How to manage high co2 in the home on the cheap.

I am not a Certified Industrial Hygienist. That being said, I'm passably familiar with air assessments, and one thing that stands out to me is that you don't know it's the CO2 causing your symptoms -- you just know that higher CO2 correlates with your symptoms. That's totally understandable, because it's the CO2 that you can measure. Indoor air folks generally use CO2 measurements as a useful way of gauging roughly how much stale air is building up over a day -- you measure CO2 in the morning, let in the humans, and then measure how much higher it gets. Rapidly rising CO2 is a sign of poor ventilation. From that, I'd say you have evidence that your house is poorly ventilated. However, that doesn't mean that the CO2 is what is causing your health problems. There are a lot of other things that will build up in a poorly ventilated space -- VOCs from offgassing building materials, mold spores, dust, particulate, etc, etc. Any one of those could be either the primary cause of your symptoms, or could at least be contributing. So one thing I would suggest is looking into other possible causes. Depending on your insurance situation, you could get tested for allergens that might be contributing. (Even if you've been tested before -- I once managed to develop a new allergy in about 12 months, as confirmed by two scratch tests and observations. It happens.) You can also just take the approach of allergen reduction without testing. Inspect your house for water infiltration or mold growth. (You mention that part of your house is subgrade, which raises your chances of getting mold.) In your bedroom, vacuum, dust, wash all bedding in hot water and dry on hot. (That last one alone makes a huge difference for me.) Run a cheap Wal-Mart air filter (available for $30) to cut down on dust and particulate. If you use any scented products, candles, air fresheners, etc., remove them. Remove any bedding with feathers. Try keeping one room pet-free. Alternatively, you could bring your scale to a friend or family member's house, and see if you can find someone else with indoor CO2 levels as high as yours. If you can be exposed to similar CO2 levels in someone else's home without showing symptoms, that raises the probability that the CO2 is just a proxy measurement for the real issue. Incidentally, I'm not writing off CO2 as a potential cause -- it totally is, and there's http://newscenter.lbl.gov/2012/10/17/elevated-indoor-carbon-dioxide-impairs-decision-making-performance/, and certainly at levels that you're seeing. HOWEVER, I do think it's worth considering other potential causes, if only because some of them may be easier and cheaper to address than the CO2 issue.

It sounds like your options are 1. Blow your old air outside. 2. Suck in new air. 3. Remove the CO2 from your existing air. You said you think option 1 (negative pressure) is dangerous. Number 3 will drive up your heating bills and even so you'll likely be cold. I think the answer is number 3. Plants may not consume "that much." but it's not clear that you need to consume "that much." You don't need to get rid of every CO2 molecule, just to have fewer of them. Get lots of plants and you should have fewer of them. Find fast-growing plants as these will suck up the most, says the interwebs.

On the chance that some of your CO2 is seeping from the ground, I would not install a vent to blow air out the window, that would just suck more CO2 out of the ground. If you think that there might be seepage from the ground into your basement, you could look at the kind of basement ventilation units that are designed for radon problems.

When I lived in a cold climate, I would still open the windows in the bedroom at night (unless it was directly snowing inside!) and would ventilate the rest of the house once a day by turning off the heat and opening doors and windows. I actually wasn't aware of CO2 pollution indoors, but I just liked fresh air. I don't know where you live and so windows really may not be an option, but could you at least leave the door open for a few minutes a day? And yeah, definitely get plants.

[ICNH]'s other here; I know that growing plants are of questionable value, but as I see it, it can't hurt. So we use bamboo, because it's fast growing, dandelions, lettuce or squash, because we can feed it to the tortoise, and marijuana. I figure we can then sell the pot, and use it to buy a properly installed air/ heat exchanger. And if we're caught, I can hope that the police have enough of a sense of humor to let us off with a stern warning about selling schedule 1 drugs for home improvements. (kidding, of course, but I suspect getting information about how to realize one of these in our house is going to be the answer that solves this problem.)

You may be able to have an HRV installed and rent it, rather purchasing it and owning it. (I realize renting doesn't make sense in the long run, financially.) Call around. Buy a heat recovery ventilator, and just venting it in a window (some of the units can be picked up for $500-$600, which I think we could swing), and later having it done correctly. I think you need to 'balance' the fans in HRVs to avoid negative pressures and so on, along with installing inlets and outlets on various exterior walls in the right way to deal with differing wind pressures. Technicians up here are trained, probably licensed, and carry manometers. Since you guys are in a sticky spot, double check with people in an HVAC forum to see if this is feasible.

We also did discuss plants, but the general consensus seems to be that they don't actually consume that much co2 in a home scenario. You can put the sensor near your plants and see if they're helping. I'm a bit http://goaskalice.columbia.edu/indoor-air-quality-and-plants, but data is the important thing.

Don't open up a designed-to-be cold attic to the warm house, you'll damage your roof. Can you air the house out for 15 minutes a day? Open all the doors and windows and get a good cross breeze going, then just close it up and turn the heat back on.

I would try an experiment, open up the highest and lowest window in the house a bit to create a constant air leak, one which the furnace will have to supplant to keep you warm. Do it and compare how you feel along with the nightly energy usage (natural gas / electric meter) to get an idea of what this will cost you. You might find for just a few dollars more a month you get a low CO2 reading. Also do you have a Carbon Monoxide detector in your house?