ASHRAE Journal - December 2009 - 117

Gas Tankless Water Heaters By John Dieckmann, Member ASHRAE; Kurtis McKenney, and James Brodrick, Ph.D., Member ASHRAE G as tankless water heaters (also referred to as instantaneous or demand water heaters) have been gaining in popularity relative to conventional gas storage water heaters in the U.S. With energy factors of 0.80 or higher,1 gas tankless water heaters are more efficient than gas storage water heaters, which typically offer energy factors of 0.60. The majority of domestic hot water in the U.S. is supplied by storage water heaters. Water is heated in an insulated storage tank, in most cases either by an electric resistance heating element or by burning natural gas or LPG.* The heating rate, for either electric or gas storage systems, is less than the typical maximum household hot water demand. This difference is balanced, for a period of time, by the volume of hot water stored in the tank. Electric storage water heaters have electric resistance heating elements immersed in the water in the storage tank and lose efficiency primarily from heat loss through the tank insulation. Typically 5% to 10% of efficiency is lost, depending on the thickness of the insulation and the amount of hot water consumed versus the size of the tank. Gas storage water heaters have a gas burner under the water storage tank, with a flue running longitudinally through the center of the tank. Heat is transferred from the flame to the bottom of the tank. The flue provides additional surface to transfer heat from the combustion gas * A small amount, less than 5%, of domestic hot water is heated by other fuels.2 to the water, before the combustion gas flows out the flue. In addition to heat loss through the tank insulation, gas storage water heaters lose efficiency by heat loss up the flue, both when the burner is on and during standby operation. The steady-state thermal efficiency (heat into the water divided by the higher heating value of the gas) is typically 75%, with the other 25% lost out the flue. During standby operation, additional heat is lost by natural convection-driven air circulation up the center flue. As the hot water in the tank heats air in the flue, the air rises and flows out the flue, carrying with it a finite amount of heat. The combined effect of these losses yields the typical energy factor of 0.60 as noted earlier. A gas tankless water heater has a modulating gas burner that fires as needed to supply hot water at an approximately constant temperature, meeting hot water demand on a real time basis. The burner only fires when water flow though the system is detected. Beyond the heat stored in the relatively small mass of the heat exchanger and the equally small volume of water in the heat exchanger, there is no thermal energy in the heat exchanger that is subject to being dissipated and lost during periods when no hot water is being drawn. Typical products for residential applications have a maximum nominal input of up to 200,000 Btu/h (60 kW), which is capable of continuously providing 4.2 gpm (0.27 L/s) of water at a 77°F (25°C) temperature rise. This is equal to the temperature rise used in the DOE efficiency test procedure: 58°F (14°C) cold water supply temperature and 135°F (57°C) hot water delivery temperature, representative of average conditions in the field), enough to supply two showers continuously. Comparison of the energy factors of tankless and storage water heaters does not provide a completely accurate prediction of the difference in energy use that would be experienced in the field. The energy efficiency of a gas storage water heater varies with the total daily hot water consumption (increasing with increased hot water use, as standby losses become a less significant part of the total energy used), but is not sensitive to the size of hot water draws. However, the energy efficiency of a gas tankless water heater is insensitive to the total daily hot water consumption, but is degraded by frequent, small hot water draws. Field and laboratory measurements coupled with available field data on draw profiles1 shows that at typical use conditions, the effect of small water draws is to reduce the energy factor as measured by the current DOE test procedure by about 7%. For example, a gas tankless water heater with an energy factor of 82% would deliver hot water at an efficiency of 75%, about 15% higher than a typical gas storage water heater. Therefore, the gas tankless water heater can be expected to consume 20% less energy to heat a given amount of water, assuming that the 64 gallon (240 L) daily hot water consumption in the DOE test procedure is representative of the actual hot water consumption. Note that the DOE test procedure does not account for heat losses from hot water ASHRAE Journal 117 December 2009

