Reposted from i.e., the Center for Energy and Environment's Innovation Exchange blog --


CEE’s Ben Schoenbauer has been travelling around the country recently to share his work to improve the efficiency of domestic hot water systems. His current research project is evaluating the energy savings potential of integrated space and water heating systems, also known as dual integrated appliances (DIA) or combination systems (combi systems). He has been presenting this work to both researchers and practitioners. In support of his outreach, the Innovation Exchange created five animations to show how the parts of the various systems interface and to allow comparisons between them.

The first animation is a conventional hot water and space heating system where a tank storage water heater supplies domestic hot water and a forced air furnace provides space heating to the house.

The second animation shows a combi system where the tank storage hot water heater is used in combination with a hydronic air handler (hot water rather than hot combustion gases from burners circulates in the heat exchanger). The water heater supplies the hot water to the air handler to supply space heating to the house.


The third animation is a combi system where an instantaneous, or tankless, water heater has replaced the storage hot water heater. This combi system does not create the standby losses that would occur with a storage tank full of hot water.

The fourth animation shows a combi-system that uses a hydronic boiler rather than a domestic heater as the hot water source. 


This final animation, based on results from CEE's field research, lays out the factors that influence the installed efficiency of a combi system.

We hope that these animations will improve your understanding of these systems, whether you’re an appliance expert or a homeowner investigating possible energy improvements. Feel free to link to or embed these videos from this blog post, our resource page, or our YouTube site.

The motion graphics animations were designed and produced by Huma Saqib.


Related Innovation Exchange project:

Retrofitting Integrated Space & Water Heating Systems

Related posts:

Residential Water Heater Calculator

ACEEE Hot Water Forum

DIA Lab Field Trip

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Comment by tedkidd on December 10, 2012 at 7:18am
These systems definitely have their place, so long as designers understand the unavoidable efficiency sacrifice due to inability to run low water temperatures.

Conflict occurs at the water to air coil. To have effective transfer here, the water must be hot. Dan Holohan, Robert Bean, and as of last week John Straube of BSC have all published the power of running water in the 80f range, a temperature not possible when you add another heat exchange distribution step.

Ideally distribution is directly connected to combustion. This allows lowest possible temperatures, which in turn pull/wring/squeeze (imaging squeezing a sponge) the most BTU out of both the sensible and the LATENT heat of combustion.
Comment by Gordon Stone on December 10, 2012 at 6:59am

How does this solution compare in energy use to a high efficiency furnace with on-demand water heater? 

Are these hydronic air handlers a slide-in replacement for existing furnaces?

Comment by Greg La Vardera on December 10, 2012 at 6:56am

So who offers such appliances here in the US. Last time I checked, nobody.

Comment by Lester Shen on December 10, 2012 at 6:45am

Thanks for the comment, Barbara.  The system you are referring to is a common type of combi system and a high efficiency boiler with a well-insulated side arm tank is a pretty straightforward high efficiency install for hydronically heated homes. Ben has been working on a project with Sustainable Resources Center, a local low-income provider, looking at the potential for combi systems as a mechanical system retrofit with forced air systems. While it may not be the first thing most people think of, a combination system with a high efficiency water heater and a hydronic air handler has been a good solution. Ben's project is characterizing the installed performance of over 300 DIA systems with monitoring in 20 Minnesota homes and documenting the subsequent energy savings. You can learn more about the project here.

Comment by Barbara Smith on December 6, 2012 at 1:33pm

I usually think of a combination system starting with a hydronic boiler using distribution through radiators, rather than an air handler.  Then with domestic hot water through a sidearm water heater, indirect water heater, or maybe a tankless water heater.  Maybe this varies in different parts of the country.


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