August 30, 2012

Abstract: 

This guideline pertains to design and application guidance for combination space and tankless domestic hot water (DHW) heating systems (combination systems) used in residential buildings, based on field evaluation, testing, and industry meetings. As residential building enclosure improvements continue to drive heating loads down, using the same water heating equipment for both space heating and domestic water heating becomes attractive from an initial cost and space-saving perspective. This topic is applicable to single- and multifamily residential buildings, both new and retrofitted. In order to be assured of meeting the Building America savings goals, and the persistence of those savings after installation, continued sharing of lab and field testing results is needed.

Executive Summary

This guideline pertains to design and application guidance for combination space and tankless domestic hot water (DHW) heating systems (combination systems) used in residential buildings, based on field evaluation, testing, and industry meetings. As residential building enclosure improvements continue to drive heating loads down, using the same water heating equipment for both space heating and domestic water heating becomes attractive from an initial cost and spacesaving perspective. This topic is applicable to single- and multifamily residential buildings, both
new and retrofitted. Before committing to wide-scale implementation of such combination space and domestic water heating systems for high performance buildings, whether new or retrofit, design decisions and site conditions affecting performance, maintenance, and occupant acceptability should be well understood. Current performance rating procedures for this type of hot water heating system and its many variants are inadequate to provide convincing prediction of estimated savings. In order to be assured of meeting the Building America savings goals, and the persistence of those savings after installation, continued sharing of lab and field testing results is needed.

The primary intended audiences for this guideline are: plumbing and heating designers and contractors, weatherization and building efficiency retrofit program managers, new construction building efficiency program managers, water heating equipment manufacturers, building research program managers, and energy policy and advisory staff.

As Building America builder partners choose to employ gas-fired, tankless domestic water heaters (both non-condensing and condensing) in order to achieve higher overall building energy efficiency, the strategy of combination space and domestic water heating systems appears attractive. The advantages are the relatively high water heating efficiency, the high heating capacity of gas-fired tankless water heaters (TWH), the compact space-saving size, and the initial cost advantage of using an existing high efficiency water heater in combination with additional components to simultaneously provide domestic water heating and space heating.

However, these tankless water heating systems are not without important potential drawbacks, including not being able to provide consistent temperature hot water during low-flow draws and frequent intermittent (on/off) draws. Addition of a small storage tank overcomes these drawbacks compared to a reference system without a small tank, however, proper insulation of all system and distribution components is critical. There may also be efficiency shortcomings
whereby tankless heaters rated for condensing may not operate as condensing units due to high return water temperature. Maintenance issues related to pipe and heat exchanger scaling, and water heater inlet strainer clogging are additional risks to evaluate. Different water heating technologies are used in gas-fired TWHs. Some manufacturers use what is called "flash heating," which heats only some of the water very hot, which may tend to generate more strainer-clogging mineral precipitate than non-flash heating units.

From a performance point of view, combination systems utilizing TWHs are of particular interest because of the high heating capacity and low standby losses. However, consistency of supplied water temperature at low flow rates and during rapid on/off usage patterns is a concern. Storage type water heaters reduce or eliminate those concerns, but have relatively high standby losses.

Adding a small, external, well insulated storage volume to TWH combination systems provides a high value solution. Tankless water heaters also have more complex designs and water heating strategies that can impact efficiency at different flow rates and temperature differences. Intricate flow measuring and flow controlling components need to be protected from potential damage by foreign particles that may be in the water, but those protection filters can require unacceptable cleaning intervals. In order to achieve supply air temperatures of 105oF or greater, combination systems with a hydronic air handler generally require heating water at a higher temperature than required for DHW only (radiant floor hydronic systems do not). The hotter water is heated, the more potential there is for mineral scale and galvanic corrosion. All of these factors need to be considered and firm design recommendations made before wide implementation of these systems.

Combination space and DHW heating systems work best in houses with high-performance building enclosures and ducts inside conditioned space. This allows for better comfort at lower heating supply air temperatures, and for less conflict between DHW and space heating demands. In retrofit applications of combination systems, it is especially important to make sure that the existing air duct system is well insulated and air-sealed.

Combination systems may make the most sense in new construction since proper design of the total system is possible, including properly sized and insulated plumbing to avoid extended delay time in delivering water, and properly sized and sealed air ducts. In retrofit cases, the existing gas service line (either the outside utility line or in building) may not have adequate capacity to serve the high demand of a TWH or high capacity storage type water heater. In addition, retrofit venting may be more difficult, and old scaled pipes may worsen water flow or inlet filter
clogging problems.

The rating performance standards for combination systems need to be expanded and improved to encompass the new equipment and designs both on approaching the market. That is also needed to better predict actual performance by testing and modeling more realistic use patterns and a wider range of inlet and outlet water temperatures, including for solar preheat to combination systems.

