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Old 01-24-13, 10:09 PM   #351
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Hi again,

It has been a long while since i last posted to this thread, but i have often returned to read your contributions. This is really insipring stuff!

In the mean time, I have been installing my own HRV, although (as I explained in a post here a long time ago), my main goal is actually de-humidifying in winter, as I have (or had) problems with condensation and I could smell mold developing.

I first started building a HX countercurrent core using a combination of plastic frames and aluminum foil (not plates). This turned into a horrible mess. I don't think I will finish that project as I won't get it sufficiently airtight and the airflows would mix. If i ever redo this, i'll take the double cross-flow core approach and coroplast, I think.

What i did do, however, was to drill holes in my walls and simply install a tube-in-a-tube duct system, to get some ventilation (and de-humidification) with minimal heat recovery. I reconned that if I ever feel the urge to make a second attempt at building a proper HX-core, I would need the tubing anyway, and i hoped the tubes might work as a minimalistic heat exchanger while solving my humidity problem.

The picture below shows my situation at home (first time i sketch something in google sketchup, quick and dirty job).

The red bar is where i installed a 160mm diameter PVC tube, slightly tilted to allow condenstion to drain, containing a 110mm flexible aluminum duct. The space between both tubes contains the stale air. The entry point for stale air is indicated on the right end of the red tube. The interior tube with clean air goes through an extra interior wall, into the kitchen. Those two rooms are separated by an old three-piece leaky wooden door (from the 1940's), the upper part (say 80cm) is permanently closed, providing a kind of trap for hot humid air coming from the shower which is integrated with the bedroom. The total heat exchange surface is really small, 0.11m*pi*5m ~= 1.7m2. But on the other hand, this aluminum duct surface is very irregular; and thin.

This is all quite minimalistic and low-cost, but it performs rather ok, i find; at least for my specific situation. But let me make some points:

- I have only a 60m2 appartment, its just me and my wife living there, and we both work during the day.

- My appartment is not the most airtight, I have old wooden floors, not the newest windows, there is bound to be quite a bit of leakage.

So i am quite content to ventilate moderatly. If i ventilate on a low volume all day long, even when absent, i get reasonable heat recovery on those slow airflows, in spite of the small surface of the tube.

If my wife and I are around, cooking, showering, breathing,... higher volumes are required and i have to live with some cold air flowing in (although heat from condensation seems to make up for a lot). Not that we ever feel a cold draft or so. To me, that is just the price to pay to get rid of that humidity problem, its not that i was planning on saving on heating costs or anything, quite on the contrary.

I wired everything up with four sensors (cheap DHT22), hooked them up to an arduino with a wifly shield, and programmed it to send all data to my router running dd-wrt. There, the data is stored, and i mess around in php, determining the return value to give back to the aruino to tell it how fast it has to spin the fans for some given sensor values. Having a router making the decisions on how fast the fans should spin has the advantage that I can tinker with the system in a friendlier langage of choice, no memory or time-keeping issues, datalogging. I do not have to reconnect the arduino every other day if i want to change something. I can do that from Spain (where I work), while this machine is running in Brussels (where i go every few weeks).
The fans are standard 120mm pc fans that push about 100m3/h without any pressure (but then these tubes are very large, there is hardly any pressure to overcome given this type of exchanger). The fans are 4 wire which means they can be controlled via a low-current pwm signal from the arduino.

I made a small website showing the aparatus in action. I will change the domain name in a few days to avoid being hacked (its just my badly configured router running that website).

What really surprises me is that it keeps the appartment quite dry, even with these small volumes of air. This makes me wonder (in retrospect!) about many of the systems i see here and planned to build myself: wouldn't running those have completely sucked all moist out of my house even at moderate speeds? Without any moisture-recuperation, isn't running small flows the only viable way? And given that, aren't relatively small units quite ok?

Obviously I am also simply quite jealous of many of the nice systems i see here! But from my experience low humidity really would become an issue for such larger machines. Of course one could keep a nice large system and have proper ventilation, using plants or humidifying otherwise... perhaps that comodo dragon could help keep moisture levels up too!

PS there are lots of issues left to do with my system: i have some stale are leaking into my fresh air entry outside, for example (which you can see in the graphs on the website). Also: the inner tube is too large, given the specifics of how i did things (the inner tube has no curves, the stale air has a narrow spot to pass where it enters)... i have to run the inward fan much slower, not to cheat on my efficiency, but even just to get the same change in temperature (excluding condensation), from which I would derive the same volume of air is flowing (comments ?)

