DIY ventilation heat exchanger

[AC_Hacker]

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

[Ko_deZ]

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

[ThomSjay]

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

[AC_Hacker]

Originally Posted by Ko_deZ

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.

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.

Originally Posted by Ko_deZ

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.

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.

Originally Posted by Ko_deZ

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.

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.

Originally Posted by Ko_deZ

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.

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.

Originally Posted by Ko_deZ

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.

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

[Piwoslaw]

Originally Posted by AC_Hacker

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.

How about monitoring CO2 and humidity?

[Ko_deZ]

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.

[AC_Hacker]

Originally Posted by Ko_deZ

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

Good advice.

-AC_Hacker

[AC_Hacker]

Originally Posted by Piwoslaw

How about monitoring CO2 and humidity?

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

[Ko_deZ]

Originally Posted by AC_Hacker

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.

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.

Originally Posted by AC_Hacker

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.

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.

Originally Posted by AC_Hacker

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.

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.

Originally Posted by AC_Hacker

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

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.

Originally Posted by AC_Hacker

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.

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.

[AC_Hacker]

Originally Posted by Ko_deZ

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.

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

Originally Posted by Ko_deZ

If you need any help with the arduino, let me know.

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

Originally Posted by Ko_deZ

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.

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