# outdoor wood heater with heat exchanger



## CSA again (May 2, 2007)

Hi Fokes I tryed posting this thread somewhere else without much response.


I am thanking of building a outdoor wood heater, not a boiler.
My plan is to pour a 6" concret pad, 5 ft by 10 ft. on top on 2" ridged styrofoam insulation.

On top of this a fire box made from sedule 40 steal pipe.6ft by 24"(closed on one end ,doors to load wood on other)

Then lay block, aprox. 5 ft high around the primiter of the pad, encloseing the stove.( leaving the door end sticking out to load fire wood into stove) 

Now I would pile 10 ton (or whatever it takes) of granite rock around and on top of the fire box. Seal the top with a steal plate roof. And insulate the entire outside very well.

Now that I have a very large and heavy heat retaining ,heat exchanger 
I will run 2 insulated air ducks underground into by basement . One to my exzisting return air duck the other to my heat supply duck.( already in my basement with an oil funrace)

Add a inline fan to curculate the air through the hot rocks and back into my house , a thurmostate to controll the fan and thats it ...

Im thanking once the rocks are hot I would not have to build another fire for a day or so?
I read once that with 10 ton's or more of well insulated stone a system like this would only need a fire every other day?

So anyone ever here of ,or know where I could find a plan like this??
anyone ever try anything like this?
Do you think it will work?
Thanks for any comments or info..


----------



## MELOC (Sep 26, 2005)

it just sounds complicated. i have read of folks making forced air heat outside, but most folks nix the thermal bank and just insulate the heated air. i think that you would still need to fire the stove every day, even if only a little. 

i know a guy who uses a wood fired boiler outside and it pipes hot water to a heat exchanger in his ductwork. he has forced hot air heated from an outdoor wood fired boiler. he also has a heat exchanger for his hot water. that is what i hope to do someday, only i may nix the forced air and just use my radiators.


----------



## Ed Norman (Jun 8, 2002)

It sounds very close to a HAHSA. Google it some time. That uses your firebox and slab and building, but then runs pipe around inside the block building, then it is filled with tons of sand for the heat storage. Water is run thru the pipes and into the house to radiators or a heat exchanger. The sand stores and transfers the heat to the pipes. 

Rock should work but I would want some good baffles to make sure the forced air has to take a long route to get to the exit, instead of a shortcut, bypassing most of the rock.


----------



## crafty2002 (Aug 23, 2006)

Yo could put 6 - 275 gallon oil tanks over top of your firebox to store the heat in. That would be a total of 1650 gallons of water. One BTU is the ammount of energy it takes to heat or cool one pound of water one degree.
Water weighes 8.34# per gallon so you would have 13,761 pounds of water. If you allowed the to heat to 200* and used the stored heat until the water cooled to 125* that would be 75 BTU's per pound of water or a total of 1,032,075 BTU's of stored heat. 
Most oil fired furnases are rated at 100-125,000 BTU's per hour but they usually cycle on about 30% of the time so you are actually only getting more like 30-40,000 BTU's from them per hour. That would give you about 25 hours of heat. 
It you built the building higher you could stack the tanks and double the amount of water and and stored heat to make it two days on a load, if one load of wood would heat that much water. 
Just something to think about.
Dennis


----------



## CSA again (May 2, 2007)

Good Info Thanks...
I have been reading to , this is called a thermal mass heater .
some of these use heavy wall 4" tubeing inside the block walls to pipe the air through then fill the block building (around the stove and tubeing) with sand for fhermal mass heat storage.
I did not want to use water for heat storage because of freezing . rust . boiling . ect.
I could use water(with coils inside the sand) to transfer the heat into the basement ,but that would mean another heat exchanger to transfer heat into my duck work system. Plus antifreaze, pop off valves, and a pump
Any more ideas out there??
thanks eddie


----------



## artificer (Feb 26, 2007)

Just some numbers to use. 

Water has a specific heat of 1btu/lb for every degF temperature rise, as crafty2002 said. In comparison, stone/concrete has about .2BTU/lb*degF. Loose gravel has a weight of around 90lb/ft^3, and sand is about 120lb/ft^3 compared to 62lb/ft^3 of water.

