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ABSTRACT

A demonstration of the control and operation of a hybrid boiler array to increase efficiency at a minimal cost is presented for two installations. Comparisons to the previous baseline non-hybrid approaches are provided, with overall efficiency improvement and economic analysis included.

Hybrid boiler configurations integrate the operation of an array of condensing and non-condensing boilers to optimize seasonal heating efficiency, while avoiding the incremental cost of condensing boiler capacity that never achieves condensing efficiency. The boiler array has the combined capacity necessary to meet the building's winter design space heating load. During milder winter conditions, high efficiency operation is obtained by operating only the condensing boiler, controlling supply water temperatures to no greater than 150°F (66°C) which maintains return temperatures below 130°F (54°C) to assure condensing operation. As the outdoor temperature drops below a specified threshold the hybrid controller enables the non-condensing boiler(s). Further decreases in outdoor temperature result in increased supply water temperatures, up to a maximum of 180°F (82°C). As supply water temperature increases to meet building heating load, the return water temperatures can increase to as high as 160°F (71°C), preventing condensing operation and degrading boiler array efficiency from condensing to non-condensing levels.

The hybrid approach offers additional financial savings by altering the typical redundancy strategy of providing 2N, or 200% redundancy, to N+1, 150% redundancy. This reduces the capacity, and cost for the heating system, while still including enough redundancy to meet the heating load at the winter design condition, even if any single boiler in a typical three boiler hybrid array is offline.

The demonstrated hybrid array approach with N+1 redundancy provided maximum seasonal heating efficiency to be realized, while minimizing upfront cost for boiler capacity, by limiting the purchase of the condensing boiler capacity to the amount that can be operated in condensing mode during the heating season. The remaining heating requirement is provided by two non-condensing boilers.

The hybrid condensing and non-condensing boiler approach resulted in increased seasonal heating efficiency over that of the baseline non-condensing design of 15.5% and 28.2% for each of the demonstration sites, respectively, at two Illinois Army National Guard (ILANG) facilities.