Pop-Sci DIY HRV (1986)
 

Plans from a 1986 Pop-Sci... they're still readable.

-AC_Hacker

great post,

has anyone used a car or lorry intercooler radiator as a heat exchanger as these are quite cheap and designed for air heat transfer ?


Regards
p

builditinyourshed,

I'm not so sure I know what this looks like, or how it works. Do you have a photo or drawing or more detailed description?

-AC_Hacker

http://www.infinitemotorsport.com/im...ntercooler.jpg

usually used in turbocharged engines the intake air to the engine gets heated by the turbo so it then goes through the intercooler which is normally cooled by air from the front grill. Basically a radiator except for air.

OK, now I see what you mean...

Well, you are right that it is an air to air heat exchanger...

I think that it is designed for a pretty high temperature differential, because it looks to me that the surface area might be rather small.

Can you find one for cheap, from an automobile salvage yard to try out?

The cross-stacked squares of corplast can amount to some serious area for pretty cheap. Around here, they use corplast for political yard signs, and after the election, they are worthless (so are the politicians, but that's another story).

-AC_Hacker

I agree they are meant for high heat differential and realistically the largest I've ever seen are for the big rigs and they're not 4sqft in size. They also aren't exactly cheap being big specialized pieces of aluminum. Coroplost should be much much less expensive.

"Ductless" Heat Recovery Ventillator...
 

Here's something to tickle the fancy of anyone who is considering doing a DIY Heat recovery Ventillator...


Since it mounts on the wall, ducts are not required.

Interesting configuration, kind of breaks out of the 'thinking box' of other HRVs.

Product Info...

-AC_Hacker

While trying to find a way to bond CPVC pipe to coroplast I came across a link to this company that apparently uses coroplast for the heat exchanger.

Building Performance Equipment, Inc. - Air to Air Energy Recovery Ventilators - Product List

I don't see that listed on their page though

Looks pretty interesting.

Here's a drawing of the HX box... apparently they are doing a counter flow arrangement... apparently one channel is straight-through, the other, I can't quite figure out.

Any ideas?

-AC_Hacker

I searched for about an hour to find an image in a pdf which showed a counter-current fine-wire heat exchanger. The pdf can be found in Breathing Window ))) Smart Ventilation System )) Kristinsson and one can also see the set-up of the heat exchanger. A fascinating device!

I have been looking into ways that the air-exchange rate of an HRV/ERV can be controlled.


Conversations with local Green and Passive House contractors have confirmed that HRVs/ERVs with CO2 sensor controllers are being used now. But they look frightfully expensive.

So, a CO2 sensor can be used to control the speed of an HRV/ERV by varying the duty cycle of a pulsed DC wave form driving a fan.

CO2 sensors are available in various configurations on ebay.

I am investigating the possibility of using variable-speed computer fans, since they are highly evolved and are cheap. A potential problem I have recognized is that there may be a problem with using computer fans in an environment that is operating close to the dew point.


Seems that moisture can drastically shorten the bearing life of the computer fan. I have also seen ECM fans that move the motor and it's bearings out of the air flow, so bearing life wouldn't be so affected by moisture. Fans of this configuration are also called Tangential Blowers. Trick is to find one with an ECM motor.


I have also found out that speed-control chips are made for these fans, and the chips are in high-volume production and are also cheap.


So far, the control chips have various steps to control the fans, some chips have 4 steps, others have more. I have also seen Arduino projects where an Arduino board out-puts varying PCM pulses based on an input signal...


...and some CO2 sensors intended for Arduino integration.

-AC_Hacker

Hi AC

There is a couple of reasons not to automatically turn down the recovery unit, and more specifically not using CO2 alone. Say you have a hot an humid day, and a cold night, but you are not at home. You would want the unit to run at a normal rate to avoid condensing on your windows and water running down into your window frame. If you come home to take a shower before leaving again you get the same situation. Then you have the situation where you just came home and you make food. To avoid the smell all over the house for hours until the CO2 level rises, you would want it to go a bit faster. Also, if you are like me, and smell like a week old dead cow after a full day of manual labor, some fresh air does not hurt (at least that's what the wife sais :-p ). Of course, you could have both CO2 and a humidity sensor, with a manual override, but now it is starting to become complicated.

Then, also to consider, is the fact that DC motors don't have anywhere near the life expectancy of an AC fan. It is also slightly less efficient, and if you count the conversion from AC to DC, even more so. More to go wrong also, which would happen when you are out of the house for a long time.

