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3 Types of Heat Pumps and Air Conditioners

The 3 Types Of Heat Pump And Air Conditioner Capacity

One way of categorizing heat pumps and air conditioners is by how much heating and cooling they can provide.  By how much, I’m not referring to, say, a 2-ton versus a 3-ton air conditioner.  What I’m talking about is whether it provides a fixed capacity when it’s running (single-stage) or has the ability to change the amount of heating or cooling it puts out (multi-stage or variable capacity).  Most homes have fixed capacity systems, but mini-split heat pumps are starting to gain traction here in the United States, as are multi-stage systems.  So, let’s find out about the 3 types of heat pumps and air conditioners.

Fixed-Capacity Heat Pumps

A fixed-capacity (single-stage) heat pump has only two modes:  on or off.  When the thermostat says it’s time for more heating or cooling, it kicks on and ramps up to its full speed almost instantly.  When the thermostat is satisfied, the power shuts off, and the system sits idly, waiting to roar up to full capacity again upon the next signal from the thermostat.

You can think of a single-stage heat pump as a car without gears or accelerator.  When you turn it on, you’re going full speed.  The only way to stop is to turn off the engine.  That would be a terrible way to design a car, but it’s sort of okay for heating and cooling equipment.

The advantage of fixed-capacity equipment is simplicity.  It’s either on or off, so the controls needed are straightforward.  Because this type of system is so common in homes, finding people to work on it and getting parts for it are relatively easy, too.  It’s also usually the least expensive of the three types we’re discussing here.

The biggest disadvantage of a fixed-capacity heat pump is that it’s terrible at matching the capacity to the actual load on the house.  Remember the three types of heating and cooling loads:  part-load, design load, and extreme load.  A single-stage system is oversized for most of the year, sometimes by a huge amount, because the load is at part-load conditions close to 99 percent of the time.

That oversizing is the source of another downside of fixed-capacity systems.  When the system is putting out a lot more heating or cooling than a house needs, people inside the home gets blasted every time the system comes on.  That’s not a recipe for thermal comfort.

And that’s where the other two types come in.

Multi-Stage Heat Pump

Because the heating and cooling loads on a house go up and down throughout the course of a day as well as through the seasons, it makes sense to have heating and cooling equipment that can change their capacity.  The simplest way to do that is with the second of the 3 types of heat pumps:  a two-stage system.  Such a piece of equipment has two modes of operation:  a low capacity (or speed) and a high capacity.

Imagine the car from our previous example that has no gears or accelerator.  The engine is either on or off, and when it’s on, it’s going full speed.  A two-stage heating or cooling system is like that except it now has two engines.  You can turn one engine on to go one speed, or you can turn them both on when you need to go faster.

Because the preponderance of heating hours and cooling hours occur with a house under part-load conditions, a properly sized two-stage system will operate at its lowest capacity most of the time.  On the really cold or hot days, the system will shift to the high-capacity mode because the demand is higher.

A two-stage system, therefore, offers a huge improvement in efficiency and comfort over a fixed-capacity system, but it doesn’t end there.  Multi-stage equipment also comes with three, four, or even five stages.  Manufacturers do this by using clever compressor tricks.  Because heat is moved by the refrigerant, multi-stage equipment simply needs the ability to run the refrigerant through the system at two or more different rates.

When you start looking at multi-stage equipment, an important concept to understand is turndown ratio.  It’s the ratio of the highest to the lowest capacity, and it’s a good number to know when picking equipment.  For example, one 4-ton, 5-stage air conditioner runs at 1 ton of capacity on its lowest stage.  The turndown ratio is 4 ÷ 1, or 4-to-1.  That’s a good turndown ratio.  The same manufacturer also offers a 2-stage system with a low end that’s 83 percent of maximum.  With such a small difference between low and high stages, that 2-stage system doesn’t provide much advantage over a single-stage system and may not be worth the extra cost.

