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AC_Hacker 01-29-13 03:22 PM

Arduino CO2 Sensor Controls PWM Fans in HRV
2 Attachment(s)
(* The title of this thread should have been "Arduino CO2 Sensor Controls PWM Fans in HRV, not "PCM" Fans. My bad. *)

EDIT (2014-06-09): For those who may doubt the need for a HRV, or a CO2 controlled HRV, here is some interesting reading. Thanks to BBP.

A new EcoRenovator from Ireland, cerberus, tipped me off to some inexpensive CO2 sensors. In the past, CO2 sensors had been pretty expensive, but the attractive price (about $10) got me started. Thanks, cerberus!

* * *

My project idea is that HRVs are used to provide fresh air and at the same time minimize home heat loss. The heat loss from the outgoing air can be reduced, but never reduced to zero.

A 'smart' HRV would sense the level of CO2 and when the level of CO2 reached a predetermined allowable level, it would run the fans. If the CO2 level was higher, it would run the fans at a higher speed, if the CO2 was lower, it would run the fans at a lower speed, and if the CO2 was acceptably low, the fans would not run at all.

EDIT: My thinking has changed on this part of the project. I'm now considering using the varying voltage from pin 4 of the CO2 sensor as an input to one of the Analog in pins on the Arduino... and controling the Arduino's PWM pin with that. I found some examples of that HERE and HERE.

This would increase the overall efficiency because the HRV heat loss would only be incurred when required.

So far, petty simple.

This project will use an Arduino to read a CO2 Sensor (Telair 6004) and then use the CO2 readings as input to control PWM fans in the HRV.

Here is a photo of one of the CO2 monotoring boards, measuring about 2" x 2" in all it's twinkling, gold-toned beauty:

An here is a diagram of the 12 pins that seen at the bottom of the photo above...

And some documents:
So the Telaire 6004 is capable of either analog output (pretty good) or digital output (pretty accurate). But so far as I have been able to determine, the digital output is not so easy to implement.

I was able to locate the following code to read the analog output pin (pin 4) on the Telaire 6004 and to blink an LED that is connected to Arduino digital output (pin 13).


Analog Input
Demonstrates analog input by reading an analog sensor on analog pin 0 and
turning on and off a light emitting diode(LED) connected to digital pin 13.
The amount of time the LED will be on and off depends on
the value obtained by analogRead().
int sensorPin = A0; // select the input pin for the potentiometer
int sensorValue = 0; // variable to store the value coming from the sensor
void setup() {
void loop() {
// read the value from the sensor:
sensorValue = analogRead(sensorPin);
// sensorValue is the variable that stores what the CO2 reading is
// Send the value back to the computer

* * *

Since I am barely able to make an Arduino blink it's little light (which is the most basic exercise in the Arduino House of Chops), I can use help.

So, I'm open to any ideas from any of you Arduino dudes & dudettes on this project. I ain't proud.



stevehull 01-29-13 09:28 PM


Using CO2 as a trigger for an HRV (or ERV) will not work as a reliable index of home air "staleness". Here is why . . .

The average adult at rest gives off about 200 ml of pure CO2 per minute. Let's think of an really airtight house with four people in it after 7 days.

Assume a house of 1200 ft2 and a height of 8 ft (volume of 9600 ft3 or 270 meters3).

Four people at the end of one week give off 8,600 L of pure CO2 (8.1 meter3).

The increase in CO2 at the end of one week in a literally airtight house with four people is 0.03% (8.1/270)- an almost vanishing small number to trigger with.

A FAR better option is to place the HRV (or ERV) air intakes in the bathrooms running at about 10-20 CFM. When either the bathroom light or shower light goes on, then the blower speed goes up 2-3 times (with a "hang" timer to keep the higher fan on for 5-10 minutes post use).

I used this exact scheme in a home I built over 20 years ago and had indoor winter humidities between 30-35% and summer between 45-50% (no higher). No steam covered windows in bathrooms, always fresh in the home and no mildew in walls.

I looked at CO2, did the calculations and am wondering why commercial units use it. People "believe" CO2 will build up, but in even a super tight home (one air change per day), the use of CO2 as a trigger doesn't cut it.



kostas 01-30-13 10:49 AM

I think stevehull has a point here.
Maybe the problem is not the CO2 after all, but the stale air by its self. What makes the inside air uncomfortable is a mix of (ok, CO2), humidity, odors, smoke (for smokers and cooks), ecc, and that's not easy to detect by electronic sensors. I guess the best method is to try down and find the minimum acceptable air change for each environment.

AC_Hacker 01-30-13 10:58 AM


Originally Posted by stevehull (Post 27823)
...Let's think of an really airtight house...

Steve, I don’t need to 'think' of a really airtight house because I live in one right now.


Originally Posted by stevehull (Post 27823)
...CO2 at the end of one week... is 0.03% almost vanishing small number to trigger with.

Vanishing to your arithmetic perhaps, but not disappearing. This is probably why CO2 sensors have been so expensive for so long.

