Getting the most out of wood heat.

[Ronnn]

My stove is medium sized built from 1/4 inch steel plate and welded together. Has fire brick lining. The fire box is 28.5 by 17 wide by 18 inches tall at the front which slopes up  to 23 inches at the back. Don't really know if it matters but there it is.

My questions concern getting the most heat out of the wood it can gobble. The opening to the fire box is 10 x 11 so I can put pretty big stuff 25.5 inches long in it. Our home is made to circulate heat thru natural convection. It's on six different levels and the lowest is where the stove is. The two main walls that divide up the 2k sq.ft. space  stop maybe 16 inches short of the ceiling. There are large arches 13 ft. at the base in both these walls with  wide  stairs leading to each lower level. In the winter the cooled air is like a river flowing down those steps. One last thing. The place has 14 inch adobe exterior walls and 10 inch interior walls everywhere and lots of concrete in the floors with lots more under flagstone and saltillo tile. Thermal mass up the ahhhhhhhhh, wazoo!

We began by using the stove closed down alot of the time. This could just about clog the spark arrester in about six weeks. So I started thinking that I should just fill the place with as much heat as possible and let the place just soak it up. The stove is probably 40 feet away from where we sit a night.

What I don't know is if this is the best strategy or not. We tried various catalytic combusters but they were always mounted in cheap housings whose inner components burned up and dropped the element into the bottom of the six inch chimney. I really thought they got us a little more heat to move around in the house. But I just about cant find them any more. That whole portion of the chimney glowed red for hours. No its not a fire hazzard in a mud house. My concern is two fold, by burning it hot am I just pushing most of the heat up the flue or is there enough just comming off the stove to make it worth while to burn the extra wood. It's cleaner burning I know that much. I only have to clean the spark arrestor once a year. I do not yet have outside air feeding the thing so I know that is a minus right there but I could probably come up with something. I did this with our fire place but we don't use it. I haven't finished the smoke shelf and chimney yet. I know this stove can move a lot of air. But I'm unsure how I'd do an air feed now since the heavy mansonry floors are all finished. That would have been the best way. I hadn't planed on putting the stove where I did when we did the floors. If anyone has Ideas I could post pictures of the area where is sets.

Sorry for the rambling questions.

Ron

[hydrosun]

A short hot fire will burn cleaner and more completely than a long cool fire. But where is this heat going? If you control the air to the fire to just enough for the fire and then close it down after the main burn, less will go up the chimney. The heat radiated from the stove will be absorbed by the surrounding surfaces. And air will convect the heat around the house. Since the house is hotter during the burn than with a slow low heat the house will lose heat to the outside quicker. How much quicker depends on how well insulated the house is. But since the damper is closed after the burn less is going up the chimney all day.

I've got an underground house with lots of mass so I'm in a similar situation to you. I've decided to burn short hot fires when I use the wood stove for heat. But the temperature is less even, with it warmer when the stove is going and 5F cooler by the time we fire up the stove the next evening.

Chris

[dnix71]

Can you weld plates on the fire box like fins on a heat sink and radiate heat that way?

Or build a parabolic reflector and place it above the stove to bounce heat back down to the floor?

[hiker]

ever hear of a stack robber...

hooks up to your stove pipe -just a box with a lot of tubes running throu it..

than place a fan behind it to blow the heat out..

once your fire is going good --turn the air intake down plus the outlet..

makes for a long lasting hot fire--not really a fire but hot coals -which are

hotter than a flame fire--and your wood lasts a lot longer..

sure you could pile all the wood in there that it could hold and force air in there with a blower and have a huge flame --with the stack wide open...

well all your heat goes up the stack!!plus you waste a lot of wood!!

best one my dad built years ago for the shop was twin 100 gallon drums..

top one with a big hole in the center[sheet metal? welded] with fan behind..

for the house before we had the gas furnace hooked up -- he hooked up the hot water radiators to a copper coil he had inside on a cut in  half 50 gallon barrel stove..

natural circulation..two story house....we cooked on the top half of the stove.[flat metal welded on tophalf]

you cant beat simplicty with technolagy--you think!

[hiker]

ps:

the fire burns really clean-damper on low-plus air intake!!

just my experance on the old homestead--you really want the most you can get out of the wood...when you have to go out and get it yourself-plus cut to length and chop..

hay i miss those days!!!!!!

thought i might be able to get out of this town someday..now im raising my grandkids.

i hit the trails with them!!http://www.kenaihiking.com/wonderings/162/never-too-young-never-too-small/

hiker....................

[Ronnn]

I knew this was the right place. Thanks for some good solid ideas. Some of these raise questions.

Hiker, on the stack robber. I'd heard long ago that these could cool the stack enough to increase creosote build up. Have you not found this to be the case? I like the idea otherwise.

dnix71 I like this idea too. It's pretty doable right away.

hydrosun, I haven't been doing my burns like you suggest. I'm going to see if I  can implement your suggestions. What do you consider the end of the primary burn? In other words what are you looking at when you open the door? Logs that now look like big embers or a bed of coals? I'm guessing the former. Thank you for the great suggestions.

