Larry Lile

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since Aug 16, 2014
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Recent posts by Larry Lile

Thanks, Derek - So if I understend you correctly you have a watertube copper heat exchanger inside the flue? Didn't quite catch your drift. Sounds like it is something of a creosote problem, but you are handling it by frequent cleanings. Is that right?
4 years ago

Vlad Alba wrote:I guess I don't understand what you're talking about.

I was just meaning that I want to brick over my little stove. Cover it with bricks. Surround it with bricks, leaving the front door of the stove open and available but covering the top and sides.

I was wondering though if a firebrick would refract the heat rather than just soak it up. Also, I was wondering if firebricks are way expensive.



Yes, firebricks are expansive, and no, you don't need firebricks. Any brick will do. Firebrick is for very high temperatures, inside the stove. I've seen several very successful installations where a conventional stove is used inside a brick archway or surrounded by a brick wall, and these structures can hold heat for days. It is a great idea.
4 years ago

Jared Blankenship wrote:Has anyone completed any of the projects mentioned here yet?



Still working on it.

I am building an outdoor stove out of firebrick and lightweight concrete block, will put it in a small well insulated steel shed (with steel studs! Can't burn!) Completed the foundation before it started freezing this winter, masonry is on hold until thaw.
The stove will look a lot like a rocket stove, with a secondary preheated air supply. Although it will have a steel door, inside it will be all firebrick lined. It will accept 3 foot logs to reduce woodcutting effort. I can ut plenty of wood off my place that doesn't need to be split, just cut it to length. By the numbers one charge of wood should heat the tank.
I have a 450 gallon insulated wood tank, lined with EPDM roofing for a water storage tank.
Welded up a watertight firetube heat exchanger. This is a 20 gallon drum with firetubes inside. The drum is filled with water. the assembly goes inside a 55 gallpon drum, with 6" tube on either end, and baffles that make the heat go through the firetubes. Tested airtight to 30 PSI.
Water side is all closed loop. The tank is open, but that water isn't pumped,does not contact any steel, all the pumped water is inside closed heat exchangers in the tank. After studying open and closed loop, I concluded that open loop systems create a lot more corrosion and require more expensive pumps.
.
There will be several safety controls. Of course, PT relief valves. There will be one near the heat exchanger at a high spot, and one near the tank. Another I am considering is a spring-loaded diverter that closes a flue damper if there is no power to the pumps. A damper motor keeps it open to the heat exchanger, but if power fails or temperature goes to high the damper closes off heat to the flue heat exchanger. This would help prevent no-flow problems during power outages. I have several ways I could put this on backup power, so the pumps run when the power is off, and I will probably do that as well.
The loop to the stove has *no* inline valves, no strainers, nothing that could potentially clog up or be forgotten and block flow.

I weighed the pros and cons of putting this stove inside, and decided that, although I will engineer everything possible to prevent a steam explosion, the remote possibility of such a catastrophe inside the house would be a deadly disaster, but outside in a shed it would just be a mess. The big stove goes outside.

This is a complex project, and it'll take a year to complete. However, I've built stuff this complex before, and been successful after a few tweaks, so I'm not worried. My wife, however, is about to call the guys with the loonie truck.
4 years ago

Mike Schofield wrote:Hi I've got a length of galzanized steel ducting that I was going to use as a flue pipe on my woodburner. Good idea or bad idea?



The other guy says there is "Controversy". No controversy here - don't do it! All welders know that zinc gases are poisonous, and heating a galvanized part is dangerous. The idea of burning off the zinc *inside your house* is not something I would recommend. Galvanized pipe does not belong in wood heat period.
4 years ago

Jeff Thorpe wrote:Larry,

Thanks for the reply, I'll check out your links later. After doing the calcs I realized that water in an open system really isn't capable of storing a vast amount of heat like I thought it would. I've been wondering a similar thing as one of your ideas - if it wouldn't be easier to bury some coils in firebrick or cob? Storing the heat in solid form instead of liquid - the bricks can be heated up to 800 degrees or whatever, then just pumping water through the coils should extract that heat fairly well. Obviously preventing a steam kaboom would be important, and you'd have to protect the copper from both flue gases and cement.

I haven't thought about this much, but another idea has crept into my mind - everybody is afraid of steam, but what if we just acknowledged the power of steam and built a steam system? Instead of trying to prevent steam, using steam to our advantage? People have been building steam systems for 150 years, and you can readily get old steam radiators pretty cheap....?



