A/C Compressor Speed: One, Two, or Variable

footer-logo

 

The article is about A/C compressor speeds: One, Two, or Variable speed.  Most brands of Central A/C have various models — with different models having One, Two, or Variable compressor speeds.   The speed an A/C compressor runs at affects; interior comfort, humidity removal, and energy efficiency & usage.

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.

 

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

 

A/C Compressor Speeds: One, Two, or Variable Speed

central air conditioner outside unit

Image Source: DreamsTime

 

The compressor is located inside the outdoor unit.  It moves refrigerant throughout the HVAC System.  While the refrigerant is in the outdoor unit (the condenser) — the refrigerant is a liquid.  The outdoor unit’s coil allows its fan to pull air through the coil — to remove heat from the refrigerant.

Then, the refrigerant moves inside the home — and into the indoor cooling-coil (“evaporator” coil”).  The indoor coil converts the refrigerant into a gas that becomes quite cold.  The furnace blower-fan pulls hot & humid air through the indoor cooling-coil.  The heat plus some humidity is removed from the air as it passes through the indoor cooling-coil.   Then the refrigerant-cycle repeats for as long as the A/C runs.

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

(Note: The compressor is shown in the left front corner of the diagram of the outdoor unit.)

 

DFW Weather During One Entire Year

During summer — Dallas’ weather is considered: “sub-tropical”.   The air will be hot & humid most of the time.  During May to September — the weather is often be described as “oppressive.”

During winter — Dallas’ weather is considered: “intercontinental”.  This means (cold & dry) air-masses can come from Canada — and (warm & humid) air-masses can come from the Gulf Of Mexico (or Mexico) — at any time.

In DFW — once it gets hot for summer, it stays hot.  But, during spring and fall (as occasionally during winter) — a one-speed A/C may have too much cooling capacity.  With a single-speed A/C, this may cause 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 may cause the house to feel: “cold and clammy”.    But that doesn’t mean a single-speed A/C is a bad choice in DFW.   It only means that indoor-comfort may not be optimized during cooler weather.

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

Image Source: ShutterStock

A/C compressor Speed — Single-Speed

A single-speed (called: “single-stage”) has one A/C compressor speed.  The unit is either on or off.  Single-Speed A/C are the most common because they are the lowest priced.   Because the compressor runs at one speed — it always cools at its maximum capacity.

During cooler outdoor temperatures — there may be times when the A/C output exceeds the home’s cooling-demand.   That doesn’t mean the A/C is oversized.   The A/C is sized to provide comfort at the hottest time of the year.  If it’s running most (or all) of the time when it’s hot outside — it’s likely correctly size for the home.

When the A/C’s cooling capacity exceeds the home’s cooling-demand — a single-speed A/C will cycle on & off frequently.  This may cause the humidity to be higher than you prefer.   You can reduce humidity by setting the thermostat to a cooler setting for a temporary period of time.   But, that may trade one problem for another — if the home feels too cold.

These problems are solved with a variable-speed A/C.  It adjusts its compressor’s speed to match the home’s cooling-demand.  As cooling demand rises or falls — the A/C compressor speeds up or slows down, so your home is the temperature you want.  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 will do a better job of removing humidity.  The slower air moves through the indoor cooling-coil — the more humidity is removed.

 

A/C Compressor Speed — Two-Speed

A Two speed (called: “two-stage”) has two A/C compressor speeds.   On cooler days — their lower speed is 60-70% of the 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 when they are running at their lower speed.  This minimizing starts & stops, and more frequent need to recool attic ductwork.  These factors lead to better energy efficiency and comfort.

It takes more electricity when the A/C turns on and off (running at full speed) — versus running longer (at a slower speed).  After a central A/C turns on — it takes up to 10 minutes before it reaches maximum energy-efficiency.  An A/C running for longer periods of time (at a lower speed) usea less electricity than one that is cycling on & off.

Longer cooling cycles provide improved humidity removal.  As compared to a single-stage A/C, a two-speed & Variable-Speed A/C removes more humidity by running at lower speeds during cooler weather.  The slower that air is pulled through the indoor cooling-coil — the more humidity will be removed.  This is an important feature in locations where humidity levels are high — like DFW.