Variable Capacity Heat Pumps

As you go from two stages to several stages, the capacity difference between any two stages gets smaller and smaller.  If your mind goes to calculus when you hear about gaps getting smaller, you may be thinking about that gap becoming infinitesimally small.  And yes, it’s been done.  That’s the variable capacity system we discuss now, the third of the 3 types of heat pumps and air conditioners.

Compressor cut-aways for inverter-driven mini-split heat pumps
Compressor cut-aways for inverter-driven mini-split heat pumps at the Mitsubishi training center

In a variable capacity system, instead of having a discrete set of stages, this type of system can vary its capacity continuously between its low end and its high end.  They usually modulate with special electronic controls on the compressor, and the term “inverter-driven compressor” applies to most variable capacity systems.  Theoretically, these systems can have turndown ratios as low as 10-to-1.  In the real world, getting down to 4-to-1 is good.

Perhaps the most common type of variable capacity heat pumps are mini-split heat pumps.  In addition to being able to vary the capacity continuously, mini-splits also come in smaller total capacity than do conventional fixed-capacity systems.  Conventional systems generally don’t come in sizes smaller than 1.5 tons (18,000 BTU/hr), but you can get mini-splits as small as 0.5 ton (6,000 BTU/hr), and smaller ones are coming soon.  And that’s good because it allows for better zoning in a house.  You can use one system to heat and cool the bedrooms, one to do the common areas, and a third for a sunroom with lots of windows—and thus lots of cooling load.

Aside from the advantage of smaller capacities, mini-splits have surprisingly quiet outdoor units.  You have to get right next to them to hear anything, and even then it’s barely audible.  Another advantage is efficiency.  The ductless units have SEER ratings and HSPF that beat most conventional fixed capacity or multi-stage system.

The biggest disadvantage of variable capacity heat pumps is cost because these systems can be significantly more expensive than fixed-capacity systems.  Also, even with the low-capacity units available, ductless mini-splits are often still too large to put one in every room because they’ll be running at the bottom end of their capacity range.  One final potential drawback is that some of the ducted mini-split air handlers have blowers that can’t produce much pressure.  They can work fine, though, with proper duct design and installation.

One other issue to mention regarding both multi-stage and variable capacity systems is air flow.  When the compressor modulates the refrigerant flow, both the indoor and outdoor blowers need to be able to modulate as well.  Ideally, when the system is operating at 50 percent of its full capacity, the blower also would be at 50 percent.  That doesn’t always happen, though.

So now you know.  Next time you need a new piece of heating and cooling equipment, remember the 3 types of heat pumps and air conditioners.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a popular book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to our newsletter and follow him on LinkedIn.


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This Post Has 32 Comments

  1. Reminded me of trying to recharge the A/C on my wife’s car (2003 Chevy) and my car (2013 VW). The Chevy was easy – single stage, clutched, on-off-on-off-on-off… The VW drove me nuts because the clutch never disengaged if the A/C was on. Discovered eventually the the VW compressor is a variable capacity one; it just runs and pumps (compresses) when it needs to. So I guess cars are mobile mini-splits!

    1. Indeed Walter… In most VW products the computer controls the compressor swash plate angle. There’s a clutch but it’s always engaged when the A/C is running. It’s sad how much more sophisticated automotive HVAC can be compared to most home systems.

    1. Maven: I don’t know that there’s a general answer to this question. It depends mostly on the particular pieces of equipment you’re comparing. Look for the latent capacity for different models and see how they relate.

  2. I have a mini Fujusute SP 9000 btu/hr cooling 12000 Btu /hr heating and use it to cool heat 1150 sq ft house most of the time. Milton

    1. Milton,
      Where is your house? Is it up to the sort of insulation/air-tightness usually discussed here? I ask bc that’s the size house I have in East Texas so I want to make a case for that size unit here. Thanks

