# air to air heat exchanger



## gobug (Dec 10, 2003)

Moonriver's thread about solar air panels has gotten me thinking. I asked Solar Gary how often air should be refreshed. The idea of taking the warm air from the inside of a habitat and throwing it out got me thinking about air to air heat exchangers. I have looked for those on line in the past. They are available.

Does anyone here use one?
Has anyone made their own?


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## gobug (Dec 10, 2003)

I just read that I should be replacing air inside my small mtn living space (if it is airtight and super insulated (NOT)) about once an hour to insure it is clean and healthy inside the living space. 

If true, that increases the importance of air to air heat exchangers.


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## gobug (Dec 10, 2003)

further inquiry turned up a goverment recommendation of 6 air changes per day in a household of 4

there are also 2 books of ventilation requirements for EPA, apparently to qualify for some goverment subsidies one must comply. ASHRAE 62.0-2010 and 62.1-2010 are the rules.


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## Jim-mi (May 15, 2002)

All the air to air units that I have seen are very spendy.......

And they are built to handle far more air then that 'pop can' thing will produce.


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## PastTense (Mar 22, 2010)

I would expect that 95% of the readers here have the opposite problem--that their houses are not tight enough, so they are having too many air exchanges--resulting in higher heating and cooling bills.


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## Ross (May 9, 2002)

HRV's are great for airtight houses but kinda wasteful everywhere else. Not terribly efficient either, the air exiting is warm and the air coming in is cool. All they have to exchange heat is a small aluminum core abour 8 inches square...... or cubed. Maybe you need an air filtre.


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## gobug (Dec 10, 2003)

Air filters are common on the heat exchangers I could find. There are many areas in specifically older houses that leak air. They are actually more dangerous to personal health than a tighter house (according to the EPA), but both still need filtered fresh air. Good air circulation from every room is an important component in a good air to air heat exchange system. There is no doubt it is especially good to eliminate all kinds of pollutants that are created inside the house. Now bathrooms and kitchens are supposed to be vented outside. This is all new to me.

I have drawn a home made air to air heat exchanger plan using pop cans which will be placed in a window well. Some of those on the market can be very expensive and very heavy. The design is not shared, so I don't know a lot about their expense. I recall a small one for a house, not a big building, which was about $250 on ebay some years ago. I haven't looked again there, because I am not ready.

This whole issue is getting to be more interesting to me. Envelope houses are essentially giant air to air heat exchange systems.


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## Windy in Kansas (Jun 16, 2002)

Ross said:


> HRV's are great for airtight houses but kinda wasteful everywhere else. Not terribly efficient either, the air exiting is warm and the air coming in is cool. All they have to exchange heat is a small aluminum core abour 8 inches square...... or cubed. Maybe you need an air filtre.


Hm, I thought I had read of 85%+ efficiency so I just looked at the site for one company and their specs show 60% and less depending up which model is chosen. A model with only 40%+ would indeed be pumping a lot of heat outside, so would a 60%+ model.


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## cmcon=7 (Mar 7, 2010)

I designed one 12 years ago and sold plans in popular mechanics, I'll put up a web sight and post a link in the next few days.
My design uses cardboard and is better than 95% efficient, I put a 1000 watt hair dryer in one side and the output was room temp.
It uses the holes in the corrugation for one air Chanel and the smooth outside for the other in a cross flow arrangement, while paper is not the best heat transfer material, the vast surface area makes up for it, and cardboard is free, you just need a few computer fans for a window unit. It is easy to build to the size/shape you need.


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## Off Grid Rving (Jul 25, 2010)

I have been designing in my head a solar air heater with a propane backup on thermostat.

not totally the same as an air 2 air heat exchanger as far as pulling fresh air from outside,

I plan on pulling air from the floor area and exhausting to the ceiling of my RV.

I will mount a thermostat inside the RV and have a propane burner/pilot light in a burn chamber outside of the RV and exhaust through a flue in the solar air heater box.

so when sun is not shining then the propane heater will be heating the air in the solar air heater box as an air 2 air exchanger what do you guys think


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## gobug (Dec 10, 2003)

cmcon - thanks for the reply. 
You say 95% efficient. How do you measure efficiency? Was the 1000 watt hair drier involved in your measurement? 
The air2air exchangers I have seen online have 2 inlets (fresh and warmed) and 2 outlets (warmed fresh and cooled house air). So if your 1000 watt heated air was cooled to room temperature, what temperature was the household air out?

Did you use a fan? What watt and cfm? You said "vast surface area", how big is vast?


