tool and your water heater has enough overhead clearnace. Rods cost less than $20.

Power anodes last much longer but they cost a LOT more (see waterheaterrescue.com), so IMHO, they're only worthwhile if a passive anode wears out so fast that it becomes a hassle to replace. In that case, I'd be looking at a polymer heater (e.g., Marathon). Heat pump water heater tanks should last much longer than standard electric since the element is only used a fraction of the time.

Allison
Bailes

Thanks for the detailed reply, Mac. One correction though. You wrote:

"The integrated type you mentioned probably uses a condensing heat exchanger inside the tank rather than pump water out of the tank, through the condensing coil and back to the tank."

I would have thought that, too, but at the house I visited on Sunday, the indoor tank and outdoor heat pump were connected with water pipes, not refrigerant lines. I was surprised because it seems you'd get better efficiency by using the condensing coil in the tank as your heat exchanger but that's not what they do. I think it must be to keep the heat pump all in one unit. No charge adjustments necessary.

Also, the refrigerant they use is carbon dioxide. I think we'll see more CO2 units in the future, for both space heating & cooling and for water heating. It's got a Global Warming Potential (GWP) of one and it doesn't destroy ozone.

The installation manual for the Sanden specifies heat tape to protect those water pipes from freezing. That may put a dent in the cold-climate efficiency, and is certainly a concern during power outages!

I haven't had one of the newer units apart to see how they work so I appreciate your observation. There's at least one company using a very long and very small diameter copper tube inserted in the tank as the condensing coil (high-side heat exchanger). It would need to be double wall and vented since it's in the potable water system, unless perhaps it uses CO2 and some food grade lubricant....interesting. This type of heat exchanger wouldn't be as efficient as a brazed plate style but also wouldn't require a pump.

There's another technology from the 80s I haven't kept up with but seems logical for the deep South where every home has AC. The outdoor unit of an AC system rejects heat to the outdoors, and it's very simple to reject a portion of that heat to the domestic water system using a refrigerant-to-water heat exchanger upstream from the condensing coil. The superheat would be extracted and it may or may not fully condense the refrigerant. Oil and liquid management aren't difficult and as long as the water pumping penalty isn't great, the system should provide pretty cheap hot water. Are these desuperheater heat exchangers common?

Allison
Bailes

Yeah, I saw that, too, Peter. It would be interesting to monitor the energy use of those tapes to see how much they use over the course of a year. In Seattle, where I saw that one, it shouldn't be much. In Minnesota or Vermont, it could be an issue for certified passive houses.

@Mac, desuperheaters (DSH) fell by the wayside after the switch to R410a, but last I checked, all the GSHP manufacturers still offer them as an option

Since the DSH's can't satisfy 100% of DHW demand, a primary water heater is still required. Moreover, you also need a buffer tank for the DSH. It's plumbed in series ahead of the active tank, acting as a pre-heater. OTOH, if the DSH is looped through the primary heater, there's no way to keep the primary heater from satisfying the aquastat ahead of the DSH since the DHW demand doesn't necessarily coincide with cooling calls. Also, since DSH efficiency is inversely proportional to temperature, a buffer tank increases efficiency since it acts to minimize the average water temperature entering the DSH.

Bottom line: the additional cost and complexity of a DSH is hard to justify, especially for high performance homes with small cooling loads.

Does calling the outdoor part the heat pump (because it really does the pumping of heat for both heating and cooling) and the indoor part the heat exchanger (exchanges heat for cold or cold for heat) make sense? Even if it does, I suspect indoor unit/outdoor unit will be easier for most folks to remember.

Allison
Bailes

John, I wouldn't ascribe the name "heat pump" to just one part of a split system. Yes, the outdoor unit is where the compressor is and that's what pumps the refrigerant, but the indoor components are just as important to the overall pumping of heat. Similarly, the outdoor unit also has a coil that acts as a heat exchanger. The indoor unit is often called the air handler unit (AHU).

I'm a bit of a stickler for terminology - I call it out if our guys refer to a heat pump as a condenser. Likewise if its coil is a "condenser coil" or the indoor coil is an "evaporator coil" Both reverse role during heating mode, so I try to avoid the limiting / ambiguous terms.

To me, a split heat pump system consists of a heat pump (the outdoor thingy) and an air handler (the indoor thingy).

