A dryer's efficiency is published as Combined Energy Factor in pounds of laundry per kilowatt-hour, and the ENERGY STAR threshold a ventless electric compact 240-volt model has to clear is 2.68 lb/kWh, against 3.93 lb/kWh for a standard-size electric dryer, according to the ENERGY STAR clothes dryers criteria established May 19, 2014. Those two numbers are not comparable as printed, because the same page sets the test load at 8.45 pounds for a standard dryer and 3 pounds for a compact one. The condition that changes the answer is the load size: divide energy by the pounds actually dried, not by the cycle.
Combined Energy Factor is pounds of laundry per kilowatt-hour
The federal test procedure at Appendix D2 to Subpart B of Part 430 defines CEF as the bone-dry test load weight divided by the per-cycle combined total energy, which includes standby and off mode as well as the drying cycle itself. Bone dry has a definition in the same appendix: the load is dried at maximum temperature in 10-minute periods until its weight changes by 1 percent or less. The cycle being measured starts with the load at 57.5 percent ± 0.33 percent moisture content and ends when the load is between 1 and 2.5 percent, and a run that finishes above 2 percent final moisture content is invalid.
The ENERGY STAR criteria attach a minimum CEF to each product class, along with a maximum test cycle time of 80 minutes:
Scroll sideways to see the full table →
| Product class | Minimum CEF (lb/kWh) | Bone-dry test load |
|---|---|---|
| Vented gas | 3.48 | 8.45 lb |
| Ventless or vented electric, standard, 4.4 cu ft or greater | 3.93 | 8.45 lb |
| Ventless or vented electric, compact 120 V, under 4.4 cu ft | 3.80 | 3 lb |
| Vented electric, compact 240 V, under 4.4 cu ft | 3.45 | 3 lb |
| Ventless electric, compact 240 V, under 4.4 cu ft | 2.68 | 3 lb |
Almost every heat-pump dryer sold in the United States sits in one of the compact rows, because Appendix D2 defines compact as a drum capacity under 4.4 cubic feet.
The test room is 75 °F, and the annual figure assumes 283 cycles
Drying testing under Appendix D2 is run with the room held at 75 ± 3 °F and 50 percent ± 10 percent relative humidity, and ventless machines have to start each cycle at that same room temperature, which the appendix suggests reaching by leaving the machine at ambient for at least 12 hours between tests. The per-cycle standby and off-mode energy is spread across a representative 283 clothes dryer cycles a year until March 1, 2028, after which the same appendix switches the figure to 236. Every kilowatt-hour-per-year number on a spec sheet or an EnergyGuide label is that per-cycle result multiplied by the cycle count, not a measurement of a particular household.
Ventless drying puts the heat and the moisture in the room
ENERGY STAR's clothes dryers page states that heat pump dryers "use around 70% less energy than a conventional clothes dryers" and that, because the machine does not push air outside, no dryer vent is needed. The second half of that sentence is the ownership fact. The heat the compressor rejects and the water the condenser drains both stay indoors, which raises the load on whatever else conditions that room — the reason a laundry closet and a dehumidifier often end up on the same shopping list.
Example: 123 kWh a year on a 4.0-cubic-foot machine
Bosch's published specification sheet for the 500 Series 24-inch Compact Heat Pump Dryer, model WTW87NH1UC, lists a 4.0 cubic foot drum, ventless operation, energy consumption of 123 kWh a year, 900 W, 208/220-240 V on a 15 A circuit breaker, a silence level of 65 dBA and a net weight of 125 lb. The figures are the manufacturer's own.
Convert them with the rules above. At the representative count of 283 cycles a year set by Appendix D2, 123 kWh works out to 0.435 kWh a cycle. The compact bone-dry test load is 3 pounds, so the implied CEF is 3 ÷ 0.435, or 6.90 lb/kWh — well clear of the 2.68 lb/kWh floor for its class. Per pound of laundry that is 0.145 kWh. A standard-size electric dryer sitting exactly on the 3.93 lb/kWh threshold uses 8.45 ÷ 3.93, or 2.150 kWh a cycle, which is 0.254 kWh per pound. The heat-pump compact therefore uses about 57 percent as much energy per pound, a 43 percent reduction against an already-certified conventional machine rather than the 70 percent quoted against an uncertified one. At 17 cents per kilowatt-hour, the rate assumed here, 123 kWh a year is $20.91. This is an example of how the published figures combine, not a recommendation. The 240-volt circuit on that sheet is the other install fact, the same question raised by an induction cooktop.
Before you buy a heat-pump dryer
- The Combined Energy Factor in lb/kWh together with the product class it was measured in, since compact and standard classes use different test loads.
- The drum capacity in cubic feet, which decides whether the machine is rated as compact under 4.4 cu ft or standard.
- The estimated annual energy in kWh, and the cycle count behind it — 283 a year through February 2028.
- The electrical requirement from the specification sheet: voltage, breaker size in amps and the plug type supplied.
- The noise figure in dBA, and the drain arrangement, since a ventless machine either pumps condensate to a drain or fills a reservoir.
- The condenser cleaning instruction in the manual, which is the maintenance step that decides whether the published energy figure holds.
- The warranty document with its exclusions and the parts list for the lint filters, which on a ventless machine are more numerous than on a vented one.