OT: Wood Stove Design

[RMinMN]

"In order to have a Healthy House, it takes a certain number of air exchanges PER HOUR."

The expert who wrote that didn't live where it often gets to -40 each winter. Every time the air changes you need to heat the incoming air by ~110 degrees and then add sufficient moisture to make that air livable.

[deltap]

Air changes usually refers to air circulation within a space. Ventilation air is also required for health. In cold climates incoming air can be tempered with a heat exchanger where exhaust air heats incoming air. I have used an idea I saw from Europe to heat incoming ventilation air. I buried 100' of pipe underground into furnace area. The ground heats the air from -20 to about 45 to save energy. In older homes there was plenty of air leakage. In newer tight construction air must be ducted in for ventilation.

[A.K. Boomer]

radiant heat exchange is to the FOURTH POWER of the temperature difference. Suggesting that heat extraction is better if the wood burner is hot. PLUS, radiant heat travels across any open space regardless of distance, and carries no draft (or dust) .

It's the little details that make this topic interesting :-)

This is good info for sure but there's drawbacks in having to wake up in the middle of the night to keep stoking the fire, depending on the type of wood he's using that's surrounding his house this can be a critical factor,,,, keeping a fire at peak burn rate consumes allot of fuel,,,

also --- yeah pulling to much heat off of a stove can reduce the combustion temperature to some "degree" but also keep in mind that's what fire brick lining is all about --- it's an insulator to both protect the metal of the stove from crystalyzing but also help to keep box temp insulated from dropping too low especially in stoves that are double boxed (like mine) and fan induced...



So just real quick, incomplete combustion is not a great thing either - but there's a few workarounds - a couple honorable mentions that have not been mentioned - two different things that can help with this problem is a catalytic converter, these are "temperamental" devises that take some know how just in how to use them for maximum efficiency and also to keep them from getting plugged up,

every single fire you start you need to rip-roar it to get them up to temp, this is how you keep them clean, but once you do you now can "back off the throttle" some and do it without building all kinds of creosote inside your stove pipe,,, once the cat is "alive" it can actually survive on smoldering wood as it contains lots of unburnt hydrocarbons,,, what this means is efficiency gets another "kick in the pants" and it does it right on top of the stove where the energy can still be milked back into the house before it just goes up the flue, and the flue temps will be lower as a result --- but the creosote factor will be a far lesser issue due to it already being ran through a high temp combustion grid,
IF you master how to use this "devise" you will also drop wood stove pollution particulates drastically... it's a far more efficient burn...

Another mention is devises like the Magic Heat, they mount onto the stove pipe directly above the stove,,, these are NOT catalytic converters but they do milk some of the waste heat off of the stove pipe before it goes out the flue, What they can be good at doing it taking a smaller wood stove and make it possible to get more heat out of it, especially if said stove is not blower induced, I would still recommend you keep the fire fairly hot, because if you don't you will build up creosote in the stove pipe but there is a type of operating principle that even helps with this,

Anywhere there is energy exchange other than in the very hot firebox there is a place for creosote to build - kinda like radiator cores getting plugged with minerals in the key places where the cooling takes place,,,
the Magic heat device is no different, dead simple - just a bunch of smaller tubes that the waste heat has to travel through before going up the rest of the stove pipe,,, it also has a thermostatically controlled fan, so these smaller tubes are getting heating up on one side and cooled on the other,,, they catch creosote deposits like crazy, but they have a simple centrally located knob that when you pull it and push it back and forth a few times it cleans the smaller tubes,,,

so kinda like with the cat stoves, always upon start up get the magic heat systems working full throttle big heat and lots of CFM going through the system - then pull/push the cleaning devise back and forth, this will send the small particles up and out of the pipe completely,
Used correctly the Magic heat system will actually keep the rest of the pipe "a little cleaner" due to it being the first vast surface area of major temperature change therefor creosotes "first stop" at collecting on anything.....

These systems are actually pretty amazing on poorly designed stoves that let allot of energy go to waste out the flue - the worse the design the better this system will work and I can attest they will roast you out if your not careful....

They could also be the perfect solution for someone who likes their stove but thinks it's too small to get the job done...

[JoeLee]

I don't know a lot about wood burning stove technology or where it stands now, but 30 years ago or so the guy down the street from me made one out of 1/4" plate. It was about 3' high by 3' deep and about 30 in wide or so. He had coils mounted to the side of it and had a slow speed circulation pump hooked up to the coils. He had radiant heat in the house. A couple logs would burn all night. I thought that was pretty efficient. He had another set of coils on the other side that pre-heated the water going into the hot water tank. If the fire went out and the temp dropped the boiler would kick in. His monthly gas bill was about $20 back then. The basement around the stove looked like a submarine with all the plumbing and valves.
He was a plumber / steam fitter by trade.

JL..............