We abbreviate the heat pump as ODU (OutDoor Unit) and AHU (Air Handling Unit) Alternatively, "Outdoor Section" and "Indoor Section"

I'm similarly picky about use of pressure terms like "low side / high side" since components of those change roles, too. Same with the fat copper pipe connecting the in and out....it's not a suction line but instead a vapor line.

Allison
Bailes

Thanks for jumping in here, Curt. As I said to John Mattson above, I prefer calling it the outdoor unit. Calling only the outdoor unit a "heat pump" would be like calling the condensing unit of an AC the "air conditioner." Do you do that? Both actually need the indoor and outdoor parts to make up a heat pump or air conditioner.

But calling the outdoor unit of an AC the "condensing unit" is likewise incomplete as said unit does more than just condensing the refrigerant. Yeah, the compressor is a necessary part of condensing but it also is the pump that moves the refrigerant through the system. Even more interesting is that you abbreviate heat pump not as HP but as ODU.

I don't know if I've heard other contractors referring to the large pipe as the "vapor line" before but I like it better than "suction line" for two reasons. First, since the small line is called the "liquid line," it makes more sense to call the large one the "vapor line." Second, suction doesn't really exist. I explain that in classes when we discuss pressure.

I admit I'm guilty of using the term 'suction' line even though I understand that term is meaningless in a closed loop (how many die hard wet-heads use suction to refer to the input side of the pump in a closed hydronic loop). Also, I sometimes refer to a heat pump outdoor unit as a "heat pump", as opposed to "heat pump system," which includes the indoor coil and/or air handler, etc.

But if you want to be totally pedantic about terminology of the trade, why not call a reversible system an 'air conditioner'? Doesn't it condition the air? Or similarly, why not call a straight cooling system a 'heat pump'? After all, it pumps heat from indoors to outdoors, no?

The trades are replete with imprecise terminology that have become meaningful, even precise, in practice. For example, I have no problem referring to the outdoor unit of a cooling-only system a condenser. Do you thing anyone in the trade would be confused by that? There are a lot of things about this industry that are wrong but that's not one worth fighting! Just don't let me hear anyone call the outdoor unit of a heat pump a condenser! :=)

All other technical issues aside...isn't the Sanden outdoor unit blocking the stairs that it is mounted above?

Allison
Bailes

Lee, it's hard to tell from the photo I posted, but the stairs are wider than they look. Also, the unit isn't really a skull-cracking danger to inattentive people because the rail you see keeps them away from it.

Here is one manufacturer's perspective. The outdoor unit is an "outdoor unit" and it can either be a heat pump or air conditioner (AC). The indoor unit is either a "gas furnace" or an "air handler". If you have a gas furnace for an indoor unit, you might also have an "indoor coil" if you are using it with an outdoor unit (heat pump or AC). Unfortunately, some of my friends in California also call a furnace an air handler, but I find that confusing.

As for refrigerant lines, we call them liquid and gas (not vapor) lines. It's a minor difference, but some stuck-up engineers with Ph.D.'s consider a "vapor" to be a gas that is at saturation conditions (not superheated).

Here is an interesting twist--Carrier used to make a triple-split system with an indoor unit, an outdoor unit (coil and fan only), and a separate compressor unit which was located indoors, usually near the indoor unit. Performance-wise it makes sense to put the compressor indoors, especially on heat pump units, but there are noise issues and it requires more field connections (electrical and refrigerant). It didn't last long on the market. I had one and liked it.

I don't like the term "coils" for heat exchangers, but that will probably never change. Heat exchangers used to be coils of tubing, but you don't see that much any more except for Trane's spine-fin "coils".

Fair point about coils, but it's such a handy term. I'm 3-for-4 on the characteristics mentioned above - stuck-up pedantic engineer, just no Ph.D.!

I wonder how the Ph.D.s refer to the moisture mixed with air...isn't that water vapor? But it's only rarely saturated

And while I'm being pedantic and we are on the topic of air heat exchangers, consider the flat thingy towards the front of most cars through which engine coolant passes... Radiator? I don't think so. "Convector" would seem to be much more accurate. It transfers very little heat via radiation.

I would simply call the out door part of a heat pump. The heat pump.

DaKyoob!

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