Ron

[fabricator]

If you don't have ceiling fans you need to get you some, all the hot air the stove is making is up there, you are living in the coldest air in the house, you need to circulate the air to break up any stratification.

[birdhouse]

ron-

you're basically trying to get more usable btu's out of your stove per fire wood used.  first off, an outside air kit is a must.  it turns your wood stove into a closed system and no longer draws its combustion air from within your home.  newer stoves claim ten to fifteen percent more efficiency by adding one.  imagine the pile of wood you'll cut, chop and burn in a winter.  then imagine ten percent of that.  it's a lot!  

fabrictor is right about ceiling fans!  it sounds like your place has a few stories?  could you find a stud bay to suck hot air all the way back down to the wood stove level?  that would be super helpful!

i've been heating my home for ten years plus off a wood stove and love it!  snow storms hit and my friends get these gnarly bills just to keep the house at 65 yet i'm chillin at 72 for little money at all.  

my home is kinda like a giant horseshoe of rooms.  i cut a hole with a fan that connects the rooms in a circular pattern.  small fan, but makes all the difference in eliminating cold spots!

birdhouse  

[hydrosun]

I'm basing it on the technique used with high mass masonry heaters. My house has the mass built in. I don't always get back to the stove but when I do I just wait until the temp lowers before damping down on the inlet, not completely closing it.

chris

[scottsAI]

Ron,

First question is why build your own stove?

http://www.woodstovewizard.com/woodstoveplans.html

Check out Rocket stove. Many things are calling themselves rocket stoves.

The key features:

Thermal mass to capture heat and release it over time.

Fast burning.

Outside air.

Secondary burning with secondary air to complete burn. (missing from most designs).

Not using Outside air is huge mistake. Hot air goes out, cold air comes in through cracks/openings. As a kid we used a fireplace. Thermostat was in the room, rest of the house froze... Room with fireplace was the only warm place.

We decided the fireplace was NOT saving us $ on heating.

Have fun,

Scott.

[WoodstoveWizard]

Hi Ronnn,

I spotted that another commenter had linked to my site on wood burning stoves and mentioned rocket stoves as a possibility.  I hope you don't mind me joining and adding some thought of my own!

If you have the scope to build a large thermal mass structure from scratch in your home then these are a fantastic option, especially in an adobe structure as you describe. Sadly I don't think I'd get away with one in my 14C first floor flat, otherwise I'd be building one myself right now rather than sitting here typing.

Rocket stoves are really well optimised:

• The burn is fast and complete, with plenty of intake air



• The heat riser design ensures sufficient draft to the fire, even with a convoluted flue



• The convoluted flu, ducted through a large mass of cob, extracts the maximal amount of heat possible from your exhaust gases



• The heat is released gently into the room - no massive swings in temperature between when the stove is lit and not

As far as optimising your existing situation goes there are a few things you could do. As you say, burning with the inlet air shut right down slows the fire, but it also makes it very polluting, sending a significant amount of smoke (unburnt fuel!) up the chimney.  For both environmental and efficiency reasons I would suggest burning with more inlet air available and/or providing secondary air.

Your second issue appears to be with heat distribution within your home - you mention a passive convection principal which sounds really interesting. One issue that we have had with our wood stoves is that there is only a limited distance from the stove where it is comfortable to sit. If you are too close you overheat, and if you are too far way  the cool air drawn towards the stove will chill your back, feet etc... (you mention a river of air - we get the same in my parents large home).

Directing external directly to the firebox will help reduce the cold air flowing in and chilling the living space. This doesn't need to be plumbed in to the stove directly, a simple open ended pipe resting within a few inches of the stove air inlet will help substantially.

To distribute the heat further still from the stove you should consider a getting a stove fan, such as the Airplus Ecofan 802. We have used fans similar to this for a number of years and they make a substantial difference to how far the heat from your stove can be distributed - some estimates are that they can improve stove efficiency by around 30%. I know that qualitatively the room "feels" warmer with one of these fans running, and the warm air spreads further through the whole house as a result.

If you can improve warm air distribution to the more remote parts of your home I suspect that you would find more of your thermal mass kicking in to play - the warm air will spread further and contact more of the wall and floor surfaces than at present - so you will improve long term heat storage as well.

key points

• run your stove with more air



• get a stove top fan to improve efficiency of heat distribution from your stove to the space

I suggest trying both of these points first and then seeing if you notice an improvement - you may find less wood is needed to heat the further parts of your home.

Long term - consider upgrading to a rocket stove. With some drive and time they can be built in a few days for under £100.

Please do let me know how this works out for you!

Mike

[ghurd]

For roughly the same 200-300ma the muffin fan uses, can move a lot more air.

Or for about the same cost as the larger EcoFans, can build a PV system to power a fan.