[oxford]

I had a magic heat on a small wood stove I had in the garage for a while. It easily doubled the effective heat output in the small space and worked great in that application.

I remember as a kid my dad had one hooked up to the flue pipe on the oil burner in the basement for many years.

[OaklandGB]

I bought a place out in the country and inherited a wood stove. Far from being wood heat reliant, my shop heat is wood. An old Southernaire wood stove. Consists of a ~30" round steel pipe, about 36" long, serving as the fire box capped at both ends with square steel plate with the one end fitted with the air tight access/loading door. The fire box is lined half way up its sides with fire brick. There is then a steel "box" cabinet surrounding the fire hold that is welded to the square steel end plates with a 12" duct opening in the top of the cabinet. There is a squirrel cage slow speed fan under all this controlled by a heat sensor.

I was skeptical of this set up for sure. But, as it got colder, I thought what the heck, I'll try a small fire and see if this works at all. Well, it does! May not be the most efficient burner but it will put out a warm breeze via that blower whose 45 degree outlet I can direct along the wall to maximize the use of the warm air. Only takes about 20 to 30 minutes to heat the shop after fire is established. But, I would not leave it unattended, or fired without the extinguisher near as it is really old.

I guess I can see how this might have once been used as a forced air wood stove, but it would take very regular reloading to maintain the fire, or it was a coal based stove. Hard to say since it was obviously some kind of surplus unit obtained for heating this old shop space that used to be a small barn.

[A.K. Boomer]

Semi-related story Joe,,, this happened in a small town about 8 miles from me, many moons ago an old woman got a modern hot water heater, she previously had an old kitchen wood burning stove that she still kept but had the plumbing removed that fed water lines to it, some of those old kitchen wood burners actually had a water jacket inside the stove back to preheat water,,,

well for some reason, the plumber that removed the pipes going to the stove wanted to complete the job by installing plugs back into the stove,,, seeing anything wrong yet?

yeah - water remained inside the jackets, the stove was now an unstoppable bomb and that's exactly what it did - the first time she built up a good fire the stove blew up,,,

it was so long ago I can't remember if she got hurt or not, I don't think she did I think she was in a different room at the time but a good portion of her kitchen got destroyed....

[CalM]

"In order to have a Healthy House, it takes a certain number of air exchanges PER HOUR."

The expert who wrote that didn't live where it often gets to -40 each winter. Every time the air changes you need to heat the incoming air by ~110 degrees and then add sufficient moisture to make that air livable.

This is a prevalent misconception.

"Air" holds very little heat energy, and requires very little energy exchange to warm it or cool it. A person can be comfortably warm in a TENT in winter with only a small heat source.
Radiant heating technology will offer abundant examples and reference. For comfortable living, "Don't heat the air in the room, heat the objects (occupants even) in the room.
Radiant heat is better at that than is convection. but has one drawback. If you are not "in sight" of the warmth source, you feel cold. Much like when a cloud scuds across the warm sun on a chilly winter day. The more "objects" that are warm around YOU, the more even will be your comfort. And "heat" always travels from a warmer object to a cooler object. Never the other way.

Humidity is an issue. I have tried many things. As an effort, I even fitted a mister to a location near the wood stove. It could pump enough air into the house to sweat every window, the condensation would freeze and the ice make looking out impossible. IIRC it was still difficult to get the RH up to 50%. There was the additional ,intolerable aspect of a fine white dust EVERYWHERE in the house. Minerals in the water, carried by the stove convective air currents.
Now I just keep one of those "night stand" ultrasonic misters next to the head board, and feed it on store bought distilled water. Runs through about 1/2 a gal. per night, and sure aids sleeping. No plugged noses or scratchy throats. But that is only for the bedroom area, and only through the night. It makes NO Change in the overall house humidity levels. Or if it does, the hygrometer does not indicate such. But I run a leaky house, and happily do so.

[CalM]

Here is a "hydronic heat distribution "rube goldbrick" rigg that was put on about ten years ago.



Two cast iron radiators on a simple support. plumbed in parallel. Thermo siphon feed to on similar sized c.i. radiator in the upstairs dining area.
The system has been wet over all these years without drips or issues. (there is an open top expansion tube in the system, so it needs about a pint of water added each year)

It started out, I just wanted a "capacitor" to spread the stove heat out. but then the plumber's nightmare in me got creative.
The colder it is outside, the hotter the fire in the stove, the better it all works, and quite a bit of comfortable heat gets moved up into the active living space, and quite a bit of the over heating near that large output stove gets extracted . All without any pumps, fans bells or whistles. :-)

Just thought you guys might like to see what evils come to an idle mind.

OH! The "cap stones" on top of the "heat absorbers" are just that. Left over soap stone from when I remade the kitchen counters.