Water 62BTU/ft^3/degF
Sand 24BTU/ft^3/degF (I've seen 36BTU listed as well)
Gravel 18BTU/ft^3/degF

Taking your 10 tons of sand (using duct work to channel the air) and 90 to 200 degrees as the temp extremes, you get: 4000 BTUs of storage. If you go with the higher amount, you get 6000BTUs. Our house has a 92% efficient 75,000BTU/hr furnace. If its running 1/2 the time, I would only have enough heat stored for 10 minuets in the cold of winter in Wisconsin.

Whats the coldest weather you typically see in NW CA? How big is your furnace, and how long does it run? How well insulated is your house?

Your dimensions give 3500lbs of concrete, and 22,500lbs of sand, if you only use 25% of the space for the burner, flue, and ducts. That gives 5200 BTUs of heat storage.

An exciting (to me at least) idea is to use a phase change material like Glauber salt (sodium sulfate) It melts at 90deg F, and has 108BTU/lb of latent heat. 

Using water to store heat isn't that much more complicated than what you are doing. I think you should give it another thought before rejecting the idea. None of the issues you raised (freezing . rust . boiling . ect.) are hard to plan around. I think the pros outweigh the cons: precise temp control, more heat storage, more efficient heat transfere from the burner...

Michael

A good resource on solar heating and heat storage.


----------



## clovis (May 13, 2002)

I like your idea!!!! Why didn't I think of that?

Here is what I would like to know: How are you going to draw air thru the system? How will you know when to start pulling the warm air thru? Just by walking to the system and checking, or will you put a thermostat/auto start mechanism on it?

Neat idea!!!! Thanks for the learning experience!

Clove


----------



## CSA again (May 2, 2007)

thanks Clovis ,

I'm still in the planing stage But yes i would put a snap type thurmostate on the inside of the thurmal mass ( like on a wood heater ) that would not let the cuclariting fan come on untill the temp is at least 120 or so.. The fan would curculate the air through tubes burried in the sand and back underground to my basement through a short insulated ditch. Or in the case you are useing rock as thurmoal mass then the air would simply curculate through the spases between the rock.


----------



## raymilosh (Jan 12, 2005)

I'm a bit unsure of the details, but I'll chime in anyway.
I wonder how much of the wood heat would escape outside. I'm thinking it would be significant. Consider insulating the thing heavily from the outside or...
With the amount of work you're considering putting in, I would recommend lookng into constructing a masonry stove inside your house. It is basically a massive fireplace, but the smoke has to pass through a maze of passageways inside the masonry and it looses all of it's heat before going out the chimney. Creosote builds up, it has chimney fires and absorbs tht heat, too. No big deal. I hear folks fire them with one hot fire every 2 days. Installing a bread oven over the firebox allows you to make bread for up to 8 hours after the fire.
Make it central in the house. It is the heat source for the house. no ducts, fans, etc.
install a tank in it to make hot water for the house, too

disadvantages are that it takes a few hours to get warm and a few days to cool off. if the outside temp fluctuates a lot, like it does here in the Piedmont, masonry stoves don't react fast enough. a solution is to have a conventional woodstove next to it. 

Just my two cents.


----------



## CSA again (May 2, 2007)

Yes Raymilosh the masonry stove is the way to go in new construction , but I live in a 100 year old house and it is really not feasible to do that. I also do not have the room inside.

I was thinking of filling the block with insulation plus 6" on the outside covered with metal sideing and metal roof

I will also add a copper coil in side the sand, while I'm at it, and run that into the basement also for some hot water later on.

I beleave this thurmal mass with sand would weigh 20-25,000 lbs and should hold heat for a very long time ...I work in a plastic injection molding plant ,and we heat 8000 lbs. of plastic pellets up to 350 f. this non-insulated mass will stay hot enough to burn you for 18 hours or more. 

come on lets here more ideas..

thanks eddie


----------



## CSA again (May 2, 2007)

I've had some people PM me with interest about this post so I thought I would bring it to the frount once more to see if there are any more Ideas out there.

I still plan to do this project at some time , its on my todo list.But doe to some health problems I did not get started in time this year.

I am still looking for ideas and or links to more info.


----------