Here are ASHRE & OSHA specs:
  • normal outdoor level: 350 - 450 ppm
  • acceptable levels: < 600 ppm
  • complaints of stiffness and odors: 600 - 1000 ppm
  • ASHRAE and OSHA standards: 1000 ppm
  • general drowsiness: 1000 - 2500 ppm
  • adverse health effects expected: 2500 - 5000 ppm
  • maximum allowed concentration within a 8 hour working period: 5000 ppm

I also previously came across some studies of German shool children that indicated a much elevated level of performance when levels of CO2 were reduced well below ASHRE standards. I just did a search for that information I had found previously, and am not able to locate it... However, I did find this:


Elevated carbon dioxide may impair reasoning

Insufficient ventilation allows exhaled gas to build up indoors, diminishing decision-making abilities

By Janet Raloff

Web edition: October 16, 2012

Carbon dioxide has been vilified for decades as a driver of global warming. A new study finds signs that CO2, exhaled in every breath, can exert an equally worrisome threat — impaired cognition — in nearly every energy-efficient classroom, meeting hall or office space.

The work assessed decision-making in 22 healthy young adults. Their performance on six of nine tests dropped notably when researchers raised indoor carbon dioxide levels to 1,000 parts per million from a baseline of 600 ppm. On seven tests, performance fell substantially more when the room’s CO2 was boosted to 2,500 ppm, scientists report in a paper to be published in Environmental Health Perspectives.

These data are surprising, says Roger Hedrick of Architectural Energy Corp. in Boulder, Colo., because “1,000 ppm of CO2 used to be considered a benchmark of good ventilation.” Hedrick, an environmental engineer, chairs the committee that drafts commercial ventilation standards through the American Society of Heating, Refrigerating, & Air-Conditioning Engineers.

Carbon dioxide levels are often substantially higher in buildings than the 350 to 400 ppm typically found outdoors. Indoor values of 600 ppm are considered very good. But depending on how many people inhabit a room and how many times per hour its air is exchanged with outdoor air through ventilation, “there are plenty of buildings where you could easily see 2,500 ppm of CO2 — or close to it — even with ventilation designs that are fully compliant with current standards,” Hedrick says.

“We’ve seen higher CO2 levels associated with increased student absences and poorer performances on school-type tasks,” says study coauthor William Fisk of Lawrence Berkeley National Laboratory in California. “But we never thought CO2 was actually responsible. We assumed it was a proxy for other [pollutants].”

His group recruited college students to spend much of a day in a room with computers. Individuals could read or do what they wanted for much of the time. But for part of each 2.5-hour period, participants completed role-playing tests on a computer that required them to manage an organization as it underwent a series of problems or crises. Throughout each of three test segments, conducted in random order, room ventilation was kept very high. Only carbon dioxide levels varied by segment: 600 ppm, 1,000 ppm or 2,500 ppm.

The role-playing tests are more complicated than most used to measure cognitive abilities, notes epidemiologist and coauthor Mark Mendell of Lawrence Berkeley. But they offer a gauge of important real-world skills, he says. “And the magnitude of effects measured at 2,500 ppm was astonishing — so astonishing that it was almost hard to believe,” he says.

If these trends are confirmed in follow-up studies, Hedrick says, “it would be very strong evidence that ventilation rates need to be increased.” Carbon dioxide standards were developed largely with the aim of controlling body odor, he notes. High levels of CO2 were viewed as suggesting occupancy levels were climbing to where “a place may begin smelling bad.”

Classrooms are fairly densely occupied, he notes, so their carbon dioxide frequently exceeds 1,000 ppm. With an increasing push to reduce heating and cooling costs, “there are plenty of school districts using lesser amounts of ventilation,” he says. “So I would not be at all surprised to find 2,500 ppm in a lot of school districts.”

In fact, “CO2 levels are not that hard to control,” says Jack Driscoll of PID Analyzers in Sandwich, Mass. Building managers just need to measure them on a regular basis, he notes. At the American Chemical Society’s fall national meeting in Philadelphia, he described a new rapid, hand-held CO2sensor designed for classrooms and office buildings.


J.N. Driscoll et al. Wireless indoor environmental quality (IEQ) monitoring in classrooms and laboratories (VOC's, CO, CO2, and T). American Chemical Society meeting, Philadelphia, Aug. 22, 2012. [Go to]

U. Satish et al. Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance. Environmental Health Perspectives. doi: 10.1289/ehp.1104789. [Go to]
Here are relevant specs for the Telaire 6004:
  • Measurement Range = 0 to 2000 ppm
  • Accuracy = @ 72F (22C) when compared against a certified factory reference 40 ppm + 3% of reading

My design parameters are:
  • FANS_ON @ 1000 ppm (0.1%)
  • FANS_OFF @ 300 ppm (0.03%)

I'm sure I'll be tinkering with that.


