Inefficient oil/wood combo

[RickieBlue]

I have an older oil/wood combo furnace that is pretty inefficient at capturing heat. There appears to be quite abit of heat being lost up the flue and I would like to try to recover some of this. I understand flue temperature dictates draft and creosote formation and will need something easy to clean when it gets coated up, and I doubt draft will suffer too much. I have seen commercial units available to put in the stack, called "Magic Heat', but would prefer to make something myself. It doesn't necessarily have to be a hot air exchanger, it could be a liquid heat recovery unit. Does anyone have any valuable ideas to help me with this project? Any constructive ideas are welcome! Thanks!!!!!

[MelTx]

  I dont have a wood burner so this is just an idea.Some times you can take a solution from one venue to the next problem.In hot water heaters they use a sacrifice rod to attract the corosion,there-by saving the rest of the tank.

   What I am getting at is maybe you could roll up some fine mesh screen wire into a tube and insert it down into the chimmny.Maybe some of the creosote would stick to the wire tube.Then it could be taken out and cleaned ever so often.

         Product #314 Cresso-Tube Pat Pend             MelTx

[kell]

Let's say you have 8" stove pipe.  You could insert a 4" stove pipe inside it and do a little sheet metal work to have the ends of the inner 4" duct exit the main duct cleanly.  Since the area of a 4" circle is only one fourth the area of an 8" circle you won't be blocking the duct much.  Put a 12 volt muffin fan (the kind used in computer power supplies) at the end of the 4" duct.  For future cleaning, you might need to make it so you can disassemble.  And of course, the seams of the inner duct need sealing.

[Volvo farmer]

I've got a heat scrubber type of thing that was on the flue of our old wood stove. Same idea as yours except it used a 12" pipe on the outside of the 8" pipe. The section was 24 or 30" long. It had a bimetal switch to turn the fan on and off. Only problem was that AC squirrel cage fan was really noisy. This is hard to describe so I drew a picture.

[hiker]

sounds like all you need is the old tried and true   STACK ROBBER  

buy one or make one its just a box with a bunch of tubes welded in..

with a fan in back to push the heat out..........

[RickieBlue]

Volvo farmer, I like that.....just a thought..how about I introduce a little air into the pipe at this point? I suspect there could be a secondary ignition of the wood gas, as I would put this pipe right on the outlet of the furnace..at any rate, Thanks for the Great idea! Thats what I like about this forum, a person will get many good ideas, and for sure, will get one that meets their needs and expectations!.........Rick

[Volvo farmer]

Uhhhh, I think the stovepipe is not where you want any kind of secondary ignition going on. A device like this tends to cool down the flue gasses and clog up the works. Add "secondary ignition" to "creosote" and you might find yourself with a guy like DanF in your driveway holding a firehose.

Back when we used this thing and our old 1980 vintage Earthstove, I'd go to clean the flue every fall. The 8" pipe would be down to about 6" at the cap, We'd fill half a trashbag with creosote when I brushed it out. We burn some pine and sometimes a lot of pinion pine, very pitchy. But this thing was not a clean burning stove. The theory then was to shut off the air, let it smoke, and keep something warm in the firebox all night.

We recently got a ~5 year old earthstove with a catalitic combustor in it. I'd never go back to that bohemoth, wood-eating, 8" flue, monster we used to have. I'd much rather have a stove designed with a secondary air combustion, because cats tend to wear out after a few years, but I'll take what I can get used for a couple hundred bucks. The neighbors are probably much happier as well (less smoke). BTW, our wood consumption wend down ~25-35% when we got a better stove.  My theory is to let the stove designers design stoves, I don't know enough about it to make a safe efficient appliance and they seem to be much better at it than they were 20 years ago.  

Anyone who has an 8" flue and wants this old heat scrubber, I still have it laying around here. The modern stuff is all 6" and I don't ever see myself going back to an 8" flue.

[BoneHead]

Yeah, the same concept as my fireplace insert. Just some steel tubes heated by the source with the fan blowing the air inside back out. The fireplace here is big so this unit is probably smaller, but the insert does a really good job... It's just a tad noisy though. With a newer fan, you could probably cut almost all the noise out. I saw one a few years back in a really old house that looked like it was made of exhaust manifolds or something...lol. If you can weld, you can make one easy.

[Clide]

I've been around wood burners for 30 or so years but don't quite understand the newer wood burners and I definitely don't know about catylitic heaters. Some observations are:

1. Torrid Aire, magic heat, etc motor driven fan heat extractors (like the drawing in a previous post) work very well.
   
2. 9 out of 10 woodburner owners shut their stoves down to spread the fire out over time and go to that heavy creosote, smudgy, smoldering fire.
   
