Measure Guideline: Condensing Boilers - Optimizing Efficiency and Response Time During Setback Operation

L. Arena

Research output: NRELSubcontract Report


Conventional wisdom surrounding space heating has told us a couple of things consistently for several years now: size the mechanical systems to the heating loads and setting the thermostat back at night will result in energy savings. The problem is these two recommendations oppose each other. A system that is properly sized to the heating load will not have the extra capacity necessary to recoverfrom a thermostat setback, especially at design conditions. The implication of this is that, for setback to be successfully implemented, the heating system must be oversized. This issue is exacerbated further when an outdoor reset control is used with a condensing boiler, because not only is the system matched to the load at design, the outdoor reset control matches the output to the load undervarying outdoor temperatures. Under these circumstances, the home may never recover from setback. Special controls to bypass the outdoor reset sensor are then needed. Properly designing a hydronic system for setback operation can be accomplished but depends on several factors. Determining the appropriateness of setback for a particular project is the first step. This is followed by proper sizingof the boiler and baseboard to ensure the needed capacity can be met. Finally, control settings must be chosen that result in the most efficient and responsive performance. This guide provides step by step instructions for heating contractors and hydronic designers for selecting the proper control settings to maximize system performance and improve response time when using a thermostat setback.
Original languageAmerican English
Number of pages40
StatePublished - 2014

Bibliographical note

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

NREL Publication Number

  • NREL/SR-5500-61151

Other Report Number

  • DOE/GO-102014-4369


  • ACCA Manual J
  • AFUE
  • baseboard convectors
  • boost control
  • Building America
  • combustion efficiency
  • gas-fired
  • hydronic systems
  • indirect DHW system
  • mass storage
  • modulating condensing boilers
  • night setback
  • outdoor reset control
  • recovery period
  • residential
  • residential buildings
  • right-sizing


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