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Building Science

Using Hot Water to Heat Air with a Hydronic Furnace

Increasing safety, comfort, and efficiency in an old, leaky condo

Image 1 of 4
Common-venting a natural draft water heater and an atmospheric combustion furnace overrides an important safety feature in the furnace.
Image Credit: Allison A. Bailes III, PhD
Common-venting a natural draft water heater and an atmospheric combustion furnace overrides an important safety feature in the furnace.
Image Credit: Allison A. Bailes III, PhD
A hydronic furnace uses hot water to heat the air that heats a home.
Image Credit: Firstco
The Rinnai hydronic furnace uses hot water, which could come from a tankless water heater, to heat air.
Image Credit: Rinnai
The Rinnai tankless water heater can be matched up with their hydronic furnace to provide safe and efficient hot water and heat to a home.
Image Credit: Rinnai

I’m embarrassed to admit it, but yes, that photo shows the gas furnace and water heater in my condo. (I used to live in a very green SIP home that I built, but that’s another story.) It’s an 80 AFUE (80% efficient) furnace and natural draft water heater.

Notice also that they’re common-vented, which can be a combustion safety problem. National Comfort Institute combustion safety instructor David Richardson calls this “one of the most dangerous installations allowed by code.”

The good news is that soon I’ll be replacing both of these combustion appliances to improve the safety, energy efficiency, and comfort in our home. What I’ve decided to go with is a hydronic furnace (sometimes called a ‘hydroair’ system) that gets heat from a water heater, probably a condensing tankless model. It’s a nice configuration that makes a lot of sense, especially for us. I’m not going with a heat pump—mini-split or otherwise—because the condo association pays our gas bill, but we pay the electric bill.

How a hydronic furnace works

Here’s how it works. The water heater does two jobs — it heats the water that we’ll use in the house, and some of that hot water is piped over to the hydronic furnace to heat the air passing over the hydronic coil. The second image, from Firstco, shows the setup.

The schematic here shows a conventional, tank water heater, which works fine for this application. You just have to make sure the capacity of the water heater, whatever kind you choose, is great enough to handle both the domestic hot water needs and the heating load of the house. This won’t be a problem in our home since our heating load is currently about 25,000 Btu per hour, and I’ll be making some improvements to the building envelope as well to reduce it further. Even Rinnai’s smallest hydronic furnace (45,000 Btu/hr) and tankless water heater (157,000 Btu/hr) will give us more heat than we need.

To prevent stranded water in the cooling season from becoming nasty, the pump will circulate water through the coil periodically. In the Rinnai model, the pump runs two minutes in each six hour period and even learns the schedule of hot water use to maximize efficiency. If we go with a tankless water heater, we’ll put a small buffer tank into the system, which keeps the tankless water heater from coming on for each and every little hot water demand.

So, instead of two combustion appliances, this type of combo or integrated system means we’ll need only one. (Although it may be possible to do this with electric water heating, it’s not a good idea because of the amount of current required.) The two main companies that I know of that make hydronic coils, available by themselves or packaged in an air handler to make a hydronic furnace, are Firstco and Rinnai. I haven’t decided yet which one we’ll go with. You can see a photo of the innards of the Rinnai hydronic furnace below.

Safer and more efficient heat and hot water

I really like the idea of this type of integrated system. It’s simple, and in our case, it’s going to be much safer and more energy efficient than what we have now. We could use the blower in our current furnace and just add a hydronic coil, but we’re already using all the vertical height available in the room. By replacing the whole gas furnace, we’ll get a new, more efficient blower as well. I’m not sure about the Firstco model, but the Rinnai has an EC motor (sometimes called an ECM motor, which is equivalent to saying ATM machine or PIN number because the M already stands for motor). The electronically commutated motor (ECM) can be much more energy efficient than the standard Permanent Split Capacitor (PSC) motor.

My strongest motivation in wanting to do this is to get rid of the natural draft water heater, with its combustion saftey issues and possibility for backdrafting. I really don’t want to wake up dead* some day because of carbon monoxide poisoning. So, what I’d like to do is replace it with a high efficiency, condensing, tankless water heater. I’m not a huge fan of tankless water heaters just for the sake of ‘being green.’ They may or may not save energy because the fact that they can keep pumping out the hot water continuously means that you may end up using more hot water than you would with a tank. When it’s part of an integrated system like this, though, I think it makes a lot of sense.

