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 Corey Johnson on December 24, 2013 at 1:38pm

I wish that I would have had a better design plan when we built our first house.

<a href="”>Maxxsale</a>


Comment by Ben Schoenbauer on December 14, 2012 at 11:23am

We are optimizing for a max return water temp of 107 F, typically we get about 100 F.  But, we are forcing the systems to delivery 115 F or hotter air temperatures to be comparable to a condensing furnace.  If we had better designed duct systems and didn't have to worry as much about cold blow, we could easily reduce air temperatures and return water temperatures. 

Comment by tedkidd on December 14, 2012 at 10:54am

Great comment Bill

Ben, what is the return water temperature range you are seeing?

Comment by Ben Schoenbauer on December 14, 2012 at 10:46am

I think there has been some confusion in the comments.  We are looking at replacing existing forced air systems with forced air combi systems.  So the installation costs are only from the water heater and air handler.  In the retrofit homes we are using the cooling equipment isn't changed.  

Bill, the animations weren't supposed to convey all the detailed information, just give an introduction.  We are publishing the results from the project in a set of Building America reports.  The first one detailing the laboratory testing and optimization is available at

I agree that better controls to that can modulate the fan and pump will significantly improve these systems, these controls are part of our 2013 research plan.  Currently our systems are setup up to operate at their optimized performance.  The air handler kicks on when the thermostat calls for heat and turns off when the space has been heated.  We only see a couple degree change in return air temperature during a cycle.  We have a larger enough water temp drop over the air coil (about 40 degrees) that return water temperatures stay low throughout the cycle.  

Our systems are open loop,(no external HX). The high efficiency equipment use stainless steal or other corrosion resistant materials.  We have worked with the manufacturers and are confident in the durability as long as water quality (ie softness)  is maintained.  

Comment by Bill Bradbury on December 14, 2012 at 10:18am

Ben, these animations are great, but they lack real information like; what kind of control system are you using, the second image in the optimal installation hits 85 % efficiency when running optimally, but what happens when the house warms and return air temps increase and water return temps increase? Now you are back at 120F water return temps. I am using 3 speed manual selecting pumps because I have not found a way to control a var speed blower and pump that is still cost effective. I think that is all that is lacking to make these systems feasible. Also are you using a flat plate HX to separate systems so you can run anti-corrosion/calcification chemicals or are you just cleaning the systems often? I am very excited about these systems, but I don't quite have it dialed in yet. I anxiously await your reply.

Patrick, the one down side of combi-systems is they don't play nice with solar thermal. Caleffi's new 25 gal solar storage tank gives you an indirect tank with electric back-up and 2 internal heat exchangers, this allows one for solar and one for a tankless water heater. Here is a link to their Idronics manual for solar combi-systems

As for emitters, Runtal radiators outperform all others. Radiant floors are nice but they are not as efficient as modern flat panel radiators since the radiators also cause convection in the tubes in the back of the radiator. Think of a heat source that only radiates and is only separated from a perpetual heat sink by R-10. Not so great.

Comment by tedkidd on December 11, 2012 at 6:30pm

Current system baseboard or radiators?  Floor - Expen$$ive.  I have it in my bathrooms, it is awesome.  I can run higher temps because there's not much surface area, so it doesn't boil you out.  If you have an extra $20-30k in the budget, awesome!  

How will you cool and dehumidify?  Probably with a heat pump, right?  A device likely to cost less to run than your boiler.  A device that can handle fresh air needs, humidification, and dehumidification.  

That house on mini-splits, or a standard split, in VA, if I had to guess would take 2-3 tons and probably consume 5000-10,000kwh year for heating and cooling.  

So if the in floor is a luxury item you really want, definitely go for it.  (Hopefully it won't mean you shortcut the heat pump quality.)  I don't know your KWH rate, but if it's like ours, at $500 a year you are not going to cut any meaningful money out of your energy bill.  