      1. House is located in St. Louis. I have not calculated Manual J but ceiling dry wall R 19,floor over crawl space R30 walls drywall R 16 . Long wall face east and west 24 feet but large walnut tree to shade south,16 foot wall, and east. West wall has one window, 10 sq ft double pane. I try to base load heat pump and use fan to blow air into main part of house. I know crude by works for most of day. Cloudy rainy days reduces load. I have a thermometer on main west wall in a bedroom, original house, and after sun goes down it reads 82 to 83 degrees. I set central air conditioner to 79-80 degrees during day. One ton unit that is on or off, simple unit. Heat pump unit is on south wall so it get shaded by walnut tree most of day. Unit is very quiet. Fujusute, SP, unit, 600 watts on cooling and 500 watts heating. Heating works at 5 degrees. I would recommend that you look at NREL website for free energy software and videos. Roof color on house is GAF light color, See CRRC website. House siding is vinyl light blue. Milton

      2. Kelly, based on Milton’s 3/4 ton unit experience in St Louis and knowing East Texas, I think you would undersized. It is dependent on a lot of factors beside insulation and air tightness. Glass to wall ratio and building orientation greatly affect the cooling load also. 3/4 ton for 1150sqft, that’s over 1500sqft/ton!

        1. It is certainly more than we have now 🙂 Some window units that are boring a hole in my auditory cortex!
          We are mid-90’s today with, bizarrely, a North wind and humidity of 60% (sounds low to me), heat index is 109. I have 1 small and 1 medium window unit on and it is 77 degrees in here. The house is passive solar and situated with the roofline on an E-W axis with the garage on the West End – deep enough eaves and arbors on the South so no direct sun comes in windows. Everything is ca. 1986 on deep clay soil so nothing is tight any longer – need windows, reseal them and doors and walls. It is daunting. I always enjoy this, thanks .

          1. Anyway to know the window units capacity? The nameplate may have the amp draw or wattage or possibly BTUH. Probably the two together are already a little more than a ton since the smallest units I find are around 6000 btuh. You’re medium size unit may be about the 9000btuh capacity that Milton has installed (or it could be a 11-12000btuh, I see a few of these that operate on 120v). So say you have a 9000 and 6000, you’re at 15000btuh (1.25tons). You’re not to peak weather conditions and are only able to maintain 77F, therefore I’m still thinking a 9000btuh single unit will be undersized. The passive solar design seems fairly sound, so as you mention air tightness is probably you’re major concern. Maybe with the improvements you mention you’ll be able to get by with a 1 to 1-1/2ton unit. Nothing beats having a good HVAC load calc done though to be sure.

  3. We have a two story new home with a single air conditioner. The ac can be felt upstairs but can’t cool it enough. Could a second outside ac unit be installed with a second air handler in the attic that ties into existing flex ducts to provide dedicated ac?

  4. Hey Allison, what are your thoughts on the ducted inverter systems using the horizontal discharge outdoor units and a conventional single stage air handler. The costs on these typically rebranded Chinese units seem to be really low. Is the single speed blower a big hindrance to comfort?

    1. Tim: I’m not clear what kind of system you’re talking about here. But if it consists of an inverter-driven compressor with a single speed blower, that doesn’t sound like a good idea to me.

      1. I’m referring to the Gree Flexx and the Midea Central Ducted series that are sold under those names but private labeled for many US known brands like Carrier, Lennox ,GE and also sold online by a bunch of different marketers.(Blueridge and Mr Cool are examples) Generally about 20 SEER systems but with a pretty basic air handler. Mitsubishi and others now have traditional US style air handler systems too. I’m having a GE 2 ton system installed this week in an 1850 sq ft above code house I’m building.

        They seem very cost effective compared to some of the big dollar variable capacity systems in the marketplace but am concerned about the the airflow not ramping while the compressor will.

  5. Outstanding tutorial on Heat pumps and air conditioners. So easy to understand and see real world applications. All new information to me so I learned so much.

  6. I’d rather see a smart thermostat that is actually smart and let me set the temperature differential between actual and target. The simplicity and lower cost of the single stage system is very attractive, especially if you spent the money on air sealing and insulation. I am using Sensi which I like except it has the heat pumps cycling on/off at a ridiculous rate even at the widest differential setting. Unless you are big money, getting the thermal envelope correct seems to be the best investment before you splurge on fancy heat pumps.

