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Hot rodding a wood stove


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#1 xjmark

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Posted 18 December 2011 - 12:51 AM

Finally got around to starting my wood stove powered hydronic supplemental heating loop.
A little background: When I bought my house, it was all electric baseboard heat upstairs, and a couple of propane infrared heaters in the main garage. Over the years, I installed a wood stove and a high efficiency warm air furnace in the garage, and a ductless heat pump upstairs.

As the weather gets colder, the heat pump in the house can't keep up, and the electric heat comes back into the picture. I can get the garage nice and hot using either the wood stove, or the furnace, but I didn't want to bring the garage air into the house. That's where the idea of transferring some of that wood stove heat into the house using water came from.

I've been scrounging components and materials for a while, and now had enough stuff to get started. The wood stove is an All Nighter Big Moe, and they came with a recessed section in the flue pipe area for an optional water coil. Problem was, the water coils are scarcer than hen's teeth, and if you do find one, it's beat up and astronomically priced.
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To build my own version, I used 3/4" corrugated 304 stainless tubing that is normally used for natural gas lines. It has a 1500# min burst pressure rating, and 2400* melting point... good enough for me. I wrapped a section around a piece of smokepipe, and put it into the recess of the stove.
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After that was in place, I mounted up a circulator pump on a stand, and started piping things together. The safety components and most of the fittings I bought brand new.
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Not knowing if this thing was going to work at all, I temporarily hung and connected an old water to air coil with hoses for the test run. This coil will eventually be located in the house with a fan to extract the heat.
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Filled the loop with water, purged the air out and started the pump. Fired up the wood stove, and ran it real hot like I usually do to get a good bad of coals. Within 45 minutes, the water temp was up to 180*. Not bad, but that's with no fan blowing across the coil.
As the stove stabilized, and ran it as normal, the supply water temp stayed between 120* - 150*. Putting a fan to the coil dropped the temp, but it maintained pretty well.
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All in all, it looks like it's going to work. Next step is piping it across the garage rafters into the house, and then starting on the controls. The battery backup system will be the last step. I'll update this as I get things done on it.

Edited by xjmark, 18 December 2011 - 12:56 AM.

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#2 SnowJeeper

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Posted 18 December 2011 - 01:40 AM

I like it :up:
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#3 CJKarl

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Posted 18 December 2011 - 05:19 AM

Nicely done. I wonder if that's enough contact area for the heat transfer. What if you just laid a 10' coil of copper line right on top of the stove? Along with the stainless on the stack.
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#4 GSSFC

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Posted 18 December 2011 - 06:58 AM

What about drilling 2 holes right into the fire box and mount the coils...plus more even, right inside with bulkhead fittings to connect your plumbing up?

That does seem like an awesome idea to use the woodstove like you are.

Tim
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#5 wayoffroad

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Posted 18 December 2011 - 07:33 AM

I love this discussion. I have been designing a similar system and collecting parts for years. the area I got stuck was the water temp should stay under 200 degrees as long as its circulating, but what happens if you loose/run out of power, or have problems with your circulator? What happens if the water just sits there and gets above 400? I'm afraid I would be unable to use my wood stove when I need it most, when I loose power and exhaust my battery backup.
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#6 wayoffroad

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Posted 18 December 2011 - 07:37 AM

What about drilling 2 holes right into the fire box and mount the coils...plus more even, right inside with bulkhead fittings to connect your plumbing up?

That does seem like an awesome idea to use the woodstove like you are.

Tim

This is what I want to do. I would run a 3/4" SST pipe in, and back out of the fire box.
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#7 rvgjeep1

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Posted 18 December 2011 - 08:01 AM

My buddy built a wood fired pool heater by building a big coil inside the fire box with bulkhead fittings on the back. I knew it would not work on the pool, but it worked well enough to pull heat from a coil. He used 1" copper and as long as there is water you wont melt the solder. With 1/2" he could have got more heat.
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#8 massxj

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Posted 18 December 2011 - 08:06 AM

put a vent tube on that PRV
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#9 TJ

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Posted 18 December 2011 - 08:28 AM

my $.02 add a tank for the hot water, use a hot water heater with a blankie, and you need more contact area. Build a stainless or copper tank that fits the top of the stove, in spare time that is knowing full well how much time you have on your hands :rolleyes:
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#10 FGZ

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Posted 18 December 2011 - 09:13 AM

my $.02 add a tank for the hot water, use a hot water heater with a blankie, and you need more contact area. Build a stainless or copper tank that fits the top of the stove, in spare time that is knowing full well how much time you have on your hands :rolleyes:



I think that's what Belben's setup looks like IIRC, he's put pics up somewhere before...maybe in Mark's "penny-pinching" thread.

