mold & active HRV
 

Found this dew point calculator.
It's the first one I've found that actually addresses
the issue of mold growth.
These guys are in the document preservation business.

Dew Point Calculator



While I've got your attention...I've been catching up on the
active HRV issue. I'm puzzled by the talk about cobbling
together a fridge or dehumidifier.

The action of an HRV loses heat that must be replaced.
If your primary heat source is as efficient as you know how
to make it, why not use that to replace the heat?
Conversely, if it's not, put your effort into improving the
efficiency of the primary heat source.

What am I missing?

You are correct, an HRV loses heat indeed.
What you are missing is that places where people recide need ventilation.
Now in short you have 3 options (I don't want to sound blunt but I want to keep it short, I know you are not stupid.)
1) a leaky house where wind blows through the walls while it is freezing outside.
2) a well sealed and insulated house with 2 windows open, while it is freezing outside.
3) use a somewhat complicated apparatus that inserts and extracts roughly equal amounts of air and recovers a substantial amount of energy from the warm air extracted, again while it freezes outside. (The inbound air is now almost roomtemperature, this saves energy and raises comfort, if done well.)

I agree a focus on the efficiency of the primary heatsource is important, it's good for the environment and the wallet but it is another subject. For maximum profit one should consider both: don't let the warmth of your primary heatsource slip through the walls or an open window, keep it where you want it while also refreshing the air you breath.

I think that an active heat exchanger can have a place, but the niche is a pretty small one.

If your house is so tight that it requires ventilation, AND your insulation is sufficient that your heating needs are extremely low, then the heat that would be lost from a very efficient HRV could be salvaged by an active HRV. In fact, a mathematical formula could be developed to determine the range where an active HRV could be useful.

I have also seen them used in industrial settings where ventilation requirements, due to airborne pollutants & particles, are many times higher than in a house. Active HRVs are useful there.

-AC

Well, actually you hit the hammer on the nail (pun intended).
While cooking you produce some seriously bad air with excessive moist and fat, like Kostas mentioned, but that can be filtered, if done right.

Then again while cooking you produce some (a lot) extra heat that could be reclaimed, instead of being extracted to the outside by the cookinghood.

So Piwoslaw, it's not only possible to do but I designed my system to be the cookinghood. We have a livingroom with open connection to the kitchen, smells from the kitchen always reached the livingroom (and the rest of the house). Now with the HRV in action that is much and much less.

I didn't post for a few days because of work etc., but in the off-hours I did some work on my apparatus/surfing the web/playing with Google's Sketch Up:

http://ecorenovator.org/forum/attach...0&d=1358027749

You see the ground floor is is 44m^2 in total (490-ish sqf) of which around 33sqm or 75% is livingroom, it's volume is 86 cubic meters (3000qf), the kitchen is about 1/6th of that.

More details later. Point is: I hooked up the ducts temporary without thermal insulation, and expected realistic results. Not true.
I was disappointed by primary figures but things were working fine, in the situation. Hooking things up proved the concept of ventilation, as expected but the heat regained was lower than expected.

Playing with it the results got better, without changing the HX-core or the cabinet, just the ducting.

More details will follow later. Today while preparing dinner the temperature in the livingroom was 20C, the FAI was 23C (Reclaimed heat from the kitchen, pans on furnace with boiling vegetables and potatoes) so quite soon it felt a bit warm.

Answers don't seem to address my question, so I'll try again.
I'm addressing the advisability of adding an active element
to your HRV.
Start with the tightest, most insulated house you can manage.
If I build the most efficient passive HRV I can, I still lose some heat.
Say, that I lose 1BTU/HR.
That 1BTU/HR has to be made up somewhere.
Option one is to use the primary heat source to replace that
energy loss. If my primary heat source uses the most efficient
technology available, how can adding what amounts to a heat
pump in the HRV be more efficient...even ignoring the
cost of the additional plant?

Stated another way, wouldn't improving the efficiency of the
primary heat source give me more bang for the buck?
Any increase in efficiency applies to the entire heating load,
not just the amount lost by the passive HRV.

Stated yet another way, I can't understand why anyone
would consider cobbling a dehumidifier or the guts of
a refrigerator onto a passive HRV. Seems like the cost/benefit
ratio is higher than your other alternatives.

Ah, I see I misinterpretated your question.
The main heatsource is used to provide heat when needed, and thus it will not run continiously. Usually it will produce large amounts of heat in bursts.

