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Comment by rektide | Hacker News Reader

It does matter for sure. At least in a modern/energy-efficient "tight" (as in, airtight) build.

You can be right that turning on circulation makes the number go down. But that doesnt matter. Over time the concentration can go up and up and up, even with h"v"ac running.

Few residential HVAC systems actually ventilate. The focus on energy-efficiency has been a bit blind to the need to get rid of od co2 & take in oxygen. You can get fancy energy-recovery ventilation (ERV) devices that actually do air-exchange with thermal coupling, to cool or heat the incoming air. But this sort of system is alas exceptionally rare.

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I installed a broan nutone ERV this past summer and it's made a tremendous difference with the CO2 levels in my house. We retrofit spray foamed the house (don't, dumb idea, definitely not the attic if you're reading this) and CO2 would build up in the nursery over night well past 1400ppm according to an AirThings Wave+. Now at night it averages like 800ppm in there.

With an ERV, the best thing that I found to do is wire it to my furnace such that when the furnace kicks on, it will trigger the ERV to start. Otherwise you'll get furnace blower cycles separate from ERV cycles and it won't be as effective.

Seconding the large impact of an ERV, the air inside feels much fresher when it's on vs when it's unplugged.

I've taken to just running the HVAC fan at all times, regardless of whether heating/cooling is happening. This tends to even out the temperature throughout the house, as well as CO2 concentration, it keeps the air filtering through the HVAC HEPA filter, and the ERV can run continuously, so rooms rarely get above 600 ppm CO2. Unsure about the energy use impact, but the air quality difference is palpable.

> We retrofit spray foamed the house (don't, dumb idea, definitely not the attic if you're reading this)

Can you elaborate? We are considering moving into an older house and renovating. Adding insulation being part of the idea. Our current home has full foam insulation with erv so I am curious what difference you experienced.

There's two issues with spray foam insulation:

First the off-gassing. It emits a lot of harmful compounds as it cures and I understand the emissions to go on for a long time. So if you mind about air quality it wouldn't be my choice.

Second is the potential for damp to build up in the building fabric. The foam is airtight so make sure the ventilation is right on both sides of it.

A bonus third is the pain of removing it when you need to do building repairs. With board or batts you can lift them out; with spray foam you have to cut then scrape off.

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