Advancing the Condensing Boiler’s Outdoor Linear Reset

Posted by Bryan Haag on Wednesday, March 25, 2015

line spacing image

When it comes to condensing boilers, the lower the water temperature, the more efficiently the system will run, with some systems 96% efficient and above, under optimum conditions.

Today’s boilers use the outside air temperature to determine the water supply temperature that the boiler needs to produce. This is done with what is known as an outdoor reset – typically using a linear correlation, based on the outside air temperature, known as the linear reset curve. As the outdoor temperature decreases, the water temperature supplied is increased, and vice versa.

The linear reset, is a default industry standard when installing hydronic heating systems. However, every building is unique; factors like usage, hydronic coils, and insulation affect how each building will respond to changing outdoor temperatures. In a recent decentralized condensing boiler design project at a university campus, we worked with the system’s Direct Digital Controls (DDC) manufacturer to program a unique sequence of operation for the entire DDC system to learn a reset curve for each building, based on actual building data, instead of outside temperature.

The boilers receive and analyze real-time internal space or return water temperature feedback and automatically adjust the supply water temperature for that building, overriding the default linear reset. Furthermore, that data is stored and the system learns and anticipates the building’s unique heating needs, resulting in lower energy costs and improved space comfort

To present the data simply, we grouped and averaged the individual campus building’s data into two data sets; (1) buildings without outdoor air input into their HVAC systems, and (2) buildings with outdoor air input into their HVAC systems, since the outside temperature would have a greater effect on the latter group.

Boiler Performance Graph

The data set results are still preliminary at this point, but as you can see from the chart, in the coldest days of the heating season, both groups are supplying significantly cooler water while meeting space heating set points, than would have been supplied with the default linear outdoor reset.

Although currently there is only data available for a part of the year, it shows the boilers are producing lower water temperatures, which will generate energy savings, because the boilers are operating more efficiently, when compared to the standard linear reset. In fact, the boilers are in condensing mode the entire year, operating at 90+% efficiency (condensing boilers operate in the 88% efficiency range when in non-condensing operation.

If you have an existing condensing boiler, or are installing a new condensing boiler, these low or no-cost advanced controls can increase your boiler’s efficiency and save you energy dollars. If you have questions or want information on compatibility with your building or campus heating system, contact us & we’d be glad to help.

Authored by:
Bryan Haag Author Photo

Bryan Haag, PE


contact request a free evaluation join eNews

Categories: Buildings & Campus

Tagged: Energy Planning & Management  |  Mechanical

Add a Comment