Commercial Condensing Boiler Optimization

Commercial Condensing Boiler Optimization The availability and high market penetration of high efficiency condensing boilers provides significant cost-effective savings potential for commercial building owners and CIP programs. However, the actual savings achieved in the buildings is less clear. The sensitivity of condensing boiler efficiency to field operating conditions is widely recognized in the industry, but information about the impact of actual field operating conditions in Minnesota buildings has generally been limited to only varied anecdotal sources. With funding from a CARD grant, the Center for Energy and Environment undertook this field research study in order to gain more insight into the performance of commercial condensing boilers under real operating conditions, and to identify methods for utility programs to improve their energy efficiency.

This field study involved long-term monitoring at 11 buildings representative of commercial condensing boiler installations in Minnesota. Key characteristic variations were building type (e.g. education), boiler system size, boiler system piping arrangement, type of controls, and whether it was a condensing boiler system exclusively or a hybrid system.

Figure 1: Three wall-mount condensing boilers in a commercial building (Source: Center for Energy and Environment)

Operating efficiencies as found

The observed condensing boilers confirmed the fact that on average they are achieving just over half of the expected savings. Figure 2 below shows the actual and potential efficiencies of each system, compared to the 80% baseline for non-condensing boilers. The green bars represent the annual average operating efficiency of the boiler plants at each site while the red bars indicate the rated condensing boiler efficiency. The study found that the average boiler plant efficiency of 87.2% is 6.8 percentage points below the average rated efficiency of the condensing boilers.1

The use of the non-condensing boilers in four hybrid systems (at sites ED1, ED3, MF1 and MF2) reduced the overall boiler plant efficiency by 1.8 to 5.7 percentage points below the operating efficiencies of the condensing boilers alone. On top of this, the actual efficiency of these condensing boilers was 88.4%, which is 5.6 percentage points below the ideal efficiency of 94%.

The largest single factor impacting operating efficiency was that the water temperatures entering the condensing boilers were far above the 80°F value used in laboratory efficiency rating tests. However, suboptimal burner tuning (i.e. extra excess air), burner firing rate control, and cycling control also contributed to performance below rated conditions in nearly every buildings.

Figure 2: As operated, potential and rated efficiencies of condensing boilers and hybrid systems

Potential to do better

The study estimated savings for a number of control, burner adjustment and piping changes that improve boiler efficiency. While they should be implemented at the time of boiler installation, most can be done afterwards. Savings estimates are shown in the blue bars in Figure 2. Note that in all cases the maximum achievable efficiency is still below the rated efficiency. This is primarily because the return water temperature of 80°F is too low to provide adequate heat to the buildings through the radiators and/or air handlers throughout most of the heating season in Minnesota’s climate. Despite the efficiency limitations related to the return water temperatures needed for adequate heating, the researchers found that all of the sites have low-cost opportunities to improve boiler efficiency, and at some sites the potential is very large. The best savings measures, in order of savings, are:

 Improved outdoor reset control  Preventing or correcting sub-optimal piping  Improved staging control  Improved burner tuning

About 80% of the identified efficiency improvements can be achieved with a simple cost-payback period under five years. On the other hand, improved variable speed control of building loop pumping was also evaluated, but its estimated impact and cost-effectiveness was significantly lower. The hybrid boiler systems in the study have the largest potential for increased efficiency through better optimization— especially in the areas of staging control and piping.

CIP program implications

The most important study findings related to CIP programs are listed below. The first three highlight opportunities to increase CIP program impacts while the last three suggest that current CIP program impacts may be less than previously assumed.

1) Additional Savings Potential--Greater savings can be achieved through various control, piping, pumping and burner tuning optimization--especially for hybrid systems. CIP program enhancements, such as new requirements and/or services, can potentially achieve greater savings among current and past condensing boiler rebate program participants. 2) Cost-Effectiveness of Changes—About three-fourths of the identified savings potential can be achieved with a payback of five years or less. Burner tuning, outdoor reset control, and staging control are most common and cost-effective. CIP programs could potentially achieve the vast majority of identified savings potential by focusing program enhancements entirely on burner tuning and boiler system controls. 3) Perceived Value Possible CIP Program Enhancements--Local industry contacts surveyed (trade allies) recognize value in a number of possible CIP program service or requirement changes that improve operating efficiency, even when such enhancements are more complicated than current program approaches. This was especially true for commissioning, training and support for changing of existing installation control settings. 4) Market Penetration--Condensing boilers dominate the commercial boiler market across all building types and sizes. CIP program evaluation should consider that a high percentage of participants would have installed condensing boilers in the absence of a program. 5) In Field Efficiency--The operating efficiencies of commercial condensing boilers tends to be far below their rated efficiencies. CIP program savings estimates should be based on conservative estimates of savings. 6) Tune-Up Savings Magnitude--While the same degree of tune-up adjustment to a condensing boiler has more impact than for a non-condensing boiler, the potential savings identified in the study boilers was still far below current CIP program assumed savings for tune-ups. A fresh evaluation of boiler tune-up program savings is needed.

Details are available in the full report, “Commercial Condensing Boiler Optimization,” on the Commerce website. In addition, CEE will summarize the results of this study in a webinar on May 4 (see separate announcement in the Webinars section of this newsletter). For more information, contact project manager and CARD program administrator Mary Sue Lobenstein Reference Hybrid systems use both condensing and non-condensing boilers. They are used to provide capacity for low outdoor temperatures.