Modifying a wood furnace to increase efficiency
 

My cousin just dropped me off a nice little wood furnace for my garage. Yep, another freebie! But, it does need work. From the name plate on it it looks like its from the early 80s. It is VERY simple which is great. However, it does not have a dedicated inlet for combustion chamber air. Its also pretty small so I'm guessing a lot of heat will escape out of the flue.

I am looking to refurbish this furnace as well as modify it to increase the efficiency. I am looking for ideas! It'll obviously need a dedicated intake for the combustion air. I'm not sure what can be done with the flue to increase efficiency so I'm looking for ideas there. I am also thinking of running some plumbing to it to work as an auxiliary water heater since I'm sure it'll provide more heat than is needed to heat my garage. Anyway, I don't know a lot about furnace design or anything so if anyone has any links to info or anything it would be great.

Here are the pictures:

Front of the furnace. The rust is not only from use, but sitting outside for a while.
http://ecorenovator.org/pictures/woodfurnace01.JPG



http://ecorenovator.org/pictures/woodfurnace02.JPG


Here you can see the problem with the unit. It has several holes in the back. I'll just weld them shut.
http://ecorenovator.org/pictures/woodfurnace03.JPG


This is the back side. Again you can see the holes.
http://ecorenovator.org/pictures/woodfurnace04.JPG


A nice perspective shot. The one hole under the exhaust flue is fairly good in size. Nothing that can't be fixed though.
http://ecorenovator.org/pictures/woodfurnace05.JPG


Here is the unit without the cover over it. There is a blower that attaches under the unit and blows air over the combustion chamber and the hot air exits the top via a large duct. Pretty simple design.
http://ecorenovator.org/pictures/woodfurnace06.JPG


Here is where the blower attaches on the bottom.
http://ecorenovator.org/pictures/woodfurnace07.JPG


This is where the air comes up from the bottom and around the sides.
http://ecorenovator.org/pictures/woodfurnace08.JPG

Woohoo! Someone's getting a heated garage :)

Very commercial/industrial looking.

I found out today I may also be on the receiving end of a "new to me" wood stove to replace the worn out old thing currently heating the workshop.

I've done wood stove repairs before when they devolped cracks like that and the best thing I can suggest is to put a patch over and zig zag the welds or you will have the cracks reopen, lining the inside with fire brick is really the best idea, welding some angle iron in to hold the fire brick would be a good idea, this keeps the fire box hotter without deforming or burning the steel sides of the box, a hotter fire burns cleaner and burns the fuel more completely.
If you want to keep it simple, have your chimney as it exits the back drop down instead of going up, the downward loop extends down a 2-3 feet then goes up, to get a fire started you are going to need to install a "T" with a solid baffle so the smoke can go straight out and up, this will help extract a great of heat out of the smoke while still in the fire box by only letting the coolest of the smoke exit, more modern furnaces have the chimney opening at the bottom to create the same affect.

I am also a big fan of devices like the Magic Heat http://www.transoceanltd.com/applian...ages/magic.jpg

I've seen a number of these devices in place and everyone seems really happy with them as they not only help move the warm air but they pull more heat out of the smoke and they have a baffle cleaner built in to keep it cleaning and working.
If you want to heat water a single loop of stainless pipe should work as a slow preheat otherwise wrapping a coil around the chimney that then goes through the fire box should heat up faster so you can heat a tank or floor faster.

If you plan to locate the stove near a wall (or walls in a corner), you might want to think
about using a reflective shield over any insulation or fire-proofing.

A reflector, will keep the radiant heat inside the garage. Without wasting fuel/heat on the
interior studs etc and exterior of nearby walls.

16-18 gauge galvanized steel on about 3/4-1" standoffs makes a effective heat shield. We has this around a wood stove, and on the other side of the wall, you could easily find the edge of the shield by feeling the temperature difference. The shield should go several feet beyond the extents of the stove.

The stand-offs / air space is very important to get good reflectance.

YouTube - Radiant Barrier Under Shingles Won't Work Without An Air Space

Is that a goats leg in the lower right of the first pic?:eek:

The goat with a pink collar in the 7th pic??

Ha! I missed that one. :o

Never seen a goat in a collar, in a pink one? :p

OT (does not mean On Topic).
 