Hydronic heat is great supplemental heat source for heat pumps, too

In our case, the hydronic furnace will be our sole source of heat. If you’ve got a gas water heater and a heat pump, though, hydronic heat works great as a source of supplemental heat. Most heat pumps use electric resistance heat to supplement the output of the heat pump when the temperature drops below the balance point. Electric resistance heat isn’t very efficient and can run up huge electric bills.

Dual fuel systems, which use a furnace as a backup heat source, act as a replacement source rather than a supplement. When the temperature drops and the system calls for more heat, the heat pump shuts off and the furnace kicks on. The heat pump’s evaporator coil is usually installed downstream from the furnace, so to run both simultaneously would cause the refrigerant to boil, probably damaging the compressor. Putting the evaporator coil before the furnace would allow both to run at the same time, but furnaces generally have way too much capacity anyway, so, unless it were a modulating furnace, it wouldn’t make sense to do that.

Because of the disadvantages of electric resistance and dual fuel systems, supplemental hydronic heat makes a lot of sense. Putting the evaporator coil upstream of the hydronic coil allows both to run simultaneously and is a great way to provide the supplemental heat for heat pumps. David Butler, probably the most knowledgeable HVAC designer I know, wrote a couple of articles on this topic earlier this year: Just Say No to Furnaces in High Performance Homes and Heat Pumps and Hydronics – A Great Team for High Performance Homes.

Those of you in cold climates may well be familiar with hydronic heating systems, but they’re usually attached to hydronic distribution systems, too. In cooling climates where everyone uses forced air because of the desire for air conditioning, hydronic systems are rare. I think that these hydroair systems could have a lot of benefits and look forward to getting ours installed. I’ll let you know how well it performs.

*Tip of the hat for that expression to my 10th grade industrial arts teacher, Mr. Jackson, who liked to lecture to some of the guys in our class about their behavior.

Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.


  1. homedesign | | #1

    Southern Perspective
    Your Energy Vanguard Blog is one of my favorites.
    GBA is good for all around Energy Efficiency... but GBA does NOT understand air conditioning or the Southern Climate
    Kudos to you
    AND I agree with your opinion of David Butler.
    Kudos to David Butler

  2. homedesign | | #2

    never mind
    I re-read the blog

  3. homedesign | | #3

    I realized my mistake while you were posting

  4. GBA Editor
    Allison A. Bailes III, PhD | | #4

    Response to John Brooks
    That's a good question about how many others have this metering arrangement. It definitely makes a difference in how I look at improvements. If I wanted to game the system, I could put in a gas dryer, gas range, gas refrigerator, gas toilet... (Uh, well, maybe not all of those.)

    I'm actually going to help lower everyone's bills by putting in higher efficiency equipment because we're probably going to install a 0.96 EF condensing water heater. I'm also president of the board so I'm trying to figure out ways to help everyone go this route. With gas rates so low, though, it's difficult. We pay less than $0.60 per therm for gas because we're a bigger user.

    Thanks for the compliment, and yes, anyone who reads David Butler's voluminous postings on LinkedIn or in the Energy Vanguard blog should be able to see that he's an HVAC expert.

  5. homedesign | | #5

    Safety Trumps
    The safety issue is paramount

    When I first read your blog it sounded like you were "gaming" to what was best for you.
    Then when I re-read ...I realized you were probably helping your neighbors.

    What would be the best strategy for the "neighbors" collectively?
    If everyone else were to follow your example.....
    Would you still rule out heat pumps and mini-splits?
    are you setting the best example?
    1. safety
    2. first cost and operating cost?

  6. homedesign | | #6

    Carbon Monoxide detectors
    Many older homes/condos do not have detectors

    Allison... It sounds like you and your neighbors have an elevated "risk"
    do you have a detector?
    How about your neighbors?

  7. user-917856 | | #7

    Lifebreath air handler + HRV
    Life Breath also makes a hydronic air handler with a built-in HRV. Made in Canada.

  8. seDUbRtWbM | | #8

    Build America work on combi-units
    You might be interested in some work being done by the Build America Program with regard to dual integrated appliances.

    If you want the system to come close to its rated efficiency you'll need to keep your discharge temp low and return temp even lower. To over simplify you need as large a coil as you can get and then push as much air over it as possible. If not you'll loose the energy benefit from condensing and the system will operate at an efficiency close to what you have now. Sending out 120-130 degree water and getting back something below 105 would be best. This only gives you a 106 degree or so discharge air temp hopefully all your ducts are inside and they don't aim at you. The Rheem spec sheet has some examples

    The other thing to worry about is stuff in your water clogging up the small very fine filters. To cut down on maintenance a large pre-filter can be installed.