And, by the way, you will need some serious PC fans to get anywhere near the airflow and pressure you should have in a ventilation system. The AC ones used in professional PC racks would be more than sufficient, but I have seen none that is DC, and certainly none with a normal noise level.

At least here in Norway, you don't get this CO2 control for the HRV. I can see the efficiency benefit, but I suspect that it has more to do with selling point than actual usability. If you have a good recovery system there will be very little energy loss anyway. I actually have a problem with mine being too efficient (93% approximately). The air coming in to our bedroom when there is 0C outside (32F?) and normal temperature inside is <2C (4F) below the outgoing air. This keeps most rooms close to the average, which is a bit too warm for me to sleep in. If it is above freezing, we have to open a window a little. Also, air has little energy capacity, so a little change in airflow will not really affect things that much. I would suggest finding other places to save energy than reducing the amount of fresh air. My recovery unit is a medium setting now. I put it in low if we leave for a long time, and high if we have more than 8 guests. it is about 30% up and down in airflow for the different settings.

Ko_deZ

I found this pdf about the fine wire hx http://architecture.ucd.ie/Paul/PLEA...er_ref_188.pdf

Well, I have considered what you have said, and I think you are right that CO2 alone may not be the best parameter to control fan operation. On the other hand, as I have looked further into advanced ventilation approaches, CO2 monitoring seems to be firmly in the development path, as far as very high efficiency goes.

There has been considerable research into Electronically Commutated Motors (AKA: ECM), so the carbon brush thing is not so much a factor. DC motors do offer the promise of inexpensive speed control. When you combine the advantages of ECM and variable speed/power, the advantages are considerable.

Also, as you probably already know, varying the speed of AC motors can be done. But thanks to the development trends in electronics, variable speed AC motors are becoming commodity items... there's one in my Chinese heat pump (outside), whirring economically away right this moment... continuously variable. I just don't know if a variable speed AC motor & controller would be worth it for a home project.

Well, that brings up another interesting point, do we really need central air ducting? Running fans to push air through all those many feet of ducting uses up quite a bit of energy just to get the air from one place to another. I am seeing emerging trends to use small HRVs in each room, directly venting stale air to and getting fresh air from the outside, this avoiding duct losses. I have been looking into air volume rates that are required for rooms in my house, and the volume rates are quite small. In fact, my greatest problem is finding fans that are small enough. That is one of the reasons I am looking into computer fans. For a single room, without duct losses, they are just about right. And the tangential blowers are very quiet. That is what I have in my mini-split (inside), and I usually can't tell if it is operating or not, except for the fact that I am warm.

I don't think that CO2 sensors are common anywhere yet, but I think that the world is about to go over an energy cliff, and a lot of research is taking place now to reduce energy.

Well, right now I don't have a recovery system at all, and I need one. I figure if I'm going to do a DIY recovery system, I should do the best one I can.

Regards,

-AC_Hacker

How about monitoring CO2 and humidity?

What about smell food smell? Gas leaks? Should the ventilation run at maximum when there is a fire? Temperature? Certainly, a non-humidity recovering system like the window unit mentioned earlier should not be running if the outside temperature is below freezing since there would be a huge buildup of ice in the outgoing path. That would actually have to measure outside humidity as well, as that is an important factor in the efficiency of the HX and would determine the lowest temperature possible.

There are too many factors for this to be a good system. Sure, efficiency will be higher, but how much higher than >90% will you get? If you manage to cut the energy loss in half again, that sounds like a lot, but if your energy loss was just a few watt-hour anyway? Is it worth significantly reducing the experienced indoor climate (smell) because there is only one person in the house?

Also worth considering is that you without a ducted system will have to have one in each room, making the one in the bathroom much more troubled with icing up. Having a normal fan vent in this room would be throwing away a whole lot of energy since the hot and humid air during and after showering contains a whole lot of energy.

If your ceiling height is high, then it is very efficient to pull out energy from the high area and regenerate the heat to other parts of the house.

Not having a close to average temperature in your house will possibly cause moisture problems in your wall. The humidity-block (directly translated from norwegian. We use plastic for this) is inside the wall, thus at a cooler place than the inside air. Humidity has a tendency to even out troughout the house, but relative humidity will be higer in cooler rooms for this reason. If the humidity then also spreads into the walls, as it does, it might hit dew point before meeting the humidity-block in your wall. So a evenly heated house is important in cold climates.

