Modifying a wood furnace to increase efficiency - Page 2

Xringer · 11-28-10, 11:44 AM

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..

Ryland · 11-28-10, 03:13 PM

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.

Daox · 11-29-10, 07:40 AM

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.

Xringer · 11-29-10, 09:36 AM

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..

Ryland · 11-29-10, 10:18 AM

Quote:

Originally Posted by Daox

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.

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.

Daox · 11-29-10, 10:28 AM

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?

Piwoslaw · 11-29-10, 12:42 PM

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).

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...

Ryland · 11-29-10, 06:09 PM

Quote:

Originally Posted by Daox

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?

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.

abogart · 01-06-12, 08:03 AM

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!

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

) 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.

abogart · 01-17-12, 09:02 AM

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

. 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.

11-28-10, 11:44 AM	#11
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	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..

11-29-10, 07:40 AM	#13
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	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. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-29-10, 10:28 AM	#16
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	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? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

11-29-10, 12:42 PM	#17
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 964 Thanks: 189 Thanked 111 Times in 87 Posts	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). 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... Attached Thumbnails

11-28-10, 03:13 PM	#12
Ryland Master EcoRenovator Join Date: Mar 2009 Location: Western Wisconsin. Posts: 913 Thanks: 127 Thanked 82 Times in 71 Posts	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.

11-29-10, 09:36 AM	#14
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	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..

01-06-12, 08:03 AM	#19
abogart Efficiency Afficionado Join Date: Jan 2012 Location: Michigan, US Posts: 68 Thanks: 21 Thanked 6 Times in 4 Posts	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! 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 ) 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.

01-17-12, 09:02 AM	#20
abogart Efficiency Afficionado Join Date: Jan 2012 Location: Michigan, US Posts: 68 Thanks: 21 Thanked 6 Times in 4 Posts	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 . 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.