Love the idea. I guess if you're not supposed to let the water get above 200, you're about where you want to be with the heat exchanger. With good insulation on the supply lines to the air exchanger I don't see why you couldn't get some good comfortable heat into the house with that.
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#11 rvgjeep1

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Posted 18 December 2011 - 10:34 AM

I'm a believer, but I read a lot about it and they say you just cant make enough hot water. I think a air/air heat exchanger would work better with a fan (blow it into the house), or put your coil directly into a heat catcher in the pipe.
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#12 xjmark

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Posted 18 December 2011 - 09:03 PM

What if you just laid a 10' coil of copper line right on top of the stove? Along with the stainless on the stack.


What about drilling 2 holes right into the fire box and mount the coils...plus more even, right inside with bulkhead fittings to connect your plumbing up?


I think a air/air heat exchanger would work better with a fan (blow it into the house), or put your coil directly into a heat catcher in the pipe.


I looked into all of those ideas, but since the All Nighter stove had the recessed flue pocket already built in, I might as well use it.
The corrugated stainless tubing was plan "B". The original heat exchanger was built out of six 3/8" copper tubes, brazed, triple wrapped, and manifolded into 3/4" headers. I used a piece of pipe that was a little smaller than the 6" smoke pipe as a pattern to make the triple wrap. I figured it would give me a perfect fit because of spring back on the tubing. Well, it didn't spring back as much as I thought it would. I tried to loosen the coil up, but the work hardened copper didn't cooperate, and the fawking thing turned into a big pile of scrap. :censored:

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The air/air exchanger would have gotten more thought if the stove wasn't 30' away from the wall of the house.
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#13 xjmark

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Posted 18 December 2011 - 09:11 PM

put a vent tube on that PRV

I'm tying the pressure relief vent pipe right back into the inlet side of the pump.
It's a DSI (direct steam injection) system that will achieve 100% efficiency when the relief valve pops. ;)
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#14 SKEETER

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Posted 18 December 2011 - 09:26 PM

I love this discussion. I have been designing a similar system and collecting parts for years. the area I got stuck was the water temp should stay under 200 degrees as long as its circulating, but what happens if you loose/run out of power, or have problems with your circulator? What happens if the water just sits there and gets above 400? I'm afraid I would be unable to use my wood stove when I need it most, when I loose power and exhaust my battery backup.

One option would be a dump valve wired in. It would be placed as low as possible and wired so when powered it would be closed allowing water to circulate. When power is lost the dump valve defaults to an open position which allows water to drain out into a catch basin or drain pipe. This system should have an auto water feeder and auto air purge to fill back up once power comes on.
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#15 xjmark

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Posted 18 December 2011 - 09:49 PM

I love this discussion. I have been designing a similar system and collecting parts for years. the area I got stuck was the water temp should stay under 200 degrees as long as its circulating, but what happens if you loose/run out of power, or have problems with your circulator? What happens if the water just sits there and gets above 400? I'm afraid I would be unable to use my wood stove when I need it most, when I loose power and exhaust my battery backup.


Good questions, those are the same ones I had.
When it's all said and done, The system will be fully controlled by electro/mechanical temperature switches to turn the pump and fans on/off automatically. It's all going to run off of 115v house current during normal operation, but if the power goes out, it gets plugged into the power inverter dedicated to the pump and fans. For now, I have some car batteries sitting on float chargers for emergencies, but eventually want to build a bank of 12vdc storage batteries with solar back up for extended outages. There is also the generator.

If all of that fails, or the pump craps out, I would drain the water down and leave the drain valves open to prevent a high pressure condition. At worst, the water would boil out of the tubing.
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