A ducted system also keeps a positive pressure on the incoming air, and a negative on the outgoing as long as you have set it up correctly. Total should be a small negative pressure in the house. This will avoid pushing moist air out trough holes in the humidity-block, avoiding dew and humidity damage. A per-room based system must also have pressure sensors on both indoor and outdoor to make sure it does the same.

This is becoming a whole lot of work, with a whole lot of possible errors and bugs, and all it does is save a tiny bit of energy. Yes, I agree, make it as good as possible, but also consider the amount of time and effort, and investment, compared to the gain. I considered making my own system too, but I decided that it was not worth it.

Example of my house:
93% efficiency, 300m³ per hour, 20C inside 0C outside.
The outgoing air will have a temperature of 0+(20*0.07) = 1.4C. Difference is 1.4K
The specific heat capacity of air is about 0.001297J/cm⁻³k⁻¹.
My energy loss per hour is 1.4K*300000000cm³*0.001297J/cm⁻³k⁻¹ = 544J. That is not very much. 1W for one hour is 1J/s*3600s = 3600J.
If I run my ventilation unit at minimum, I have 150m³, and at max, 600m³. It would still not be a whole lot at max. Actually the fans use more power than that.

I suggest spending time and money insulating and swapping windows instead.

Good advice.

-AC_Hacker

That's where my thinking was leading me, too.

Humidity sensors are not too expensive, about $15.

The CO2 sensors are a lot more expensive. I've seen some units on ebay for about $50 to $500. That's pretty high for DIY.

It seems like an Arduino might be suited to monitoring the sensors, but I have no idea yet how to integrate the data and act upon it effectively.

The single-room HRV idea seems really interesting to me from the standpoint of dealing with local concentrations of humidity, cooking odors and also CO2. It seems that if the kitchen is a source of humidity and cooking air, why purge the whole house. Also, I have had my thinking aimed at sealing my house against leaks for so long, that the idea of intentionally making two holes in the wall of each room makes me uncomfortable. I mean, if there was a strong prevailing wind, which is not an unusual occurrence where I live, I think that the net result would be that there would be a mass airflow through the house, taking the heat with it.

But I am seeing evidence, especially coming from Scandinavia, that direct-vent HRVs are receiving serious consideration.

So far, the passive house approach uses a central HRV. I don't know yet if one approach is ultimately more efficient than the other.

-AC_Hacker

I suggest increase fan speed, or decrease fan speed to keep them both stable. Say 1% fan speed per 1% RH, and something similar for CO2. Then you use the highest of the two. I also suggest adding a "vent" button that will run the fans at 100% for 20 minutes or something like that.
You do not purge the whole house. You place one exit valve in the kitchen and in each bathroom/washing room. If you have an area with higher ceiling, you place one there too. Last, place one in the top floor as high as possible. Humidity goes up, so you need to vent from high up to get the humidity out. Input air should be in living room and bedrooms. There are different end valves for ceiling input and wall input (the wall input is the same as the output valve in both ceiling and wall). What this configuration does is that it moves air in to and out trough the ducts in both bathroom and kitchen, reducing the spread of smell and humidity. If you have single room HRV, the air will not be travelling into these rooms, thus you will allow much more of the smell and humidity to spread. Not necessarily a problem, but at the least a small inconvenience.
You are very right, your indoor doors will not hold air pressure very well. A 10m/s wind does generate a very high pressure difference on different sides of the house. Before we started making our houses so air-tight the wind would blow straight trough the walls. You could use some very simple electronic properties to avoid, or at least reduce, this. If you put a flap on both input and output that the fan will blow so it opens, you could use an electro-magnet to hold it shut if it starts to get windy. Sparkfun has wind speed readers that would suit this perfectly, but it will affect the efficiency of the fans. A servo that moves something like a blinder to close the opening could be better, but not so easy to get tight. I suggest also using temperature sensors, so that you can avoid icing and automatically start cooling if it gets very warm. If you use the LM35 (iirc), you can just connect one leg to +, one via two normal diodes to -, and the last one one analog input on the arduino, and you get the temperature in steps of 1C per 10mV, or something similar with a F version. The resolution is about 0.5C, which is more than enough. I did just this for the termostat for my floor heating, and it is excellent.
The swedes are looking into this a lot I know, and the danes with their brick and concrete houses don't have so many other options for refitting. For the swedes, the main reason seems to be that their normal heat recovery the last years seems to be having air-vents in the bedrooms, and then a suction system with a heat pump, pulling air in trough the holes in the walls and out from the kitchen and bathrooms, and then recovering energy from the outgoing air. There are a few disadvantages to this, one major one is icing on the cold side of the heat pump which significantly reduce the efficiency, another is the cold area that you get around the vents, when closed there will be dew and other nasties. Also, with very high dT the heat pump cannot manage to exploit the outgoing air energy very well. To "fix" this problem i houses that are already built, single room HRV is the easiest way to accomplish an improvement. Also, Sweden is not very windy. Take Norway as the opposite. We have 10m/s regularly, so this single room system would not work at all. Danes have it worse with regards to wind, so this will be very interesting to see how plays out.
There must be comparisons done to determine this, but in the long term I do believe that total cost of ownership will be significantly less with a central system. Easy to maintain, efficient, and expensive components will make the lifetime a lot higher. Production pollution also goes into the calculation, and something that last longer is usually more environmentally friendly. Also, if something more efficient comes along, you can always change a central unit, not so easy with 10 singe room ones.