An open issue for me is also how to control the unit. Simply by relative humidity? dewpoint? difference between inside/outside dewpoint? absolute humidity? relative humidity seems ok for a first approach, but say temperature drops at night, relative humidity increases, and the unit starts to speed up exactly when ventilation needs are low (and outside temperatures are coldest). Any ideas on this?

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Old 01-25-13, 01:23 AM   #352
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Welcome back Pladijs Congrats on building a working HRV.

I took a look at your numbers on the website and I noticed the following:
Stale air entry 50.3 RH
Stale air exit 99.9 RH
Should these two be switched? The RH should be lower after going through the HX.
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Old 01-25-13, 01:37 AM   #353
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Originally Posted by pladijs View Post
...What really surprises me is that it keeps the appartment quite dry, even with these small volumes of air. This makes me wonder (in retrospect!) about many of the systems i see here and planned to build myself: wouldn't running those have completely sucked all moist out of my house even at moderate speeds? Without any moisture-recuperation, isn't running small flows the only viable way? And given that, aren't relatively small units quite ok?...
What is the volume of your apartment?

I'm not an HVAC technician. In fact, I'm barely even a hacker...
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Old 01-25-13, 02:15 AM   #354
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Originally Posted by Piwoslaw View Post
Should these two be switched? The RH should be lower after going through the HX.
Actually those figures look fine to me.
The stale air from the appartment is at 50%RH when it is entering the HX as warm air. Then it starts to cool down and when it exits the HX and is blown outside it is at 100%RH as colder air can hold less moisture.

The stail air is withdrown from right under the ceiling, which means it is a few degrees warmer than average roomtemperature, so on average the RH in the rest of the appartment will be higher than 50%, which is a bit moisty indeed.
Also if this stale air is at 100%RH when being blown outside it is likely there was condensation in the HX (this last week it was fairly cold over here in Belgium and the Netherlands.).

So Pladijs, those numbers indicate that your build is doing what you want, dehumidifie your appartment, and at low cost.
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Old 01-25-13, 03:01 AM   #355
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Originally Posted by AC_Hacker View Post
What is the volume of your apartment?

I could tell 60 sqm x average 3mt hight= 180 m3 more or less

My congrats for the job, a quick solution with minimum costs and work.
Could you better explain how your Arduino control system works? I guess many of us are interested about it but (at least myself) have no clue about the argument

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Old 01-25-13, 05:50 AM   #356
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Thanks for your support!

To your questions:

Piwoslaw, Fornax is right; the moist hot air cools down in the HX, reaches 100% RH, and condensates. You can tell some water is removed from the air as the dewpoint (which is more of a measure of absolute humidity, keeping pressure constant), drops significantly. But then again this dewpoint also changes for the incoming air, which i cannot explain... apart from the fact these sensors are quite cheap and not precise. Or there is some air mixing. As I wrote before, the entry and exit of air outside my house are not separated well yet. In any case: there is quite a bit of water coming out of that HX!

The appartment is about 180m3 indeed. The replacement rate would be way to low if 1/ there would be no leakage and 2/ would the appartement be occupied for a larger part of the day. Now it is only moderately too low, I guess. But before there was no "real" ventilation at all, save from opening a window.

On the arduino, I am not sure how much details you would like. Also, I have no background whatsoever in electronics beyond what I learned the hard way, so i am not sure I'm the one to listen to on this. But anyway:

The DHT22 temperature sensors can be setup as described here (times 4)

The fans get their main power from a 12V, 1A rated power supply, which also feeds the arduino.

The fans are 4-wire, the system is described here:

I bought these el-cheapo ones (not so sure why)

For a replacement (if ever), I was thinking of some sanyo-denki fans, possibly water-proof ones. I like these axial fans. Somewhat more powerful ones to push the air through some filter (I already notice there is a lot of dirt going through those tubes, good thing they can easily be cleaned or (for the interior one) replaced).

What is great about that 4 wire system is that one of the wires gives back a "tacho" signal (see point 2.1.3 in the document), and one cable is used to control the speed of the fan (point 2.1.4).

To connect the tacho signal, I connected that to one of the digital inputs, and then (using a resistor!) to the 5V supply on the arduino. As the document states, the fan will connect the 5V to ground once per half revolution (open collector), the resistor makes sure only a small current can flow; the arduino will detect the voltage drop. I use the "pulseIn" command to count the pulses.
For the pwm, that works in the opposite way: the fan provides 5V, this needs to be connected to a digital input (using a resistor!), which will connect this to ground at a certain frequency. To get that frequency, a small hack is required. IIRC I run mine at 32khz or so, out of the specification range, but it works just fine.

This all seems quite a bit simpler compared to soldering up everything yourself to do the pwm-ing to the main power of the fans, at least to me.

