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Posts: 2,240
Reply with quote  #1 
I finally decided that my ventilation system deserves an upgrade...

All feedback welcome, preferably before I start hacking up my Styro Air Collector (-:


Attached Files
pdf AIR-AIR_HEAT_EXCHANGER_FOR_HRV.pdf (66.41 KB, 129 views)

(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...


Posts: 135
Reply with quote  #2 
Looks like a decent design to me, certainly easy to implement.

If you could find a way to achieve counter current heat transfer that is generally most efficient and typically the way mother nature does heat, or moisture recovery in living systems. For example a camels nostrils are designed to recover over 90% of the moisture during exhalation/inhalation If I recall my high school biology correctly. I believe commercial units use a vapor permeable material in the exchangers that allows moisture to exchange as well as heat. This reduces condensation issues inside the enclosure as warm interior air cools. Here in Colorado humidity is a good thing in winter, so preserving it is a significant benefit. I'm sure in other places it is not the case.Typically humidity is very low here, indoors and out.

Perhaps a duct-within a duct could be devised where the hot air is passed out through a duct that is located inside the center of the fresh air intake duct, where the fresh air is traveling the opposite direction. Or vice versa might be better with the exhaust the larger pipe? I don't see a real advantage yet either way the heat transfer surface area is the same. That area available for exchange is small in such a design and extending the length may be necessary for a good exchange efficiency.

If the sizing can be arranged for approximate equality that would probably be best. I don't know how they size ducting over on the isles but a 12" duct has an area of 36xpi inches, an 8" duct has an area of 16pi, slightly under half but in the ball park. 4/5 ratio. Same ratio is seen with a 6"duct area of 9pi, compared to a 4" duct area of 4pi.

Mattie pointed in another thread to this link from Gary that should be added to this topic too

In this fine example, as many commercial units, they use cross current flow, different than counter current, but still similar principle of passing the hotter exhaust first over the air that is about to enter the living space, then on down the line until the coolest air about to exit is passing by the coldest air just entering. I think the main reason for the cross current flow as opposed to counter current is that they can accommodate a lot of exchange surface in a very compact space.

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Posts: 283
Reply with quote  #3 
Heres an interesting thread regarding HRV i have not had a chance to look through it all yet, it certainly sits well with the DIY ethos of this forum.

Theres a more compact version of the double pass crossflow HRV here on Kostas post.


Hopefully can give others a few ideas.
Regards Mattie

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Posts: 2,240
Reply with quote  #4 
Hi, Joe,

Thanks for the feedback and kind comments !

If the sizing can be arranged for approximate equality that would probably be best.

Here is how I worked out my tube sizes:


Cross-section of box = 25 cms x 60 cms = 1500 sq.cms

÷2 = 750 cms for fresh air and 750 cms for gas

taking 3 pipes : each pipe would therefore be around 250 cms2 in section

250  ÷ π = 250/3.142 = 79 = r2  √ 79 = 8.9 x 2 = dia. = 17.8 cms

say 17 cms dia : /2 = r = 8.5 x 8.5 = 72.25 x π = 227 cm2 x 3 = 681 cm2 for 750

error = -10% = near enough...

So use 3 x pipes of dia. 17 cms.
I agree that counter-flow would be optimal, but I was just aiming for a bog-standard design for evaluation, that would gain me even just a degree or two.



Thanks for the additional links - it is all grist to the mill [smile]

(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...

Posts: 135
Reply with quote  #5 
I thought you mentioned, perhaps is the SHCS thread, a 3*C deltaT? that hardly seems worth trying to recover to me, since I have to deal with a deltaT of ~46*C (-28/+18). It will really be hard to get more than a degree out of such a small differential.

Posts: 243
Reply with quote  #6 
Hi GH,
A couple more to look at.