Well, there are two schools of thought on this. Solid materials don't leak, corrode metals, turn into steam, or go kaboom. Many people have built high-mass stoves using cob or brick successfully. Others have simply built a conventional wood stove into their house and surrounded it with a nice arch of brick - I know some beautiful and successful homes built on this principle. But the physics of heat are in favor of water. Water has a high capacity to store heat per a given weight or volume. You'll need 4-5 times as much volume cob or brick to store the same amount of heat as water. Your brick isn't going to get to be 800 degrees, not after the first inch anyway, it will be warm, maybe hot to the touch near the heat source, so mostly it will be about the same range of temperatures as a water heat storage system. You'd control your water system so it doesn't get more than 180F, dropping to maybe 110F. Water *IS* capable of storing a lot of heat, and that is exactly why many people use it to communicate energy from an outdoor boiler to a house. Water is also a lot more portable than bricks, being easy to pump from point A to B.

This is a cat that can be skinned several ways, and thar ain't no one right way to do it. (Well, there are wrong ways. KABOOM! )

My great-grandfather died in a steam explosion at a homemade steam-powered sawmill. That's plenty of information for me about steam engines. A steam engine is a HUGE technological leap over a wood boiler, with high pressures, moving parts, tolerances, etc. Plenty of guys idle away many hours with this pleasant hobby, the ones that are not in the burn ward anyway, but I would not recommend trying it.

4 years ago

Jared Blankenship wrote:Anybody have any ideas? I'll buy the DVDs if I feel pretty confident that this will work, but I hate to spend $100 and not be able to use them.



I've started posting what little I've learned on this thread.

4 years ago

Jeff Thorpe wrote:I would like to build a RMH in my home and store the heat in water, and then connect to my baseboard system for whole house heating - has anyone done anything like this?



I am on the same kind of project, researching, and experimenting with the idea. I have built some crude wood fired boilers, some rocket stoves, and am currently building a large rocket stove intended to heat the house as a boiler. Have done some interesting firing experiments with cordwood as well as wood chips, quite successful but so far the stove is not hooked up to anything, just running out in the driveway.

{and yes, I am competent to build a wood fired boiler that won't explode or burn down my house. An important consideration}

Buffer tank sizing is a little complicated, but not overwhelming. I went through that in this post on Hearth.com, read it and then come back here. The short answer is you are sizing the buffer tank to absorb the full output of one firing of the wood stove (if it is batch fired) and then sizing the system so that so-many-firings of the wood boiler will provide so-many-btus daily for a house heating load. I ended up with about 450 gallons needed to allow 2 firings per day. That's typical.

The question I am pondering right now is how to make a good heat exchanger to absorb heat out of the flue gas from a big rocket stove. There are about three good ways to go:

Coils of copper pipe (water tube)
Steel or stainless steel tubes inside some sort of flue box (water tube)
steel or stainless steel tubes inside some kind of water tank (fire tube)

Copper melts at low temperatures and corrodes easily, but is a great heat transfer medium and is easy to work with. Copper does not get along with flue gases or dissimilar metals. I am imagining using copper coils inside a layer of castable refractory cement (to separate it from the corrosive flue gasses and provide a heat transfer medium) adjacent to a flue. Haven't thought this all the way through.

Water tubes inside some kind of flue box have been done by several hackers. There is a company that sells stainless steel tubes for this purpose. Simple but expensive answer would be to buy 3 of their coils, install them in a 55 gallon barrel, and plop that on top of my rocket stove. The rest is plumbing.

A fire tube heat exchanger is also interesting. If one was a good welder one could weld tubes into a tank watertight, fill it with water, add a circulating pump (and safety controls and temp/pressure relief!) and pump hot water to your buffer tank. A 30 gallon barrel, with fire tubes inside, inside a 55 gallon barrel, arranged so the flue gases went through the fire tube pipes, might make a pretty good heat exchanger.

All these heat exchangers are heavy and require independant structural support. Fire tube is the heaviest.

Water treatment is an important consideration often missed by us hackers. All of this stuff (flue gases, water) is corrosive. These guys sell all the chemicals and test kits you'll need to keep your pipes from rusting out in a year.

Open vs. Closed system? Open systems seem simpler at first, have less (but not zero) potential for a steam explosion. But they evaporate water, requiring makeup water, they introduce oxygen, which accelerates corrosion and thus use more chemicals. Pumps in open systems may have higher head requirements and may need to be self-priming. Closed systems solve about half these problems and introduce several more - need expansion tanks, perhaps more overtemp/overpressure relief valves, and so on.

What I am finding out is, the hydronic end of this will be more expensive than the wood stove itself. Pumps are $200 a pop, copper or stainless is outgrageous, aquastats and overtemp/overpressure valves are needed, expansion tanks aren't cheap either.

4 years ago