Renewable Energy Questions/Discussion > Solar (heating or electric)

Passive solar air heater - for home

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DJ:

Yes and you can measure that Condensation point so you avoid it. Also burning good Timber with a low moisture content and burning it hot rather than having it starve for air and giving off tars etc also helps eliminate the problem. There are additives you can put on the fire also to help with the cleaning.
You don't have to have the flue stone cold but there is a heck of a lot more energy available than what most people harness.

As far as gas goes, Modern heaters to get best efficiency ratings must have a flue temp of 40o C or under. They often have PVC flues.

MadScientist267:
I haven't directly messed with high temp exchange for exhaust, as the truck uses a ventless system that is very efficient (tho I apologize up front, I don't reveal the details on that particular design, not to keep secrets, but because I'm breaking just about every rule for LPG, and while I have no idea the extent I could ever be liable, I'll let the "destructables dot something" work that out lol)... anyhow, not the main point...  ::)

It brings two things to mind... I'm not sure where the sulfur part comes in, but I can say "American propane" as I burn it, there's no sign of it in there. I only pick up on sulfur when I'm equalizing the batteries. There is however, definitely plenty of water... and I still need to work out exactly where it goes with the very latest scheme, but I can tell you that it no longer rains inside, at the very least.

That said, and really mostly to feed into the original physics of air to air heat transfer, I've found, much to my dismay in this case, that PVC can make a rather "efficient" air to air interface. I ended up exploiting and using it to my advantage, but as originally intended, part of its purpose was to circulate air from the floor to the ceiling, which it will do, but it's heat content is gonna change non-trivially by any stretch, if there's any kind of delta to outside.

Ok, so I told you that to ask you this... It would seem the trick would be then cooling the exhaust just enough to keep the poly safe, and then let the condensation "run wild" in a downhill slope... You're cooling the gas as it travels, if done right, I'd imagine both would flow just fine and leave the outside vent with all natural physics, so no moving parts either... The question, lol: "Yes?"

[MOD NOTE:]
I might see about forking this thread somewhere back because it's all good stuff, I'm all about deep critical thinking... but we've derailed this one pretty good, and I myself for one would actually be very interested in what might come of the solar-centric version originally posted as well.

DJ:

Condensation only occurs when one medium is cooler than another.
There may be some condensation at startup but that would quickly dry out as the flue in this case heats up. Same effect as a car exhaust.  Also like a car exhaust there is likely to be a layer of carbon on the metal which will stop or reduce the direct contact of any formed acids with the metal.

 I would also suggest that any moisture is in the flue and the heater and once driven out will not keep developing.  After warm up where the moisture is quickly driven out,  I see no process that is going to create a steady stream of moisture as seems to be proposed.
I know Brivis gas heaters have a flue temp of 40o c or under and don't create any substantial condensation and the heat exchange cores are made from Mild steel.  I had one which was many years old and it was in fine condition only replaced because of the better cost efficiency of reverse Cycle AC.
Any flue is going to get condensation on start up but in the as case of a car exhaust, no matter how much you cool it once running it's not going to be dribbling water like a tap.

Commercial heaters disprove the condensation acid theroy.

MadScientist267:
You're forgetting a very key component of combustion, which very most certainly continuously produces water... the key attribute to most fuels we burn is that they are all hydrocarbons in various forms... looking up wood, specifically, I quickly found this basic core equation, via big G fu "wood combustion process":

Organics (CH2O) + O2 ?CO2 +H2O + Heat

This applies to all of them, appropriately, say, propane:

C3H8 + 5 O2 ? 3 CO2 + 4 ?H2O + heat

... you get the idea...

Yes, there is water, it is generated by the combustion process itself (in *addition* to whatever liberation from previously existing water trapped in say, wood).

You're correct in that the steam will not condense on a surface that is above a momentary sample's dew point temperature, but if you are talking about bringing superheated steam in exhaust gases thru a long pipe where it reaches room temp before it leaves the pipe, all of the moisture generated by the combustion that is taking place, will condense somewhere along the line as it loses its ability to hold the water anymore at the lower temps. Standard flue pipes, like car exhaust, don't stay cool enough to allow this condensation to form, with the one exception (as you said): when it is starting up cold.

If you took that exhaust pipe and extended out in a zigzag or even long straight line, eventually the exhaust will give up all the heat it is going to, (only limited by the outside temp and length of the pipe), and in the process, release all its water vapor because it can no longer hold it in the cooled stream.

What you then have is cool carbon dioxide + liquid water (ignoring any other additional components and contaminants that might be present as well).

That water will go *somewhere* and not if, but *when and as* the heat gets extracted, whether that's inside the pipe, or after it has left the vent and cools sufficiently while mixing with the cooler outside air.

frackers:
Its common here for a wood burner flue to have a clip-on heat radiator between the stove and the ceiling - mine certainly has.

I've seen older (euphemistically called character) homes with a log burner or open fire in each room that gets lit in April and burns all through winter to October without a break. In all of those, a chimney rather than a flue was the idea of the day (pre 1st war) which means there wasn't much gain from the exhaust gases (and being brick most fell down in the earthquakes!!).

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