I do think this is a very interesting project, even though I have very little faith in it ending up as a cost-efficient solution. If you need any help with the arduino, let me know.

Btw, pretty much all passive houses that where built 15 years ago (as experiments), seems to be getting humidity damage and rot. I talked with a few people that helped putting those houses together. Not much has changed in the building process. Some materials have gotten better, and the humidity block quality and awareness has improved but that is about it. I am dreading how it will end with my house after the next 20 years.

I'll do my best to help you to see the error of your ways...

Very generous of you. It is very likely that I will need help on that part.

So, I take it that your house is a passive house? When was it built?

When you say "humidity block" do you think that is the same thing as "vapor barrier"?

..and what part of Norway do you live in? I'm curious about the Heating Degree Days there.

-AC_Hacker

Hi, I was checking links to my website and found this forum, its been interesting reading!
I build Passive Houses in Ireland and the UK and we make our own HRV units that fit completly into the external wall. We fitted close to 50 units this year and they seem to be working well with higher return air temperatures than some of the PH Certified Paul units. We use Copper fine-wire heat exchangers that have 20km of wire in each heat exchanger.

Welcome to the site Viking House. You make what we dream of! :) Would it be possible for you to share plans of your HRV units with us? Do you have any idea how efficient they are?

I have some data from a data logger but the file is too big to upload. I remember last January when it was -13 degrees C the return air temperature was +17 degrees C, and the house temperature was 19 degrees.

That sounds very hi-performance to me! But, that fine-wire hardware
might not be something that a DIYer could duplicate in a home shop.. :(

If it turns out that I have to install an HRV unit in my basement,
I hope it won't turn out to be too expensive.

Well, I suppose I could sell you some heat exchangers at cost (€150 each plus transport) if you want to try them out?

You certainly have my attention.

Will there be issues with US electricity?

-AC_Hacker

That €150 is just for the FiWi heat exchanger, not the box, motors or electronics! You can use US fans and control them with a potentiometer and make your own box, have you looked at EC fans?

OK, according to today's exchange rate, that would be $202 US. So how much do you suppose shipping might be?

Also, how does someone clean one of these units to remove fine dust, etc?

-AC_Hacker

Viking House,

I just noticed that on your Viking House page, you said,

I just looked it up and the speed of sound is 343.2 metres per second, so your Heat Exchange Ventilator appears to be Super-Sonic.

Do you have problems with the sonic boom when the unit is operating?

Concerned,

-AC_Hacker

I got the vapor equalization speed figures from one of the science guys on the UK Green Builders Forum who is usually pretty reliable, I tried to verify the by doing some Google searches but failed so I'll probably reword it and leave out the "at speeds of + - 500m/sec" bit!

It does seem a bit fast.

But I am interested in the FiWiHex...

You didn't comment on the dust issue. I'm using a mini-split to do some of my heating here, and just the other day I did see that there was considerable dust build up, so my 'dust consciousness' has been activated. Have you had any issues with dust, regarding the FiWiHex element? Can it be washed?

Also, I'm intrigued by your proposed use of low temp solar for space heating. Do you have more information on the water-to-air FiWiHex? It would bve a perfect fit for the diffused sunlight we have here (very similar to your own winters).

-AC_Hacker

Hi AC, check this link Welkom bij Vision4Energy / Vision4energy
Regarding dust; the air/air ones can easily be removed and washed, the air/water ones could be vacuumed.

