Roger Floyd

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since Oct 14, 2013
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Recent posts by Roger Floyd

Wow, Randy, that's great you recaptured a lot of waste heat!

In the rocket mass heater (thermal battery) cobb bench, they recommend a minimum amount of turns for the exhaust pipe. Maybe ash could just be vacuumed out when the stove is off?

Thanks for posting, especially the 4 pictures!

11 years ago
Hi Thekla, the toolbox is that part you pasted into your post. Just click where it says MY PROFILE to change your settings, up at the top of this page.

Once again, I got to thinking... Pellet stoves have that (strong) fan forced exhaust. Air out = Air in. What goes out, must come in. If it couldn't pull outside air, it could deplete your oxygen for breathing.

If you had about a foot of concentric pipe, then peeled open the intake layer, and create your own ductwork just for the intake, so it can still pull in outside air. It would still pre-heat, for that 1 foot.

And your hot exhaust would be isolated, ready for sending thru a cobb bench, then out the wall, in the style of a rocket mass heater. I think this was your original idea, other than the concentric issue
11 years ago
Hi Thekla, I think your idea could work. If you're careful not to block the cold air intake, your idea of extending the hot outdoor exhaust, down into the slab could work fine.

You're thinking of scabbing cement (with your newly extended hot exhaust pipe in it) onto the outer edge of your concrete slab floor?

Normally getting hot exhaust to flow downhill won't work, but like you said, you have fan forced exhaust. Get some second opinions for sure, I've never tried it.

And make certain you can't ever be breathing the fumes, of course.

Modern slabs are likely on styrofoam insulation. My house is from the 1950's so the slab is right on the earth, makes it damp.

Here's another idea, if you could SPLIT APART the hot and cold pipes, like somehow peel open the outer concentric layer. Then run the hot thru a cobb bench and out the wall like a rocket mass heater. The cold air intake could pull from indoor air, if your house isn't overly "tight" - Maybe that's what you were thinking, originally?
11 years ago
Hi Thekla, I got to thinking, I remember when I first saw the concentric exhaust pipe. I was putting siding on a new building, and read the chrome plate on the outside of the wall. It said caution, hot-and also, do not block, air intake. I was fascinated by that for a really long time, the dual purpose of encircling the exhaust for fire safety, and heating the intake, to make more heat indoors, to make the exhaust hotter again, and on and on. Just hard to get out of your head for a while. "Round - Like a circle ever spinning, in the windmills of my mind" (words from a song from the 60's) Just saying, don't worry, eventually you'll get over it, and life will get back to normal
11 years ago
Wow, I love these forums, way more fun to use the brain, than say, watching pro sports on TV! The trouble with putting a coil in the pipe is it would create an obstruction, reducing the air flow. The dual direction blood flow in water foul is amazing, I never knew.

My certified woodstove has a horseshoe shaped coil in the fire box, close to the top of the fire box, just below the flue. It is 3/4 inch pipe, maybe copper, not sure, both ends plumbed thru the side of the stove. I got it second hand, so unsure if it had been altered. I read somewhere that certified stoves are not allowed to have a coil in the fire box, as it does cool the combustion, making it less efficient, and adding to air pollution.

I used to commercial salmon fish, on various 58' boats, 6 cylinder diesel engine, nice hot engine room. Our hot water was heated by a copper coil wrapped around the exhaust stack above the engine. The stack was actually wrapped with insulation first, not sure why, but then the home made copper coil was twisted around that. Steaming hot water for the galley sink, worked great. There was also an expansion tank, like your electric hot water heater might have. I knew a guy with a woodstove in a step van, told him about this, and he fashioned a working shower in his step van, wrapping the coil around his stove pipe. Cold water in at the bottom, hot out at the top.

I've not been able to figure a good place to do this on a rocket mass heater. A bath tub in the cob will crack it's finish if empty. Don't want to cool the fire box. Might wrap the barrel, as the exhaust is probably too cool, but maybe the exhaust could work.

Anyway, no, not too technical. The best illustration is the old days wood-fired kitchen cook stove, with the coil in it, and a water tank close by. As long as the hot coil water molecules can travel upwards to the tank, it will work. They had no electricity back then!

Here's another example. If a hot water tank is in your basement, you can have instant hot water (no waiting) at all your taps, as long as EVERY horizontal pipe run goes at an upward grade. The hot molecules want to move upwards, and will, if they are allowed to.

