DIY ventilation heat exchanger

[Ko_deZ]

Originally Posted by Drake

I have only found large commercial "wheel" ERV's in the US. Anyone know of a "residential" sizes one? Ko deZ what is the brand name? 93% of what? How can two passing air flows do more than reach equilibrium? The same with relative humidity of the air flows. Without a HP extracting it.

In the US, I have no idea. In Norway, Villavent are quite popular.
93% heat recovery. 0C outside, 20C inside gives me incoming air at approx 18.5C.
piwoslav got it right. Countercurrent flow. Check out heat exchanger in wikipedia, loads of good info there.

[Daox]

Originally Posted by Ko_deZ

Just poking my head in here too. This cross heat recovery unit has a major drawback. It sends moist air out, and dry air in. With a rotating wheel you will condense the moisture from the outgoing air, and evaporate it to the ingoing air, so you keep a decent humidity level indoors. Also, this helps against the unit freezing up. If only the outgoing air is dehumidified, there will be a build up of water, which might freeze if the outdoor temperature is cold enough. We regularly have -20 here, so even the wheel based models have some issues. My unit is made by ener(.no), and they have two large aluminium fins and a 3 way air walve that directs incoming and outgoing air back and forth between them. To my mind this should work better when it is really cold, as the hot and cold side will alternate. Also, the unit should be somthing like 93% efficient when running in low mode.

-Ko_deZ-

Do you have any diagrams of how a unit like this works? Its pretty easy to see how the others work, but I'm not understanding a rotating wheel heat exchanger. I'd certainly LOVE to have a 93% efficient unit though!

[Ko_deZ]

Indeed I do: Thermal wheel - Wikipedia, the free encyclopedia

There are brushes between the two sides that helps avoid that much air is resirculated. Poor maintenance of these often results in bad odors returning back into the house.

It is very important that you position the input and outputs at the right places, and that you make extensive use of noise traps on all incoming places (bedrooms, living room). Usually it is enough with two noise traps in series at the heat recovery unit shared by all outgoing places (all bathrooms, washing room (because of damp air) and kitchen. Have a separate output for the cooking place with a non return valve on it in addition). Make sure that there is a little lower pressure inside the house than outside (pull fan and exit valves more open than for incoming). This to make sure that you don't push hot and moist air out trough the leaks in the walls where it will condense and give you rot and fungus problems. If outside is hot and humid most of the time, and indoors is cooler than outdoors, do the opposite.

[DoctorDoctor]

I actually have been thinking about this very problem, a diy, inexpensive air to air heat exchanger. I would like to do this for my crawl space. I am not a big believer in radon risks. I thought about doing a traditional heat exchanger with lots and lots of sheets of aluminum. One could do this in a rectangular duct.

But I thought that this would be simpler and very cheap: Take a three inch round aluminum vent pipe and put it in a four inch round vent pipe. Wrap it in insulation and make it pretty long, say 20-25 feet. The cold, outside air goes in the center pipe. The warm, moist inside air exits through the large vent pipe outside of the center (the "annulus" part). There would be condensation developing on the inside pipe, dripping down, so it would have to be inclined down somewhat with a collection pan.

One couldn't move the air too quickly, but it is my contention that with a sufficiently long pipe (again about 20 feet) the temperature of the air in the center pipe would be coming out warmed to almost the temperature of the inside air temperature.

Other variations: two three inch pipes in a six inch outer pipe. This would match the areas inside the smaller pipes with the outside remnant area. Or seven three inch pipes in a honeycomb pattern inside a nine inch outer pipe.

One would want the inner pipe to stick out somewhat so that one isn't sucking in what one is blowing out.

[Daox]

I think your solution would work, but how do you keep the smaller tube centered in the larger tube?

I also think the DIY rectangular setup would be superior just because you can have so much more surface area per amount of area the exchanger takes up.

[Piwoslaw]

Originally Posted by DoctorDoctor

Other variations: two three inch pipes in a six inch outer pipe. This would match the areas inside the smaller pipes with the outside remnant area. Or seven three inch pipes in a honeycomb pattern inside a nine inch outer pipe.

• I'd go with the honeycomb pattern, since it would have much more surface area than just one concentric tube. It may not be as efficient as the layered version, but may be easier/cheaper to build.
• Make sure that the tubes are spaced from the outer tube and from each other. There won't be good (if any) airflow/heat exchange in the vicinity of where they touch.
• Have the inbound cold air in the outer layer. That way the warmer, outbound air doesn't lose heat through the outer wall - more of it can be absorbed by the inbound stream.
• Of course, both air streams should be flowing in opposite directions. But that is obvious.

Originally Posted by Daox

I think your solution would work, but how do you keep the smaller tube centered in the larger tube?

Add fins/spacers. They would also slightly increase the HX area.

[Xringer]

I've heard of this kind of rig before. It used iron pipe buried with stone under it
and had drain holes drilled in the bottom of the entire run of pipe.
But, the system was a basement dehumidifier.

[DoctorDoctor]

Yes, I was thinking of using fins to hold the center pipe. My point is to use K.I.S.S. (keep it simple stupid). Sure one can design and make a air to air heat exchanger with near perfect numbers - outside air is heated to almost that of inside air and vice versa, so that no heat energy is lost. Surface area is indeed the critical factor for heat exchange. The ideas like the "thermal wheel" look great but no one is going to build them. If you want better heat exchange, just make the exchanger longer. Not sure that the honeycomb is actually better than just adding another section of aluminum vent pipe to make it longer.

One would probably want an air flow so that the entire volume of air in the crawlspace (or basement) is pumped out at least once a day. If one increases airflow too much, one doesn't get as good heat exchange. Also, the humidified air in the outside part of the larger pipe will start freezing on the inner pipe (if the ambient temp is less than freezing).

One issue is that one doesn't want the fresh outside air that you have just pumped in to be pumped out. So the out-take can't be too close to the intake. I was think that a 90 degree corner on the outer tube with a hole cut out so the inner tube goes through it. One would seal all around this hole. I wish I could draw a picture.

[DoctorDoctor]

Piwoslaw writes, "Have the inbound cold air in the outer layer. That way the warmer, outbound air doesn't lose heat through the outer wall - more of it can be absorbed by the inbound stream."

My reason to have the inbound cold but warming air on the inside and the warm and moist but cooling outbound air on the outside is that there would be condensation on the surface of the inner pipe. This would drip down and if the whole thing was slightly declined, one could collect it and get rid of it.

[ThomSjay]

I just had an idea!
Have the outer pipe on a slight decline as it exhausts the air. This would allow any condensation to drop off. Now, here is my brain wave, haha....the intake would be a single length of down spout pipe to give a slight pre-heat to the air before entering the inner pipe of the heat exchanger.
The only down side to this is in the summer when one wants to have air conditioning.