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Old 12-17-2018, 02:25 AM   #1
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2019 23' Flying Cloud
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Any engineers? Furnace exhaust heat. That's a lot of heat!

So my science-y side of me got the better of me. You know high school stuff. I reinstalled the good batteries yesterday and did a visual checklist making sure everything was up to snuff and operating as it should. Turned on all fire related gadgets and bang everything turned in automatically on que. Fridge, furnace etc.
One option was to take a Peltier device (thermoelectric device) and somehow convert that lost heat into electricity however small it may be. Make a small box and whilst ruithe furnace one could recharge a cell phone maybe. I wish I had a heat gun to measure the heat output directly at the vent because it is near untouchable. So there exist enough temperature difference using two opposing heat sinks to create electricity.
The other, using a safe method was to keep the underside of the trailer warm-er directing the heat under it to keep the cold draft at bay. Maybe using a water bag it would heat it up that in turn use that warmed water via hoses to heat the floor up.
Doable?
Well, if you've come this far reading I appreciate it.
Cheers.
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Old 12-17-2018, 02:57 AM   #2
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[ . . . 30 minutes later from another night owl . . . ]



It is all lost heat. Welcome to the world of RV travel -- full of contradictions, inefficiencies, and miracles.

Go to bed and get a good night's sleep . . .



Glad you got the batteries sorted out, Brandon!

Peter
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Old 12-17-2018, 05:35 AM   #3
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Just remember there's some poisonous CO gas in that furnace exhaust. Any device to capture the lost heat must be prevented from diverting that into the air you breathe.
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Old 12-17-2018, 07:58 AM   #4
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That's the downside of an RV furnace, they blow a lot of heat and propane right outside. You can't redirect the air without running a risk of CO, and too much pipe it might not exhaust properly

If you reeeeaaaalllyyy cared, my best bet would be a liquid cooled heat exchanger, and pump the liquid somewhere you want warm (like underneath). Water risks boiling though.

Bottom line is you're looking at a LOT of work, for returns that are quite diminishing.

My musings. You could theoretically create a safe and effective heat exchanger as a freestanding structure outside the RV, but it wouldn't be portable. Anything portable will require hundreds of hours of design time. Anything portable not requiring hundred of hours of time will recover so little heat as to never pay for itself. Your thermoelectric generator for example, will produce a lot less electricity than a tiny solar cell.
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Old 12-17-2018, 11:25 AM   #5
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Furnace exhaust heat

Ever look into adding one of those catalytic heaters like the Big Buddy? All of the heat generated goes into the coach since there is no exhaust. I'd be concerned with co emissions and the heater using up all of the oxygen inside the coach for combustion. Would have to keep a window cracked open. I've seen these catalytic type units installed in some of the cheaper trailers in the past but am not aware if they are still being installed in the newer trailers. Might be an option.
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Old 12-17-2018, 11:45 AM   #6
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Ever look into adding one of those catalytic heaters like the Big Buddy? All of the heat generated goes into the coach since there is no exhaust. I'd be concerned with co emissions and the heater using up all of the oxygen inside the coach for combustion. Would have to keep a window cracked open. I've seen these catalytic type units installed in some of the cheaper trailers in the past but am not aware if they are still being installed in the newer trailers. Might be an option.
I shyed away from catalytic heaters for that reason. Something about the propane catalytic reaction occurring in my breathing air seemed like a really bad idea.

Plus you get water buildup, which is already a big enough problem in the winter

There is a company that makes vented catalytic heaters, I don't know what percentage of the energy is lost to the outside air in that case, but I've heard good things.