Something like CmeBrew's floor model, or this box fan,

http://www.fieldlines.com/story/2007/3/10/23942/1876

http://www.fieldlines.com/story/2008/7/8/172317/7334

G-

[Adam T]

  Many good ideas, some things I would take a different path with.  I have been around wood heat with fireplaces and stoves 40+ years.  The engeneering that has gone into wood stove design has led the industry to a point where the non-cataletic stoves can seriously compete with the cataletic equipped stoves.  As far as a home built stove is concerned, it would be cool to say "I did it", but just because I "CAN" do something, doesn't mean it's a good idea in every respect.  But I will leave that between you and your insurance agency.  A good stove is worth every dollar you invest in it (Don't think of it as "spending").  And it can litteraly last you a lifetime.

  Before I forget, I reference this site often, I have no affiliation with it - www.woodheat.org - they aren't selling anything except safety, and that's free.

  The things you put on the stove pipe to pull off extra heat, come at a cost.  Increased creasote deposits in your chimney will come hand in hand with the heat you steel from it.  Use the suggestion of taking the heat directly off the stove with a fan.  If your house can form a natural convection cycle as you suggest, that will help enormously in distributing the heat.  Another thought is if you have central heating, peridically flip the fan to "on" from "auto".  It doesn't run the furnace, just circulates the air wherever you have ducts.  Some programmable thermostats even have a setting that does this in pulses for you.  On xx minutes, off xx minutes.  Also as you mentioned, providing a supply of outside air to the "vacinity" of the heat source is a good idea.  Any air provided this way will not be promoting air infiltration at locations like windows, electric recepticles, celing fixtures, doors, etc.

  Fireplaces are lovely and beautiful, but don't come close to the efficiency of a stove, excepting maybe if you are using a fireplace insert.  There are always exceptions, it's not a rule unless you have exceptions.  One exception is a very poorly built stove.  The best use of a fireplace, is for dating, because it's not the fireplace that is keeping the room hot.  LOL

  Just curios what region you are in?  What kind of wood do you burn?

[Ronnn]

Chris and fabricator, I do have three cieling fans in the main areas of the house. I am grateful for all the help

Ron

[fabricator]

If you ever remodel and want to do thermal mass right, make the stove a boiler and build it into the fireplace, then  pour a thin concrete floor with radiant tubing in it, that would be the ultimate use of thermal mass.

[Ronnn]

Fabricator, I built radiant heat into the house from the beginning. Firm believer it's the best kind of heat. My system was down this winter.

Adam T I'm with you on sticking things in the chimney, except a catalytic converter maybe and I can't find a good one these days.

Rocket stoves.... I have Yanto's book liked the idea but don't like all the dickin around with the wood that seems to be necessary. Long straight poles of just the right diameter was my impression. I have a place I could build one into a long  circular bench that will be made of cob. But I'd need to be around one to get the feel for alternatives to the wood thing.

Our stove is 40 feet away from the hangout areas and we can burn it hot as we need to. It's right next to and adobe wall and very close to two others. I saw a variation on the rocket idea at an earth building seminar I attended over a few days up in Toas. They used to heat a stock tank hot tube. The stove and the hot tub were made of cob the the chimney ran right under it. Right where the flue turned vertical they had a secondary burn chamber to heat the flue before lighting the business end. They just had in a small fire place with a door you could put in front of it with an air inlet. It worked pretty well. I only just remembered that and that would be a way I could do the rocket idea. I could even build the burn chamber to take bigger wood maybe. Tho the one they made used mostly what I would  call kindling.

Ron

Ron

[troy]

I too have heated with wood for many years.  I was a trained wood stove techie back in the day.  Overall efficiency means squeezing the max BTU's out of a given supply of wood.  In the real world, pretty much any non-EPA rated woodstove (which would include virtually all homebuilts) are doing really well if they hit 50% efficiency.  35% or worse would be average.  Wood stove design turns out to be unbelievably complex. 

When the Feds put in the EPA rating requirements, literally dozens of little wood stove companies disappeared because they couldn't pass the test.  Even after trying repeatedly with many different designs.  These were stoves from companies that were well respected and had good reputations.  Compared to a terrible stove, they were good.  By modern standards, they were half of what they could be.

So, a good EPA rated wood stove can be close to 80% efficient, and those with catalytic converters may break 85% if the wood is good and they are operated well.

In theory, if yours is home built, and you switch to a good EPA rated stove, your wood consumption to heat your house could drop by half.  Half the pollution, half the wood cutting/chopping/splitting/carrying.  Half the deforestation, etc.


The Rocket stove (of which there are several designs) are a bit hazy on overall efficiency.  They are unquestionably light years better than the horribly inefficient open fires that they often replace in 3rd world countries.  Compared to a modern airtight EPA rated stove, I doubt very much that they break the 50% barrier.  An open fire or a fireplace is typically 5% efficient, or worse.  Bad examples of open fireplaces can actually have negative efficiency numbers, actively cooling the house, on average.


Secondary combustion is the key to these very highly efficient wood burning appliances, and it's way tricky to achieve.

The big advantage to a catalytic stove is it can attain secondary combustion conditions and temperatures very quickly.  Typically with the first load of wood.  A non-catalytic stove will take longer to hit secondary combustion.  For a stove that is operated more or less continually, the difference is trivial.  For a stove that has a short hot fire in the morning and another in the evening (like in a superinsulated house) the cat wins hands down.


HTH,

troy