New factory-supplied total systems are needed to overcome mixed supplier conflicts. Improved design and control methodologies are needed to maximize combination system benefits. This includes predicting and achieving better consumer comfort and energy savings, for example, by providing stable water temperature throughout the range of common flow rates and use patterns, assuring consistent condensing operation, fully understanding the pros and cons of adding small storage volumes to combination systems using TWHs, and adding solar preheat to combination
systems.

Acknowledgment

The author acknowledges the valuable participation of NYSERDA in providing access to, and support for, two combination space and tankless DHW heating systems (combination systems) for evaluation and monitoring.

1 Introduction

This guideline pertains to design and application guidance for combination space and tankless DHW heating systems (combination systems) used in residential buildings, based on field evaluation, testing, and industry meetings. As residential building enclosure improvements continue to drive heating loads down, using the same water heating equipment for both space heating and domestic water heating becomes attractive from an initial cost and space-saving perspective. This topic is applicable to single- and multi-family residential buildings, both new and retrofitted. Before committing to wide-scale implementation of such combination space and domestic water heating systems for high performance buildings, whether new or retrofit, design decisions and site conditions affecting performance, maintenance, and occupant acceptability should be well understood. Current performance rating procedures for this type of hot water heating system and its many variants are inadequate to provide convincing prediction of estimated savings. In order to be assured of meeting the Building America savings goals and the persistence of those savings after installation, continued sharing of lab and field testing results is needed.

The primary intended audiences for this document are: plumbing and heating designers and contractors, weatherization and building efficiency retrofit program managers, new construction building efficiency program managers, water heating equipment manufacturers, building research program managers, and energy policy and advisory staff.

This guideline is important to the building industry because the successful application of gas-fired TWHs has the potential to significantly reduce hot water heating energy use compared to standard power-vented water heaters. Combining these tankless water heating units in systems that also provide space heating may free up building funds to invest in other improvements that further contribute to the overall success of high-performance housing.

Overall, the goal of the U.S. Department of Energy's (DOE) Building America program is to “reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes and preretrofit energy use for existing homes).” To this end, Building America teams conduct research to “develop market-ready energy solutions that improve efficiency of new and existing homes in each U.S. climate zone, while increasing comfort, safety, and durability.”1

2 Home and/or Document Inspection

As Building America builder partners choose to employ gas-fired, tankless domestic water heaters (both non-condensing and condensing) in order to achieve higher overall building energy efficiency, the strategy of combination space and domestic water heating systems appears attractive. The advantages are the relatively high water heating efficiency, high heating capacity of gas-fired TWHs, compact space-saving size, and the initial cost advantage of using an existing high efficiency water heater in combination with additional components to simultaneously provide domestic water heating and space heating.

However, these tankless water heating systems are not without important potential drawbacks, including being unable to provide consistent temperature hot water during low-flow draws and frequent intermittent (on/off) draws. Addition of a small storage tank overcomes these drawbacks compared to a reference system without a small tank, however, proper insulation of all system and distribution components is critical. There may also be efficiency shortcomings whereby tankless heaters rated for condensing may not operate as condensing units due to high return water temperature. Maintenance issues related to pipe and heat exchanger scaling, and water heater inlet strainer clogging are addition risks to evaluate. Different water heating technologies are used in gas-fired TWHs. Some use what is called “flash heating,” which heats only some of the water very hot, which may tend to generate more strainer-clogging mineral precipitate than non-flash heating units.

3 Tradeoffs

3.1 Measure Selection Criteria

The implementation of hundreds of combination space and DHW heating systems during the mid-1990s to early 2000s led to some lessons learned. These systems were primarily installed by production builder partners in the markets of Las Vegas, Albuquerque, and Houston. They employed ducted hydronic air handlers and storage-type, natural draft, natural gas-fired water heaters, where water heater input capacity was about 75 kBtu/h and the water storage capacity
was in the range of 50 to 75 gallons. In isolated cases these were high output (100 kBtu/h), high efficiency (95% condensing combustion efficiency) units, but in most cases, these were standard to slightly higher input capacity and standard efficiency (EF=0.56 to 0.62) units. Those combination space and DHW heating systems worked well when the space heating load was less than about 35 kBtu/h, which is normal for most Building America projects.

Design issues were studied related to DHW priority control, control of intermittent flushing of the heating loop to avoid water stagnation in the off season, and design optimization of the storage capacity, the storage temperature and the heating output capacity. Few problems were experienced with these systems with one exception. In a small percentage of cases, a serious energy waste problem occurred whereby collection of debris under the integral check valve in the circulator caused a natural thermo-siphon flow that would send hot water through the space heating coil when space cooling was active. A stronger spring-loaded, more positive shut-off check valve or a powered solenoid valve could have eliminated that problem; however, other market forces were already at work to cause a move away from those systems. The price of high efficiency furnaces (93% to 95% AFUE) was falling and the furnaces were more readily
available from a number of manufacturers.