I guess these comments will teach the prospective poster about posting pictures.
Never post a picture with your pet in the frame. Or, anything that's more interesting
than the real subject of the pic!

Anyways, since I'm posting about goats.. Show Goat Supplies

Maybe because I lived in south Texas as a child.. Where I saw thousands of goats. Some with bell collars.
They called em 'Lead Goats', because a sheep herd would follow them. Odd as that sounds.. :o

The other idea for pulling more heat out of your stove is to get a larger chunk of ducts work that fits around the chimney and have a thermostat and a fan that blows air around it as it gets hot, thus cooling the chimney, doing anything like this is going to require that you seal the chimney well, that you clean it once a year as soot will more or less condense on the side of it, but you can lessen the need for those jobs if you burn good hot fires as they burn cleaner with less residue.
So any time that you make a bend, make sure that it's possible to clean that bend.

Hmmm, I really like that idea Ryland. Perhaps I could duct the inlet from the blower that goes on the bottom to suck air through this larger diameter duct work over the exhaust.

Back in the late 70s, I saw an basement wood-stove installation that had a double-walled SS chimney.
Right above the stove, there were air holes punched in the outside wall.
Up about 5 feet, there were some matching exit air holes. (These were factory holes).

It worked using natural convection. And it worked very well.
The air was sucked into the bottom holes and came out the top, a lot hotter.
I think the air gap between the pipes was about 3/4 inch..

The homeowner was very happy with the performance of the scavenger..

I would think you might get more heat out if you have two blowers as the air entering either point is going to be cooler as it enters so it is going to be able to take on more heat.

After doing a bit more reading I'm more inclined to do as you originally suggested and line the inside with a fire brick. This would insulate the fire box and make it burn hotter. Then, I could pull all of the heat out of the flue instead of trying to steal it from the fire box?

I found the attached drawings in Producing You Own Power (edited by Carol Hupping Stoner), in the chapter on cooking and heating with wood. It's a wood stove made out of two 55 gallon drums, the bottom one for burning, the top one is a heat chamber. The latter can have a smaller (35 gallon) barrel inside, fitted with a door, and used as an oven, or just for extracting more heat from the flue (more surface area).

http://ecorenovator.org/forum/attach...1&d=1291053471

http://ecorenovator.org/forum/attach...1&d=1291053471

So I'm thinking you could add something like that to your stove's flue. I imagine you could put bricks or water in the upper barrel to keep heat stored for longer...

You will still get alot of heat from the fire box walls but you will also get another 10+ years out of the fire box because you will not be getting the steel red hot.
Just like the heat traps on a hot water heater you want a heat trap of sorts on your wood stove and as long as the air intake is lower then the lowest point in the chimney you should not get a back draft.
If you look at the natural convection in the barrel stove the heat is going to rise and rise and rise until it's up the chimney and out of the building, if the upper barrel had the chimney leave the lower part instead of the upper part of the barrel you would get more heat out of it and because it's relatively sealed the chimney will still pull a draft with the still warm smoke going up it.
The only way you are going to not have a draft is if there is a blockage, if the smoke cools to the point that it's close to the out door temp or you have an extreme low pressure weather system.
Masonry mass stoves can cool the smoke to the point that it is hardly warm, maybe 120F and it still drafts well, their key tho is burning an extremely hot fire, small dry wood that burns up within an hour or two, heat up the mass and that mass stays warm for 12-24 hours.

I have an EnergyMate woodstove almost exactly like that one in my garage. It was sitting in the basement of the house when I bought it. Looks like somebody had it hooked up at one time, but replaced it with the gas furnace, which now occupies the chimney flue. I removed an old oil burner from the garage but left the 6" vent pipe intact. Bingo bango, I dragged the woodstove out of the basement and in to the garage (it's probably a good 200 lbs. or more), got a couple sections of black stovepipe and I was heating the garage with wood! :thumbup:

I later "installed" a flue thermometer (actually from an old propane grill, reads "warm, ideal, and hot") which is crude but works to roughly judge how hot the flue is and the rate at which it is heating up or cooling down depending on how I fire the stove or when I add more wood.

