Water heaters are not water delivery temperature control devices

There are two concerns relating to this point:

1) Accuracy of the delivery temperature.
2) Heater storage temperatures resulting from trying to use the heater to control the delivery temperature.

1) Accuracy of delivery temperature.

Mixing valves control the temperature of delivered water to as tight as ±3°F(±1.7°C). Several well considered, extensive and widely referenced standards for mixing valves exist (including ASSE1017, ASSE1016 and CSA B125). These standards have been developed to ensure the safety of hot water users by specifying and allowing the verification of devices specifically intended to accurately control water delivery temperatures. These standards consider issues such as supply pressure and temperature changes and flow rate variations. The goal of the standards is to ensure accurate control of the delivered water temperature.

Water heaters simply CANNOT provide this function. The thermostat of a water heater is a static control device intended only to allow the heater to perform its intended function as a hot water generator and storage device. The thermostat can be “set” with an accuracy of ±5°F(±3°C) at best. And, significantly, it is positioned such that the temperature it senses is not the delivered water temperature. Depending on the heater design, the thermostat may be positioned in the lower portion of the water, or may even be positioned externally, sensing the temperature of the heater’s steel surface. These positions are perfectly acceptable for the thermostat’s intended function, but are totally unsatisfactory for monitoring the heater’s delivered water temperature.

Further, thermostats have “on” and “off” positions; this is fundamental to the way in which they work. The “off” position is the point at which the thermostat instructs the heat source to turn off (i.e. when the heater has reached the “set” temperature). At that point the temperature within the heater begins to fall (slowly by heat loss over time and rapidly as the hot water is drawn off). The temperature will continue to fall until the temperature reaches the thermostat “on” point, at which time the heat source will be turned on. The difference between the “off” and “on” points can be as much as 9°C (15°F). So, a heater set to 120°F (49°C) can reach 104°F (40°C) before re-heating will begin. Given the tolerance on the thermostat set point, this temperature could be as low as 100°F (37°C). (This argument makes no allowance for any additional over- or under-shooting of the control mechanisms.)

At these low temperatures the water heater clearly is not performing its intended function as a hot water generating device. There must also be a question over the efficiency of a heater operating in this manner. With such low storage temperatures the available hot water will be used very quickly, resulting in the heater switching on more regularly and remaining on for longer each time.

This situation is clearly undesirable. There should be no requirement to set the heater to 120°F (49°C).

Inevitably, when the end user is faced with water temperature that may be below 104°F (40°C), they will turn the thermostat to a higher position (either themselves or via their plumber). In doing so the whole intent of any temperature control requirement is negated.

While it is tempting to somehow limit the maximum heater thermostat setting to 120°F (49°C) this simply will not provide a satisfactory situation.

2) Heater storage temperatures resulting from trying to use the heater to control the delivery temperature.

The above discussion shows that attempting to set the heater thermostat to 120°F(49°C) can result in storage temperatures as low as 100°F (37°C). Water stored in this temperature range must be considered dangerous in that it provides an ideal environment for the Legionella bacteria. This temperature in combination with the bio-film commonly found in water heaters provides conditions in which the bacteria can thrive.

By way of reference, please refer to The Centers for Disease Control and Prevention (CDC). The page highlights that water within the temperature range of 90°F - 105°F (32°C - 41°C) provides ideal conditions for legionellae bacteria to reproduce to high numbers (under the heading “Where is the Legionella bacterium found?”)

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