    If your primary motivation is safety buy a low level CO alarm like the one NCI sell. You can do combustion safety testing and make sure you have a good cap so nothing can get down the chimney as Richardson called. I've seen a duck, squirrel and bird pulled from flues but none of them had a cap. I'm all for getting rid of natural draft DHWs but there is something special about it's simplicity and that it works when the powers out. I'm all for the science project and wish you the best with yours and look forward to the updates.

  9. GBA Editor
    Martin Holladay | | #9

    More information from Building America
    For more technical information about the difficulties of using an on-demand water heater in a hydro-air system, see Michael Chandler's report from a Building America Program expert meeting: Stuff I Learned at Joe Lstiburek’s House, Part 1.

  10. AndrewInChelseaQC | | #10

    Don't count on cheap gas

    I wouldn't count on cheap gas!


  11. kim_shanahan | | #11

    life breath clean air furnace
    Glad that Kris mentioned the Life Breath Clean Air Furnaces.

    I liked them so much I put them in 27 homes I built in 2007-2008 here in Santa Fe, NM. We coupled them with Noritz on-demand water heaters and a 4x10 solar panel. We had 80 & 120 gallon solar storage tanks that did not have electric resistance heating coils. The Nortiz kicked in if there wasn't enough solar heated water. We employed a mixing valve to get the domestic hot water cool enough for human usage because the Lifebreath was most efficient at higher temps.

    Like the furnace you are contemplating, it periodically flushes the lines during no demand for heating times. The added benefit was definitely the built-in HRV, which we also ducted to bathroom and laundry exhaust locations for a balanced whole-house ventilation system.

    The homes were all affordable homes in our market and had HERS ratings in the mid-50s. The HERS rating was somewhat problematic because hydronic furnaces are not easy to input with existing software programs like REM-Rate. We had to default to the rating of the Noritz water heater.

    We never made claims that we had solar-heated homes because occupant load varied from house to house, even though all homes were identical. A single person almost nevery had the Noritz fire, even in the dead of our 6000 HDD winters, althought the family of 5 living next door with three teenagers couldn't get through the morning showers without the natuaral gas back-up. Our solar tank also made the system better than a strictly on-demand only arrangement.

    I have come to think that the hydronic furnaces make even more sense for retro-fits than new construction because as we make our homes tighter the need to eliminate gas-fired, poorly combusting equipment becomes even more of an issue. The advantage of the hydronic furnace is that it can be put anywhere in a home with no sealed mechanical closet and the heating sources can be anywhere, even outside hanging on the wall!

    The other obvious advantage is that it makes the home solar-ready for space heating. Heating our living space and our hot-water via solar is the critical low-hanging fruit. The abandoned penetrations that were providing combustion air for the furnaces can also now be utulized for the supply and exhaust necessary for the HRV.

    I look forward to the day when this type of retrofit becomes standard practice and not just a discussion on your enlightened blog. Keep up the good work and lets get our friends at GBA on this bandwagon

  12. user-991559 | | #12

    an update on performance?
    Thanks very much for this article, I'm wrestling with a decision to go to this type of system right now.
    I have a small semi-detached 2-storey house in Toronto, built in the 1930's - no insulation whatsoever.
    I bought the place 4 years ago, knowing that the NG furnace was already showing signs of slow death - I'm at the point now that I need to replace it.
    At the same time, I have a basement apartment in the house. When replacing the furnace I'm also thinking of replacing the conventional NG tank water heater - as it doesn't exactly meet safety codes to have a combustion appliance near a sleeping area.
    I've got a few quotes from contractors to replace the furnace with a new 95%+ dual-stage ECM furnace. I also have a quote to replace the tank water heater with a Rinnai tankless.
    A friend of mine just a couple years ago installed a Rinnai hydronic furnace, which got me thinking that I should just do the same. The Rinnai tankless RU80i paired with a AHB60 hydronic furnace comes out to only a couple hundred dollars more to install than the Rinnai RL75i and an American Standard Gold ZM furnace.
    I'd like to know from you how your system is working out?
    Some of my worries are...
    - the 'putting all your eggs in one basket' argument. If the tankless goes down, no heat & no hot water
    - warranty is only 5yr parts/ 1yr labour for Rinnai. American Standard furnace is 10yr parts, 10yr labour if I get yearly servicing (which should be done anyway).
    - Rinnai hydronic furnace is not dual-stage. Our house is quite drafty in winter if the furnace isn't on it gets quite chilled in parts of the house.

