Published: February 12, 2014


Condensing combination boilers, often referred to as condensing combis, are used in residential buildings to provide both space heating and domestic hot water (DHW). They are attractive for their high operational efficiencies and for their dual or “combi” heating functions which, when paired with a tankless DHW delivery method, significantly reduce the space required by the system to meet an occupant’s space and water heating needs.

Residential Combi Boiler in Madison, WI

Condensing combi boilers are tankless DHW systems that also provide space heating. They are particularly well-suited for hydronic or radiant floor heating. The principle components of a condensing combi boiler are a natural gas burner, three heat exchangers (primary, condensing, hot water), and piping for space heating water or steam and domestic hot water. Cold water enters the system and is warmed by flue gases from the boiler as it passes through the condensing heat exchanger. The heated water then passes through the primary heat exchanger to capture even more heat. If it's a steam system, the steam either leaves the boiler to provide space heating or passes through a third heat exchanger where it is condensed into hot water to supply DHW.

We want to acknowledge our appreciation for the efforts of Tim Lindstrom, Nick Funk, and Dan DelVescovo (students at UW-Madison) for their involvement in the research of this technology



Implementation costs

$3,000-$7,000 installed cost per system

Potential energy savings

20% savings for both heating and domestic hot water needs

Condensing combi boilers have been established in European markets for as much as two decades and are prevalent to varying degrees throughout the continent. In England, for example, over four million homes had condensing combis in 2012, representing 18.2 percent of English households. In contrast to Europe, condensing combi boilers are just beginning to emerge in the United States. No studies exist that provide data on condensing combi sales either domestically or in North America.

Condensing combi boilers are most appropriate for smaller homes that have no more than two bathrooms, although they can be used in larger buildings with a zoned system. Tankless DHW can have trouble providing consistent water temperature during periods of simultaneous hot water draws so works best where there is less likelihood of multiple hot water draws at one time, such as in homes with fewer bathrooms. Issues with temperature consistency have also been observed for activities that call for low-flow draws (such as hand washing) or during periods of short intermittent hot water delivery.

Combi boilers would be easier to retrofit in homes that already have a hydronic heating systems, since the circulation system of piping could be maintained.


Energy savings: When comparing a condensing combi boiler to a conventional non-condensing boiler (~78% efficiency) heating system and typical tank domestic hot water (0.57 Energy Factor), the combi boiler can offer a homeowner considerable savings, with a combined system efficiency of over 90%. Depending on the system arrangements and existing conditions/usage, a household might see between 20-30% savings in both heating and domestic hot water heating.

Challenges and market barriers

Cost: First cost can be a deterrent because the system includes both a domestic hot water heater as well as a boiler. The non-installed costs can range from $3,000 to $5,000, depending on the manufacturer (as determined through a web search of retail costs). Installed costs would likely range between $2,000-$3,000 (per conversation with Slantfin representative).

Operational issues: There have been some reports of water temperature inconsistencies during low-flow draws like short hand-washing periods.

Lack of experience with installation: Since the technology is not widely adopted in the United States, installation contractors may not be familiar with installation or may not be able to best market this combi system as an option for homeowners. Manufacturers like Navien are working to address this shortcoming through trainings with installation contractors. (

Statewide energy savings

We took a high-level look at the potential energy savings in Wisconsin from condensing combination boilers The estimate is meant to provide a sense of scale showing the impact this technology might have on Wisconsin energy customers.

To estimate statewide impacts, we assumed that this would be a replace-on-burnout opportunity to install a condensing combination boiler in residential homes that are currently heated by natural gas boilers. We applied a technical savings rate of 30% to the a household's space heating and water heating consumption, for the approximately 13% of Wisconsin households that currently use a boiler to heat their home.

All data used for these estimates are from the Wisconsin Energy Statistics (2012) and Department of Energy's Residential Energy Consumption Survey data (2009).

Financial incentives

There are not currently financial incentives available for the condensing combination boilers, and at this point, there are no custom incentives for residential buildings through the Focus on Energy program.

Madison home

A Madison homeowner installed a Quietside boiler which has a 90% efficiency rating. The unit was installed about 10 years ago and is typical of units installed in Europe and Asia. The system includes a self-modulating gas valve that adjusts the amount of gas per the demand for heat. The heat is delivered to the house either through radiant flooring, radiant ceilings or baseboards.

Expert Meeting Report: Recommendations for Applying Water Heaters in Combination Space and Domestic

summary This report summarizes a meeting held in July 2011 of building and heating experts discussing the design, performance and maintenance of combination systems. The goal of the meeting was to develop foundational information that could be used to develop a Building America measure guideline. This goal is important because current rating procedures inadequately predict estimated savings. The meeting discussed both new and retrofit buildings, in both residential and multi-family applications.
citation Rudd, et. al. Expert Meeting Report: Recommendations for Applying Water Heaters in Combination Space and Domestic. US Department of Energy, Building America. June 2012. LINK
note Link to full presentations can be found here.

Optimizing Hydronic System Performance in Residential Applications

summary This report examines three types of natural gas-fired residential hydronic systems, including a condensing boiler with on-demand domestic hot water generation, a high-mass condensing water heater with an external brazed plate heat exchanger for supplying space heating, and a modulating, condensing boiler with a standard primary/secondary loop and an indirect DHW tank. The researcher sought to determine the optimal combination of components through both modeling and monitoring of installed systems, specifically trying to understand the effects of variable-speed pumps, boost controls, reduction of cycling due to presence of mass and alternative DHW options.
citation Arena L. and O. Faakye. Optimizing Hydronic System Performance in Residential Applications. Department of Energy, Building Technologies Office. October, 2013. LINK

Performance of Combination Hydronic Systems

summary This ASHRAE Journal article details a range of combination hydronic heating systems to "help identify design features and practices that can promote reduced fuel usage." It should be noted that this article does not specifically discuss the condensing tankless combi boiler, but is a useful reference for various other systems.
citation Butcher, T. Performance of Combination Hydronic Systems. ASHRAE Journal. December 2011. LINK

Economic and environmental comparison of natural gas fired conventional and condensing combi boilers

summary Taken from the abstract: "In this paper, energy efficiency and life cycle cost economic analysis of conventional and condensing natural gas fired combi boilers in the individual heating systems are evaluate. The exit flue gas temperature of a conventional gas fired boiler is usually high and a great amount of heat energy is lost to the environment. In the flue gas, both sensible heat and latent heat can be recovered by adding a condensing heat exchanger. Thus, the condensing combi boiler efficiency can be increased by as much as 10%. For this purpose, equations of annual fuel consumption and condensed water vapour quantity are identified and also life cycle costs method has been used to analyse the feasibility of both combi boilers. It is found that condensing combi boiler can provide fuel saving y8% in comparison to conventional combi boiler in individual heating systems. In addition, the life cycle cost analysis showed that condensing combi boiler is cheaper than conventional combi boiler."
citation Comakli, K. Economic and environmental comparison of natural gas fired conventional and condensing combi boilers. Journal of the Energy Institute, 2008.