Author Topic: Passive solar air heater - for home  (Read 2020 times)

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Offline rossw

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Re: Passive solar air heater - for home
« Reply #15 on: May 23, 2017, 06:22:14 pm »
There may also be an element of scale at work here, Steve.
I suspect (no, I know) that your interpretation of space heating is different :)

Offline MadScientist267

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Re: Passive solar air heater - for home
« Reply #16 on: May 23, 2017, 06:29:06 pm »
Ok, yes, you're absolutely correct.

And I won't claim a propane pilot light or throttled back crock pot will heat a house... but one might be amazed at what can be done with not much.

The main thing is, "you know what they say about assumptions" ;D

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Offline eidolon

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Re: Passive solar air heater - for home
« Reply #17 on: May 23, 2017, 06:43:41 pm »
I made an apartment in a house I owned. In one room all I had was a thermostat relay, and a top burner element from a stove sitting on some bricks running on 120V.  The room was well insulated. The heat would not come on if I was watching TV.  That TV was about 150W.  It is all relative.  Your blue sky is not my blue sky.

Offline DJ

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Re: Passive solar air heater - for home
« Reply #18 on: May 24, 2017, 09:17:57 pm »

I doubt I'll convince you otherwise, but while I haven't gone along with the whole "tea light" thing in whatever cultish forms it takes, I can attest that the pilot light of a propane heater is a viable source of heat under the right circumstances.

Actually that would take no convincing at all.  I'm not smart but I can do a bit of research on the net and what I have looked at  tells me that the pilot light on a water heater is worth a good number of KW per day.
I wondered about this when thinking about how the surface area in the heater which is used to transfer heat to the water could also radiate it back the other way and had a nice Chimney effect to do that. I pondered if the pilot light would offset that and tried to find something on it and there it was.
I forget the numbers now but I do remember it was significant and much more than one would assume.

If this was an indoor heater as my old aunt used to have and the room was small, then this could easily make a difference to the comfort level of a place.
Co levels may be an issue in a confined space depending on setup but that's another story.

And that's the thing, You don't have to be smart with this stuff, any pelican like me can look it up and find the truth even if it is different to what most people would assume.

One thing I see and strikes me a lot is with Slow Combustion wood heaters.  People often Duct the flue out through a wall as soon as possible or straight up through the ceiling.  No matter how efficient the firebox of the heater, there is still a HUGE amount of energy in the flue.  -IF- I had one of these heaters, I'd run the flue to the other end of the room and back along the wall and then go up. I'd also put a small fan blowing along the length of the pipework to make sure the max amount of heat was extracted.

I verified my belief on this last year. Went and looked at a house where the owner had ducted the flue straight up from the ground floor  through the floor in the first story and then out the roof instead of going out through a wall.  He'd put a piece of stainless ( nicely Polished) around it leaving a gap forwards and had a couple of small fans on the back of the metal.  The heat coming off that flue into the top floor was warming it beautifully.  I think a lot of people stopped and looked at that and got a bit of an education. :0)

Quote
All that said, I'll leave it up to you to figure out how a whole 25W of battery power at night can let me cut the main propane burner off at night and let the main box cool down to save energy (and by extension in that case, *money*), all while making the difference that allows me to remain cozy in the process.

I guess we may get into semantics here but on this we may have a difference of opinion or terms.
And I have not looked this up but pretty sure the numbers would prove that 25W, the  heat output of a tail light globe or around that of a candle, would not raise the temperature of a room in degrees as displayed on a thermometer on the wall,  even a single degree in an average 9x9ft room with typical and realistic airflow.

Now, If you are using that 25W in heated clothing such as they wear on Motorcycles or for hiking,  directly convecting that heat  to your person, Different matter. As you mention battery power and I don't take you to be a person that believes in any of this fantasy over reality make believe, I suspect thats what you may be talking about and it is a realistic and creditable outcome.

I have asked all these proponents of tea light candles I have engaged with to tell/ show me how many degrees they raise the temp of a room on a thermometer.  Not a single one has done that.  Warming to me is doing that, demonstrating a temp on a devise made for that purpose. A thermometer.
If the source won't do that then to me, it's not doing it, belief is. If I can stick a thermo in say electric clothing and the thermo goes up, then to me that's realistic and factual.  Electric blankets on beds are low powered as well but they too can keep you toasty warm at night and again, if you threw a thermo in the bed and measured the before and after temps you would see a difference.

Someone saying something does keep them warm but can't be demonstrated with a thermometer to me is just pure and utter crap.

I remember some years back going away for a baseball Tournament for my son. We booked this quaint little guest house just round the corner from the place that was also quite cheap. Unfortunately the place had no heating and the weather was getting down to a degree or 2 above freezing. Anymore than a single fan heater would trip the breaker and that wasn't nearly enough to heat the place to any level of comfort. After the 2nd time of resetting the breaker, I looked at the fuse layout and saw while all the lights and power were on one circuit, the Stove and bathroom heater were on separate circuits.

Came in, opened the oven door and turned that on and all the elements as well. Went in and turned on the bathroom heater and the place did start to become more comfortable.  After a couple of hours the place had come up to 20oC so I backed the stove off a bit and stabilised the place at that.
as said, heat is heat and it's cumulative whatever the source.


