Bottom Tank Heating for Legionella Control in Electric Water Heaters
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Solution Overview
Problem
Electric water heaters with dome-shaped bottom walls create cavitated zones where sediment deposits form, promoting Legionella bacteria growth due to lower water temperatures, and existing solutions like additional heating elements or tank redesigns are costly and impractical.
Innovation Solution
Incorporating controllable resistive heating elements, such as heating films or wires, directly in contact with the dome-shaped bottom wall, along with a thermostat and temperature sensor, to maintain water temperature above 140 degrees F in the lowermost region of the tank, preventing bacterial growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the bottom heating element is lowered to place it close to the bottom wall of the tank, then the water temperature at the bottom of the tank increases, but the element becomes inaccessible for replacement or repair and requires a large access panel that causes heat loss
Solution Approach 1:
The heating element is nested within the insulating support base structure, specifically housed in a cavity formed in the base. This allows the element to be positioned close to the tank bottom wall for effective heating while being accessible through a small access panel in the base, eliminating the need for a large heat-loss-inducing panel.
Solution Approach 2:
The insulating support base acts as an intermediary structure that houses the heating element in a cavity. This mediator allows the element to be both thermally effective (close to tank) and maintainable (accessible through base), while the base itself provides thermal insulation to minimize heat loss through the access panel area.
2Reliability
If a third heating element is added into the tank to periodically raise water temperature to sanitizing levels, then bacteria proliferation is prevented, but the system becomes costly
Solution Approach 1:
The existing bottom heating element is made multi-functional by controlling it to perform both its primary function (heating water for consumption) and a secondary sanitizing function. The controller enables the element to periodically raise temperatures to sanitizing levels, eliminating the need for a dedicated third sanitizing element.
Solution Approach 2:
The bottom heating element continuously maintains water temperature in the lower region at or above 140°F, which both prevents bacterial growth and eliminates the need for periodic shock heating. This continuous action at elevated temperature achieves sanitization without requiring additional elements or complex periodic cycling.
3Reliability
If cold water flow is directed through the lower region of the tank to flush deposits, then bacteria growth is prevented, but the tank design and conduit network must be redesigned
Solution Approach 1:
The solution extracts the sanitization function from the water flow system and places it in the thermal system. Instead of modifying the tank design and conduit network to direct cold water flow through the lower region, the invention uses the heating element to thermally sanitize the lower region, requiring no changes to the water distribution infrastructure.
4Temperature
If a heating element is placed on the outside of the tank against the bottom end of the outer sidewall, then water temperature at the bottom increases, but the element is located in an area filled with insulating foam that is not practical to access
Solution Approach 1:
The heating element is nested within a cavity formed in the insulating support base, positioning it against the tank bottom wall for effective heating while maintaining accessibility through a panel in the base. This resolves the contradiction by providing both thermal effectiveness and maintenance accessibility.
Solution Approach 2:
Instead of placing the heating element on the external sidewall (one location), the invention relocates it to the bottom region through the support base structure (another dimension/location). This dimensional change provides both the required thermal contact with bottom water and accessibility for maintenance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively sanitizes the lowermost region of the water tank by maintaining temperatures lethal to Legionella bacteria, reducing the risk of bacterial proliferation while being cost-effective and practical for integration into existing water heaters.
Implementation Method 1
a controllable heating element interposed between the insulating support base and the outer surface of the dome-shaped bottom wall to heat water in the tank in the immediate area of the dome-shaped bottom wall to a temperature sufficient to sanitize the tank lower region
Data Source
AI summary
An electric water heater is described and wherein the bottom portion of the water holding tank is provided with various forms of electric heating elements to heat the water in the lowermost region of the tank adjacent the dome-shaped bottom wall to a temperature sufficient to prevent the proliferation of bacteria growth such as the Legionella bacteria in such lowermost region. The insulating foam support base of the water heater also provides a thermal barrier to the heating elements while biasing the heating element on the dome-shaped bottom wall in a region to insure excellent heat transfer to the cavitated zone surrounding the dome-shaped bottom wall where sedimentary deposits occur to create a culture medium for bacteria growth.