CH-18-C034 -- Field Evaluation of Condensing Heating Rooftop Units (RTUs) in 100% Outside Air Applications

Condensing combustion technology has been applied to indoor equipment for years, primarily to space heating product lines from natural gas-fired home furnaces to hot water/steam boilers in the residential through industrial sectors. However, its introduction into outdoor, packaged gas heating and electric air conditioning rooftop units (RTUs), used for space conditioning low-rise commercial and institutional buildings, has been slowed by certain economic and technical challenges. Economically, sufficient net savings in operating costs must be demonstrated to payback the cost premium of the condensing RTU. Gas cost savings during the heating season will be reduced by increased electricity consumption costs year round for the RTU supply fan to overcome the additional in-line pressure drop of the secondary condensing heat exchanger. Additional installation and any ongoing maintenance costs for ancillary equipment for combustion condensate management must also be accounted for in payback calculations. Technically, that combustion condensate management must demonstrate both adequate freeze protection in colder climate rooftop environments and code acceptable sanitary sewer disposal, including any local requirements for neutralization of its acidic content. Only recently have some heating, ventilating, and air conditioning (HVAC) manufacturers started to apply condensing combustion technology into limited RTU product lines. Their product literature often targets larger ventilation or make-up air fraction applications for their emerging, condensing RTU product lines. Previous monitoring activities by the authors of conventional, 80% thermal efficiency (TE), non-condensing heating RTUs clearly support these high outside air load applications as an early market entry point for cost effective use of higher 90% TE, condensing heating RTUs, especially while the initial lower volume production costs dictate even greater premiums for condensing RTU equipment.A specialized class of RTUs, known as dedicated outside air systems (DOAS), used to condition 100% outside air for building ventilation, became a focus for a follow-on evaluation of condensing RTUs. Over the heating season, a DOAS warms cold outside air for code required building ventilation and operates continuously during building operating hours. DOAS in "big box" retail stores, in particular those open 24 hours/7 days a week, exhibit both high heating loads and long runtimes. Based on a field evaluation that was conducted over the 2012/2013 heating season of two non-condensing DOAS retrofitted with condensing heating modules at a 24/7 "big box" retail store in the Chicago area, a condensing DOAS provided gas savings of 11% or 2400 therms (253 GJ) with 1285 kWh of added fan electricity over a year for an annual net energy savings of $1444 with gas and electricity prices of $0.65/therm ($6.16/GJ) and $0.09/kWh, respectively. Based on an installed cost premium of $6085 for the condensing DOAS and its condensate system, plus a projected annual combustion condensate neutralizer maintenance cost of $65 for calcium carbonate replenishment, a simple payback of 4.4 years is estimated.