An HRV will run 24/7. If it freezes hard then on the cold side of the heatexchanger ice will build up and that has to be removed or prevented. Removing the ice requires running defrostingcycles during which no heat is recovered.
Preventing the forming of ice would require heating up the incomming cold air to a point where no frosting will occur. This requires less heat than the main heating source provides but demands a continious heatsource.
Rather than putting a resistive electrical heater there people get creative with heatpumps etcetera since they are more efficient than plain electrical heating.

I hope this is a more suited answer to your question.

I agree with Fornax.
As a preheating stage (and even for a post-heating one, in order to gain the lost temperature of the HRV) I was planning to use a reclaimed car heating radiator like this one:

http://img218.imageshack.us/img218/9...torestufa2.jpg

It could be plugged ti the main heating apparatus, or better more, to a closed circuit that will use the heat of the gas boiler's exhaust pipe



A small 12V pump and a thermostat will do the job ;)
The only think I do not know yet is the flow/pressure drop it causes.

I spent a lot of time looking into Passive House design and the rationale behind some aspects of the design.

Passive House decided NOT to opt for a ZERO energy house because the additional amount of sealing/insulation to get from Passive definition to Zero Energy was too high to offset by saved energy in a reasonable time.

Usual ASHP COPs are in the neighborhood of 2.5 to 3.7 average.

However, when the source temperature is raised, the efficiency rises considerably. I measured efficiencies of up to COP=6 with elevated temps, in my experimental setup.

An active HRV would exploit this kind of elevated temperature.

There aren't many heating sources with a COP=6.


By the way, your 1 BTU/hr loss example could be offset by keeping a pet reptile, maybe a Komodo Dragon (a cat would certainly be be too much heat).

-AC

You could use a cooking hood with a recycling function, usually economic models have that. Adding some good active carbon filters will certainly help, so you will have some clean and warm air to feed your HRV.
I also noticed that you haven't installed an outlet duct in the bathroom, I guess it's for the constructing difficulties but extracting the stale air from there will definitely help further more.

I see, it's about freezing up.

My toy HRV has been much colder than I remember.
I had it on the ToDo list, but this thread made me go measure it.
Long story short, it was frozen solid. Never had that problem
last year, so never thought about it as a possibility.
Washed it out with hot water. Now, the inside exit temp is
up by 15F.

I can't get my head around your COP=6 argument. The heat
to get the elevated temps has to come from somewhere.
Sounds like using one heat pump to raise the temperature
for the second heat pump.
I'm not saying you're wrong. I'm just stuck back here
at conservation of energy and not able to convince myself
that the gain is worth the cost.


OK, so now, I gotta think about efficient defrosting.
Adding a heat pump is outa the question. What else?

Simplest option is to put the incoming fan on a timer.
That reduces the input flow and allows the core to warm up.
I'm still worried about the resultant negative pressure, but
that may be ok in winter???
OR
Add a fan or vane that bypasses the core for incoming air.
OR
Figure out what to measure.
If the output end of the core is what freezes and reduced
air flow is the symptom, I can measure air flow or plenum
pressure or optical ice detector as the sensor.
Maybe can sense the fan speed as it increases with higher
back pressure. I'm using computer fans with the sense wire.
THEN
I can use that to control the fans.
OR
I can use that to open a leakage path between the two
ports on the outside end. The defrost heat isn't lost,
except for that to melt the ice, but the excess heat gets
recycled. You can let the core get very warm without
wasting much during the defrost cycle.
There is less fresh air coming in.
AND
all of the above uses the primary heat source.

I have a vision of a vane in the outgoing air path.
As the flow goes down, the vane allows a leakage
path between the two ports on the outside end
to let the core
heat up for defrost. You don't want it running in
steady state at half flow, so need to have some
mechanical hysteresis to let it finish defrosting.
Or maybe hold it open with an electromagnet until
the core temp gets above freezing.

Another thought.
If you're using cross-flow heat exchangers, you
can't expect much better than 50%. So, you put more
than one in series.

Imagine several cores in series...number TBD.
In steady state, there should be at least one
core who's temperature is above freezing,
but below the dew point. You drain out most of the
water while it's still wet. Should be much less ice
to defrost.

Whatchathink?

At this point in time the bathroom and toilet (kitchen inlet is blocked) are connected to the old whole house exhaust fan in the attic. That one will be decommisioned.
I'll connect the HRV to the existing ductwork so that toilet- and bathroom air will pass through the HRV. For this I made it so that when it's colder outside more air will be extracted in total than is inserted into the livingroom. (When it is not colder then several windows will be open, enough ventilation)

Our livingspace is the warm area, upstairs can be colder. The warm area is well sealed, floor insulation is so-so, which will be my next project. The goal for the warm living area is balanced ventilation with HRV. Upstairs will have some negative pressure, colder fresh air will trickle in, especially via the attic which is actually a useable room (In the Netherlands an attic is livingspace or a storageroom, we insulate the roof instead of covering the floor with a foot of cellulose)

So the HRV will take over from the whole house extraction fan. That single fan (anno 1984) uses more electricity than the 2 in the HRV together, 20,- gain per year on that.

