Hydronic Furnace & Tankless Water Heater — A Great Combo!
I'm embarrassed to admit it, but yes, that photo is the gas furnace and water heater in my condo. 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. 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 safety, energy efficiency, and comfort in our home. What I've decided to go with is a hydronic furnace that gets heat from a tankless water heater. It's a nice configuration makes a lot of sense.
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 image at right, from Firstco, shows the setup.
The schematic here shows a conventional, tank water heater, which works fine for this application, too. 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. (In case you're new here, I'll point out that Energy Vanguard does HVAC load calculations, so if you're in the market for a new system, we can help you.)
Instead of two combustion appliances, this type of combo or integrated system means you need only one. (Although it may be possible to do this with electric water heating, it's not a good idea.) 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. At left is a photo of the innards of the Rinnai hydronic furnace.
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). I'll write another article sometime on the difference between the electronically commutated motor (ECM) and standard Permanent Split Capacitor (PSC) motor, but all you need to know here is that the ECM can be much more energy efficient.
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, as shown at right. 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.
So, that's my plan. I hope to be able to do it sometime in the coming year and will definitely write more about it once I get going with the project.
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*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.