ENGS 89/90 Reports



Year of Graduation


Project Advisor

Benoit Cushman-Roisin


Solomon Diamond and Rafe Steinhauer

Document Type


Publication Date



Significance: There is seemingly nothing more frustrating than running out of hot water during your morning shower or having to wait minutes for the water to heat up. Despite these shortcomings, water heating is the average homeowner’s second largest energy expense, accounting for almost 20% of total household energy consumption in the US. Additionally, roughly 12,000 gallons of a household’s annual water use is wasted in waiting periods for adequately hot water, representing over 22% of the household’s yearly water usage. The most common residential Domestic Hot Water (DHW) systems today focus on low upfront costs. EverHot’s optimized system has a higher upfront cost but decreases operating costs through increased energy and water efficiency, leading to a cheaper lifetime cost as well as offering environmental benefits.

Objective: To confirm energy and water efficiency improvements of the proposed hybrid heat pump, tankless heater, and smart control system design with computer and physical modeling and testing when compared to currently marketed DHW systems.

Innovation & Approach: The first novel aspect of the EverHot DHW system design is the combination of a hybrid tank-based air-source heat pump and on-demand tankless system. Modern heat pumps are up to four times more efficient than current DHW systems. Heat pumps have not been broadly adopted due to their prolonged recovery period, meaning it takes substantially longer to heat the water than with a traditional DHW system. The hybrid system is able to take advantage of both the efficiency of the heat pump and the capacity of a tankless system. The heat pump storage tank works much like a battery and is able to heat the water in the tank during periods of low demand and also take advantage of reduced electric energy rates during off-peak hours. Through an extensive system modeling using TRNSYS, a state of the art DHW software, EverHot has been able to accurately predict subcomponent energy usage in different demand scenarios and understand system sizing requirements. Based upon the analysis, the heat pump will be able to meet the majority of DHW needs with limited intervention from the tankless system. The physical system mock-up allows for validation of the model and a more thorough understanding of how it performs in a real environment. The second novel aspect includes a smart control system that leverages deterministic (infrared occupancy) sensors to improve the overall efficiency of the system and eliminate wait times for hot water.

Deliverables: The main deliverables are a physical prototype set up in Thayer and a corresponding computer-modeled system in TRNSYS. From the prototype and computer model, EverHot collected data on energy use, water use, and pipe insulation. As part of the physical prototype EverHot built a functional smart control system for the recirculation pump. Information from the models and analysis informed system information and sizing for a new construction multi-story single family house in San Francisco and for a retrofit of EverHot’s sponsor Jack Oswald’s house. Additionally, EverHot considered novel aspects of their design for intellectual property purposes. Finally, EverHot created a hot-water system consumer education brochure.

Impact: Based upon economic analysis, an average family of 4 in California can save approximately $376/year using EverHot’s proposed hybrid DHW system and should have a capital payback period of about 5.3 years as compared to an electric storage tank system. Based upon TRNSYS modeling, the EverHot system consistently outperforms conventional DHW system heating elements for the same hot water usage. EverHot’s consumer brochure will allow consumers to make more informed and environmentally friendly choices when choosing their DHW system as well as serve as a marketing tool. Incorporating the design into a multi-story single family house and into our sponsor’s house will provide EverHot with information on how the design will perform under normal use conditions.

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