Measure Guideline: Condensing Boilers - Control Strategies for Optimizing Performance and Comfort in Residential Applications

Research output: NRELSubcontract Report


The combination of a gas-fired condensing boiler with baseboard convectors and an indirect water heater has become a common option for high-efficiency residential space heating in cold climates. While there are many condensing boilers available on the market with rated efficiencies in the low to mid 90% efficient range, it is imperative to understand that if the control systems are not properlyconfigured, these heaters will perform no better than their non-condensing counterparts. Based on previous research efforts, it is apparent that these types of systems are typically not designed and installed to achieve maximum efficiency (Arena 2010). It was found that there is a significant lack of information for contractors on how to configure the control systems to optimize overallefficiency. For example, there is little advice on selecting the best settings for the boiler reset curve or how to measure and set flow rates in the system to ensure that the return temperatures are low enough to promote condensing. It has also been observed that recovery from setback can be extremely slow and, at times, not achieved. Recovery can be affected by the outdoor reset control, thedifferential setting on the boiler and over-sizing of the boiler itself. This guide is intended for designers and installers of hydronic heating systems interested in maximizing the overall system efficiency of condensing boilers when coupled with baseboard convectors. It is applicable to new and retrofit applications.
Original languageAmerican English
Number of pages38
StatePublished - 2013

Bibliographical note

Work performed by Consortium for Advanced Residential Buildings (CARB), Norwalk, Connecticut

NREL Publication Number

  • NREL/SR-5500-57826

Other Report Number

  • DOE/GO-102013-3895


  • AFUE
  • baseboards
  • Building America
  • condensing boilers
  • Consortium for Advanced Residential Buildings (CARB)
  • controls
  • heating
  • optimiziation
  • outdoor reset
  • residential
  • residential buildings


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