   

Can the smudgy, smoldering fire be accomplished on the new "reburner" stoves? I recently purchased a Waterford Ashling stove and intend to find out. But I'll make some predictions:

There is no way around cleaning the stack and with the heat extractors they must be removed and cleaned as well.

The most serious draft limiting problem in the past was creosote buildup in the stack.

Draft in the newer stoves is probably more important than in the old days because there are more constrictions in the air and gas flow in the reburners.

If efficiency (the most heat) is the objective, radiation from a single wall stack into the room along with a stack robber (#1 above) will be hard to beat.

The objective of a clean stack is not worth the heat lost by using double wall pipe in the room (raises the temperature of the exhaust gases to avoid creosote buildup) unless you have a surplus of perfect dry hardwood and don't mind throwing the heat away. (the double wall pipe may be required by insurers)

It has been discovered that by burning the air from a heated room that the entire room content of heat and air goes up the stack every 15 minutes. Drawing air from outside is more efficient. My compromise will be to draw air from the back room to circulate some heat back there.

Clide

[kcrompton]

I am new to the board but I was reading here about your stove efficiency problems and thought this may be a great alt. I found this article a while back about building this Scandinavian design fire place/ heat sink and they claim it can keep you house warm with 1 fire all day. I have no real experience with this but it is a good read if nothing else.

Ken

[kcrompton]

Sorry I forgot to add the link to my last post and didn't see a place to edit my post. Here is the link for the fireplace.

http://www.hollowtop.com/cls_html/do-it-yourself/masonry_stoves.htm

Ken

[Wolvenar]

Well may I offer another point of view, and some observations.

I have been taught a few things burning wood for heat most of my life that seems to go against most common advice I hear today.

I am by NO means an expert at this stuff!! just what seems to make sense, or works for me.

First we normaly let the fire go out or at least let the house cool over night and use heavier blankets, although the bedrooms tend to stay quite warm. Each morning the fire gets re-lit when needed again and allowed to blaze as hot as possible until the house  is back to norm. I've been told this is just asking for a chimney fire, however I have never had much build up to worry about this way. It seems to either burn out  while there is only very little to worry about, or re-vaporize and dissapate outside the flu doing this every morn. But the flu always gets a cleaned regular no matter if it seems to need it or not.

 (let me know if I am totaly off my rocker)

Disclaimer to try to cover my butt

This is what has worked for me and my family under known long present conditions, I cannot tell you this is best or even a good idea for anywhere else.

I wouldnt recommend this if the stove is in any way around anything that just might be flammable while abnormaly hot, old or thin stove pipe, or a lot of flu buld up already.

Second thing that seems to help is a LOT of solid heat storage.

Air heats fairly fast but is displaced, and/or cools fast, so you cant burn hot and hope it lasts long. if you heat something solid ( rock, fire brick , sand, cast iron, etc, etc)  it can take a hot fast burn to minimize smoldering wood that builds crap in the flu. The solid heat storage gives off the heat relativly slowly over a longer period into the air to help with losses of heat in the air.

This last one because VERY appartent in how our oil furnace worked compared to wood heat. The oil furnace burns hot, but has a large air flow past a thin heat exchange. The heat from this never seemed the same. I spent a long time trying to figure this out, so I tried a few experiments. Finaly I added a rack (made from cast iron) inside the heat plenem on top of the furnace for firebrick, which allowed air flow around each brick. After that the furnace came on a LOT less and used somewhat less fuel. The house also lost the cold spots that were present with only the oil furnace.

I am placing my bets on the heat convection that continues after the fan is off always moving the air in the house much like the wood furnace.

Another trick I just have done with this wood furnace is I added a 12 inch pipe around the 8 inch from the stove to the chimney, placing a "T" where the elbow to the chimney is, then piping that strait up to a vent in the floor above, the convection from this is amazing but some thought had to go into isolating it from flammables    (and kids) cause its pretty hot. I also keep a close eye in this and check that 8 inch pipe when I clean the chimney for and troubles just in case. This keeps the area around the stove a lot less hot in just one spot.

I am playing with an idea of adding the forced air option (to firebox) to this eood furnace, but piping in ouside air to the blower. I really dont have a clue how this will affect anything yet.

Wolv

[Laylow]

Some more information about your setup would be very helpful.

By principle, I am against using flue stack heat exchangers.  You stated that you are aware of the relationship between flue temperature and draft but I just have to state that for the flue to draw properly you need to send a lot of heat up the pipe.

Here are some things that I would start with:

Make sure that your flue is well insulated once it leaves the heated space.  The better the insulation the less btus you have to spend on draw.