A friend of mine in AC was telling me about a chain of Jewelry stores he had as a client. He had done the AC in a number of their shops some years back but they were now getting complaints the places were too  hot even in winter with the AC on full tilt.  He couldn't understand it at first how multiple units must be failing all at once. 
When he went to look at the problem, he found they had remodeled said stores and were now running up to 20KW of incandescent lighting in these places completely changing the heat load he calculated when he installed the AC. What he had put in was now over taxed on the lighting let alone any other outside heat sources like people and weather.  He said he always leaves margin for the inevitable overly hot days etc but  more than doubling the lighting load was not something he took into account.
They didn't want to cut back on lighting or go to fluros so he installed another AC unit. Not cheap in a couple of the stores as new mains had to be put in to meet their power demands.

Hate to think what the power bill in these shops must have been!

Offline rossw

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Re: Passive solar air heater - for home
« Reply #19 on: May 25, 2017, 04:05:48 am »
-IF- I had one of these heaters, I'd run the flue to the other end of the room and back along the wall and then go up. I'd also put a small fan blowing along the length of the pipework to make sure the max amount of heat was extracted.

I applaud your enthusiasm, but some of what you say is badly tainted with lack of understanding of real-world facts.
There is a reason gas-fired flues are ducted out as soon as practical, and although you may extract somewhat more energy, there is a limit.

Combustion products of many readily available fuels include sulfur and water. If you cool them too far, the water condenses from a reasonably safe vapour to liquid... and captures sulfur. Result: H2SO4. Sulphuric acid. It'll eat out your heat exchangers, flue, duct etc in no time. Oh, I suppose you could re-plumb it in something that won't be affected, but it's an interesting balancing act - withstanding heat AND acid. And cost effective. And not too high an expansion coefficient.

We need to balance all our grand ideas with reality and practicality.

Offline DJ

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Re: Passive solar air heater - for home
« Reply #20 on: May 25, 2017, 07:00:04 am »

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.


Offline MadScientist267

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Re: Passive solar air heater - for home
« Reply #21 on: May 27, 2017, 02:08:28 am »
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.
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Offline DJ

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Re: Passive solar air heater - for home
« Reply #22 on: May 27, 2017, 03:20:18 am »

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.

Offline MadScientist267

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Re: Passive solar air heater - for home
« Reply #23 on: May 27, 2017, 04:17:54 am »
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.


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Offline frackers

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Re: Passive solar air heater - for home
« Reply #24 on: May 27, 2017, 06:33:37 am »
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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Offline eidolon

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Re: Passive solar air heater - for home
« Reply #25 on: May 27, 2017, 08:22:59 am »
Older than that. Pioneer chimneys were massive so they could heat up all day and radiate through the night as the fire waned without maintenance through the night.

Offline DJ

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Re: Passive solar air heater - for home
« Reply #26 on: May 27, 2017, 09:51:54 am »
Older than that. Pioneer chimneys were massive so they could heat up all day and radiate through the night as the fire waned without maintenance through the night.

The russian Fireplaces being a prime example.
A ton or more of thermal mass designed to extract a lot of heat from the fire and store it.  A long criss crossing path for the exhaust gasses to traverse and give up a lot of the heat normally discharged and wasted to atmosphere.

I don't know if the emissions were room temp but they were certainly a lot cooler than in conventional designs and massively more efficient that's for sure. .

Offline DJ

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Re: Passive solar air heater - for home
« Reply #27 on: May 27, 2017, 09:57:47 am »
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,

No, not thinking of cooling it that much and I suspect it would be actually quite difficult to do that through a conventional flue due to the boundary layer of the gasses and other thermodynamics. I also think that would take an impractically long amount of ducting.
That said, there would certainly be a LOT of heat that could be extracted before the gasses cooled enough to generate the condensation problems.

As mentioned, commercial heaters MUST have a flue temp of 40o or below to gain the max efficiency ratings so it must be possible to get this sort of efficiency without major problems.
When I get a chance, I'll try and look this up to see what I can find. always intersting to find the facts on this sort of thing because rarely is it what you would think.

Offline MadScientist267

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Re: Passive solar air heater - for home
« Reply #28 on: May 27, 2017, 02:35:52 pm »
Thing is, you're talking about taking it down to 40C... and condensation happens *during* cooling.

All comes down to the amount of water produced... which for wood, again, I can't say exactly what the dynamics are, but with propane, it's non trivial. However they're dealing with it, the only thing certain in my mind is that they *are* dealing with it hehe
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Offline lighthunter

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Re: Passive solar air heater - for home
« Reply #29 on: May 27, 2017, 03:03:07 pm »
The heat exchanging isnt as difficult as building a combustion process (at least for wood) that burns very hot and clean. Clean burn is important otherwise heat xchanger will plug.  If i remember correct a company called "garn" uses a refractory ceramic tube to finish the hydrocarbon/oxygen reaction before water immersed flue in their owb. Probably the simpest design ive seen. I dont have so i cant verify. Many others use downdraft to get the combustion temps to 2500F, some of those can b a headache.
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