Something to put wet mittens on ;-)

[A.K. Boomer]

It's not the purdiest thing but can't argue with that - lots of thermal mass storage there for when the fire dies out too... ah what the heck - paint it all high temp flat black and it "kinda" looks like it could belong...

[CalM]

Air changes usually refers to air circulation within a space. Ventilation air is also required for health. In cold climates incoming air can be tempered with a heat exchanger where exhaust air heats incoming air. I have used an idea I saw from Europe to heat incoming ventilation air. I buried 100' of pipe underground into furnace area. The ground heats the air from -20 to about 45 to save energy. In older homes there was plenty of air leakage. In newer tight construction air must be ducted in for ventilation.

Underground "ventilation" systems are noted for being very effective growing environments for all sorts of "fuzz and mushrooms".

Send a bore scope through there from time to time. They might be ideal for the arid regions wher nothing grows, but not something I would live with here in the North East.

[CalM]

It's not the purdiest thing but can't argue with that - lots of thermal mass storage there for when the fire dies out too... ah what the heck - paint it all high temp flat black and it "kinda" looks like it could belong...

The only things looking at it are a bench lathe and a milling machine. The Upstairs heat radiator IS prettied up nicely ;-) "wimmin" and the like, SHMBO! ;-)

eta

It wouldn't take high temp paint either. the units never get above a temp too hot to touch. Call it 150 degrees F

Upstairs, the warmth is just right to set ones rear end on (if desired) ;-)
The return is always around "room temp". Just a sensation of cool to the hand.
Self regulating and all as it is. Temperature Density and all that scienterific stuff.

Water, as opposed to air, takes a LOT of HEAT to change it's temperature.

[Doozer]

I am honestly not up on the temperature requirements of a wood stove chimney
to keep creosote from forming. But when I had a house with a gas furnace,
I was surprised how hot the furnace vent pipe was that exited up the chimney.
This was before the direct vent furnaces with an additional heat exchanger.
So what I did was run the vent pipe out of the furnace and down a 10 foot length
of 5" duct work, and 10 foot back to go and exit to the chimney. This radiated
a good bit of free heat to the basement, which would have normally gone up
the chimney. As I said, had this been a newer and more efficient direct vent
furnace, I would have not needed to do this. But it was a fun experiment that
did work. I know they have "Magic Heat" heat exchangers with a fan that you
can install in the stovepipe of a wood burning stove to gain some additional
heat for the room. Maybe look in to one of those.

--Doozer

[A.K. Boomer]

Water, as opposed to air, takes a LOT of HEAT to change it's temperature.

For sure - but that's why it's so great for thermal storage and why once you "finally do" get the whole system up and running you get to coast for such a long time,,,,

only trade off is getting it there, as in "damn I built a fire an hour ago why is it still cold up here" lol

[jdunmyer]

I heated my shop with wood for about 20 years, using a barrel stove. After a short beginning experiment, I built a stove with s Sotz kit, which was made of stamped sheet metal and sealed very well, especially after adding additional bolts around the door frame. Added a Sotz catalytic converter and a Magic Heat. The best part was adding a computer to control the draft. Used a Radio Shack Color Computer (CoCo), which had 4 A/D converters (joystick ports) and a relay that was used to start/stop the cassette recorder for program saving/loading. I had a thermistor to sense room temperature, and 2 gas water heater thermocouples in the stack, one below and one above the cat.

The draft control was a 24 VDC solenoid that pulled a 1-quart paint can lid off the door by about 1/4", with a spring to close it. The lower t-couple did the actual controlling to keep the stove at setpoint. The setpoint was changed based on the room temperature, running the stove "hot" if below setpoint, "idle" if above. The thermistor and thermocouples were connected to a quad op-amp to amplify the signals to the 0-5 volt level needed by the CoCo's joystick port. The computer had a 6-bit converter, so putting in a 32-degree offset enabled room temperature readings from 32 -> 95 degrees; the stack temperature was read in 15-degree increments. The CoCo's cassette relay powered another relay to operate the solenoid, and I had a battery backup on the computer to keep it from crashing from a power bump. If I had an actual power outage, I could open the manual draft on the stove.

This all worked very well, I ran it for at least 15 years, using the same barrel. All electronic parts came from good ol' Radio Shack, back in about 1985. The program was written in BASIC and was 60 or 80 lines long.

The stove sat directly below the chimney, which was a MetalBestos solid-pack 6" SS design. I had a 5-gallon bucket with a piece of muffler tubing brazed into the bottom that I propped against the bottom of the chimney and ran the sectional cleaning rod through it, cleaning the chimney from inside. However, I found that if I had dry wood, seasoned and kept under cover, I had very little trouble with creosote.

I eventually replaced the stove with a gas furnace so I could travel during the Winter. I have a water system in the shop, so it must be kept heated. I do still have the stove, computer, and interface box, but have removed the chimney.