Measurement

Modulation range

The span between the lowest and the highest output a compressor or a gas valve can hold steady, written as a pair of percentages of rated capacity. A system quoted at 25 to 100 percent can settle anywhere in that band; a single-stage system has no band and only runs at 100 or off.

Also called modulation band, turndown range, capacity range

Direct answer

What is the modulation range of an HVAC system?

Modulation range is the low-to-high output band a system can hold, given as percentages of rated capacity. A wide band such as 25 to 100 percent lets output track a light load without cycling.

Modulation range: how it works

A building's heating or cooling load is below the equipment's rated capacity for most of the season and only reaches it on the design day. A system that can lower its output to meet a light load runs longer and steadier, which holds temperature and humidity closer and avoids the losses of frequent starts. The low end of the modulation range is what sets whether that is possible: if the smallest output the system can hold is still above the load in the room, it has to cycle on and off regardless of how high the top end goes. The high end only matters on the coldest or hottest hours.

Where you meet the term

  • On a variable-speed spec sheet, where a figure such as 30 to 100 percent is quoted for the compressor.
  • In a short-cycling complaint on an oversized system, where even the lowest stage overshoots a small room.
  • In a zoning design, where the smallest single zone has to draw at least the system's minimum output.

How to check it

  1. Find the low and high figures on the submittal. The manufacturer's product data lists minimum and maximum capacity for each outdoor model, often as a percentage and as a BTU per hour figure. Owner check
  2. Compare the minimum output with the smallest load it must serve. A technician checks the lowest steady output against a room or zone load so the system is not forced to cycle on the mildest days. Technician
Read this twice

Modulation range: what it is not

  • A wide modulation range does not fix an oversized system. If the minimum output still exceeds the load, the system cycles as a single-stage unit would.
  • The headline number is the top of the range, but the useful figure for comfort is the bottom of it.

Modulation range questions

Why does the bottom of the modulation range matter most?

Loads sit below rated capacity for most of the year, so the question is how low the system can idle. A high top end does nothing on a mild day if the lowest stage still overshoots the room.

Can a two-stage system modulate?

Not continuously. It has two fixed points, a low around two thirds of capacity and a high at full, and it steps between them rather than settling anywhere in a band.

Does a wider range always mean better comfort?

Only if the equipment is sized correctly to begin with. A right-sized variable-speed system holds conditions well; an oversized one still cycles because its minimum output is above the load.

Where this definition comes from

Written from manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • Compare single-stage, two-stage, and variable-speed AC systemsCarrier · manufacturer support articleCarrier comparison table for residential air conditioners: single-stage described as 100 percent on or off, two-stage as high at 100 percent and low at about 65 percent, and variable-speed as modulating between 30 and 100 percent; humidity control graded standard, enhanced and premium with a claim of up to 400 percent more humidity removal for variable-speed against a standard system in worst-case conditions; efficiency graded standard, high and highest SEER2; and upfront cost graded from lowest to highest across the three.
  • Single stage vs two stage vs variable speed air conditionerLennox · manufacturer support articleLennox comparison of air conditioner staging: single-stage running at full capacity only, two-stage with a low setting given as typically 60 to 70 percent capacity and a high setting at 100 percent, and variable-capacity described as offering sometimes as many as 700 different levels and running almost continuously at very low speeds, with humidity control described as basic, better and exceptional across the three and sound described qualitatively rather than in decibels.