Yes, the concentric pipe is an attempt to harvest heat on it's way out. I still think your easiest harvest point is that hot air blast exhausting outside.

You can put your copper heating coil anywhere. I did one in the middle of my yard, building a little fire in the middle of 8" diameter coil, fed by garden hose, closed valve on the hot top side of the coil. Wait for it to heat up, then open the valve, worked great!

I read that water is actually a better thermal battery than cobb. I could maybe draw you a sketch, but you could probably google thermo-siphon. As long as your coil is downhill from your tank, it'll work. Your coil belongs outside, in that hot blast of waste heat!
11 years ago
thank you Johnny, good to know!
11 years ago
I think pellet stoves pre-heat the intake air to increase combustion temps. If you cobb the pipe, you'll be cooling the intake air. Might not be a problem, but it could change how the pellet stove burns. That hot exhaust blasting outside is where it's at!

If the pipe exterior inside the house is quite hot, yes more length indoors (moving the stove) would help. Thermal mass, close to, but not touching the pipe, shouldn't affect the intake air temp.

Another experiment I did a few years ago; Woodstove water coil plumbed to slightly elevated 60 gallon stainless steel water tank, no pumping required. All that hot water in the tank (thermal mass) gave me a warm living room in the morning, unattainable previously. It really really works! I have removed the woodstove, replacing with rocket mass heater, that firewood thing was just way too hard on my back.

Back to the concentric exhaust pipe with preheated intake air - I think this is true for pellet stoves, gas water heaters, propane stoves (the woodstove style ones) or anything where the exhaust may go out through the wall, excepting rocket mass heaters
11 years ago
AFTERTHOUGHT - - Pellet stoves have a concentric exhaust pipe, the inner pipe is the hot exhaust, the outer circle is the air intake. This is to keep the hot, inner pipe safe and fireproof, as well as to pre-heat the (outer pipe) incoming air, for a more efficient combustion. You shouldn't put a coil inside the flue, nor should you pack cobb around it.

Also, instead of the open pail like I used outside, you could have some copper pipe or tubing in front of the outside exhaust. This would feed into an indoor tank, just like the old school wood-fired kitchen cook stoves with the copper coil in the fire box, thermo-siphoning to a large, slightly elevated hot water tank. The secret to this set-up was, an extra piece of vertical pipe, tee-d in next to the hot water tank, externally connecting the hot side with the return side. This "kept things moving". Make sure the tank is widely, openly vented at the top, so pressure doesn't build up. You'll probably have to add some water once in a while. Send the boy out to the well! haha - The thermo-siphon effect is caused by the hot molecules in the water rising upwards in the water, towards the hot water tank. The water isn't moving like a pump, but the hot molecules move. Some folks call this a phenomenon, but the best thing is, it is passive, requiring no power. A final caution, don't let the water freeze, drain it out, or keep it hot. I bet it will run upwards of 150 f. degrees. Remember the longer you run it, the return side will keep getting hotter and hotter.
11 years ago
My bro has a 5th wheel trailer with a propane furnace, vented thru the wall. The exhaust is really hot outside! This is waste heat, just like your pellet stove, or a rocket stove, a rocket mass heater, etc...

So, I set up a platform, topped with a styrofoam sheet insulator. Used a one gallon aluminum cook-pot full of water as a test, about an inch or 2 in front of the exhaust. We measured it at 125 f. degrees, it may have gotten hotter if left longer. You could thermo-siphon (a passive, energy-free way of moving hot water heat) into an equal-height, long trough of some sort (a metal rain gutter?) inside your house. You might want something enclosed, more like a big horizontal pipe, or tank, so water vapors wouldn't evaporate indoors. But don't trap the steam, it could over-pressure (doubtful but you never know) so vent the top, to indoors or outdoors, that's up to you.

This (hot water) large thermal battery could also be used to heat an outdoor shed, or greenhouse, etc... It will recover exhaust heat, but without the toxic fumes. You could even electrically pump from the large hot water reservoir, into radiant heating. A distant room (insulated water pipes/hose for the distance) or in-floor hydronic heating system (pex tubing, a hot-safe hose, Home Depot) Many, many options here! Good wishes all, thank you open source attitude!
11 years ago