Consider adding some height to your flue.  You might benefit from a stronger draw with less heat.

If you do use something like Magic Heat try to go to a larger flue diameter in that section so that you aren't losing any cross sectional area.  You may also have to control how much heat it is exchanging if you start having draft problems.

If you were to somehow add a powered vent to your flue you can try to extract all of the heat from your flue gasses.  Just make sure that you aren't burning your fire hotter than what the stove was designed for.

I had a woodstove that had a sheet metal body.  More like a wood furnace I guess.  I wanted to set a pan of water on it for a little humidity but the body of the furnace never got hot enough.  My solution was to run a piece of copper tubing from the pan of water and wrap it around the stove pipe with the end pointing back at the pan of water.  I would draw a syphon in the tubing and when the water heated up it would shoot out jets of steam and boiling water back at the pan.  **WARNING** Never draw the syphon with your mouth on a hot stove!  You will burn your face off!!!

The syphon idea worked for steam although sometimes the syphon would break overnight.  It could be difficult to get the syphon design just right.  You could probably try something similar with larger tubing.  You could even run the tubing inside the stove.

But really, I wouldn't suggest the syphon or even most of the other stuff above.  I think you should just dig out your old box fan and point it straight at your stove.

Also, one more suggestion for any combustable heat source: Pipe the combustion air directly from the outside straight to the fire.  That combustion air has to come from somewhere and unless you pipe it straight from the outside it is going to come in via your window, doors, and mouse holes.  And that air is cold!  On the other hand, cold air is good for combustion!

Well, I didn't mean to write so much.  Just don't anybody burn their faces off please.

[Wolvenar]

When I read this I wondered why not have this piped from the water container to the heat source below the container via the copper pipe. In this case coiled around the stove pipe, then back to a point slightly above the container or water level. I would guess that the water pressure from the container would give enough back pressure to force any water circulation in one direction back into the container from the end just about the container, if not pure steam coming strait out of the open copper pipe. at worst case a easy flow, maybe high temp check valve at the bottom of the tank.

[Laylow]

I never really understood how it worked or it didn't work.  It worked fine while the stove was hot but the syphon always broke overnight, presumably when the fire went out.  Changing the shape of the syphon also had a big effect on performance.  Sometimes it shot out straight steam and other times it would spit water as well.  I thought about running the tube inside but didn't want to start cutting into the heater.  Problem I would worry about there is how well the copper would hold up.  As long as your going inside the stove though, you might as well just set a little boiler in there.  Some kind of closed container with a tube going outside to vent the steam and pressure; no syphon needed.  Make the tube big enough so that you can fill it from the outside.  Be sure you consider any safety problems.  You don't want to build a boiler that can blow up.

[RazDude]

I use a Quadrafire wood stove.  It has gas reburning before the hot gases go up the flue.  It is very efficient.  I have had to clean my stack only twice in nine years of use.  My technique is to do as one post mentioned -- in the morning I start a new fire and, let the fire get very hot before cutting it back with the two controls it uses.  It uses outside air for combustion which, I feel, is a must.  

As efficient as it is, I am still going to rob some of the heat to heat water in my boiler which, in turn, will distribute the heat throughout our home via underfloor and baseboard heating.

I do not believe in stack robbers for same reasons already posted on this discussion.  I am going to run a water coil through the fire box and allow the hot water to collect in a pressure water tank (with PTR valve).  From that hot water in the tank the boiler will pickup hot water through a second set of coils in the tank which act as a heat transfer system.

1944 my dad did this in, of all things, a fireplace.  We had so much hot water we used it to steam clean the snow and ice off our sidewalks and steps.  My dad used four runs of ordinary 3/4" galvanized pipe as "and-irons".  The pipe was held off the bottom of the fireplace two inches with legs welded to them.  It was one inch higher toward the front of the fire pit.  As the water heated it rose in the sloped pipes.  No pump was used.  At the time, plumbers he asked told him it would not work; that the water would not circulate; that the pipes would quickly burn out, etc.  That was 63 years ago.  The system is still working fine today.  Not one pipe has burned out or leaked.  We found that the hotter the fire the faster the water circulated thus self-cooling the pipes.  

The key to this is sloped pipes so the heated water can rise and form a circulation in the adjacent tank.  

It works!  And, with this system there is no robbing of heat from the stack.  

The downside is that, in a wood stove you lose space in the firebox.  As my firebox is limited for vertical space, my choice is going to be a V or W shaped pipe in a vertical position on the side of the firebox.  Cold water will run in at the bottom of the fire box, hot water out through the pipe at the top of the firebox.  Both hot and cold water pipes enter via the back of the stove which will entail passing through two layers of steel.