DIY ventilation heat exchanger

[AC_Hacker]

I'm working out the details of the design now.

From reading product literature, it looks like condensation will be an issue. There seem to be various schemes involving sensors and circuitry and "drying cycles", but for now I'm going to plunge ahead and ignore condensation to see just how big a problem it really is.

Since my realization that I have greatly over-sized my core, I have decided to go with a dual-core design. I'll divide the core stack in two, the cores being in series, with the air passing from one core to the next. This will double the distance over which heat exchange will take place, and should approximate a counter-flow heat exchanger.

So the next design issue will be duct sizing... I recall that Piwoslaw posted this:

Originally Posted by Piwoslaw

As for air flow, I believe the minimum is 0.5 house volumes per hour, the maximum is either 2 or 4 house volumes per hour (don't remember). When replacing more than 2 (or 4) house volumes per hour the heating/cooling/recovery system may not be able to keep up, leading to noticeable drafts and energy loss. Also, the ducts should be sized to keep air speed below 5 m/s to keep noise down.

There is a wonderful little program called CurveExpert, that is true shareware and does a great job of curve fitting and formula generation.

I tried inputting value tables from HVAC sites, in order to generate an automated formula for you, my reader. Unfortunately my first attempt failed so I found some online calculating pages that will calculate duct sizes from CFM.

Here one for metric

Here's one for US measurements

And here is a table just to double check your results.

The issue here is that if the air in the duct flows too quickly, it goes into turbulent mode and can make a lot of noise. A friend of mine re-did the ductwork in his house and neglected this issue. Silence really is golden.

The ventilation rate I previously calculated was about 15CFM, at least that's what I rounded it up to.

Using the online calculator for US measurements, and using the value of 15 CFM, and using PVC as my duct material, I get a diameter of 2.85 inch ID.

So anything bigger than 2.85 inch will be nice and quiet. Three inch looks to be a winner so long as I keep my runs short. Four inches will just cost me more money and it will require more insulation to be efficient.

Bigger isn't always better.


Regards,

-AC_Hacker

PS: Next is fans

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[Xringer]

I've been thinking about how to install a unit, so the core is sitting with
the (filtered) outdoor-air-input (carrying the moist air) at the best angle.

I would want the (moist) air coming into the top of the core, so it will
have gravity (and air pressure) helping push the water droplets down and out of the core.
(Into a drain pan with a hose on the bottom).

The longer water is allowed to sit in those passages, the more likely
that water is going to collect airborne dust particles, pollen & etc.

Once a water droplet filled with particles evaporates, the residue
left from the contaminates can build up and be difficult to remove..

The dew point temperature have been so high these last few weeks,
anything that is cold is instantly wet when left out..

~~
Instead of making more holes in my house, I've been thinking of building
my unit like a window AC. (Custom sized for my windows).
So, it could be installed easily and easily pulled out to be tweaked or cleaned.
The drain hose would just water the flowers below the window..

I like the idea, but how am I going to get that good drain angle with a window rig?? Humm...

[Piwoslaw]

Originally Posted by AC_Hacker

PS: Next is fans

I've read that centrifugal or crossflow fans are most efficient for household ventilation. Also, they should be DC powered:

• AC powered fans "hum" at the grid's frequency and may cause the whole installation to vibrate,
• AC fans can cause electromechanical interference,
• DC can be easily tied into an RE or back-up power installation.

[AC_Hacker]

I came across information about a Passivhaus (AKA: Passive House) that was built in Illinois.

• Here is a link to an HTML site.
• Here is a downloadable PDF.

The house uses an HRV that is almost the size of a furnace, but uses only a tiny fraction of the energy (NOTE: there is an integrated 1 kWatt resistance heater for severe occasions).

This is the first time I have seen the kind of serious HRV that a Passivhaus would utilize. This really alters the definition of HRV... Now all the others seem like toys.

In this particular project, the Westaflex WAC 250 HRV utilizes a 100 foot ling, 8 inch diameter buried earth tube. The WAC 250 unit has sensors and dampers and can switch the incoming air between earth tube air or straight outside air as the situation demands (wow!).

This PDF has the picture shown below

It looks like there is a real-deal counterflow heat exchanger at the heart of it all (or could it be a dual-crossflow?).

I think I finally understand why someone would build a house around a HRV, especially if it was one of these.


Regards,

-AC_Hacker

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[Daox]

Wow, that is sweet lol. Thanks for posting that info. I've seen that house before and love it. I didn't know of the monster HRV in it though.

[AC_Hacker]

Originally Posted by Piwoslaw

I've read that centrifugal or crossflow fans are most efficient for household ventilation. Also, they should be DC powered:

• AC powered fans "hum" at the grid's frequency and may cause the whole installation to vibrate,
• AC fans can cause electromechanical interference,
• DC can be easily tied into an RE or back-up power installation.

Yes, quite right.

At this point, since my application is so undemanding, I'm considering a pair of 3 inch fans.

But I'm also looking into cross flow fans (AKA: tangential flow).

I also investigated Pulse Width Modulated fans...

...but the energy used by the above fans is so small that energy savings would be very small.


Regards,

-AC_Hacker

[Xringer]

Since I'm thinking small (and no new holes in the walls), I looked around and found this.
Amazon.com: Bionaire BAP336-U EverFresh Air Exchange System: Kitchen & Dining

I'm not sure how this works, and the from factor is 'out there',
but it shows that my idea of a DIY window mounted unit, wasn't very original.

But that's not going to stop me from trying to figure out how to make one..

[AC_Hacker]

Originally Posted by Xringer

Since I'm thinking small (and no new holes in the walls), I looked around and found this.
Amazon.com: Bionaire BAP336-U EverFresh Air Exchange System: Kitchen & Dining

I'm not sure how this works, and the from factor is 'out there',
but it shows that my idea of a DIY window mounted unit, wasn't very original.

But that's not going to stop me from trying to figure out how to make one..

I noticed that the latest Mini Splits are featuring "Fresh Air".

Search for the "Fresh Air" term, download the appropriate PDF, you might be able to do a "Fresh Air" hack on yours.

-AC_Hacker

[Xringer]

No way my wife is going to let me touch that Sanyo! She loves that thing!

Plus, there is no venting as you well know.. Only an Ion generator (whatever that does).

Your dual core diagram above has given me a clue about core mounting
(in a window unit) that will allow the core to drain..

[AC_Hacker]

A design that is different from the cross-flow types.

Klingenburg GmbH - Energy Recovery - Counterflow plate heat exchanger

Appears to be very similar to the core used in the Westaflex mentioned earlier.

Don't overlook this link, it has lots of interesting literature.

Here's a grab from one of the the documents above.

The company appears to be in the business of selling high efficiency counterflow cores.

Regards,

-Jim