So far I didn't mention the bathroom, it's upstairs and the sketch a few posts above only covers the groundfloor. I did include it in my plans though, it is a place that needs ventilation after all, need to remove the moist from taking a shower, and can recover heat from that.

Here's what I'm planning to do for the ducting.
I live in a two-floor house (total area 90 sqm), ground floor is another property, on the first floor I have the entrance, an open space with the kitchen and a slightly separate living room, a bathroom and the stairs that lead to the attic. There I have a small corridor, another bathroom, a studio and two bedrooms.
Behind all this there is a small place (25 sqm) used as a closet. I will install the HRV there as I can easily reach every room using short distance ducts (max is 5 mt long). I will plug the main inlet and outlet to the outside via the roof right above the machine (the drawing is quite schematic, in and out ducts will be much more distant from each other).
On the first floor I will have fresh air in at the living room while taken the stale air from the kitchen area and the bathroom.
On the attic I'll plug the fresh air in the bedrooms and take it back from the bathroom and the stairs arrival (in that way I'll get some of the kitchen odors that inevitably go upstairs + max hot inside air).
I think it's quite balanced like that. The only problem is that I'll have to make three 120mm diameter holes to the attic towards the first floor, that's 45 cm thick! :(

Notice two pairs of ducts: (bathroom & livingroom) and (bedroom1 & kitchen). In both pairs one intake and one exhaust duct run next to each other for 2-4 meters. Can you turn each pair of ducts into a long countercurrent heat exchanger? Use rectangular ducts instead of round ones and have them touching along the larger sides, then wrap the pair in insulation.
Putting smaller HXs ahead of the main recuperator will increase the efficiency of the whole setup, like having a larger recup.

Great placement!

Yes, that could be a good idea but I have some concerns:
First some dew may occur to the exhausting duct, so I'll have to put another condensation pipe.
Second the heat exchanged by those ducts will only affect the destination rooms and not the whole system.
Sincerely I have serious doubts about the effectiveness of this solution.
But I am speaking as a non-expert, so maybe I'm wrong.

Speaking about the aforementioned ductwork I was thinking of another improvement:
In the first floor I have the internal AC unit (Daikin dual split 18000Btu Inverter) placed between the stairs and the bathroom (AC1 in the drawing).
In the summertime I could deviate the fresh air supply right above the unit so that the incoming air gets immediately chilled and distributed with major speed by the fans.

While above the kitchen I could add some more recessed intake points in order to catch all that precious heat generated by cooking in winter time ...

http://ecorenovator.org/forum/attach...1-ductwork.jpg

What do you guys think about it? :)

Having a second thought of it, conveying the FAI duct to the AC in the summer seems to be quite mandatory...

Excess Air: New England: HRV or ERV?

BTW, the guy's blog has an interesting and useful duct construction guide here:

Excess Air: Rules of Duct Design, Why?

There shouldn't be much condensation since the temperature difference across the HX will be no more than a few degrees. In some cases there might be some, but it will be in the duct already going to the main recuperator unit, so slightly tilting the duct down towards the recup should let it drain into the main unit, which will already be handling condensate.
I did a few quick and dirty calculations (assuming outdoor temp 0°C, room temp 20°C and all HXs 80% efficient) and the effects are:

1. The fresh air which goes though those extra heat exchangers is warmer when it arrives at its destination room (18-19°C instead of 16°C).
2. The fresh air which does not go through a second HX is slightly cooler than if the whole system had only the main recup unit (14-15°C instead of 16°C).
3. The stale air exhaust is slightly cooler (3.5-3.7°C instead of 4°), which means that the efficiency of the whole system is higher (~82% vs 80%).

So, there are gains to be had, though during most of the year probably not worth the extra work. I could do a more detailed calculation with better input data.

Yes, you could be right about it.
But my guess is that you'll need something more that just two adjacent ducts to exchange some serious amount of heat.
Theoretically speaking you could put a number of some CPU heat coolers on both channels (side to side) to enable heat transfer. Something like this without the peltier chip: http://web.tiscali.it/fsguegl/immagi...onditioner.jpg

But then, you could always built a more efficient HX by adding some more PP sheets!.. ;)

Please do more calculations, with more details and please post the entire calculation process here in the forum.