I added a flue damper (after several red-hot runs :eek:) and from what I can tell, the door for the sliding ash pan seems to function as the only combustion air inlet, as there are no other air dampers on it. The little blower on the bottom doesn't push much air, but it seems to do a decent job of heating the garage within a few hours of run-time. I have been able to fire it so that there is little or no black smoke coming from the flue. I have also successfully burned anthracite coal in it (love the smell, maybe I'm just weird) and I personally prefer it to wood. It burns longer and hotter.

I, also, have had some of the very same ideas as you for the stove. Combustion air pipe, flue heat scavenger, and even hot water heating. More on those ideas to come soon if I get some free time. Also, I bought some of those Ecologs that are made from recycled cardboard and wax the other day and tried a couple of those, not very impressed though. That could be another thread I think.

Back in my days as a foundry worker, I operated a natural gas fluidized-bed calciner furnace which ran at a constant 1500°F. It was equipped with cross-flow exhaust heat recuperators that heated the incoming combustion air from the exhaust being drawn out of the furnace. They were basically stainless steel boxes with tubes for the exhaust to flow through. The combustion air was introduced to the bottom half of the side of the outer box, where it was forced across the bottom half of the tubes by a divider, then up and back across the upper half of the tubes, where it exited to the insulated hi-temp combustion air pipes. Once the recuperator temp reached about 500°F it had sort of a turbocharging effect on the furnace temp, making it rise very quickly from 1000°F to 1500°F. The combustion air exiting the recuperators was usually 450°F to 700°F during normal operation. The combustion air was responsible for not only providing oxygen for the natural gas to burn, but also fluidizing (bubbling) the sand inside the furnace.

I'm sure some of you can see by now how this could be implemented on a wood stove. My idea is to use an air-to-air HX to preheat the combustion air from the flue gases. The most practical method is to put metal studs in a section of 6" stovepipe, surrounding that with a section of 8" duct. A divider should be run down the pipe on both sides so that fresh intake air from outside comes in one end, runs down the length of the HX, then moves across and up the other side of the pipe to the outlet on the same end as the inlet. The flue pipe up to the HX, the HX itself, and the heated combustion air should all be insulated. I would simply remove the ash pan on the bottom of the stove and make an insert in the end of the combustion air duct to fit inside the opening, able to be moved out of the way for removal of ash. Also, a damper would be located in the combustion air duct somewhere close to the stove to control firing.

The goal is to get the combustion air entering the stove to 400°F, which should be sufficient to activate pyrolysis of the fuel on contact, creating the "turbocharging" effect that I'm looking for.

I like the aforementioned expansion chamber idea too. Maybe that could be a use for the extra 55 gal. drum that's been sitting in the garage :thumbup:. The HX could be mounted between the stove and expansion chamber. I'm thinking that after that the exhaust should be pretty well cooled down. Any creosote buildup due to low temps should happen in the expansion chamber and not in the flue beyond that point, hopefully.

I have been toying with the idea of adding secondary air to this woodstove, it has no damper in the firebox door like most do. There seems to be more than enough area from the open bottom grate to provide both primary and secondary air for the fire. What about putting the expansion barrel right behind the stove so that the firebox exhausts directly into the expansion barrel? The primary air pipe could run right through the barrel, preheating it directly from the exhaust gases. A secondary air pipe could be run through the barrel to discharge directly into the exhaust from the stove. That way, if the exhaust is hot enough, secondary burn could take place inside the expansion barrel. Instead of damping down the flue, secondary air could just be opened up more when the flue temperature heats up. Hotter flue temp = more secondary air = hotter primary air. Primary air could be used solely to control the primary burn in the stove.

Are there any ways to improve efficiency of zero-clearance fireplaces? I have one in the house I bought and would rather improve its efficiency than replace with a wood stove. It has a fan for the heat box and I have connected it to outside air, including an inline fan. I have also made the glass doors more airtight by applying high temp caulk to the folding edges of the glass so they seal when closed, and added door gasket material to the outer edges of the door. I still don't always get good combustion and would like to improve it.

What is the problem with the combustion? Are all the flames going up the flue, or is there a lot of smoke coming from the chimney?

