Using Hot Water to Heat Air with a Hydronic Furnace
Increasing safety, comfort, and efficiency in an old, leaky condo
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 AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. (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 loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. of the house. This won't be a problem in our home since our heating load is currently about 25,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour, and I'll be making some improvements to the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. 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 backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney.. 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 pointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit.. 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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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.
- Allison A. Bailes III, PhD
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