I would like to see how you are going about it.

-AC

How about this: I'll share my rough calculations with comments and tell what and where can be modified. Then anyone can plug in their own parameters to get a better approximation of their own system.

Since there are many different temperatures to keep track of, I'll use a drawing to explain which is where:
http://ecorenovator.org/forum/attach...1&d=1358285990
So there are two heat exchangers: All of the air goes through HX1, but only part goes through HX2.
Fin: Fresh air in,
Out1: Fresh air leaving HX1,
RF1: Fresh air entering Room1, temperature equal to Out1,
In2: Fresh air entering HX2 temperature equal to Out1,
RF2: Fresh air exiting HX2 into Room2,
RS2: Stale air entering HX2 from Room2,
Out2: Stale air exiting HX2,
RS1: Stale air from Room1,
In1: Stale air entering HX1, temperature is a weighted average of Out2 and RS1
Sout: Stale air exiting HX1 to outdoors.

My assumptions:

• Fin=0°C,
• RS1=RS2=20°C,
• both heat exchangers are 80% efficient,
• exactly half of the fresh air goes to Room1 and the other half goes through HX2.

In order to get a first estimate of Out1 I assumed for a moment that In1=20°C. Since HX1 recovers 80% of the heat, and In1-Fin=20°C, then Out1=Fin+0.8*20°C=16°C, while Sout=In1+Out1-Fin=4°C.
So, since Out1=16°C, then this is the temp of the fresh air going to Room1 (RF1) and going into HX2 (In2).
HX2 recovers 80% of the heat, and the temp difference of its two inputs RS2 and In2 is 20-16=4°C, then RF2=In2+0.8*4°C=19.2°C, and Out2=16.8°C.
In1 is the average of Out2 and RS1, since equal amounts of HX1's air go through HX2 and go to Room1. So In1=(16.8+20)/2=18.4°C.

OK, now we are back at the first step (where we temporarily assumed that In1=20°C). Now we do everything over again starting with In1=18.4°C. This will change most of the numbers and after a few steps we get yet another value for In1. With each iteration the changes become smaller, so 3-4 steps are enough.

I did this again assuming that the Room1 and HX2 split HX1's air not equally, but 1:2 (ie HX2 gets twice as much air as Room1). The only real variation is that In1=(RS1+2*Out2)/3 instead of a regular average.

As I mentioned earlier, these calculations were meant to be quick and dirty. There may be a better way to do it, but I am not John von Neumann to sum up infinite series;) Just an estimation was enough in this case.

Anyone who wants to do this in more detail should plug in his own numbers: each HX may have a different efficiency, each room may have a different temperature, etc. Moisture in the air will also skew the results. Have fun!

heat sinks
 

I tried the computer heat sink method.
Problem is that the thing is designed to get rid of about
100W with a temperature differential > 60C.
The heat pipe temperature differential was relatively small,
but, at low temperature differentials, it boils down to coupling
to the air with tiny delta-T. I didn't save the test data
because it wasn't even close to feasible in my context.

I don't think there's anything wrong with the concept
if you can make it BIG enough.

In order to get more than 50% efficiency, you need
a temperature gradient over the transfer path.
That's gonna require multiple heatsinks and heatpipes
in series.

I tried to solve that problem with a bunch of butane cylinders
poked thru two adjacent ducts. Same problem. Heat pipes
work fine, just need a way to get the heat out of 'em
into the air.
I had some emotional issues with my HRV being a bomb.
Would work great for water to water heat transfer.

I thought about spot welding some fins, but decided a live
butane tank wasn't the best choice for welding.:)

Hi ham789, interesting experiment.
Have you tried putting fans on both of them?
Anyway, those heatsinks are quite expensive, I recon, so it seems not very convenient in the first place...

Yep, I did some bench experiments with controlled temperatures
and fans on both.
I picked these up cheap at garage sales. To get reasonable
efficiency, it would take at least three pair of 'em.

Some of the homemade heat-pipe stuff got posted here
about a year ago. Maybe by AC_HACKER.

I never found a cost effective method (cheap) of sealing 'em.


I'm not highly motivated, because the coroplast HRV is working
well enough.

I totally agree with that :)

Another issue that I'll have to handle is the RH output.
During this month of testing I noticed that inhouse RH never raised over 35%, and that's not a big deal.
I was thinking of using a humidifier placed in the inlet plenum.
There are two alternatives:

A vaporizer, cheap and easy to use, but very high current consumption (at least 1200W)

An Ultrasonic humidifier, more expensive, delicate to use and not so healthy, as it creates water drops that carry impurities that are in the reservoir, including minerals from hard water (which then forms a difficult to remove white dust on nearby objects and furniture), and pathogens growing in the stagnant tank.

