HPAC Magazine

READER FAVOURITE FROM HPAC’S ARCHIVE: An Integrated Hydronic Alternative

June 2, 2014   By ROBERT BEAN

It has always made some sense that if you use hydronics to provide space heating, you ought to also use it for generating domestic hot water. This is especially true if space heating loads exceed domestic water loads and especially when traditional systems are located in cold cli­mate regions. However, we should ask whether the same logic applies when domestic water loads and storage tank temperatures exceed space heating loads and tempera­tures. This recent reversal in loads and temperatures is a re­sult of increases in enclosure performance and the use of low temperature space heating systems. The reversal is also driving the combination system configurations in North America. It is attracting the attention of those responsible for CAN/CSA-P.9-11 – Test method for determining the per­formance of combined space and water heating systems (combos) and ASHRAE Standard 124-2007 – Methods of Testing for Rating Combination Space-Heating and Water- Heating Appliances. 

Thomas A. Butcher, Ph.D. from Brookhaven National Laboratory (BNL), and other North American researchers have done and continue to research the efficiencies and per­formances of traditional boiler-based integrated space heat­ing and domestic water heating configurations, and the evolving water heater-based combination (“combi” or “combo”) systems.

This article will review the results from Butcher’s 2007 pa­per, Performance of Integrated Hydronic Heating Systems, and his 2011 ASHRAE Journal article titled Performance of Combination Hydronic Systems. Readers are challenged to consider an alternate system that fits somewhere in the BNL results, while offering a “design for failure” feature for cases where clients desire stand-by emergency heat.

Butcher looked at several system configurations including the traditional boiler with internal coil; a boiler with an indi­rect tank; a separate boiler with fuel-fired tank type water heater; and a fuel-fired tank type water heater for both DHW and space heating. In all 14 studies have been presented with results tabulated and graphed (see Table 1 for report sample). It is not possible in this article to fully articulate the outcomes of the past and present studies, but it is sufficient to say that combustion efficiency, oversize factors, load pro­files and idle losses make for some less than simple re­search with interesting and useful results.

In looking at all the possible combinations of traditional and developing configurations, one method fails to show up in the studies. The “missing” system combines a smaller but high efficiency fuel-fired space heating boiler with a larger high efficiency dual-purpose tank type fuel-fired water heater, connected through a single wall plate heat exchanger (Figure 1). The first time I saw a version of this system in operation was in a design/build that our business unit occu­pied for many years. The system was developed by industry colleague Brian Wheeldon, who proposed that in the heating season such a system could generate domestic water using the space heating boiler and a plate exchanger as a pre-heater and re-heater; using the volume of the gas-fired water heater as a thermal storage tank. During the non-heating season when the boiler was off, the gas-fired burner of the water heater would kick in. In this manner during the heating season there are no idle losses in the water heater attribut­able to the water heater’s combustion. Likewise, there are no cycling and idle losses typical of a boiler used for generating domestic hot water during the non-heating season. Essentially, when the appliances are combined it enables the optimized benefits of both systems throughout the year.

In 2011 Butcher and BNL presented outcomes for com­bined systems that might be possible when properly config­ured and controlled. Again the method described above was not evaluated but it would be reasonable to suggest that such a system could enable the higher efficiencies afforded by configurations studied and presented in Table 2.

In addition to enabling peak performance of individual ap­pliances under a combined configuration, the alternate sys­tem affords the building owner an additional benefit: the ability to switch to a standby heating source in the event of an appliance failure. For example, should the boiler fail or require service at any time during the heating season, the tank type water can be engaged to provide a temporary heat source for space heating. In residential applications where space heat­ing loads and temperatures are becoming lower than domes­tic water loads and temperatures, using the water heater as a “back up” is an elegant solution. Likewise, if the water heater fails or requires service during the summer months, the boiler can provide the necessary power at least until the tank is repaired or replaced. In the case of the latter, an open bypass valve would allow the boiler vis-à-vis the plate heat ex­changer to act as a tankless instantaneous heater.

There are many configurations of combined space heating and domestic water systems. Several are the subject of on­going studies including water heater-based combo systems. Though the configuration discussed here is not included in the current research it does overcome some of the chal­lenges with traditional systems including meeting the re­quirements of “combi-systems” using dual purpose water heaters. It allows the opportunity to operate with the lowest idle and cycling loss for higher annual system efficiencies and with an emergency “back-up” benefit that clients would appreciate if and when a problem arises. <> 

Robert Bean, R.E.T., P.L.(Eng.) is president of Indoor Climate Consultants Inc. and a director of www.healthyheating.com. He serves on ASHRAE Committees: T.C.61. (CM), T.C.6.5 (VM), T.C. 7.04 (VM), SSPC 55 (VM).

i Butcher, T.A., Performance of Integrated Hydronic Heating Systems, Project Report BNL-79814-2008-IR, December 2007

ii Butcher, T.A., Performance of Combination Hydronic Systems. ASHRAE Journal, December 2011