Research Project Summary Information
Condensing Boilers and Low Temperature Baseboard Convection Space Heating(ST10927-1)
Steven Winter Associates, Inc.
High-efficiency, condensing boilers have been available in the U.S. since the 1990s and are now common in the residential market, but in many instances they are not achieving consistent condensing performance levels due to high return water temperatures. This is particularly true with hydronic baseboard heating systems and their traditional sizing and design methods. In high performance, low-load homes, a change in the baseboard sizing and overall system configuration is warranted. In response to this need, the Contractor will team with Ithaca Neighborhood Housing Services (INHS) and Brookhaven National Laboratory (BNL) to devise a three phase research study to evaluate the performance of condensing boilers using baseboard convector delivery systems.
The first phase of the project involved monitoring boiler performance in six existing homes in Ithaca, NY. This research began in late January 2009 and was concluded in August of 2009. Information gained from the first phase was used to design and size systems for three new homes also located in Ithaca, NY. Bench top research was conducted by BNL (Phase II) in conjunction with follow-up testing and monitoring of the new homes.(Phase III) has been conducted to further define the best design parameters to ensure maximum boiler efficiencies, included was the development of a 'Best Practices Guide to Implement Condensing Boiler, Low Temperature Convection Systems' that will be circulated through trade organizations.
Measured/monitored data indicates that boiler output temperatures are consistently set too high to promote condensing of boilers meant to condense, and flow rates are too high through the respective zone valves. These finidings are being presented to installers and manufacturers. Optimizing outdoor air reset including proper orientation, helps maintain the return water temperature, improving the boiler's heat exchanger performance. Overall findings point to methods for operating boilers closer to their design and stated efficiencies, ~ 95% when condensing actually occurs.
Phase I produced the following findings:
(1) condensing boiler systems and baseboard convectors are frequently installed no differently than systems with a conventional boiler
(2) boiler supply temperatures are typically left at manufacturers settings of 180 degrees F or higher
(3) primary loop configurations that are recommended by manufacturers appear to reduce the frequency of condensing
(4) baseboards are sized using high output temperatures, meaning there is too much distribution
(5) circulators move the water at speeds 2-3 times higher than necessary
(6) all systems studied experienced extreme delays in recovery from temperature setback.
Phase II BNL work produced the following conclusions:
(1) any control technique that reduces return water temperature will significantly improve the boiler's achieved efficiency
(2) condensing boilers can be operated with flow rates significantly lower and temperature rises higher than manufacturers recommend, to achieve greater performance
(3) manufacturer's have proprietary controls, making it difficult to standardize on an optimum/efficient setup at installation
Distribution systems are typically too complex, adding wasted pumping energy, and slow return times that affect operating system temperatures.
Steven Winter Associates, Inc.
50 Washington Street
Norwalk, CT 06854
Heating, Ventilation, & Air Conditioning
NYSERDA Contact Information
R&D - Buildings Research