Here's a useful table for figuring duct size relative to airflow.


-AC_Hacker

I don't know if you guys have seen this tiny package or not,
MANHR100W Heat Recovery Fan Heat Exchanger Ventilation Controls Condensation | eBay

http://www.alertelectrical.com/uploa...d/7_1692_e.jpg http://www.fans4less.co.uk/pub/files..._w450_h400.jpg

It looks like it might be installable in a window, (Even a very small window)
to give at least one room some fresh air..
It would have to be inside an insulated window box, (and it uses 220vac).

Anyone know about the core used in the small Manrose products??

No, not passive house, but not too far away. The U value of my walls is about 0.15W/Km^2, which to my knowledge is the same as a passive house. In Germany a passivhaus makes more senes than here. It gets so cold here that a passive house just means less heating. We still need some heating here.

Vapor barrier! That sounds like the words I was looking for. Excellent!

I live close to Oslo, but a bit higher so subtract 2C from the Oslo temperatures. If you find the Heating Degree Days here, please let me know =)

Viking House:
Was there much icing in the heat exchangers when you had those really cold temperatures?

Here is some info from your area (Fagernes Leirin) for "Heating Degree Days (base 65)".
History : Weather Underground

It does look colder than Rygge.. :)

Ko_deZ,

I was going to put your location information into a form that is available on the web. They use local historical information to calculate Heating and Cooling Degree Days.

So your description is too vague, so is not very useful for me to be able to be able to generate relevant information.

So, this is the Web Page: Heating & Cooling Degree Days - Free Worldwide Data Calculation

You go to that page and follow the instructions,

"Enter a weather station ID if you have one, or search for any city, state, ZIP code, or airport code."

The rest should follow easily.

I'm interested in what your average heating degree days figure is, in F degrees.

For instance, my local HDD is:


Regards,

-AC_Hacker

I did not find any nearby, so I used Oslo. To be more specific, IOSLO5.

Month starting HDD % Estimated
12/1/2010 1507 0
1/1/2011 1174 0
2/1/2011 1211 0.2
3/1/2011 1062 0.8
4/1/2011 538 9
5/1/2011 513 0
6/1/2011 260 0
7/1/2011 164 8
8/1/2011 245 4
9/1/2011 389 0
10/1/2011 666 0
11/1/2011 791 0

Those are the numbers. I have no idea what they mean yet, but I will look into that when I have the time. The sum seems about twice of yours AC =) Then again, Oslo is at the same latitude as Greenland, so... Thank God for the gulf stream!

I drive past Fagernes on my way to our mountain cabin. I find our local weather gurus (norwegian metrological institute) better at statistics and displaying it (works outside norway too actually)
Weather statistics for Fagernes, Nord-Aurdal (Oppland)

But of course, we have done the clever thing and gone for the SI system, so it is all in Celcius (which is Kelvin offset to 0 at the freezing point of water, not the absolute zero)

=)

Just a couple of questions. was it 18,50C or 19,49C indoors? What about return temperature? 16,50 or 17,49? If we take the most extreme cases, the best one gives ( 100 * ( 17,49 + 13 ) / ( 18,50 + 13 ) ) = 96,8% efficient, while the worst case gives 90,8% efficient. Both very good, but the last is about three times the energy loss of the first. (first one is 3.2% lost, second is 9.2% lost). I did not also calculate the outside temperature insecurity, but that would only push it even further apart.

Also very important for the total efficiency is the power usage of the fans. I guess they are both placed before the exchanger to recover as much of the heat as possible, but what wattage where they during the calculation? If they where running very hard because of frost, firstly you would have overpressure inside, which you really do not want, also, you would have a lot of heat from the fans contributing to the "efficiency" of the unit. Of course, all units have this, but the numbers at the time of measurement are important.

Also very important to the numbers is the relative humidity indoors and outdoors, and the rate of airflow. With that high an efficiency, and no humidity recovery system, I can see no way there will not be frost on the outgoing circuit. Of course, with no airflow, the efficiency would be very high indeed, but there might be a defrost safety system?

-Ko_deZ-

EDIT: The reason for there having to be frost is that the outgoing air would have to be about 2C apart from the incoming air, so -11C. That is about 22 below freezing in F. Unless the indoor air was remarkably dry, there would be condensation, and at below freezing temperatures, there will be ice. There is no way around that.