Central A/C Speeds: 1, 2, or Variable

footer-logo

This Article Is About Central A/C Speeds.  Al’s Plumbing, Heating & A/C in Plano, Texas provides maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps.  Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (same company as American Standard), and Coleman HVAC (same company as York HVAC).

Al’s also provides full-service plumbing maintenance, repairs and replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Al’s is near your home in Plano, TX; Allen, TX; and Frisco, TX.  We service all homes in southern Collin County, TX and Denton County, TX with no travel charges.

Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience.

Central A/C Speeds: One, Two, or Variable

Image Source: Shutterstock

Central A/C speeds mean how fast the compressor (in the outdoor unit) — is capable of running.  There are 3 speeds:

  • Singe-Stage (One Speed)
  • Two-Stage (Two Speeds)                    
  • Variable Speed — Runs at any speed needed to meet the home’s cooling demand. 
  • A/C Speeds Up / Slows Down as cooling demand changes. 

 

Most Central A/C Brands Have Various Models

Different models offer different Central A/C speeds.  The speed an A/C compressor runs affects; interior comfort, humidity removal, and energy efficiency & usage.

Most new residential A/C installations are single-speed because they are the least expensive.   The capacity of the A/C is sized for the hottest summer temps.   During spring and fall, when outdoor temps are mild, a single-speed A/C’s capacity is sometimes too large for the current cooling demand.  This causes the A/C to cool the house very quickly and shut off — and doesn’t allow enough time for humidity removal.  This may leave the house feeling “cold and clammy”.   During these times, the air coming from the vents may be uncomfortable cool.

A Variable-Speed A/C increases or decreases its cooling capacity — by adjusting the compressor’s speed.   This allows it to run longer (while using less electricity) — giving it more time to remove more humidity — while not cooling the house too quickly.   This can be a great feature for areas, such as DFW, where humidity levels are often quite high year-round.

 

Want To Learn Ways To Reduce Cooling Costs Without Replacing Your Central A/C?

Click For Part 1 Of Our 4 Part Article: Als Plumbing And A/C 75 Ways To Reduce Cooling Costs

 

DFW’s Weather During The Year

During Summer — Dallas’ weather is considered: “Sub-Tropical“.   From May through September, it’s hot & humid most of the time.

During Winter — Dallas’ weather is considered: “InterContinental.   Cold & dry air masses come from Canada.  Warm & humid air masses come from Mexico or the Gulf Of Mexico.  During winter, DFW’s outdoor temperature & humidity change frequently.  It depends on which way the wind is coming from —  and wind direction can change one or more times during one day.

During DFW Spring & Fall (and occasionally during Winter) — a Single-Speed AC will have too much cooling capacity for the outdoor temperature.  This causes the home to cool off quite fast — then the A/C turns off.   If it’s humid, the A/C may not run long enough to remove enough humidity.

This Doesn’t Mean A Single-Speed A/C Is A Bad Choice in DFW.   Only that indoor comfort can’t be optimized during cooler temperatures (because the A/C is sized for DFW’s hottest weather).   During milder outdoor temps –it may seem the AC’s output air is too cold and/or indoor humidity is too high.

With Variable-Speed A/C — the outdoor unit’s compressor speeds up & slows down — to match the home’s current cooling demand (using less electricity at slower speeds).  As the cooling demand changes, the AC changes its compressor’s speed (and the furnace’s blower-fan speed).  When the AC is operating at slower speeds, it runs longer — allowing more time to remove humidity.  It also keeps the AC from producing uncomfortably cold output air.

central air conditioner outside unit

Image Source: DreamsTime

A Central AC’s Compressor Is In The Outdoor Unit (called “condenser unit”). 

 

When The AC Runs — This is What Happens In The Outdoor A/C Unit:

  • While in the outdoor unit –the refrigerant is compressed into a liquid.  Then it’s released under pressure into the outdoor unit’s coil.
  • The pressure makes the refrigerant hotter (than before it’s compressed).  This is because the refrigerant must be hotter than the outdoor temperature in order to release the heat within it.
  • The outdoor unit’s fan pulls tremendous amounts of air through its coil — and removes the heat from the refrigerant.

While The AC Is Running — This is What Happens INSIDE The House:

Shown: (on right side of image) Indoor Cooling Coil (evaporator) Inside a Furnace

Image Source: ShutterStock

  • The refrigerant heads (inside the house) & into the Indoor Cooling-Coil (called: “evaporator” coil”).
  • NOTE: This coil is inside the furnace or ductwork & can’t be seen without removing a panel.
  • This coil converts the refrigerant into a gas — causing the refrigerant to become quite cold.
  • Next, the furnace blower-fan pushes hot & humid air through the indoor cooling-coil.
  • The heat and some of the humidity are removed from the air as it passes through the coil.
  • Last, the cooled air is distributed throughout the house.

