Use HX in hot air plenum to produce DHW?

  • Active since 1995, is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.


Aug 1, 2018
Cascade wisconsin
The only issue is if yours did not come set up for it, you have take the air jacket off (at least at the back) enough to weld the mounting studs on the back of the firebox...not that bad to get to once the blower housing is off...
i do have the studs installed for the coil. Just no coil yet. I wanted to see what my monthly cost to run our natural gas water heater and if it was worth adding a tempering tank and so on…. We’re paying roughly $25/month for NG and that also runs the dryer, and stove. At that rate, it wasn’t worth it for me.
Brother firefighter, I'm going to just generally advise you not to go this route. You MIGHT be able to make it work, but I think you'll probably be disappointed. I think your biggest problem is in temperature differentials. It's going to be difficult to transfer the heat to move a lot of BTUs, and in your situation, you're trying to transfer twice, once to the water, and then back to the garage air. And, I'm guessing that in WI, your garage is going to need a lot of heat. I've seen it get to -25 in my attached garage, and that is going to take a lot to overcome. You might be able to keep it above freezing if you ran it all of the time, but I wouldn't expect to be able to bump the temperature to comfortable unless you start a day ahead of time, or had a lot of stored hot water, and even then, I'm not sure.

Because of the temp differentials, it would be a lot easier to do this if your goal is to keep the garage at 40. If you're looking for 70, ever, I think you'll be disappointed.

I'll respectfully disagree with Brenndatomu on the issue of the factory hot water coil. I ordered one with my stove, and. just can't see that doing this job. I returned mine. From what I've seen and done, if you want to move a lot of the heat from your VF to some other use, the duct heat exchanger is the way to go, as long as you have cold water going through it - which is why 40 is going to be so much more doable in the garage than 70.

I guess I'd throw out that a VF100 doesn't make a lot more heat than the average woodstove. If you consider that a lot of people heat their average home with an average woodstove, that makes sense. The VF is sized to do the same job as the average woodstove . Most folks heating a garage, also have an average house woodstove in there doing that job. So, depending on the heat needs of your house, and your garage, you're still roughly one woodstove short on a cold day.

That said, if you're finding that your VF is a little big for your house needs, it could be an advantage to be able to move some of that heat to some other use, like the garage, while keeping the easy full reload program going without cooking you out of the house. I do find that option useful with my setup, and, while it would never work for anybody where it actually gets warm, I get a side benefit of being able to have a hint of air conditioning on hot days by using my system to cool the house by running my incoming water through the heat exchanger system before it gets used for irrigation or whatever.

If you were using this to heat or preheat your DHW, and dump excess heat to the garage when desired, it might make sense, but I think you should plan on having some additional source of heat for your garage.

If you think you want to do this, and would like to talk to me, I'd be happy to talk to you about it FWIW. I don't know if you can PM here. If not, post again and I'll get you my business number. There's a lot of little things like size of heat exchanger depending on the water temp you're shooting for, that I've learned. There's a lot to tweak depending on your specific situation.

And, now I have to go do something about the Type 3 engine with the two inside dual tires that didn't survive the last forest fire. Never Forget!
Well. I’m just looking for the best way to extract heat out if my vf100. I have no problem overheating my house by filling the firebox to 3/4 full even with the damper dial set to the lowest setting and the thermostat set to 50 degrees. So yes, if I can dump some heat off the plenum, the better.
I’m not really looking for a constant 70 in the garage either. 40-50 would be nice. I can get it up to “working temp” with my portable propane heater. As far as outside temps, I’m in south east wi about an hour north of Milwaukee. So our average temp don’t go below 0 that often. We stay in the teens to twenty most winter.

E Yoder

Minister of Fire
Jan 27, 2017
Floyd, VA
It would be a lot easier to start out with a wood boiler. :)


Minister of Fire
Jan 8, 2014
Wisconsin Dells, WI
i do have the studs installed for the coil. Just no coil yet.

Let me know if you are going that route. I have the coil off of mine I am no longer using as well as the air shield that goes over it. They both are just sitting down stairs collecting dust.

I am down by the Dells.
  • Like
Reactions: Firefighter44


Burning Hunk
Apr 19, 2019
SW Montana
If you're looking for 40 degrees, and not worried about getting DHW to 120-130, go with a 22x22 heat exchanger. I think mine is a 4 pass. The 22x22 will allow you to capture more of the heat output. If you prefer the 22x24, that's ok, but with the 22x22 the end without the plumbing connections can be contained inside the plenum, so you don't have to insulate it or cut a hole in the opposite side, just put an angle iron shelf in there for that end to sit on. I recommend checking the tubing direction on the exchanger so that it won't allow trapped air. Some have an arrangement that can be fed from the bottom, with the exchanger horizontal, and will have an upward path for air or steam to escape from ,any point. Some are plumbed such that the water would have to go up, and then back down, and I wasn't comfortable with that. I can probably find the model info on the exchanger I used if you ask.

Use a baffle to divide the plenum to the rear of your heat exchanger so that there's a 2" gap to the rear for the rather cold air that comes
over the top of the back of the stove to be diverted up into the plenum behind the heat exchanger. My baffle comes down to within 1/2" of the stove top.

I think you will not have enough btu's for a Modine heater to blow anything more than cool air. It'll work, but it's probably major overkill. If I had one laying around, I'd use it, but I'd probably have to find a way to tone the fan down a lot. Ideally, you'll want to pick up the cold air off the floor of the garage. I'd think about using a similar heat exchanger to what you're using on the stove, perhaps adapted in a box that uses a small squirrel cage blower for air circulation. A fan roughly equivalent or a little smaller to those portable squirrel cage Stanley and the like blowers they have at WM and HD seems about right to me. Personally, I'd get one of those and use it to feed the exchanger enclosure - they're cheap, quiet, and have 3 speeds to choose from. I'm not familiar enough with radiators to have an opinion, except that I wonder if it would transfer heat well enough with the small temp differential you'll be working with. I know that the heat exchanger and fan will. In theory, the lesser temperature differential is going to be at the garage end, so you'd want to have more heat transfer capability there, but I'm not sure how much difference it's going to make in reality.

I recommend the TACO 006e3 for a circulation pump.

If you're going to have plumbing in the garage, you're probably going to want to run some kind of antifreeze in the system. That will decrease the efficiency, and complicates the matter if you decide you want to do DHW tempering also. If antifreeze, will you have an open system with a small reservoir, or ??? If it's closed, don't forget to incorporate a relief valve - just in case.

Just my 2 cents - if you use RV antifreeze, get the more expensive propylene glycol stuff, not the ethanol stuff that's always on sale cheap in the fall. The ethanol stuff is corrosive, flammable, and only good for toilet traps IMHO. FWIW, we've found that it will chew on an aluminum fire pump while the PG stuff doesn't.

So that's my best collection of answers, tips, and thoughts based on what I've read you're doing so far. Please let us know what you build and how it works!