    1. I have two Sensi thermostats for the two zones in my home , and like the simplicity and overall functionality but agree that the temperature swing is too narrow and results in short cycles even at widest differential setting.

  7. I’m looking at adding a mini split into my master bedroom. Considering units like Seville, Pioneer or Cooper-Hunter from Amazon or directly from these companies. Any thoughts on them?

  8. Inverter driven mini-split heat pumps are dominating residential new builds especially those built to the passive house standard. For retrofitting existing houses with gas fired hydronic heating, the popular choice is air to water mono block heat pumps, often replacing the first floor radiators with under floor heating PEX pipework.

      1. I am looking forward to your thoughts about monobloc designs, which I think make so much sense. ASHRAE likes them in 228, with the smallest leakage factor. But a Canadian expert told me that in cold climates the antifreeze can really drive up the cost!

  9. We are installing Mitsubishi cold climate heat pumps like I think Tim Koelker was referring to. Variable to 30%. Our winter design temperature is 10 degrees and there is no need for backup. They’re heating capacity at 5 degrees or less equals their rated cooling capacity. There are municipalities that are starting to require either these or GSHPs when going electric. Would be great to see an article on them.

  10. I purchased a Black and Decker portable A/C unit and found that it has a single vent so it uses conditioned house air to cool the compressor coils and vents it so create a vacuum in the house that then sucks in outside hot air into the house. It is terribly inefficient so I am surprised that the EPA allows this type portable unit. It also runs for 3 minutes, shuts down 3 minutes continually short cycling because of the half degree or 1 degree thermostat setting to turn on and turn off the unit. The turn on and turn off temperature setting for air conditioners should be adjustable for the customer; I would set it for 2 or 3 degrees and short cycling or oversized units would not be a problem.

  11. I have a question here that I know for sure can be answered! I got a new Daikin Fit unit and it has performed amazing for the most part. Its a little oversized at 2.5 tons but duct is in the attic and the old unit was a 3 ton.

    My problem is that the house used to be mostly balanced with temperatures. With the new Daikin fit I find that the house is more lopsided then before. My living room/dining area is kind of one big room and my kitchen has 2 open doorways one to each room. During the winter that area is warmer then needs to be and during the summer its also warmer then it needs to be. The thermostat was never moved nor was the return air but I added in a whole extra trunk line and filter for the new unit. Both filters are 20×25 and I’m using honeywell filter grille filters so they are essentially 3-4″ media MERV 11 filters. Thermostat located in hallway where 3 bedrooms come together. Return filters are located at opposite end of hallway (one grille almost in the kitchen).

    Does having an inverter variable speed unit make the placement of thermostat more critical? Should I have it moved? In my mind it should be moved.

    There are TONS of setting on the Daikin Fit also. No sure if someone can help me there but I have no clue if my dealer set it up properly for my house or not.

    1. Shae, I’ve had 2 Daikin Fit variable speed inverter heat pumps installed almost 2 years ago in a 2-story house built 2008 in Atlanta GA. and find that if you use the mobile app and play with both the fan speeds and Comfort – humidity range settings, it can help you tailor settings to your house. The dealer didn’t know about these and only after living with the system can you get a feel for what’s right since there are so many variables involved. Not sure about the thermostat as I’m pretty balanced except for the kitchen getting a little cooler compared to the other rooms. It may help to use ceiling fans selectively?
      In my case, one dealer correctly oversized by half ton, my lower floor unit in the basement and it works great with the higher average temps we have been experiencing country wide. The other dealer undersized my attic Daikin Fit and used the regular design temp which makes it struggle to keep the upper floor cool when it goes over 97 degrees outside, even with giant pine and maple trees providing some shade, something that is happening with more regularity as the world warms. I have open cell spray foamed the attic and put in attic and basement dehumidifiers and exhause fans after reading everything on this site but feel the design temps need to be updated to a few degrees higher.