http://ventedcatheater.com/2.html
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Old 12-17-2018, 12:17 PM   #7
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In my old 1980 Airstream, I tried heating the floor with hot water in the winter using the hot water heater and PEX tubing under the floor. It also heated the water tank. It did keep the water in the tank from freezing even during some really cold weather, but the cost was high in propane. It never really worked as well as I thought it would and I later abandoned the idea. Later, I installed a wood stove. I captured the heat from the stove and used it to warm the water in the tank and floor. That worked really well as it didn't need a pump running to circulate the water. Never had to generate electricity as I had solar panels doing the heavy lifting. In the 2014 Flying Cloud I'm living in now, I find the furnace does an excellent job preventing freeze up. Some nights, it drops to 11F in Utah, but nothing is freezing up.
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Old 12-17-2018, 01:44 PM   #8
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In my old 1980 Airstream, I tried heating the floor with hot water in the winter using the hot water heater and PEX tubing under the floor. It also heated the water tank. It did keep the water in the tank from freezing even during some really cold weather, but the cost was high in propane. It never really worked as well as I thought it would and I later abandoned the idea. Later, I installed a wood stove. I captured the heat from the stove and used it to warm the water in the tank and floor. That worked really well as it didn't need a pump running to circulate the water. Never had to generate electricity as I had solar panels doing the heavy lifting. In the 2014 Flying Cloud I'm living in now, I find the furnace does an excellent job preventing freeze up. Some nights, it drops to 11F in Utah, but nothing is freezing up.
How did you capture the heat and use it to warm the tank, without a pump?
I have a woodstove, and am curious about using it to heat water, but haven't noodled through the best way to do it.
The stove has threaded bosses to accept bolts, so a plate with tubes on it isn't difficult to do, but my concern is water in the tubes boiling
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Old 12-17-2018, 01:47 PM   #9
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If you reeeeaaaalllyyy cared, my best bet would be a liquid cooled heat exchanger, and pump the liquid somewhere you want warm (like underneath). Water risks boiling though.

ahhhh... ok... but it takes more k energy to heat water up than it does converting air temp... and why they use AIR in the furnace....

As to why not duct it under the trailer... easy... did it tried it... and what a icecicle patch did that make.. you can't imagine how much moisture comes out of the heater once the air cools... (about a gallon of water per gallon of propane someone said) ...

So what the engineers did was exhaust the burner gass's while still warm and get rid of the moisture out in the open... away from the RV....

also... the temp in the burner is about 800 deg... with the cold air input... it has a hard time maintain that 800 deg in the bellows unit of the heater... so as to getting any voltage out of the heater burner... you would be cooling it down even further...

Best to leave it alone... and remember that the HP the elect motor develops.. which is converted to watts... that is converted to amps.. used... So if you want to have a more effecent unit... start with the blower motor... and instead of the large air exchange.. go for a more slower speed... less current... and let the temp sensor work in the burner/bellows are...to turn on and off the gas to the heater while it cycles..

Of course one could do a study about leaving the vent open due to CO from other heaters.. vs.. closed while using the existing furnace...

Either way.. your out camping... not doing effecency trials... grin...
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Old 12-17-2018, 02:21 PM   #10
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Be very careful. There is a high CO level in the furnace exhaust. You don’t want to wake up dead...
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Old 12-17-2018, 02:30 PM   #11
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The exhaust flue gases should have a temperature of about 350 degrees. This gets the products of combustion out of the heat exchanger so they don't rot out the heat exchanger. The present designs have about an 83% efficiency. That is 83% of the energy in the propane goes to heating the trailer and 17% is discharged to atmosphere. Airstream has addressed this concern and is now installing hot water radiant heat. This system is higher efficiency about 90%. A lot more money for that 90-83/83= 8.4% improvement. I have been winter camping this year and have actually used 320# of propane in one year. Probably 42 days with temperatures as low as 15 degrees and night and 50 during the day. So 8.4% savings of 320#'s = 26.88# or about $20 possibly saving for a $2,000 investment. Well it is better than interest in the local savings bank.

Just a thought: How much electricity can you get out of a K thermocouple? I would guess that you could easily solder 1000 thermocouples onto the exhaust pipe and might get enough voltage to power something without affecting the exhaust temps or flow. An interesting project when hiking, fishing, swimming, picnicing, touring gets boring.



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Old 12-17-2018, 02:44 PM   #12
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The Big Buddy heater is not a catalytic heater. It is a radiant heater and produces CO.
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Old 12-17-2018, 04:02 PM   #13
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My first AS was a 22ft. FB with a SMALL unvented furnace and electric tank heaters. It put out LOTS of heat for its size and propane usage, and the interior space was small enough that the whole place got warm quickly.

There is at least one marine propane stove which has a double layered exhaust pipe that draws in and warms outside air for combustion while the other exhusts the CO, water vapor, etc. Seems to be a little less piggy than a furnace where the pipes have to warm up before the air starts heating.

I'd choose this over a non-vented catalytic heater. Trouble is where? In a 25 Eddie Bauer. Thru the front window? Right over the propane tanks.... kaboom! Anyone?
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Old 12-17-2018, 08:57 PM   #14
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Ever look at the heat in the external exhaust on your home furnace? Lots of waste heat there unless you have one of the extremely efficient furnaces.
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