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Superior Street Reconstruction

Closing the Loop - Steam to Hot Water Transition

The City of Duluth is leveraging the major investment of Superior Street reconstruction to modernize its energy system, upgrading from steam to hot water distribution. Updating this energy infrastructure provides Duluth the pathway to a modern, resilient system. This transformation will reduce the energy demands of the system and in customer buildings, also creating opportunities to reduce fossil fuel usage, reduce carbon, and integrate renewable energy.



Current System Once-Through Steam

System Transformation Closed Loop Hot Water

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The new closed-loop hot water system will greatly reduce the temperature of the water used to heat buildings along Superior Street. Additionally, energy in a loop can return to the plant in a cycle to get reheated and sent back out to customers. This saves energy and water!

The new closed-loop hot water system will greatly reduce the temperature of the water used to heat buildings along Superior Street. Additionally, energy in a loop can return to the plant in a cycle to get reheated and sent back out to customers. This saves energy and water!

  • Customers will receive reliable and cost-effective energy with the same exceptional customer service they expect from Duluth Energy Systems.
  • We are committed to keeping costs stable as we help you save energy and run your building systems more efficiently.

Customer Benefits

  • Upgrade to building equipment improves building efficiency and reliability.
  • Upgrade to distribution infrastructure reduces system maintenance and associated costs.
  • Simplifies building operations, eliminating steam trap and steam reducing station maintenance and replacement.
  • Automatic building control system improves building efficiency by eliminating the manual interface and provides real-time energy monitoring.
  • Increase in energy capacity to grow the system, which increases rate stability and financial predictability.

Environmental Benefits

  • Increases efficiency = less energy, less water
  • Reduces greenhouse gas emissions
  • Positions the system to integrate renewable energy, such as biomass (wood chips or renewable fuel oil sourced from biomass or agricultural crops)
  • Conserves 20 million gallons of Lake Superior water every year
  • The efficiency of a hot water system makes the integration of additional fuels a more cost-effective option. This past year, a natural gas pilot already proved incredibly successful, saving money on fuel and operations while reducing carbon emissions.
    • Starting in April 2019 the Duluth Energy Systems team will be replacing distribution piping located in phase two of the project, which is from 4th Avenue East to east side of Lake Avenue intersection.
    • Customer located in phase two will remain on steam until their buildings are converted to hot water in 2020.
    • The Duluth Energy Systems will continue to meet with phase two and phase three customers to discuss the technical and financial aspects of building conversions.
    • Phase three of the project is slated for 2020 and includes buildings between 3rd Avenue West and Lake Avenue.

Much of Canal Park is already on a hot water distribution system, it serves as a local example of how the hot water system can service a wide mix of buildings.

  • Inside the main Duluth Energy Systems plant are a series of boilers. Hot water is produced by combusting fuels in these boilers (predominantly gas or coal, with future potential for biomass/biofuel).
  • The water is distributed to buildings through a system of underground pipes. This piping loop supplies hot water to buildings and brings cooler water back to the plant to reheat.
  • Once the water reaches the building, it runs through the building heating system to heat the internal spaces.
  • To heat building spaces, hot water is circulated through the building pipes and out to radiators or fan coils that can warm air in rooms and hallways. Additional radiators and coils in air handling units will be installed as part of the project to accommodate the lower water supply temperatures.
  • The thermal energy (via hot water from our system) can be used to heat water for showers, washing hands, or cooking. This is called domestic hot water usage. A heat exchanger will be used to separate the district hot water system from the domestic hot water system.

More information can be found at

Closed-loop hot water systems can operate at a much lower temperature than the existing steam system. The lower the temperature of the heating medium, the lower the heat losses. This is due to the temperature difference between the heating medium and the surrounding air temperature and the rate at which the heating medium cools. The greater the difference, the faster the heating medium will give up heat, which is energy.

In a closed loop hot water system, the hot water from Duluth Energy Systems heats water in your building in an energy transfer station. The hot water that will flow through offices or living spaces in your building is separate from the hot water in the DES hot water loop. Any heat that is not absorbed by your buildings hot water energy transfer system to heat your building is returned to the DES plant to be reheated and pumped back into the hot water loop. The recycling of this water results in water savings. In an open loop steam system, no water or steam is returned to the plant, it is all wasted to the storm or sanitary sewer.

The water that is returned to the plant in a closed loop hot water system is significantly warmer than the make-up water that comes from Lake Superior. This also increases the efficiency of the system as it takes far less energy to bring this water back up to operating temperature than it does to bring the ice-cold water of Lake Superior to operating temperature.

The City’s district heating system is 85 years old, when it was designed as a once-through steam system. Energy in the steam system follows these steps:

  • 90 million gallons of treated 40℉ water is heated to 360℉ to create high pressure steam.
  • The steam is distributed to over 160 buildings downtown.
  • Once these buildings use the steam, it is cooled and condensed to 180℉. The cooled steam condenses to hot water at approximately 180℉ and is then discarded into WLSSD’s wastewater treatment system.



City of Duluth
Duluth Energy Systems
Greater Downtown Council