    I'm leaning towards the gas furnace + smaller tankless scenario, but I'm worried I'm going to kick myself in 5 years because I could have had a higher efficiency system in the hydronic furnace scenario.

    If this sounds a bit rambly I apologize - my wife just had our 3rd child a week ago, and I'm attempting to juggle what may be too many balls in the air.



  13. GBA Editor
    Allison A. Bailes III, PhD | | #13

    Did you say juggling?
    Kyle, I don't have any real answers for you because I haven't done anything about replacing my old equipment yet. You have some valid concerns, and as with everything, you've got to weigh all the pros and cons.

  14. Lane H | | #14

    Not to get all negative on hydronic heating but despite the many proven positives of hyrdonic heating there are a few cons as well, the search for a solution to the current foible having led me to this blog post as a matter of fact. Since the author and some commenters are contemplating installing hydronics in their domicile I feel I would be remiss if I didn't relay a few of the pros and cons of the system.

    I've had a hydronic heating system in our home since it was built about 15 years ago. My location is Texas near the gulf coast. Not exactly famous for long cold winters but there were two reasons, both noted by Mr Bailes, that drove me to hydronics over electric heat pumps and gas heat exchangers which are prevalent in this region. Number one was efficiency. Whatever the hot water source you have, boilder or a residentual tank heater, you are always making hot water. Hydronics harness that already created resource. It's not exactly completly free obviously as heating demand will increase hot water demand at the source as well but it's pretty close. Secondly, another point raised by Mr Bailes, is the duct burn out we all are familiar with when a conventional system is kicked on. Since it's warm humid air there is no odeous smell of months of dust slowly be burnt.

    Hydronics in residentual applications was fairly new 15 years ago and like all things new it requires some tweeking from time to time. My original system was an 100 gallon boiler (since replaced with a Takagi whole house tankless water heater) that was fabricated with (2) supply and return taps. One set for house hot water and the other for my twin hydronic units. This is what's referered to as a "closed loop" system.

    Therein lies my current problem with hydronics.

    Since the portion of the tubing, both supply and return, in my hydronic system is not "circulating" there has been a gradual increase of scale in the system that is slowly compromising (Mr Bailes mentiioned that one of the mfrs has an intermittent circulatory feature) the available heat output. After much investigation my two choices are flushing the system with a cleaner, some of which are quite toxic (remember this system also supplies house water) to flush and de-scale the system or breaking up the closed loop system by isolating the house hot water production from the house heater water production by means of an additional heater. I've since decided on the later so I can annually clean or descale the system in the off season. I will be using a small LPG (I live in a very rural area) tank heater to supply hot water for the hydronic system. This will also allow me to warehouse the gas heater in the off season since it will be dedicated to the hydronic equipment.

    Additionally, while I'm dishing out long winded perspectives, ono the subject of tankless water heaters, they too have a few minor pitfalls. Numero uno is that, especially for whole house tankless water heaters, the hot water is not instantaneous. The system is preset to kick on based on water flow. If you or your significant other likes to wash dishes by hand in the sink the cycling of the tankless heater is a little inconvenient. I got around this by installing a little electric single purpose flash heater under the sink. It takes a few minutes of running water to get hot water to all the end users around the house. Remember all the water in the piping is at ambient temperatures until the heater kicks in. Another small issue is the T-stat in the unit slowly burns out over a couple of years. I suspect that is due to scale build up to although I have a suitably sized reverse osmosis system (my water supply is from my personal water well). Oh and if you already have a NG water heater you won't be saving much money. Other than those little nits it's been great for me. It's small size is a big plus (the 100 gallon boiler had it's own room).

    Anyway, lessons learned no.

  15. GBA Editor
    Martin Holladay | | #15

    Response to Sheldon Hall
    My guess is that hydronic heat is rare in Texas -- which means that local HVAC contractors are relatively inexperienced, and as a result, hydronic systems in Texas may be poorly designed.

    Here in Vermont, I had to chuckle at your comment that "Hydronics in residentual applications was fairly new 15 years ago." Ever since Vermonters started swapping out ther wood stoves for central heating about 100 years ago, the vast majority of central heating systems here have been hydronic. This is very old technology, Sheldon -- although perhaps new to Texas.

    Most hydronic heating loops are closed-loop systems for a reason -- to prevent the scale build up that occurs when you push new tap water through the lines all winter, introducing new minerals as the water flows.

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