Another one:

And Another: 
This is an air to air heat exchanger I did to attempt to capture heat in the clothes dryer exhaust stream.
Some test data here:

While I built it just as an experiment, it continues to work well in its 2nd year.
I used some scrap (used) sheets of twinwall polycarbonate, but I think that coroplast would work at least as well.  It does provide a lot of heat transfer area.

This reference listed on the page above was the basis for it -- a good paper:

I don't see why it would not work as an HRV for a home.

If the pipe one you had the diagram of provides enough heat exchanger area to work well, then it seems like a simpler approach, but if it falls short on heat exchange area, then this one is also pretty easy to build.  


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Posts: 2,240
Reply with quote  #7 
@ Joe K
Yeah, Joe, you're right - I am hovering between 2 and 3 degrees °C difference (the house is generally at 20 or 21, and I have "caught" (!) the waste stack chucking out 23-degree C air - what a waste !)

So there is definitely a BIG difference, there - 2 or 3 degrees C is enormous !  The CMV runs all day (generally) (except if we have the wood stove on...) - so that is a fair few cubic meters of air and lots of BTUH being thrown away there...

Like I said, it is a trade-off between having a healthy level of humidity for breathing, health ROOF TIMBERS ! and a healthy electric bill !

@ Gary @BIS

Wow, thanks for all the reading, Gary ! 

And I never even *thought* about our dryer !
Will be turning my attentions to this, ASAP !
Really liked dryer installation !

Now I have a choice of either catching up on all the reading, or finishing off my in-progress works !

(guess I get to do both)


(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...

Posts: 38
Reply with quote  #8 
Here is a simple modification I made for my Electric Dryer heat and moisture recovery during cold dry weather.

Add a 4" diverter valve to the main dryer exit pipe in the basement ceiling. Add a small length of 4" hose and tie on a pair of ladies panty-hose. Problem solved.  It adds considerable moisture and heat to the winter air. In the warmer months the dryer air should be vented outside to keep the humidity out of the basement. I have a dehumidifier in the basement and typically maintain a >50%  humidity level the in the summer. My heat recovery system does draw some comments for visitors.

This should be done only for electric dryers!! Gas dryers must be vented to outside air.

Jim from IL

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Posts: 2,275
Reply with quote  #9 
Adding a bit of extra moisture to the home in winter is usually good thing. The biggest concern I would have would be the added lint and dust this would add to the home. All dryers have a lint filter, but an awful lot gets by the screen and is still vented outside. Diverting this air, even through a nylon stocking, would still not filter out the smallest particles. You could check this by using a flashlight in the dark. Remember though, wet or damp clothes won't produce lint until they are partially dry. Good reminder not to use with a gas dryer.

Greg in MN

Posts: 137
Reply with quote  #10 

Valid concern about the fine dust. But I have been doing what Jim mentioned in the crawl space for 10 or 12 years now, with little to no visible dust build up in all that time. The thing to remember is to use the double legged panty hose not single leg stockings, for the extra surface area. The panty hose are quite stretchy and hanging limp the holes are quite small. With the dryer running the PH inflate just enough to take the wrinkles out. Each time the dryer shuts down most of the captured lint falls from the sides down to the toe's. I clean it out once a year, but there are only 2 of us, a larger family may or may not need to do it more often. Or do what I did at my sister's house with 5 living there, that is to put a T on the dryer hose and two elbows so that there are two PH's hanging there.

Side notes.

Over the years there have been a number of house fires locally, blamed on the dryer catching fire. I can easily get my dryer outside. So once a year I take it out and use a 150 MPH leaf blower to blow back and forth through it to clean out any lint that may be hanging up inside of it. Then blow out the vent tubes and hoses.

The wife started complaining that the dryer was taking too long and not getting the cloths completely dry. I remembered one of those junk emails about dryer sheets plugging the lint trap. I pulled the lint trap and stuck it under running water, WELL I be darned, that screen held water not a drop got through :-o Soapy water and a nail brush cleaned it up and it works again. I wash it twice a year. OH Yah, We never have used dryer sheet.


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