Will positive pressure lower CO2 levels?

#1

Not sure this is the right place for my question, but I’m hoping it will give me a starting point. I’m a pretty capable DIY’er but know next to nothing about air quality and ventilation, so here I am!

I live in an off-grid tiny house, and I’m trying to figure out how to decrease CO2 levels inside. When I’m away (whether for hours, days, or weeks), levels are consistently in the green zone (1-2 LED’s), so I’m assuming indoor activities – rather than outdoor pollutants – are the cause. The house is ~250sf/23m2, insulated but leaky (around the windows, range hood [directly vented through the wall], and maybe other places; I’ve never formally tested).

Other considerations:

I live in the northeast US, so temperature is a factor. I don’t mind opening windows/doors to ventilate in the summer because I generate enough solar power to cool things off with the aircon when necessary (wasteful, but also kinda fun to feel like I have unlimited cooling). In the winter I don’t have that luxury.

Propane cooktop: This is the biggest contributor, since it consumes oxygen. As a rule I open the window whenever I use it, but the CO2 levels still go up to 6-7 LED’s. I end up opening the front door as well, which isn’t fun when temps are below freezing! Switching to electric is not an option at this point.

Even if I’m not using any propane appliances, CO2 levels will still go up to 3-4 LED’s at times. Dog farts may play a role, but I’m guessing it’s the small space and not enough fresh air that are the main cause.

I’ve considered an ERV (they even make them in 12V, which would consume less of my battery power), but I am not sure my leaky house would make it worth it. I can discuss with an HVAC person.

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I’ve come to the conclusion that if I maintain positive pressure in the house, I might be able to bring in enough fresh air to keep CO2 levels down. Does that seem reasonable? If so, I’m trying to figure out how best to do that. Obviously consulting an HVAC professional will be necessary, but I’d like to go into that conversation with some additional knowledge.

Thanks for your thoughts and experience on this!

#2

Any forced ventilation, whether its a positive pressure or negative pressure one, will make the CO₂ levels go towards ambient until the rate of generation indoors (from your breating, cooktop burning, etc.) equalizes with the rate at which CO₂ gets removed.

In theory since CO₂ is ~uniformly mixed with other gasses in the air, the rate should be proportional to the amount of airflow. So given a similar amount of airflow (or ACH) all systems will do a similarly good job at the task. With that in mind, you ought to consider instead is how the chosen solution affects other parameters.

Lets start with the worst possible… anything involving a negative pressure is generally frowned upon unless its a fume hood. It will result in air getting pulled in through all the leaky holes in in the weirdest of places (helpfully marked for you you with condensation – and later mold). It will also naturally pull dust and other small particles from outside to the inside.

A positive pressure helps with the dust and condensation problem. It gives you an opportunity of a singular location to put a good filter on your air intake, helping improve Indoors Air Quality further. But yeah, if you set the airflow up for a full air change every 6 hours (a minimum target for new construction in countries that bother writing this into their code; not sure about US), you will have to figure out where to get the energy to continuously heat/cool all that air indoors from “scratch” 4 full times a day! And since there is going to be a fan blowing potentially-freezing-cold air in a single, concentrated location, not adding a heating element on the intake would make living anywhere near that intake a very unpleasant experience.

Which naturally transitions us to an ERV. Here in addition to the heating element and filter one adds a heat exchanger to recover some of the heat energy from the air previously already heated. You would still set it up to be very slightly positive-pressure biased (to avoid all the problems outlined with negative pressure setups above) but instead of wasting 100% of your energy through all them leaky holes, you would waste… less. Potentially very significantly less.

Somewhat rhetorical question is then whether you have plenty of spare (free?) energy to heat a lot of air? If you don’t then ERV is likely your only option regardless. But assuming you have plenty of energy, the payback on an ERV is pretty simple to compute once you know the energy needs for a positive pressure system. That’s a number in kW any HVAC specialist will be able to compute for your specific situation as they will need to size a heater element regardless of the system.

#3

Thank you! This is really helpful.