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## gobug (Dec 10, 2003)

Off-grid
I suggest that you strive to avoid propane as a backup. There are other possibilities. If you use 2Liter plastic bottles filled with water and stored in the small crates, then warm the bottles from the energy of running the engine or solar exposure, it may be warm enough to avoid burning propane. Now if your RV does not move, or rarely is on the road, a different approach should be possible without propane. Propane worries me because of carbon monoxide. Ventilation becomes a requirement. Describe more regarding your circumstances.


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## SolarGary (Sep 8, 2005)

cmcon=7 said:


> I designed one 12 years ago and sold plans in popular mechanics, I'll put up a web sight and post a link in the next few days.
> My design uses cardboard and is better than 95% efficient, I put a 1000 watt hair dryer in one side and the output was room temp.
> It uses the holes in the corrugation for one air Chanel and the smooth outside for the other in a cross flow arrangement, while paper is not the best heat transfer material, the vast surface area makes up for it, and cardboard is free, you just need a few computer fans for a window unit. It is easy to build to the size/shape you need.


Hi,
That sounds interesting -- looking forward to seeing the details.

Gary


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## Off Grid Rving (Jul 25, 2010)

right now I have a 10,000 btu blue flame propane heater installed in my travel trailer. 

I am living in this tiny house that I built from scratch for the next year.

I want to get the combustion process outside of the travel trailer.

so I am thinking if I build a solar air heater and use it as my air to air exchanger either the sun will be heating the air for me or the propane will. and it will exchange the heated air in the panel through the thin walled aluminum downspouts to the air circulating into the travel trailer.


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## SolarGary (Sep 8, 2005)

Off Grid Rving said:


> right now I have a 10,000 btu blue flame propane heater installed in my travel trailer.
> 
> I am living in this tiny house that I built from scratch for the next year.
> 
> ...


Here is one home that uses a solar preheater:
http://www.geopathfinder.com/9535.html

Gary


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## cmcon=7 (Mar 7, 2010)

I hope you can figure out how it works from these pictures, I'm not to good a explaining. But I will given enough time.


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## gobug (Dec 10, 2003)

Cmcon-7
Thanks for the pic and posting. I think I see how it operates. I cannot read the small print on the picture though. I cannot see if any dimensions are included. If the exchanger has three corrugated levels, and is (say) 4x8 then that is slightly less than 100 sq ft. That does not seem vast to me, but who knows? What is the separation distance between levels?

I see how air out and air in have overlapping pathways. I do not see how a computer fan or 2 would cause the air to equally distribute through the cardboard corrugations. I also do not see how the fan would get all of it's air flow into the 3 levels of corrugated cardboard and not elsewhere. The air flow volume is important to me understanding your design.

Two fans or 4 fans? If two, is one out, and one in? Computer fans have a wide variety of cfm's. What are yours?

I was imagining 4" thick (computer fan size) of carboard layers with the corrugations all facing the inlet. I could not imagine how crossflow incoming air would wash over the currugated air inlet cardboard between each layer unless the cardboard layers were alternating. Then heat transfer would have to go through 2 layers of paper for each layer of airflow in or out.

Otherwise your air2air heatexchanger is fairly standard in the design you have shown. That is not criticism, just a question to further explain.

Thanks again for your reply
gary


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## SolarGary (Sep 8, 2005)

cmcon=7 said:


> I hope you can figure out how it works from these pictures, I'm not to good a explaining. But I will given enough time.


Thanks cmcon,
Please let us know what the website is when you get it up.

Does the cardboard hold up OK in service?
Do you use a special cardboard?

Gary


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## cmcon=7 (Mar 7, 2010)

One path for the air is through the corrugations, the other is on the outside of the cardboard. I got the idea while watching some cardboard burn, and the smoke was jetting out the corrugations, on a lark I blew through the cardboard and met little resistance.
My first unit was 1x2'x4" thick with 16 layers which makes for 64sq' of surface area, this was the right size for 4" computer fans, there is very little air resistance.You could easily build one to use 20" box fans or blowers.
One other nice feature is the condensation if any is wicked through and does not build up like it would in metal or plastic.

Appliance store cardboard is good because you can get very large pieces. Only use clean unbent stuff, and use a sharp knife to cut the corrugations clean so as not to impede air. I used 2 fans, you can scale this up or down to suite your needs, I think 2 length to 1 width is a good dimension for efficiency, thickness would be determined by the fan you use.
The print in the middle says" imagine the air coming out of the page through the corrugations and the air going into the page between the cardboard spaces".


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## fishhead (Jul 19, 2006)

gobug said:


> Cmcon-7
> Thanks for the pic and posting. I think I see how it operates. I cannot read the small print on the picture though.


Go to the bottom right on your screen to see if you can zoom in on the pic.


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