    2. I can’t help you with the air distribution and varying temperature issue, but we’ve had a Daikin Fit, variable speed, 2 tons ASHP installed for over two years now that replaced a 2.5 tons Carrier single speed ASHP installed in late 2000. House has been improved over the years, significantly reducing load. We also have the Daikin One+ thermostat and 2 tons VS air handler. Location is southern IN, 2,160 sf, bi-level.

      If you have any interest, there is a lot of information and user/dealer adjustable settings available in the One+ stat under: Settings/Dealer Edit/(enter last four digits of stat MAC address)/begin setup. From here you can change and/or view many things. I have reduced the T the aux. heat strips come on to -5F, heat pump to operate down to -10F, house preferences/data, filter reset, etc. You can also see technical data concerning pressures and temperatures at outdoor coil, indoor coil, fan speeds, etc. Since mine is new, I’ve taken some pictures of operating data to benchmark if I have problems later.

      Also very interesting is ‘Quiet mode,’ which can be adjusted from either standard user or dealer edit setting in the thermostat. This is a reduced power mode for the outdoor compressor and fan designed to lower the outdoor sound level. The Fit is very quiet most of the time, but I’ve noticed a lower or deeper compressor sound in colder weather when in HP heat mode and operating at full speed (800 CFM). My wife says she doesn’t notice it. I’ve also watched the operation in typical winter conditions and it will often slowly ramp up, then go to high speed, then turn off after overshooting the set point. All refrigerant levels are correct, as per dealer one year service. The solution is ‘Quiet mode.’ It essentially makes the unit run at a lower capacity, using less energy and creating less noise, during heating mode with longer run times and no overshooting the set point. But what about COP? At least with our 2 tons unit, NEEP data suggests that app. 25F and below is the temperature at which running at lower speed in heating mode is less efficient than normal/higher speed operation. So I will turn it off during really cold days here.

      Part of the sound issue I’m having, during winter, is likely due to a vibrating lineset. I need to remove some dry wall in the basement utility room ceiling and reattach the original pipe from our 1982 build. I’m sure it’s firmly attached. In addition, the installers just set the outdoor unit on a plastic pad that sits on compacted, level soil. I need to firmly attach it in some manner to help reduce vibration. I’ll probably need to use some concrete mass and rubber pads to attach the heat exchanger. Sound issues aside, I do like the idea of ‘Quiet Mode’ with the HP operating at lower capacity and reducing energy use between 25F- 55F. Funny, I found an Australian online forum (whirlpool) about Daikin HPs being too loud in heat mode from 7 years ago. Maybe the same thing? You think they would have improved it by ’20-’21 when the Fit HP came out here.

      Overall, we are very happy with the Fit VSHP. The price was moderate, the warranty is 12 years parts and labor, and the efficiency (18 SEER, 10 HSPF) and comfort have improved versus the old single speed Carrier HP (13 SEER, 8.5 HSPF). We’re using one 21’x17’x5″ MERV 15 filter that lasts a year. It’s overpriced, but I was able to order one online this year and saved $25 versus the local dealer. Good Luck.

  12. Air conditioners usually specify the room/house size it will handle but more important is how well it is insulated, how well it is sealed, window area with sun exposure, how hot the ceiling will get, and levels in a multi-story home since warm air will rise to the upper floors. With inverter variable speed compressors, oversizing should not be a problem since it should adjust its speed to the need.

  13. I was intrigued by the observation that mini-split heat pumps smaller than 6,000 BTU/hr are coming soon. Do you have any idea when these smaller models will be rolled out and by which manufacturers?

    “Conventional systems generally don’t come in sizes smaller than 1.5 tons (18,000 BTU/hr), but you can get mini-splits as small as 0.5 ton (6,000 BTU/hr), and smaller ones are coming soon.”

    I have a very small bedroom in a house in New Hampshire where adding ducts would be unpractical. It would probably be well served by single-zone (1:1) ductless heat pump if I could get one in a 3,000 BTU/hr capacity. I’m concerned that a 6,000 BTU/hr unit would be oversized for a small room.

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