We need specifics! :)

Mostly smoke going up the chimney

I picked up a newer EPA Certified wood stove with the secondary burn for cheap. I also got one for free that was made in 1985 and plan on converting it to a secondary burn stove. It looks very straight forward to do after you look at one that has been done already.

I have some pictures of the secondary burn tubes, but I am unable to resize the pictures without distorting the images.

At the back of my stove and on the bottom, there is a hole on each corner where it goes into a upside down "L" shaped tube inside the stove. That supplies the fresh and highly heated air to the secondary burn chamber on the top of the stove. Thats where you see the tubes with holes in them. There is a really good Youtube video explaining it. Secondary Combustion Explained.wmv - YouTube

On the front of the stove there is a fresh air intake and it controls the fresh air that goes in and down across the glass. You can see that in the picture I uploaded below.

That's usually a sign that the fire is burning too hot with not enough air, OR that too much air is being fed to a cool fire. You said that it had a combustion air fan, correct? Is there a damper or speed control so that you can control the amount of fresh air that is let in? If so, try playing around with it, making small adjustments and waiting a few minutes for the fire to adjust, to see if you can get it burning better. If the fire is burning hot with too little air, you'll see slow rolling reddish flames, and some black smoke coming from the chimney that smells like carbon/soot (an acquired smell). That's your cue to hold off on adding any wood for a while and give it more air. If there are short yellow flames in only parts of the wood, that is usually a sign that air is being forced into the stove faster than the fire can maintain the flames, basically blowing it out. You'll see a lot of grey smoke in this case, which should smell like whatever kind of wood (or fuel) you put in, and creosote (also an acquired smell) which is what you DON'T want in your flue. Back off on the air and maybe add more wood if there isn't a whole lot in it.

Is there a flue pipe on it or is the fireplace built right into the chimney? One thing that I feel is absolutely necessary for a wood stove/fireplace is a flue thermometer. I used one from an old Coleman grill/smoker on mine. It doesn't give any temperatures except warm, ideal, and hot; but after using it over a few burns I have a good idea of about where the needle should be. It is also a good indication of whether the fire is getting hotter or colder. I do plan to get a real one sometime though (with actual numbers on it :thumbup:). They are about $15.

Also, most wood stoves use draft fans instead of combustion fans. They have a better tendency to create a strong draft instead of forcing cold air into a fire the may not be hot enough to create a strong enough draft to handle all that air. Adding one if possible might definitely help out if you have draft issues.

I have come to realize that firing a stove is more of an art than anything, kind of an old-world technique that not many take the time or have the opportunity to learn these days.

Hopefully this helps. Let us know what you find out, more details (flame color, size, how much fuel, what kind of fuel, damper settings, etc.) would definitely help us help you out as well.

Excellent video! I have been looking for something like this because I also plan on modifying my woodstove for secondary combustion. So it's basically perforated air tubes that run up the back and along the top of the stove? Sounds simply enough.

Keep working on those pictures, we're very curious to see how these things work! :thumbup:

One thing I want in that wood stove would be an outdoor air intake piped into it. That way I'm not pulling my warm indoor air out of the house by going inside the stove and out that stack.

My thought on that is that is how leaky most houses are already. I recently pulled out the sliding window to clean it. There is a track at the bottom that has vents in it to the outside. Its not adjustable either. The vinyl windows are not an airtight unit as one might think.

Fresh air also gets pulled in when you turn on the bathroom exhaust fan, clothes dryer, stove fan, etc. They even installed a fresh air fan on the house furnace with a timer coming on every hour!

When is the house too airtight? When the stove is smoking out the front? One of my first houses I built it was so tight that you had to open a window to start the fire and keep it going. I always felt tired in that house with or without the fire. Lack of fresh air to the brain..........:confused:

You have a good point. I was reading this morning about stove pipe that is triple wall with fresh air coming down one of the sections and pipes into the stove. I remember seeing a mobile home with fresh air coming from under the house through the floor and into the stove......

Here are a couple of photos of the latest wood stove secondary burn modification. The first picture is the overhang that needed to be cut to make room for the secondary burn fresh air supply.


My other wood stove, which is EPA Certified, does not have a control for the fresh air going in the upside down "L". I was thinking of adding some kind of control to adjust the fresh air, but will wait until after I fire it up to see how well it works first.

