Water Heating Capacity
Water heating capacity is described in BTUH or by recovery rate in gallons per hour. Recovery rate figures are usually based on an 80°F temperature rise; various temperature changes are used. To convert recovery rate in gallons per hour to capacity in BTUH, use:
Water heating capacity (btuh)=Recovery rate (gal/hr) x 667 BTU / gal/hr
HPWH water heating capacity is sometimes described in tons. This is inappropriate and confusing, because "ton" is a measure of cooling capacity. This publication uses BTUH for describing water heating capacity.
Energy factor is a delivered efficiency figure defined by the US DOE test procedure "Uniform Test Method for Measuring the Energy Consumption of Water Heaters." It is calculated from data taken for a specific pattern of hot water use during a 24-hour hot water usage test. The higher the energy factor, the lower the energy consumption. The test used 64.3 gallons of hot water per day at nominal temperatures of 135°F for hot water, 58°F incoming cold water, and 67.5°F air temperature.
Details of the test procedure are available in the Federal Register. A summary is presented in Electric Water Heating News, volume 4, number 2, Summer 1991
Energy factor is not the same as water heating efficiency; it is defined only for the specific set of conditions in the test procedure. The actual efficiency of a water heater varies greatly with the amount of hot water used, inlet water temperature, hot water delivery temperature, and other operating conditions.
For commercially available storage water heaters, energy factor is generally higher for units with smaller tanks, however it varies substantially depending on design and construction details. For typical electric storage water heaters, energy factor ranges from 0.77 to 0.95, with a typical value of about 0.86. For gas storage water heaters, energy factor ranges from 0.43 to 0.86, with 0.54 a typical value. For gas units, recovery efficiency ranges from 75 to 94%. Electric units have recovery efficiencies of essentially 100%.
Table 1 lists minimum energy factors set by NAECA for various system types and tank volumes. Also included are the maximum values in each category for all models listed in the October 1997 edition of the GAMA directory.
Heat pump water heaters are much more efficient than both gas-fired and electric resistance water heaters. Values of EF for various residential HPWH models range from 2.0 to 2.5. Because energy factors defined under the US DOE test procedure consider tank heat loss, they are substantially lower than energy factors defined by the 1983 GAMA procedure, which did not consider tank loss.
|NAECA-Required Minimum Energy Factor and Best Available Energy Factors|
|Nominal Volume (gal)||Electric Resistance||Gas and LP||Oil|
a. Discontinued model; highest current model is 0.66.
b. Typical values are much lower.
Source: Minimum energy factors are from NAECA ; maximum values are from GAMA
Consumers' Directory of Certified Efficiency Ratings, October 1997
First Hour Rating
First hour rating was created by US DOE for use by the Federal Trade Commission in categorizing water heaters. First hour rating as now defined is flawed and should not be used for sizing and selecting water heaters. Recent tests show that one hour is too long a period for sizing water heaters. Hot water delivery over a 15- to 30-minute period is usually the determinant for water heater sizing.
The current US DOE first hour rating was flawed and is not in use. A revised test procedure and rating are being developed but have not yet been released.
The first hour rating commonly used and reported in the GAMA directory is quite different. It is determined by heating the storage tank to 135°F and drawing water at three gallons per minute with 58°F inlet water until the outlet water temperature drops to 110°F. The HPWH and/or elements, if any, are allowed to operate. The amount of hot water that the water heater could theoretically heat to 135°F during the remainder of the one-hour period is added to the measured quantity of delivered hot water. First hour rating is expressed in gallons.
The first hour rating definition and test procedure as they are now defined have limited applicability and practical use. The rating gives artificially higher ratings to fuel-fired units relative to electric systems than what is appropriate. For more information refer to "Water Heater First-Hour Rating vs. In-Field Performance," AT-96-18-4, ASHRAE Transactions, Vol. 102, Part 1, Atlanta, GA, 1996, Hiller, Carl C., P.E., Ph.D.  and "New Hot Water Consumption Analysis and Water Heating System Sizing Methodology," SF-98-31-3, ASHRAE Transactions 1997, Hiller, Carl C., P.E., Ph.D.
Hourly Percent Tank Heat Loss
The GAMA directory once included ratings for water heater standby loss. Heat loss was expressed as a percentage of total stored energy lost per hour. Although no longer used by GAMA, the figure remains a convenient means of expressing tank heat loss. EPRI's WATSMPL software can be used to calculate hourly tank loss for various operating conditions using energy factor ratings. Hourly percent tank heat loss is used as an input value for EPRI's HOTCALC software.
Recovery Rate and Recovery Efficiency
Recovery rate is the rate at which a water heater can heat water through a specified temperature difference. Recovery efficiency is the efficiency of the water heater during continuous operation for the heating process. Recovery rate and recovery efficiency were formerly reported in the GAMA directory for all water heaters. Recovery efficiency is still reported for gas and oil water heaters, but not for HPWHs or electric resistance units.
The Gas Appliance Manufacturers Association (GAMA) tests and certifies electric, gas, oil, and heat pump water heater performance using the U.S. Department of Energy test procedure for water heaters. The test procedure addresses all water heaters covered by the National Appliance Energy Conservation Act (NAECA). Models meeting the following criteria are included.
- Electric resistance storage water heaters - 20 to 120 gallons, < 12 kW input
- HPWH - current rating < 24 A at no more than 250 V
- Gas storage water heaters - 20 to 100 gallons, < 75,000 BTUH input
- Oil storage water heaters - < 50 gallons, < 105,000 BTUH input
Gas instantaneous water heaters - input ratings > 50,000 BTUH and < 200,000 BTUH and <180° F delivery temperature
GAMA publishes the Consumers'Directory of Certified Efficiency Ratings for Residential Heating and Water Heating Equipment which includes first hour rating, energy factor, and storage volume.
Air Conditioning Capacity
Total cooling capacity and sensible and latent capacity are stated in BTUH or nominal tons (one ton = 12,000 BTUH). These measures are similar to those used for conventional air conditioners. Latent capacity may also be expressed in pints of water removed per hour, similar to dehumidifier ratings.
Dehumidification capacity (pints/hr) = Latent capacity (kBtuh) x 0.91
Latent and sensible capacity are also described by latent fraction or sensible fraction. The fractions indicate what portion of the total capacity is provided by latent cooling and sensible cooling, respectively. The sum of the latent fraction and sensible fraction is one.
Air conditioning capacity and efficiency vary greatly with temperature and humidity. When comparing alternative units, use information based on consistent conditions. Also, for design and analysis, use performance figures appropriate to the conditions at which the heat pump will be applied. For example, a HPWH applied in a 90°F, 80% relative humidity space will be substantially more efficient and have much greater capacity than indicated by the figures for the standard ARI rating conditions.