The Refrigerant-Cycle Repeats For As Long As The AC Runs.

Click Here To See A Diagram Of A Central A/C System: Central A/C System Diagram

(Note: Look At The Upper Left-Corner Image.  The compressor is shown in the left front corner of the diagram — of the outdoor unit.)

Central A/C Speeds: Single-Speed

A single-speed (called: “single-stage”) has 1 A/C compressor speed.  It’s either on or it’s off.  It cools at its maximum capacity — regardless of the home’s current cooling needs.

During milder weather — there can be times when the A/C’s output exceeds the home’s cooling demand.   That doesn’t mean the A/C is oversized — as it’s sized to provide enough cooling for the hottest time of the year.  When it’s hot outdoors — a properly sized A/C runs most (or all) of the time.

During milder weather — a single-speed A/C cycles on & off frequently.  This may cause the humidity to be higher than you would like.  You can reduce humidity by setting the thermostat to a cooler setting for a temporary period of time.   But, that likely trades one problem for another — if the home feels too cold.

These problems are solved with a variable-speed A/C.  As cooling demand rises or falls — this A/C speeds up / slows down.  The furnace blower-fan also speeds & slows to match the A/C compressor speed — and runs longer (at a lower speed) with a variable-speed A/C.  Because the furnace blower-fan moves air at a slower speed — it’s able to do a better job of removing humidity.  The slower air moves through the indoor cooling coil, more humidity is removed.

Central A/C Speeds: Two-Speed

A Two speed (called: “two-stage”) has 2 A/C compressor speeds.   On cooler days — it uses the lower speed (which is is around 65% of high speed).  On hotter days — the A/C switches to the higher speed.  Similar to variable-speed, two-stage A/C will run for longer periods of time (using less electricity) while using its lower speed.

Less AC Cycling Increases Energy Efficiency & Indoor Comfort

  • Attic temperature can reach 160 degrees during DFW summer.
  • During cooler weather — proper attic temperature will be 20 degrees higher than the outdoor air **  Attic temperature is also affected by sun-exposure, available shade on the home, and wind speed (higher wind speeds can increase attic ventilation)
  • Each time the AC cycles — it must REcool the attic ductworkbefore fully cooled air reaches the home’s interior.

** Source: https://askinglot.com/should-an-attic-be-cold-in-the-winter#:~:text=Ideal%20attic%20temperatures%20shouldn’t,on%20your%20air%20conditioning%20system.

  • It takes up to 10 minutes before AC reaches maximum energy efficiency.
  • More frequent cycling means less time the AC operates at lower efficiency more of the time.

Longer Cooling Cycles (using less electricity) — Provides Improved Humidity Control.  

2-Stage A/C removes more humidity — because the furnace is matched with two (blower) speeds.   Air moving more slowly through the indoor cooling coil allows more time to remove humidity.  This can be an important feature in humid locations like DFW — where outdoor humidity levels can be high during cooler outdoor temps (when the wind is coming from the south).

Central A/C Speeds: Variable Speed

A Variable-Speed A/C Compressor Runs At Various Speeds:

  • It constantly adjusts its speed to match the home’s current cooling demand.
  • As cooling demand rises — it speeds up.
  • As cooling demand lowers — it slows down.
  • When outdoor temps are mild — Variably Speed A/C runs longer — at slower speeds (using less electricity)
  • Longer cooling cycles provide improved humidity removal.  
  • Longer cooling cycles provide improved indoor air quality because indoor-air is passed through the furnace filter more frequently.
  • Most DFW homes’ ductwork in the attic — longer operating times further increase energy efficiency because the ductwork doesn’t have to be recooled so often.  
  • Longer run-cycles minimizes starts & stops.  This reduces both electricity usage and wear on HVAC System.

Example:

  • A 3-Ton AC requires 7,200 watts to start running. **
  • A 3-ton A/C requires 5,400 watts to keep running **
  • The extra electricity is provided by a rechargeable battery in the outdoor unit.
  • This provides insight as to how much harder an A/C compressor works during startup.
  • Plus, it takes up to 10 minutes before the A/C is running at maximum energy-efficiency.