ASHRAE Journal - December 2009

Table of Contents for the Digital Edition of ASHRAE Journal - December 2009

ASHRAE Journal - December 2009
Contents
Commentary
Industry News
Letters
Meetings and Shows
ASHRAE Building EQ Program
Feature Articles
A Closer Look at Chiller Ratings
Cooling With Less Air: Using Underfloor Air Distribution and Chilled Beams
Cooling Concrete: Containerized Water Chilling Plant
Geothermal for Community Center
Anniversary Feature: Air-Recovery System Versus Conventional Air Conditioning
Technical Topics
New Product Preview
Products
Emerging Technologies
Washington Report
People
Special Products
2009 Feature Articles Indices
Classified Advertising
Advertisers Index
ASHRAE Journal - December 2009 - ASHRAE Journal - December 2009
ASHRAE Journal - December 2009 - Cover2
ASHRAE Journal - December 2009 - 1
ASHRAE Journal - December 2009 - 2
ASHRAE Journal - December 2009 - Contents
ASHRAE Journal - December 2009 - 4
ASHRAE Journal - December 2009 - Commentary
ASHRAE Journal - December 2009 - Industry News
ASHRAE Journal - December 2009 - 7
ASHRAE Journal - December 2009 - 8
ASHRAE Journal - December 2009 - 9
ASHRAE Journal - December 2009 - 10
ASHRAE Journal - December 2009 - Letters
ASHRAE Journal - December 2009 - 12
ASHRAE Journal - December 2009 - 13
ASHRAE Journal - December 2009 - 14
ASHRAE Journal - December 2009 - 15
ASHRAE Journal - December 2009 - Meetings and Shows
ASHRAE Journal - December 2009 - 17
ASHRAE Journal - December 2009 - Feature Articles
ASHRAE Journal - December 2009 - 19
ASHRAE Journal - December 2009 - 20
ASHRAE Journal - December 2009 - 21
ASHRAE Journal - December 2009 - A Closer Look at Chiller Ratings
ASHRAE Journal - December 2009 - 23
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ASHRAE Journal - December 2009 - 31
ASHRAE Journal - December 2009 - 32
ASHRAE Journal - December 2009 - 33
ASHRAE Journal - December 2009 - Cooling With Less Air: Using Underfloor Air Distribution and Chilled Beams
ASHRAE Journal - December 2009 - 35
ASHRAE Journal - December 2009 - 36
ASHRAE Journal - December 2009 - 37
ASHRAE Journal - December 2009 - 38
ASHRAE Journal - December 2009 - 39
ASHRAE Journal - December 2009 - 40
ASHRAE Journal - December 2009 - 41
ASHRAE Journal - December 2009 - Cooling Concrete: Containerized Water Chilling Plant
ASHRAE Journal - December 2009 - 43
ASHRAE Journal - December 2009 - 44
ASHRAE Journal - December 2009 - 45
ASHRAE Journal - December 2009 - 46
ASHRAE Journal - December 2009 - 47
ASHRAE Journal - December 2009 - Geothermal for Community Center
ASHRAE Journal - December 2009 - 49
ASHRAE Journal - December 2009 - 50
ASHRAE Journal - December 2009 - 51
ASHRAE Journal - December 2009 - Anniversary Feature: Air-Recovery System Versus Conventional Air Conditioning
ASHRAE Journal - December 2009 - 53
ASHRAE Journal - December 2009 - 54
ASHRAE Journal - December 2009 - 55
ASHRAE Journal - December 2009 - 56
ASHRAE Journal - December 2009 - 57
ASHRAE Journal - December 2009 - 58
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ASHRAE Journal - December 2009 - 60
ASHRAE Journal - December 2009 - 61
ASHRAE Journal - December 2009 - Technical Topics
ASHRAE Journal - December 2009 - 63
ASHRAE Journal - December 2009 - 64
ASHRAE Journal - December 2009 - New Product Preview
ASHRAE Journal - December 2009 - 66
ASHRAE Journal - December 2009 - 67
ASHRAE Journal - December 2009 - 68
ASHRAE Journal - December 2009 - 69
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ASHRAE Journal - December 2009 - 113
ASHRAE Journal - December 2009 - Products
ASHRAE Journal - December 2009 - 115
ASHRAE Journal - December 2009 - 116
ASHRAE Journal - December 2009 - Emerging Technologies
ASHRAE Journal - December 2009 - Washington Report
ASHRAE Journal - December 2009 - 119
ASHRAE Journal - December 2009 - People
ASHRAE Journal - December 2009 - Special Products
ASHRAE Journal - December 2009 - 2009 Feature Articles Indices
ASHRAE Journal - December 2009 - 123
ASHRAE Journal - December 2009 - 124
ASHRAE Journal - December 2009 - 125
ASHRAE Journal - December 2009 - Classified Advertising
ASHRAE Journal - December 2009 - 127
ASHRAE Journal - December 2009 - Advertisers Index
ASHRAE Journal - December 2009 - Cover3
ASHRAE Journal - December 2009 - Cover4
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