Wood Furnace w/ Air-to-Water Heat Exchanger

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JonWisconsin

New Member
Oct 24, 2021
4
Wisconsin, USA
Hi Everyone,

Looking for some advice on designing a hydronic heating system for my 2000 sqft shop. Shop is used mostly on the weekends, so the heat will be off most of the time. I've got ~2000 ft of 1/2" PEX in the slab, which I'd love to make use of. I don't live near the shop, so heating 200+ gallons of fluid is a non-starter (ie, outdoor boiler is not an option). I've looked at indoor boilers, but again, that's a pretty large volume of fluid to heat up, and most options are above my budget. As an alternative, I'm considering using an indoor forced-air type wood furnace, and sticking an 'air-to-water' heat exchanger in the plenum. The blower would pull air through the heat exchanger and circulate it directly back to the stove.

Would this work? I'm sure there must me some obvious flaw in this design, I just can figure out what it is...

Thanks in advance for any feedback.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,846
Northern Maine
If your not keeping the slab sort of warm it’s going to need some time to work.

Unit heater and a hot water tank isn’t to fancy for a shop. A friend did it with his extra garage with RFH.
 
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JonWisconsin

New Member
Oct 24, 2021
4
Wisconsin, USA
If your not keeping the slab sort of warm it’s going to need some time to work.

Unit heater and a hot water tank isn’t to fancy for a shop. A friend did it with his extra garage with RFH.
Thanks for the quick feedback Bad LP. I've got the slab split into two zones (1000 sqft each). Most of my work happens in one half of the shop, so I would focus mainly on heating that side of the slab. I imagine it would take most of the first day just get the slab up to a comfortable temp. That was another reason that I was thinking that a wood furnace might be a good way to go. I would plan to put 'bypasses' into the ductwork so that I could blow some hot air around as needed.

What do you mean by a "unit heater"?

I should also note that the shop is 100% off-grid. I've got a 4kw solar system, so I can run any electrical accessories that system has (blowers, pumps), but electric heating is mostly out of the question.
 

electrathon

Minister of Fire
Sep 17, 2015
575
Gresham, OR
Is there insulation under the slab? If not you may find that you cant dump enough heat into the slab to get any real heat gain in the room.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,846
Northern Maine
Thanks for the quick feedback Bad LP. I've got the slab split into two zones (1000 sqft each). Most of my work happens in one half of the shop, so I would focus mainly on heating that side of the slab. I imagine it would take most of the first day just get the slab up to a comfortable temp. That was another reason that I was thinking that a wood furnace might be a good way to go. I would plan to put 'bypasses' into the ductwork so that I could blow some hot air around as needed.

What do you mean by a "unit heater"?

I should also note that the shop is 100% off-grid. I've got a 4kw solar system, so I can run any electrical accessories that system has (blowers, pumps), but electric heating is mostly out of the question.
Unit heater AKA Modine.
Real world experience in Northern ME. Unoccupied house stays set at 50-55. Call up 2 t’stats on 1st floor 24 hours in advance to bring that space up to 70 when winter temps hit.
2005 built house has full basement. 2x12 TGI joists, 1/2 PEX in 1.5 inch’s of gypcrete covered in engineered wood or tile. Fully insulated between joists.

I’m positive I could not bring that up to temp from a dead cold start.
 

JonWisconsin

New Member
Oct 24, 2021
4
Wisconsin, USA
Unit heater AKA Modine.
Real world experience in Northern ME. Unoccupied house stays set at 50-55. Call up 2 t’stats on 1st floor 24 hours in advance to bring that space up to 70 when winter temps hit.
2005 built house has full basement. 2x12 TGI joists, 1/2 PEX in 1.5 inch’s of gypcrete covered in engineered wood or tile. Fully insulated between joists.

I’m positive I could not bring that up to temp from a dead cold start.
Thanks for sharing your experience Bad LP. I think this is a great 'reality-check' for me. Even if I'm only trying to heat 1/2 my slab, even the best system is going to take days, not hours, to get it from freezing up to a temp where its heating the space. Based on that assumption, it would seem like a 'hybrid' system is probably the way to go. Maybe get an oil boiler that can be started remotely the day before we arrive, and have a conventional wood stove to reduce the boiler's workload while we're there.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,846
Northern Maine
Thanks for sharing your experience Bad LP. I think this is a great 'reality-check' for me. Even if I'm only trying to heat 1/2 my slab, even the best system is going to take days, not hours, to get it from freezing up to a temp where its heating the space. Based on that assumption, it would seem like a 'hybrid' system is probably the way to go. Maybe get an oil boiler that can be started remotely the day before we arrive, and have a conventional wood stove to reduce the boiler's workload while we're there.
Do your own research but I was told and my experience agrees with what I was told that RFH doesn't respond to quick or rapid temp changes. This is the sole reason I did not pipe my detached garage floor. My plan has always been to use a LP fired unit heater mounted to the ceiling. Yes my floor and feet will be cold but the truth is I just want the ice and crap melted off our vehicles. FACT: It won't ever freeze.

You may want to think about a small glycol system if you're set on a boiler arrangement but I'll never ever own any system needing glycol. Much too corrosive for my taste. I manage a property with one is my direct knowledge of how it works and what it does to fittings.
 
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electrathon

Minister of Fire
Sep 17, 2015
575
Gresham, OR
If this was my place I would install a small heater of some sort that would keep the room at 45-50 degrees. Then a wood furnace of some sort that could heat and circulate the water. Some heat would go to the floor when the fire built, some heat to the air to speed up the heating process and freeze protection from the gas or oil heater when you are not there to build a fire. If you are there for the day and you raise the floor temp from 45 to 65 degrees, it will take another day for the floor to cool back down and the freeze protection heat to turn on. The more you are there, the better this will work.
 

Highbeam

Minister of Fire
Dec 28, 2006
19,535
Mt. Rainier Foothills, WA
I have a very similar problem. 1800 SF well insulated (including slab) shop with 1800 LF of unused pex heating pipes buried in the slab. I heat the shop air with a woodstove and it actually works very well when I'm there. The intent with the floor heat was to maintain the shop at a constant above freezing but not comfortable temperature to prevent condensation on metal things and damage to chemicals plus make it easier to boost the temperature up to 70 using the stove when I'm there.

Wood and pellet boiler systems are extremely expensive. A fossil fuel boiler seems to be a strong option with minimal electricity. I'm actually considering a wall hung electric resistance boiler (or residential water heater) for the floor heat.
 

hedge wood

Burning Hunk
Mar 1, 2009
239
Eastern NE
I really like floor heat in a shop but its like a flywheel once you get it going you want to keep it going. Before I had my Garn I used a direct vent forty gallon LP hot water heater to heat my 30x60x16 shop's slab. I would just keep the slab warm enough to keep the shop at forty degrees. I had a LP ninety percent furnace set on a timer so when I came home the shop was warmed up. Now days I use the Garn to keep the floor warm and have put a coil in the furnace and use the Garn to heat it. If you aren't going to be there all the time I wouldn't use the slab at all and use a unit heater or a wood stove. One of the last winters I worked in town I was putting so many hours in and was on call so much that winter I didn't even start the slab heat up. Just used the furnace when I would work in there.