Source: https://www.hoecoop.org/sites/default/files/GeneratorTips.pdf

NOTE: Variable-Speed & 2-Speed Central A/C Require A Matched Furnace

  • This is because the furnace must be able to adjust its blower-fan speed — to match the outdoor unit’s compressor speed.
  • Two-speed A/C requires a two-speed furnace blower.
  • Variable-speed A/C requires a variable-speed furnace blower.
  • You can’t use a single-speed furnace with a multi-speed A/C.
  • The benefits of the multi-speed A/C are negated by a furnace that can’t adjust its speed to match the outdoor unit’s speed.

Al’s Plumbing, Heating & A/C Sells & Installs 3 A/C Brands:   

  • AmeriStar HVAC brand offers;
  • 2 Single-Speed A/C Models.
  • SEER ratings are SEER-14 & SEER-16.
  • Coleman HVAC band offers;
  • 1 Single-Speed A/C models.
  • 1 Two-Speed
  • 1 Variable-Speed
  • SEER ratings range FROM SEER-14 (single speed) TO SEER-20 (variable speed)
  • American Standard HVAC brand offers multiple models of:
  • Single-Speed
  • Two-Speed
  • Variable Speed
  • SEER ratings range FROM SEER-14 (single speed) TO SEER-22 (variable speed). **

** SEER stands for: “Seasonal Energy Efficiency Rating”.  SEER indicates the efficiency rating for a specific A/C system.

Details For 3 American Standard HVAC Central Air Conditioner models (more than 3 models are available).

(We Removed American Standard’s Marketing Verbiage — Just The Facts Here.)

american standard platinum gold silver series hvac

Platinum 20 A/C has the highest efficiency.  **

  • Up to SEER-22
  • Variable-Speed cooling provides the highest level of comfort.
  • A Nest Thermostat — has the ability to measure & control the humidity level.
  • With a Variable-Speed A/C — A Nest Thermostat can lower compressor speed & furnace blower speed for better humidity removal.

** Requires a variable-speed furnace.

Gold 17 A/C  has mid-range efficiency. **

  • Up to SEER-18
  • Two-Speed cooling provides better comfort (than one-speed).
  • A Nest Thermostat can lower compressor speed & furnace blower speed for better humidity removal.*

** Requires a variable-speed furnace.

Silver 14 A/C provides SEER-14. ***

  • 14 SEER (required in Texas)
  • Single-Speed cooling provides a good comfort level.   It cannot adjust compressor speed based on cooling demand.
  • A Nest Thermostat can lower the thermostat setting to increase humidity removal. **
  • A lower indoor temperature causes the A/C to run longer — allowing more time for humidity removal.

*** Variable-speed furnace blower capability not required.

One-Speed Vs. Two-Speed and Variable-Speed Furnace Blower-Fan

  • Conventional on/off furnace blower-fans can run at one speed.  They can’t be paired with a Two-Speed or Variable-Speed A/C.
  • A Variable-Speed blower-fan runs at a wide range of speeds.
  • With a variable-speed furnace blower-fan, the motor starts at around 50% blower output.
  • After a time, the blower-fan gradually ramps up to full output when needed.
  • Once the desired indoor temperature is achieved — the blower-fan slows down — and runs continuously at a low speed.
  • The blower-fan adjusts its speed as cooling-demand increases / decreases.
  • A furnace blower uses about 400 watts when running at full speed.
  • A Variable Speed Furnace Blower-Fan uses around 80 watts in continuous-fan mode.
  • Because of their variable speed capability — those blower-fan run at low speeds much of the time.
  • This reduces annual operating costs by 25% — 75% (less than a single-speed blower-fan motor).

Source: https://basc.pnnl.gov/information/ecm-air-handler-fans

 

footer-logo

This article was about A/C compressor speeds.  Al’s Plumbing, Heating & A/C in Plano, Texas provides maintenance & repairs for all brands of Central A/C, Gas & Electric Furnace, and Heat Pumps.  Additionally, we sell and install new HVAC Systems from American Standard (same company as Trane), Ameristar (same company as American Standard), and Coleman HVAC (same company as York HVAC).

Al’s also provides full-service plumbing maintenance, repairs and replacements for every plumbing component in your home.  Al’s sells and installs Rheem Professional Series gas & electric water heaters, and tankless water heaters.  Al’s is near your home in northeast Dallas, TX; Richardson, TX; and Garland, TX.  We service all homes in southern Collin County, TX and Denton County, TX without additional travel charges.

Call Al’s today to discuss any concerns or problems you have with your HVAC System or Plumbing.  We will arrange an appointment at your convenience.