Any other ideas?

How about the ingoing air blowing over a small pan of water? This would catch some of the evaporation, increasing RH a bit. Also, the more humid the air, the less water vapor it will pick up. The up side is that it's cheap and won't increase power consumption. The water in the pan could come from the condensation going on inside the HX, so it will be more or less distilled.
Just an idea:confused:

Yes, I was thinking about something like that, too. Welding that pan directly to the post heating exchanger would be best. My only concern is messing up with the good fellas like legionella or worse. Adding a UVC germicidal lamp above it could be a definitive solution?

Here in Oregon, the outside humidity is very high in winter.
I use an ultrasonic humidifier on the occasions that it gets too
low for comfort. Put a little bleach in it to keep the diseases at bay.
Don't have much problem with hard water.

My little hrv takes out a lot of water. Depending on current humidity,
I just don't run the stove
vent or the bathroom fan to keep the released moisture inside.

Never thought much about it and assumed an steam generator used
a lot of energy. But...if it takes just as much energy to vaporize
a cc of water at 70F as it does at 212F, maybe it doesn't matter.
And if all that heat stays in the environment, the only difference
is the cost of electricity vs cost of primary heat source.

I experimented with wet towels on the bathroom rack over
the heater vent. Seemed to help, but conflicts with the primary
purpose of the towel...to dry things.

And condensation inside the HRV should give back some percentage
of the energy it took to create the humidity in the first place.
Yes???
Next time I'm bored, I'll run a thermocouple down the coroplast
and see where the dew point is located.

I keep running into diminishing returns. It just doesn't get that cold
here. Was 23F this morning, but up near 40F during the day.
Over the last 577 cycles of the gas heater, I've averaged 7,440
BTU/Hour.

Sounds like you're having trouble with low humidity.

Somewhere in this forum there is a link to a guy who made a HRV HX out of cardboard, to send the humidity back into the house.

Might try that.

-AC

Problems with cardboard include...
I don't much like stuff that stays wet forever.
Structural integrity may be a problem with wet cardboard.
Outgoing air condenses at the dew point.
But incoming air needs to be above the dew point to
get significant evaporation. That means moisture has to
move backwards. I can't figure out how to arrange it to
happen reliably in cardboard.

I haven't been able to convince myself that the thermodynamics
works. The outgoing air gives up heat as the moisture condenses,
but you have to put it back to evaporate it in the input air stream.
My gut tells me that it's more efficient to add moisture by evaporating
water at inside air temp instead of starting at the dew point.
The math may be the same, but the practical aspects of getting the
heat where it's needed suggest not to try to reclaim the outgoing
moisture.

I'm gonna have to rethink how the desiccant wheels work.
Seems like desiccation must take much less energy than condensation/evaporation.

Discussion??

I wish I remembered where the link to the cardboard HX was...


As I recall, the user reported that he had been using it for two years. Seems a stretch to me. I think that a permeable non-organic material would be better, but off the top of my head, I can't think of a readily available cheap material that would fit that bill.

But the corrugated cardboard thing definitely has its charm... as a proof of concept, it would be a dirt cheap project.

I suspect that dirt cheap projects would be ok.

I think the guy was using the cross flow configuration. I think your configuration is much better.


BTW, you doing anything today? I could drop by and show you the Fine-Wire HX that fell into my evil possession.

-AC

Here it is ;)
Izdelava kartonskega izmenjevalca | PodSvojoStreho

(woz here)

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).

pladijs.no-ip.org:8082/zip.php

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? :D

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?

Welcome back Pladijs:) Congrats on building a working HRV.

I took a look at your numbers on the website and I noticed the following:
Should these two be switched? The RH should be lower after going through the HX.

What is the volume of your apartment?

-AC

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.

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 ;)

:thumbup:

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)
learn.adafruit.com/dht/connecting-to-a-dhtxx-sensor

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:
formfactors.org/developer%5Cspecs%5C4_Wire_PWM_Spec.pdf

I bought these el-cheapo ones (not so sure why) sharkoon.com/?q=en/content/silent-eagle-se

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:
arduino.cc/en/Main/ArduinoWiFiShield
with this module
rovingnetworks.com/products/RN171XV
An alternative (perhaps cheaper?) would be sparkfun.com/products/9954

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).

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 dpcalc.org, 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?

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 :D 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.

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.

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
maximization.

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.