Dryer Testing
There are two widely used test methods in the United States for measuring the energy
consumption of dryers. The first is published by the Association of Home Appliance
Manufacturers and was last revised in 1992 [AHAM 1992]. The second is the DOE test method,
which was an adaptation of the AHAM method [Department of Energy 1981]. Both methods
require drying a carefully specified test load under controlled conditions. Both test methods
require stopping the dryer when there is about a 5% residual moisture content remaining in the
test load. This can be a difficult requirement to meet, and also means that the tests do not
determine the effectiveness of any sensors or control strategies. Thus, energy consumption is
measured only for the bulk-drying stage and not for the high-heat stage. (This stopping
procedure has been used for more than 40 years and dates from a time before dryers had
automatic termination controls.)
The DOE test method specifies a 7 pound load of 50/50 blend cloths with 4.9 pounds of
water. It uses the bulk-drying energy consumption with this load to compute an “energy factor.”
This is the number of pounds of cloth dried per kWh, corrected to represent removing water
equal to 66% of the cloth weight. DOE requires a minimum energy factor of 2.67 for gas dryers
and 3.01 for electric dryers. Test data on current dryers shows a range of 2.67 to 3.02 for gas
dryers and 3.01 to 3.30 for standard sized electric dryers. (A very few electric dryers claim up to
3.70.) These values correspond to typical efficiencies of 56% to 63% for gas or 63% to 69% for
electric (based on energy used in the bulk-drying stage only). Due to of the narrow range of
variation between models, there are no Energy Star programs, no Energy Guide labels, and no
rebate programs (except occasional efforts by electric or gas utilities to persuade consumers to
switch fuels).
In an analysis to estimate the energy used for full drying cycles, Ecos conducted its own
field testing of four different standard-sized electric dryer models. One was a “bare-bones”
model with an electromechanical timer for the control. It was a new unit of a current model, but
the design is the same as has been used for more than 20 years. The other three were also current
models, but with electronic controls and moisture sensors. They claimed to be “energy saving”
models, but offered no documentation to support the claims.
Testing included data logging of the power input and the temperature, humidity, and flow
rate of the exhaust air. Humidity was measured using two thermocouples in a wet-bulb and dry-
bulb configuration. Data sampling was taken every 5 seconds. Also the ambient temperature
and humidity were recorded
The test loads were 100% cotton bath towels, DOE test cloths (50/50 cotton-polyester
blend), or a mix of the two. Test cloths were preconditioned according to the DOE test method.
For each load, the bone-dry weight, wet weight, and final weight were recorded. The rate of
evaporation of water from the clothes was calculated by multiplying the air flow by the
difference between exhaust and ambient absolute humidity. This method was validated by 4 test
runs in which the load was weighed every 5 minutes and the loss in water weight was compared
to the calculated evaporation rate.
A total of 35 test runs were made. Each dryer was tested under conditions very similar
to the DOE procedure. Additional tests were done under conditions that more closely represent
actual use. For example, instead of stopping the drying at 5% RMC, we used the normal
9-45©2010 ACEEE Summer Study on Energy Efficiency in Buildings