Originally Posted by stevehull (Post 27823)
A FAR better option... bathrooms... winter humidities... No steam covered windows... no mildew in walls.

No steam covered windows, no mildewed walls? Maybe your standards are a bit low?

The CO2 monitor is the first part of the project, after I get that going I will integrate a DH22 (AKA: RHT03) into the controller to monitor humidity.


Originally Posted by stevehull (Post 27823)
I looked at CO2... People "believe" CO2 will build up... the use of CO2 as a trigger doesn't cut it.

People "believe"?

Doesn’t cut it?

Steve, right now I live in a house that I am radically insulating & sealing. I actually have first hand experience with what it is like to not have sufficient fresh air. I don’t need to imagine what it would be like... I am there right now. It is unpleasant, I am not just dreaming up a new project.

Steve, your research would be more interesting if it actually had a bearing on what I am now experiencing.

Do you have any experience with circuit design or software?


stevehull 01-30-13 02:16 PM


I can guarantee that CO2 is not building up. Rather, "stale air" is out gassing of construction components, the infamous body odor, cooking smells, but by far the humidity is the dominant issue.

Our respiratory control system is such that even tiny increases in the ppm of CO2 (2-3 ppm) will cause us to increase ventilation rate. It is almost impossible to withstand an environment with 1000 ppm of CO2.

Let's see what your house has in it. One way to test this is to hook up the CO2 sensor and record a week's data. All houses have air leakage. I believe the very best that I know of had one complete air exchange every 48 hours. I suspect your tight house has an exchange every 30 hours or so (very, very tight construction).

I have a lot of experience with CO2 monitoring and can help you with hooking up the analog output of the CO2 sensor to the A/D port. Biu while you are there, hook up a relative humidity detector as well.

Most homes do very well by using a constant 10-20 CFM HRV (or ERV) exchange volume all day long. The reason to kick it up is shower/bathroom use.

I used a DPDT wall switch to both control the 120 V light above the shower and to increase the air CFM to 50-60 CFM with a 555 timer "one shot" to keep the HRV fan on for an additional 15 minutes after the light went off.

The wall switch just to the bathroom turned also turned on the HRV fan to high for only 5 minutes.

Measure your CO2 and humidity - let's see what the house has in it now.


AC_Hacker 01-30-13 03:01 PM

1 Attachment(s)

Originally Posted by stevehull (Post 27833)
I can guarantee that CO2 is not building up.

Steve, I'm happy that you are offering guarantees with your opinions... this may come in handy at a later date.

In your first post, you said that:

"The average adult at rest gives off about 200 ml of pure CO2 per minute."

...and the rest of your calculations flowed from that number.

That may be a safe way to design living systems where you live, but where I live, I and most of the people I know are not content to spend their lives 'at rest'.

So, along this line, I happened across this useful chart...

And also, I looked at your ventilation rates and I calculate that your recommended rates would result in a 100% air change in 144 minutes @ 10 CFM and 72 minutes @20 CFM in my conditioned space.

I'll happily supply CO2 info when I get my circuit working, which is what I'm currently seeking help on.



Exeric 01-30-13 03:42 PM

Hi AC,
I think maybe Steve and you are talking past each other. I didn't quite realize until a few days ago what all the fuss was about HRVs. When I read an update of your thread recently about your insulation project on your house I got a real ah ha moment.

From everything I've read in that thread it sounds like the air quality problems you observe that neccessitates an HRV comes from excessive moisture tightness, not air tightness. I'm no expert but I'm very good with logic and my logic tells me that one can have near perfect air tightness and still have vapor permeability. This is the ideal. You want your home to have vapor permeability so you don't have mold problems while still making your home as airtight as possible. The opposite, perfect vapor tightness ALWAYS comes with perfect airtightness. Like I've said, I've been wrong before and may be now, but I think you are confusing the airtightness of your house as causing the air quality problems, when it is really your vapor tightness that is causing it.

There are many very good articles on the internet at both Green Building Advisor and at Building Science that explains the difference in the two concepts. I think it may also explain why a more radical solution, such as an HRV may be a good solution for you. It in fact may be the best solution for the situation you have. But in situations where the home is airtight while being vapor permeable then I agree with Steve that a simple venting plan is often sufficient.

Exeric 01-30-13 04:42 PM

Actually, Maybe an ERV would be more appropriate than an HRV because my understanding is that an ERV handles humidity as well as air and temperature exchanges. It seems like the situation AC is working with is a humid house.

Actually I'm a little confused about this. ERV's are usually used in humid climates but this isn't strictly the case in Portland, OR. Maybe someone else knows more about this, such as whether ERV's handle humidity in just one direction or both directions. If they can handle it in both directions depending on the direction of vapor drive, then an ERV seem more appropriate for AC's house, rather than an HRV.

AC_Hacker 01-30-13 05:44 PM



Exeric 01-30-13 05:58 PM

Well, that answers that. It seems to imply the humidity filtering only occurs in one direction - outside to inside. Too bad.

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