Computed from the weather station nearest your house, with your system selection and your temperature preferences. Every detail can be customized for a personalized analysis.
energy use| costs / year | total / additional savingsper yr — vs today / vs the row above | vs today · 15 yrsat today's prices |
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Humidity. In side-by-side lab testing, the same AC held indoor humidity at 50 to 52 percent on summer days when it ran in variable-speed mode, and 55 to 60 percent when it ran as an ordinary single-stage system (Kone & Fumo, Energy Reports). A Florida Solar Energy Center test house came out the same way, with the variable-capacity system holding daily humidity about seven points below a fixed-capacity unit. In a real house the gap depends on the system being set up to run long and slow, and the same FSEC study found occupied homes where it was not.
Noise, outside. Every outdoor unit is rated on the same sound scale (AHRI Standard 270). A single-stage condenser rates 70 to 75 dBA and runs at that level every minute it is on. A variable-speed condenser rates 51 to 59 dBA at low speed, which is where it spends most of its running hours, and only climbs into the single-stage range at full speed on the hottest afternoons. Fifteen dBA lower is roughly half as loud to the ear. The ratings below are from the manufacturers' own data sheets.
| Outdoor unit | Type | Min speed | Max speed |
|---|---|---|---|
| Carrier Infinity 24VNA6 | variable | 51–53 dBA | 58–68 dBA |
| Lennox SL28XCV | variable | 56–59 dBA | 68–73 dBA |
| Trane XV20i | variable | 57–62 dBA | 66–75 dBA |
| Amana AVXC20 | variable | 57–64 dBA | 71–75 dBA |
| Carrier 24ACC6 | single-stage | 72–74 dBA | |
| Trane 5TTR3 | single-stage | 71–73 dBA | |
| Goodman GSXN4 | single-stage | 71–75 dBA | |
| Daikin DX14SA | single-stage | 70–74 dBA | |
Noise, inside. The unit most people actually hear is the air handler, especially when it sits in a hallway closet or above the bedrooms. A single-stage blower has one setting: full airflow, a whoosh when it starts and silence when it stops, cycle after cycle. A variable-speed blower ramps up gently and spends most of its hours at reduced airflow, and blower noise falls much faster than airflow does as the fan slows. The fan laws put the reduction at about 50 × log(speed ratio) dB, so a blower at half speed is roughly 15 dB quieter than the same blower at full speed (ASHRAE Handbook, HVAC Applications, ch. 49). Manufacturers rarely publish a comparable sound rating for the indoor unit, so there is no table for this one.
Temperature swing. A single-stage system has one speed. It runs flat out until the thermostat is satisfied, shuts off, and the room warms back up until the thermostat calls again. A variable-speed system matches its output to how much cooling the house needs at that moment and keeps running as long as the house needs more than its lowest speed puts out, so there is no off period for the room to drift through. Manufacturers state that holds the house within about half a degree of the setting, and installers who put these systems in report it often runs tighter, a few tenths of a degree.