An Idea for a Diverter to Capture Waste Heat from a Woodstove

Still in the thinking stages on this project, but I've got an idea of what I want. This is a diverter that will sit on top of the wood stove. It will allow smoke to go right up the chimney during startup and will allow the cleaner afterburn to circulate through a masonry mass with the turn of 2 dampers. I have no idea how to size the masonry mass but think I can capture a lot of heat this way. Does anyone have advice on using something like this ?



When I'm getting ready to turn in for the night the stove is glowing coals, no flame visible. I turn the intake air off and go to bed. At this point the surface of the plate steel stove is 500f and I have to believe that 500f is also going up the chimney. I also believe that in this situation, there is no more creosote in the exhaust to deposit anywhere. If the exhaust is diverted to a rocket mass type heat sink then back to the hot chimney, I will have adequate draft to start and maintain the flow. The unknown variable is how much masonry will be ideal to soak up heat. I have enough room in the stove room to build a bench type mass like the rocket mass designs, maybe 10 feet long so it would be a 20 foot loop. The materials are cheap enough to build it - I'd like to see how it works. Any advice ? Thanks

Er yes why ? I don't see how this would be any advantage over a rocket mass heater .the heat you are trying to capture is less powerful over less time .
Also burning stuff with less air intake has the potential to form more Carbon monoxide ,not good at all, you really don't want to mess around with that . It kills silently ,no smell ,no taste, it's invisible you are dead before you know it.
David

Please don't do this. Read Erica's recent post on How Chimneys Work, for starters. If you scavenge too much heat from the flue gases, they become too cool and dense to maintain draft. Glowing coals are still burning, and what they are producing tends to be high in CO. If the draft stops/reverses, you've now got a very efficient carbon monoxide generator pumping the stuff into your living space.

If you want a masonry stove or RMH, by all means build one. But don't try to make a standard wood stove act like one by defeating a critical design element. Safety first.

Phil Stevens wrote:Please don't do this. Read Erica's recent post on How Chimneys Work, for starters. If you scavenge too much heat from the flue gases, they become too cool and dense to maintain draft. Glowing coals are still burning, and what they are producing tends to be high in CO. If the draft stops/reverses, you've now got a very efficient carbon monoxide generator pumping the stuff into your living space.

If you want a masonry stove or RMH, by all means build one. But don't try to make a standard wood stove act like one by defeating a critical design element. Safety first.

Ok thanks.

Kind of like an add-on wood stove without the blower? Or am I way off?

Karen Donnachaidh wrote:Kind of like an add-on wood stove without the blower? Or am I way off?

Not quite, I was planning on getting the heat from exhaust not convection. Will not be doing this based on advice given. Thanks for the post

Don't throw in the towel. Head back to the drawing board. You may come up with something great yet. Not everyone could come up with the design you had initially. Keep working on it.

Karen Donnachaidh wrote:Don't throw in the towel. Head back to the drawing board. You may come up with something great yet. Not everyone could come up with the design you had initially. Keep working on it.

Towel has been thrown. Drawing board put away. I don't want to die for my art.

Back in the 80's when I lived in Ireland we had a wood burning stove going up a masonry chimney.  I pumped air down through a 1 inch steel pipe  that I put down the chimney and it connected  to another pipe that came  out an unused hotplate and into the room. I think it was probably 6 to 10 liters per minute of very low pressure air.  (I used the air from a trompe in a stream.  It was probably about 1 or 2 psi (not more).  It was remarkable how long it kept the room warm after the fire went out.  (It stole heat from the chimney and put it into the living room).  A marine 200 air pump is probably too small to do this (only 3 liters per minute)  but one of the larger pumps, probably 7 or 11 liters per minute would perhaps do the trick.  My chimney went in horizontally and then straight up, so I had a maybe 2 ft piece of the 1 inch  at the bottom going horizontally with a short right angle bend on it and the one from the roof attached onto it. It worked great but the air from the stream was smelly.   If you had one of those aquarium air pumps in your attic or maybe in the living room if it isn't too noisy,  That sent air up to the top of the chimney you could have it come on when the wood stove got hot, and then turn off when the chimney had finally given out all its waste heat. I don't recall the heat of the stream of warmed air but something in the region of  70 to 90 centigrade was probably the range.   I might still have pictures and numbers.   But it wasn't a proper experiment.  I was stupider then,  didn't think to use an aquarium air pump to run the heat thief.   Brian  

You may not have to throw in the towel.