During summer, attic temperatures in DFW can reach up to 160 degrees.   This requires the A/C to run for several minutes (to cool the ductwork) — before cooled air reaches the home’s interior.   Longer cooling cycles reduce the need to frequently recool the ductwork.

Source: https://www.bpihomeowner.org/blog/two-worst-mistakes-made-attics

 

A/C Compressor Speed — Variable Speed

Variable-speed A/C compressor speeds adjust to match the home’s current cooling demand.   These A/C’s include an additional device called an Inverter.  The inverter’s job is to slow down or speed up the A/C compressor’s speed to match changes in cooling-demandVariable-Speed A/C run at many different speeds — to match their cooling-output to the home’s current cooling-demand.  Variable speed A/C continually adjusts its cooling-output — by slowing or speeding the compressor (and furnace blower-fan) speed as needed.

  • Longer cooling cycles provide improved humidity removal.  Variable-Speed A/C removes more humidity by running at lower speeds.  The slower air is pulled through the indoor cooling-coil — the more humidity will be removed.  This is an important feature in locations, like DFW — where humidity levels are high.
  • Longer cooling cycles provide improved air filtration.  Simply put, when the A/C runs longer — the home’s indoor-air passes through the furnace air-filter more times.
  • Most DFW homes’ ductwork in the attic — longer operating times further increase efficiency due to the reduced number of times the a/c must recool the ductwork.
  • During cooler weather — a Single-Speed A/C’s cooling capacity may exceed the home’s cooling-demand.   This causes “short-cycles”.   The A/C runs for 5-10 minutes — then shuts off because it’s reached the set temperature.  Because the system is frequently starting and stopping —  it puts excessive stress on A/C components.
  • Longer run-cycles minimizes starts & stops — that use more electricity & put more wear on the HVAC’s components. 
  • A 3-ton Central A/C requires around 5,400 watts to keep it running ** (actual electricity use is based on A/C efficiency — expressed as “SEER”.
  • The wattage required to start a 3-ton A/C running = 7,200. **  This extra electricity is provided by a rechargeable battery in the outdoor unit.  The battery is called a: “capacitor”.
  • This provides some insight as to how hard an A/C must work when starting to run.
  • Based on electricity usage — it’s a fair assumption is that more wear occurs during start-up than while the A/C is running.
  • Another fair assumption is that frequent stops & starts put additional wear on the outdoor A/C unit.

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

 

NOTE: Variable-speed and two-speed central A/C require a matched HVAC system (A/C + furnace).  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.  Variable-speed A/C requires a variable-speed furnace.

You can’t use a single-speed furnace with a multi-speed A/C.  With a mismatched HVAC System — the benefit of the multi-speed A/C unit is negated by a furnace that can’t accommodate it.   For instance, in cooler outdoor temps the A/C would slow down — and the temperature of the air coming from the ducts would be so warm that it would seem the HVAC System isn’t working.

 

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

  • AmeriStar HVAC brand offers: 2 single-speed A/C models.  SEER-14 & SEER-16.
  • Coleman HVAC band offers: 1 variable-speed // 1 two-speed //  2 single-speed A/C models.   SEER ratings of 14 — 20.
  • American Standard HVAC brand offers; multiple variable-speed, two-speed, and single-speed A/C models.  SEER ratings of 14 — 22. **

** SEER stands for: Seasonal Energy Efficiency Rating.  It advises of the efficiency-rating for a specific A/C system.

 

Here are details on 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 best comfort and humidity control
  • A Nest Thermostat has the ability to measure and control both temperature and humidity level.
  • 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-Stage cooling provides better comfort and humidity control 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-stage cooling provides good cooling and humidity control.  It cannot adjust compressor speed based on cooling demand.
  • A Nest Thermostat can lower the cooling temperature to increase humidity removal. **

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

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

  • Conventional on/off furnace blower-fans can run at only one speed.  They can’t be paired with a Variable-Speed A/C.
  • A Variable-Speed blower-fan (technically known as: “Electronically Commutated Motor (ECM)” — runs across 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.  Once the desired indoor temperature is achieved — the blower-fan slows down — and will run continuously at a low speed.  The blower-fan adjusts its output as cooling-demand increases / decreases.
  • A furnace blower uses about 400 watts when running at full speed.   An ECM Motor 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

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 and offer 24/7 Emergency Service.