To talk to my router, I use this shield:
with this module
An alternative (perhaps cheaper?) would be

This wifi part was actually the hardest to setup. Its not fail-proof yet, if the connection is lost, the arduino freezes. I have to add some code to handle that (possibly with a watchdog timer resetting the arduino in the worst case).

Am i forgetting something? not sure how much detail anyone here is actually interested in, so just ask. I seem to be writing such long contributions it feels like therapy somehow.

I've attached a picture of the arduino setup before it was completely finised and disappeared into my wall (that was back in may before i found this job in spain).
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Old 01-25-13, 06:06 AM   #357
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Default I think pointing it down is backwards

Been thinking about freeze ups. The previously posted
long concentric tube heat exchanger mentioned pointing
it down to drain out the water.
Problem is that with low outside temps, the output is
gonna freeze up eventually. Then it dams up all the water
and virtually all the moisture gets trapped and frozen.

What if you point it UP and drain the water from the inside?
The water is gonna collect somewhere around
the dewpoint temperature. You run it backwards and drain
it while it's still wet. Yes, the dewpoint varies along the
pipe...there will be continued condensation
at lower temperatures, but it should take much longer
to plug up. And there's no ice dam.

If you know the inside temperature and the lowest
outside temperature, to first order, you can calculate
the position along the pipe that reaches freezing.
Put a V in the pipe at that point and you drain off the
water at the lowest possible temperature that won't freeze
in worst case conditions. Not sure it makes enough difference
to warrant the mechanical complexity.

If you need a defrost cycle, just turn off the incoming air fan.
Or better yet, reverse it.

Another option is to separate the dehumidifier and ventilation.
Use a finned pipe.
Bring outside air thru it and plumb it back into the HRV
fresh air input. Control the air flow to keep the core just
below the dew point you want inside. Gives somewhat
decoupled control of humidity and ventilation. And you
can put the core in a different location to reduce the
humidity gradients.

According to, a condenser temperature of
around 50F should be plenty low to prevent mildew.
You may not lose too much efficiency by just venting
the 50F air into the space.

Are we having fun yet?
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Old 01-25-13, 06:29 AM   #358
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Yet another advantage of building a HRV with such a small heat exchange surface that it is bound to be highly inefficient: you can stop worrying too much about it freezing up the stale air never gets sufficiently cold to freeze inside of the unit. Currently the outgoing air is about 6C for an outside temp of about 0C in my system. I have never seen the outgoing air below 0C. But then Belgium does not have truly cold winters.

Something else I paid attention to is the location of the entry of fresh air: I have a small balcony and a large window. The balcony is somewhat closed off, there is a brick fence to keep people from falling off, but also on the top, there is a concrete beam supporting part of the ceiling above. I hope that this beam creates a void spot where air collects that is somewhat heated up by my window and wall; That is why i have made the entry of fresh air there. ( By putting it there, this air actually passes by the outside PVC tube, heating it up a bit further.) I should draw this, but i hope you get the idea. This is good for efficiency, of course. But given your remarks on freezing up: it also means that I am unlikely to see extremely low temperatures even at the intake of the heat exchanger. At this point, my intake is about 3C warmer than reported by the official weather station a few KMs from here. Not sure how the temperature at the intake relates to the temp at some random spot say in our garden; i'll have to measure that some day when im back home.
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Old 01-25-13, 06:33 AM   #359
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But more directly related to your post:

With upward sloping concentric tubes: wouldn't you think that most of the humidity of outgoing air condenses (and flows back outside of the HX) before it gets to the spot where it starts to freeze? So no need to think about the sport where it might start to freeze (as the amount freezing there would be quite small and melt away from time to time, say when outside air gets a bit warmer over the course of a day). I actually like your idea a lot, I would think that it almost entirely removes the need to think of freezing.
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Old 01-25-13, 07:21 AM   #360
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The colder the condensate, the more heat you've removed from it.
The V shape keeps the condensate flowing in the direction of
the air where it can have more heat removed. And since it never
gets below freezing at that point, that's the best you can do with
a static mechanical design. A gram of water drained at 0C
has a calorie less eneergy than one drained earlier at 1C.
Don't think it's worth the effort, but people like to think about

If your outside temperature doesn't get below freezing, you obviously
don't have a freezing problem.
Here,it rarely gets too far below freezing. I've got about 8F difference
between in and out air on each end. I figured I'd not have to
worry until the outside temp got below 24F. I was wrong.
Over a few days when the temp hovered between 28F and 32F, the
thing became a block of ice. Only explanation I have is positive feedback.
Ice restricts the air flow which increases the differential which
lowers the temperature and causes more ice.
The channels in the coroplast are small enough that surface
tension of water can keep it from dripping out.

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erv, heat recovery, hrv

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