What you are describing with glowing coals is actually a coal burning stove, so the naysayers are not exactly right. If they were, everyone who has a wood burning stove; set the damper closed at night, and yet had the fire die during the night, would be dead by morning. That obviously does not happen, because a fire burns in proportion to how much air it is getting, and how much air it is exhausting. This goes back to the laws of the fire tetrahedron...heat, fuel, air and chemical reaction. Whether it is 3208 Caterpillar Diesel Engine, or a wood burning appliance, they are air pumps...air in...fuel burned...air out.

Knowing what I know about coal burning stoves, what you propose MAY be possible, but coal is different in wood in that draft is often hard to maintain. That is because coal has very little heat going up the chimney because it is a radiant type of heat. That does not mean though that your idea would not work even with wood, you just have to maintain your draft.

IF you could pivot the baffles easily enough, you could connect the baffles by bimetal controls that control the dampers of many stoves. In that case, as the temperature drops to a preset temperature, the baffle opens and you stop trying to recover the heat. It is not a big deal anyway because at those temps you have no real temperature to recover anyway.

If you wanted to go high tech, you could connect the baffles via motorized baffles, temperature sensors and a plc...

There are tried and true methods to accomplish most of what you want. If you have a stove you can sit one or 2 disconnected but full hot water heaters as close to the stove as possible. Also not uncommon is to stack as much brick as is practical on top of the stove and wrap the chimney on 3 sides that way. Anything to store heat in mass. A hot water heater stripped of insulation painted black near the wood stove hooked up to water and acting as a buffer for your hot water tank is another great way to store btu's. Stoves work best in the middle range of their burn profile not tampered down or roaring. Use it to your advantage and give that high heat somewhere to go and coast between fires. Same general idea as an RMH but using what you have within its design parameters...
Cheers, David Baillie

I know that some people use a small fan placed in the opposite corner of the space from the stove that draws air down a pipe from the ceiling and blows it out along the floor, in my experience this better air circulation does more to enhance the efficiency of wood heat than anything else.

stephen lowe wrote:I know that some people use a small fan placed in the opposite corner of the space from the stove that draws air down a pipe from the ceiling and blows it out along the floor, in my experience this better air circulation does more to enhance the efficiency of wood heat than anything else.

Stephen, could you elaborate more on this?  It sounds very interesting.  If the wood stove is in the middle of a room, and you can place a small fan in a corner, do you need to have a portion of pipe (metal or PVC??) below the ceiling, and how far, down to above the small fan? or is there some way to pipe along floor wall into another room using this concept?  I am not sure if I understand completely, so if you can explain a little more it would be much appreciated.  We have used a stand oscillating fan to draw heat from wood stove into a passageway into another room, but if we could draw from near ceiling heat down to a floor piping into the other room, without it being in the way, like right up against the wall at floor, this would probably provide a better heat source as the heat rises again in the other room rather than just having the fan pull it in at waist high.

Thanks in advance.

MA Carey

One thing I have seen in very old houses ( ie 15th centuary ) is that in the ceiling of the living room was a trap door to let the warm air rise into the " master"  bed room when people retired for the night .
servants and children just got cold rooms  :-(
David

David Livingston wrote:One thing I have seen in very old houses ( ie 15th centuary ) is that in the ceiling of the living room was a trap door to let the warm air rise into the " master"  bed room when people retired for the night .
servants and children just got cold rooms  :-(
David

Ha!
When I was a child, the heat registers, as we called them, was a square grate in the 1st floor ceiling/2nd floor open to allow warm air to rise to the upstairs bedrooms.  Sounds like the same scenario, although more modern, and "anyone" upstairs received the heat, no specific class of person.  We were able to lay a heavy small rug over the heat registers during the summer to alleviate the heat of the summer.  It was still pretty warm upstairs until evening when the attic fan went on to bring in the wonderful breezes.  That was our heating and AC back in the 1960s and 70s.  ;-)

It is a very simple system: a pipe starting just below the ceiling where warm air accumulates, leading down to a fan near floor level, and an outlet pointed where you want. You can just as well point the fan outlet through a wall into the next room as back into the same room.

one day, while at an auction sale, I saw a house that had a large masonry style heater in the middle of the open main floor.  there was a HUGE grate above it to the second floor.  upstairs, each room had over 1/2 the walls cut away about 6" down from the ceiling.   and in the rooms, two or three smaller grates leading back to the main room on the outside walls, with the largest being below the windows.

Seemed nicely done with the motif downstairs, as I never noticed the grates along the walls until the tour of the upstairs rooms.


I can say this though....  NO PRIVACY