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 Bill Bradbury on December 11, 2012 at 10:56am

After installing several of these systems, I found it difficult to maintain the low return temps necessary for condensing water from the flue gases. Latent heat is the heat that steam gives off when it changes from a higher(vapor) to a lower(liquid) energy state. This heat can be harnessed in a secondary heat exchanger only if the flue gas has been sufficiently cooled in the primary heat exchanger. When return water temps are above 120F, the primary HX can not cool the flue gases sufficiently, water does not condense and efficiency drops to 80%. To maintain 120F in the DHW tank and to provide 140F to the fan coil, the return temp is going to be >120F, so condensing appliances don't make sense. If you are thinking of "cobbling together" a system like this, it is best to take Bob Blanchette's advice and use a conventional tank.

Greg - Triangle Tube's Prestige Excellence is a great product, but the $5500 price tag means I have only installed one. These "cobbled together" systems are about price point not latest, greatest.

Comment by Bob Blanchette on December 10, 2012 at 5:59pm

The combi systems work very well when the heating load is small. Perfect for those situations when you have heating loads under 30k. A conventional water heater can be used with a fan coil. Once you start buying special high BTU water heaters the combi system stops making economic sense.

Comment by Ben Schoenbauer on December 10, 2012 at 12:27pm

There have been issues with the reliability of these systems in the past.  Manufacturers have been working to improve these models and reliability has improved.  There are many manufacturers (Rinnai, AO Smith, American, HTP, Navien, and others) that develop products for this specific purpose and some of them do sell full systems.  

We got interested and involved in this project to support a weatherization project looking to install forced air combi systems as a solution to their need for high efficiency heating with combustion safety and low up front costs. It did take a large effort to optimize and install these systems. We are currently developing best practices and optimization guidelines. Going forward we are looking at several ways to improve the system by reducing the need for optimization and making these systems more accessible to everyone.


Comment by Lester Shen on December 10, 2012 at 11:48am

Thanks, Jon. That is indeed a typo. It should be CFM. We'll get that amended. 

Comment by Jonathan Beers on December 10, 2012 at 11:37am

In the final animation, what's "CPM" stand for? Possible typo for CFM? Back when the Lennox Complete Heat and the early Polaris systems were on the market, a few contractors I talked to reported bad experiences. I heard that the more recent attempts at these systems using Munchkin boilers also had some problems.

Building Science Corp put out this document several years ago:

Comment by Robert C. O'Brien on December 10, 2012 at 11:14am

In the northeast,where the majority of residential boilers are,these are referred to as hydro air systems,a combi is heat and DHW in one unit with heat delivered not using air as a transfer medium ie conventional radiation

Comment by Greg La Vardera on December 10, 2012 at 11:00am

Wait - the description in the above post says "dual integrated appliances". But everybody is talking about cobbing together systems that were not manufactured for that purpose. 

Nobody has an example of a US manufacturer offering an integrated appliance?

No offense intended, but is it my imagination that Energy Consulants are really not interested in dual integrated appliances because many earn a living advising people how to cobble together this kind of system?

Comment by Pat Dundon on December 10, 2012 at 10:47am

My best experience is with a boiler and indirect water heater in a baseboard hot water house.  I found a good way to improve it is a solar hot water system between the well and the indirect tank with circualtion between the two on a relay.  In summer it doesn't overheat, it just makes 120 gallons of 140 degree water.  in my cloudy cold winter environment it means the indirect unit is usually heating water from 75 degrees or hotter to 120, instead of well temperature. 

What I don't know about is wall hung gas fired boilers with built in indirect water heaters. Apparently Rennai (sp?) is building a unit with a 6.5 gallon onboard water heater and instant on hot water combined with a space heating boiler.  I don't see that illustrated here.  does anyone know if they are really effieient, and if so, does the family size effect efficiency because the water runs more? 

Comment by Ben Schoenbauer on December 10, 2012 at 9:50am
Great to see all the interest in this topic.  As Lester mentioned we are currently conducting a field research project (funded by Building America) to characterize the potential of these systems.  Forced air combination systems have been around for a while, but have recently started to be considered again.  There are many manufacturers that offer system components.  Most major water heater manufacturers offer a water heater designed to be the heating plant in a forced air combi system.  Several (including Rheem and Rinnai) have full systems with both the air handler and the heating plant.  
Preliminary results have confirmed the high efficiency potential of these systems. By carefully sizing system components we have been able to reduce water temps and install systems with 90% space heating efficiency.  Performance varies depending on installation, usage and the type of water heater, but our optimized systems have energy use comparable to a condensing furnace and a condensing water heater and because you only need to install a single condensing heating plant installation costs can be reduced.
Comment by tedkidd on December 10, 2012 at 7:23am
Fantastic early Dan Holohan boiler article

Building Science Theory :: a great overview from Bean

BSC Experts Session - Straub and BTU

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