From a performance point of view, combination systems using TWHs are of particular interest because of the high heating capacity and low standby losses. However, consistency of supplied water temperature at low flow rates and during rapid on/off usage patterns is a concern. Storage-type water heaters reduce or eliminate those concerns, but have relatively high standby losses. Adding a small, external, well insulated storage volume to TWH combination systems provides a high-value solution. Tankless water heaters also have more complex designs and water heating strategies that can impact efficiency at different flow rates and temperature differences. Intricate flow measuring and controlling components need to be protected from damage by foreign particles in the water, but those protection filters can require unacceptable cleaning intervals. In order to achieve supply air temperatures of 105oF or greater, combination systems with a hydronic air handler generally require heating water at a higher temperature than required for DHW only (radiant floor hydronic systems do not). The hotter water is heated, the more potential there is for mineral scale and galvanic corrosion. All of these factors need to be considered and firm design recommendations made before wide implementation of these systems.

Two combination systems were evaluated as part of a NYSERDA deep energy retrofit project in Utica, New York. A photo of these systems is shown in Figure 1. The contractor for the two combination systems was selected by sealed bid. The bid for these installed systems was about $3,500 per system, which was less than half of what the cost would have been for a boiler system replacement. The cost of the condensing water heater unit alone (EF=0.93) was about $1,000,
which is about twice as much as a power vented water heater with EF=0.62. Using the EnergyGauge USA program (FSEC 2010), domestic water heating savings were estimated to be about $150/yr, for a 3- to 4-year simple payback. However, the total combination system equipment cost savings provides an immediate payback. Investing those equipment cost savings into enclosure improvements is estimated to save another $200/year in space conditioning
energy.

Figure 1: Photo of two installed combination space and domestic water heating systems with a condensing TWH and small storage tank

The space-saving compactness of the combination system with TWH is an attractive benefit where the mechanical equipment is preferably installed inside conditioned space. A potential drawback compared to a typical furnace and domestic water heater is the risk of being without both space heating and DHW at the same time if the combination system water heater fails and repair service is delayed.

Probably the most important trade-off on the risk side is the potential for greatly increased maintenance requirements due to clogging of the TWH inlet strainer and accumulation of calcium or lime scale in the heat exchanger and piping. The inlet strainer is designed to protect the modulating water valve and flow meter in the tankless heater. However, due to the constant source of new minerals when the system is open, and the circulation of hot water and mineral
precipitate when the combination system is closed, additional pre-straining of the water just before it enters the TWH is important to avoid intolerable maintenance intervals.

3.2 System Interaction

Combination space and DHW heating systems work best in houses with high-performance building enclosures and ducts inside conditioned space. This allows for better comfort at lower heating supply air temperatures, and for less conflict between DHW and space heating demands. In retrofit applications of combination systems, it is especially important to make sure that the existing air duct system is well insulated and air-sealed.

3.3 Cost and Performance Tradeoffs

Although energy savings are expected, it is possible that overall hot water usage may increase due to the application of gas-fired TWH with “unlimited” supply of hot water. In addition, a natural reaction to insufficient or unstable hot water supply temperature may be to use a higher flow rate of hot water and/or to leave the hot water on for longer continuous periods. If these things occur, there may be some savings “take-back” tradeoffs to evaluate.

Data indicates that condensing TWH efficiency increases from about 87% at 130°F return water temperature to about 94% at 80°F return water temperature (Magande 2011). In order to maximize condensing operation, pump flow controls can be employed to control on return water temperature. In other words, if the return water temperature is too high to achieve efficient condensing operation, the pump flow could be automatically reduced. But that forces a trade-off with heating supply air temperature, since as the pump flow and the return water temperature drops, so does the supply air temperature and the hydronic air handler efficiency (meaning the ratio of heat output divided by the electrical energy input). Air source heat pumps often operate at supply air temperatures below 100°F, and geothermal heat pumps often operate at supply air temperatures around 105°F, and whole-house air circulation strategies effectively move room
temperature air, so the supply air temperature problem can be managed. But, proper duct design, and appropriate supply grille design and placement is critical to avoid cool air complaints. Larger hydronic coils can also be used to lower return water temperature without reducing pump flow or air handler efficiency, but that has an economic trade-off of higher equipment cost.

Testing from the Center for Energy and Environment (CEE) indicates that hydronic air handler coil sizes need to be much larger to achieve low enough return water temperature to provide consistently high condensing efficiency. The CEE data, averaged for a group of combination. . .

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Footnotes:

  1. http://www1.eere.energy.gov/buildings/building_america/program_goals.html