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Extending the current exponential for 20 years, we get into the 500ppm region.

I don't think that's enough to need scrubbers.

Indoor is always higher ppm (how much depends on many parameters) without proper ventilation. „Proper“ should include a „Heat exchanger“ thus you don’t need to reheat fresh air.

Still, it will add some 80ppm over the amount you have today. There's a huge amount of disagreement over how much CO2 is harmful, but it tends to happen over numbers way above 800ppm.

If your room has 2 times the open air concentration, and you are concerned if it's 2.0 times or 2.2 times, you should already be dealing with the problem.

It's not way above, it's around 700+.

So at 500 external you'd pretty much need continuous ERV but not necessarily scrubbers just yet.

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