The secondary burn modification is complete and ready for a test fire. All of the parts used to make this modification are used pieces of metal repurposed from things like a old treadmill, and some old pipe. Of coarse the stove was built in 1985 too.

Below is the "L" with the air holes that the pipe goes into. Notice that the holes are staggered.


I found that my wood clamps help when drilling the holes in the pipe. It also made it much easier to use the center punch as well.



Everything is welded, the fire bricks are in and ready for the first new secondary burn modification fire.

Very cool. How do you plan on verifying they actually do what they're supposed to do and/or what kind of improvement they've made?

You can see a better burn thru the glass and no smoke coming out of the chimney. Its REALLY obvious when you see one that is a normal wood stove and one that is a secondary burn stove. Its really incredible to see.

This Youtube video shows two stoves. One is a EPA Certified Secondary wood stove and the other is a conventional wood stove.

Advanced Woodstove Technology - YouTube

I found a PDF that has a wealth of information on stove design:
Designing Improved Wood Burning Heating Stoves

There are two that look like good candidates for your retrofit.

First, the 33 gallon in 55 gallon heat exchanger on page 21. An insulated interior barrel lets the flue gasses keep their heat. With the right air flow, this becomes a secondary burn chamber. This is basically a retrofit to turn your regular stove into a rocket stove.

A similar setup, the Library Stove on page 36, uses a heavily insulated burn chamber. You could do something similar with your stove using stove brick on the bottom and a mix of clay and perlite on the walls and top. The insulated burn chamber means higher temperatures for a more complete initial burn. The two 55 gallon drums give a large surface area for radiant heating.

There are some high-mass stoves in there as well. I don't think they would be suited for a shop since they have a huge lag from heat created to heat radiated. However, they are well suited to home heating; they are better at extracting heat since they have a very long chimney with a very large mass around it and produce a low, steady heat like a radiant floor. Permies.com has a lot of discussion on the rocket stove mass heater. They claim that exhaust from a well designed stove should have no visible smoke and be about the same temperature as dryer exhaust. I have played around with small rocket stoves with some success, but I don't have the metalworking skills to make one suitable for the house and I don't have enough space for a big cob stove.

Thanks for the info. I think sometimes I get too much cold air, when strong wind blows in my outside air inlet. I ordered a damper to deal with that. When the air is calm, it doesn't get enough air. My inline fan isn't hooked up yet, so I hope that will do the trick. I am just trying to maximize the amount of heat I get out of it. I have a free source of would if I cut it myself, but it is cottonwood so it burns fast. I have to cut it anyways, so I might as well burn it. My chimney is a multiwall steel pipe surrounded by drywall, so I don't think I can get the temperature.

Well, my secondary burn modification to my wood stove is a success. After the fire was started and burning for about 10 minutes, there is a lack of smoke coming out the chimney. Its just amazing.

In the firebox I can see the flames are now a dark red mostly and dancing. The additional openings for the secondary burn air did not effect the closing of the factory damper and in fact you can really see how much better the fire burns at the top of the stove when it is dampered.

In all it took about 4 hours to fabricate it from start to finish. The cost was.....nothing really but some electricity and some welding supplies which were minimal.

Does anybody have any idea how hot/cool the chimney flue should be? Mine is about 375 at the stovetop, 214 at 12" from the stovetop, 185 at eye level, and 100 near the ceiling. Its only the reading from the outside of the flue.

I haven't found out what the lowest temperature should be to keep the soot from building up. I have a clean burn, but I am thinking of capturing more of the heat coming off of the chimney flue.

I was re-reading Benpope's post and saw the last section, I highlighted it in bold. Thanks Benpope!

I've disconnected my dryer and only use the dehumidifier to dry my clothes, so I have no idea what that should be.

I would think that as long as your burn is very hot you shouldn't have to worry about soot at all. What you'll eventually loose is the natural draft.

I have just put a secondary air intake on my gas bottle/oildrum insulated woodburner. The first thing I have noticed is less smoke and higher temperatures on the boiler plate. I think my other problem was insufficient primary air flow, caused by having a 4" steel chimney, but only a 2" primary intake.