If you DON'T do it, put the electric mats in the bathrooms.  Not only is it wonderful, if your feet are warm in the bathroom you can keep the house cooler.  I think cold bathrooms mean thermostat bumping, they do for me.  

Comment by Patrick Michaelyan on December 11, 2012 at 4:50pm

I have been very interested in this approach to space and water heating since I got into the HP game.

I am actually trying to purchase a house, and have been spending significant time thinking through how to heat and cool it if the deal goes through.

The house is in dire need of much rehab work, but the bones are good and the current heating system is hydronic. No AC.

My preference (based on my formative years in New England) is to go DER (deep energy retrofit) on a budget. R-20 walls (above grade), R-13 foundation walls, R-10 slab, R-60 attic, and R-5 windows. Try to hit a blower door number near 1,000 CFM for a 1.5 story house containing roughly 2,500 square feet.

I want to keep the hydronic heating, but was considering one of those European combi systems (such as that from Viessman), or a condensing boiler with an indirect tank. In both cases I wanted to have solar thermal back-up.

So, I have a bunch of questions, and could really use the advice and guidance of the experts. One question is your combined thoughts on going with in-floor radiant heat throughout the house.

Thanks in advance.

Comment by Greg La Vardera on December 11, 2012 at 4:32pm

Bill, the Triangletube products seem to have stainless steel innards, look like a high-end product, but even their combination products hardly feel like "appliances" to me.

When we say "appliance" I'm thinking this is something that can live in the living spaces with a home owner. A box with lots of pipes coming out of the tops and sides is "equipment" and it usually goes in a dedicated space or closet with other such equipment. I think my disappointment here stems from the expectation the discussion would be about an Appliance.

Now, if you look at a Nibe unit from Sweden, yes, that is an applance. It looks like a refrigerator and can sit in a laundry room. They combine domestic hot water with hot water heat using heat pumps, electric, or solar and wood burning heat sources - integrated, combi, whatever you like to call it. These do everything you all are talking about setting up with so much trouble. They simply drop in next to the washer, connect a few pipes, and your HVAC is set up. No more complicated than a washer and dryer. That's an appliance.

Comment by Ben Schoenbauer on December 11, 2012 at 2:33pm

I'm not trying to say that forced air combi systems should be installed in every home tomorrow, but I think they are a good option for some homes and a very good idea to continue to develop.  

These systems work really well for low load homes.  We are installing them with higher input condensing water heaters and supplying more than enough capacity to homes with loads up to 50 kBtu/hr, as well.  My cost analysis isn't complete and their our lots of variables, but these systems will almost always be cheaper than installing a condensing furnace and a condensing water heater. 

System design is very important, the size of the air handler pump, coil and fan will make or break these systems.  I have data from 20 fulling monitored test homes where our sizing and installation specifications resulted in return water temperature between 95 and 105 degrees.  The next step is to remove the dependence on design  to ensure high efficiency.  There are similar design and installation concerns with condensing boilers and high performance air condition systems.  

Comment by tedkidd on December 11, 2012 at 2:11pm

BILL, THANK YOU!  That is a fantastic post!  

We need people to understand the importance of DESIGN!  From Robert Bean's writing: 

 Firing a high efficiency boiler to high temperatures with a programmable thermostats does not change the fact that the boiler is behaving like a mid efficient appliance.


As noted in Part I, for energy conservation it is not enough to just install programmable thermostats and upgrade ones old mid efficient boiler to a new high efficiency unit unless the boiler can actually operate at the conditions necessary to deliver its high efficiency rating.

What do we mean by this?

It means a boiler rated at say 98% only achieves 98% when it sees less than 80 deg F (27 deg C) return water temperatures (Fig. 2) regardless of the heaters Energy Star rating or use of programmable setback thermostats. This is not a trivial matter as the cost to upgrade equipment and controls can be significant and all will be for not from an energy perspective if it doesn't lower fuel consumption.

For condensing equipment to optimize we must avoid full throttle.  Optimizing heat replacement is in direct conflict with using setback.  Not sure how to convey this concept.  



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