Ask the Engineer

Why does District Energy require a return temperature of 160°F or lower for hot water?

District Energy delivers hot water to its customers for a variety of uses from heating buildings to melting snow on sidewalks and driveways. In order to keep rates down, District Energy requires its customers to maintain a return temperature that is 160°F or lower. The district system is designed for a 90°F temperature difference between the supply and return temperatures. This allowed District Energy to reduce the size of the piping network and to limit the pumping requirement for the central plant.

On the building side, typically buildings that operate air handlers, fan coils, and finned tube radiation maintain a maximum 180° F supply water temperature and a return water temperature of 150° F to the hot water heat exchanger. A proper return water temperature back to the exchanger ensures that the district’s return water temperature will be 160°F or lower. The goal is to keep the district system’s return water temperature at or below 160 °F to lower the flow velocity in the distribution pipes.

When return temperatures are high, distribution losses increase and efficiencies decrease. Capacity diminishes because more gallons of hot water are required to provide the same amount of heating to a customer. More gallons delivered to customer buildings requires more pumping, resulting in higher electricity costs. Higher return temperatures and elevated flows cause more wear and tear on the metering and mechanical equipment. All of this effort to reduce return water temperatures is producing results that benefit all customers. In 2012 we saw our return water temperature average 157.1°F for the year, 2.9°F lower than system design!

Maintaining low return temperatures is a great way customers can reduce maintenance headaches and save money, too. We recommend you review your hot water meter in your building near the service entry point of the hot water lines. The supply and return water temperatures are available for inspection to see how your system is performing. The customer service team reaches out to customers we have identified as having high return water temperatures, but always feel free to contact us if you have any questions about your system.

Why does District Cooling require a minimum 14 degree delta T?

District Cooling delivers chilled water for its customers to use to extract heat, or thermal energy, from their buildings. This energy must then be removed from the water at the District Cooling plants before it is returned to the customers for additional cooling. One way that District Cooling keeps its energy costs down is by requiring customers to maintain a minimum of 14 degree temperature difference between the water supplied to the customer’s building and the temperature of the return water. At a 14 degree delta T, it takes 103 gallons of chilled water to provide one ton of cooling. The larger the difference between the supply and return water temperature (delta T) the more thermal energy is extracted from the building per gallon of chilled water. The higher the system delta T, the fewer gallons of chilled water the District Cooling plants have to pump to provide the same amount of cooling, reducing the amount of electricity the system uses. Increasing the system delta T saves money for all of our customers.

Additional topics on the operation and maintenance of your system

All About Heat Exchangers

Biological Care of Chilled Loops

Demand Savings Through System Management & Equipment Recommissioning

Efficiency Gains with Cooling Start Up

Load Limiting: How District Cooling Meters Can Help Save You Money

Prepare Your System for Heating Season

Preventive Maintenance Checklist – Fall

Water Treatment for Your Cooling Loop

Water Treatment for Your Heating System

Learn more about energy efficiency and customer support.

District Energy St. Paul is operated and managed by Ever-Green Energy.