Effect of Temperature Stratification Near Heating Elements on the Measured Energy Factors of Electric Water Heaters.
Effect of Temperature Stratification Near Heating
Elements on the Measured Energy Factors of Electric
Healy, W. M.
ASHRAE Transactions, Vol. 114, No. 2, 1-15, June 2008.
water heaters; electric heaters; temperature effect;
heating equipment; energy factors; energy consumption;
temperature gradients; tests; stratification;
thermocouples; sensitivity; ASHRAE 118.2
Two different models of 190 L (50 gallon) residential
electric water heaters were tested to investigate
problems that arise in determining their energy
efficiencies due to the steep temperature gradient near
the lower heating element. This gradient makes the
determination of the average tank water temperature
difficult because, in accordance with current test
standard~, temperature measurements are made at only
six discrete locations within the water heater. Results
show that errors in determining this average temperature
can have significant effects on the Energy Factor by
yielding inaccurate estimates of the stored energy
within the tank. This estimate of the stored energy is
part of the correction algorithm used to normalize the
lab measured efficiency(v to the standard conditions
that define the Energy Factor. The investigation's
findings suggest that efforts should be made to ensure
that conditions at the start of a test are similar to
those at the end. For the two water heaters tested here,
this scenario was best achieved by starting with a 24
hour idling period.