They are legally required in the primary bedroom of all new build houses in Scotland, possibly also in England and Wales.

Huh? Really? Are you thinking of Carbon Monoxide detectors, perhaps? Sometimes these are incorrectly called "CO2 detectors".

Appears to be true:

> CO2 monitoring equipment should be provided in the apartment expected to be the main or principal bedroom in a dwelling where infiltrating air rates are less than 15m3/hr/m2 @ 50 Pa.

https://www.gov.scot/publications/building-standards-2017-do...

Well done, Scotland.

Monitoring equipment alone isn't really sufficient, unless it is hooked up to a fan or ventilation equipment to maintain a suitable level, and an alarm to alert if that level can't be maintained.

If the law requires just monitoring equipment, then you know that in a utility cabinet somewhere will be a screen with the data on, that nobody ever looks at, and might as well not be there.

New houses in The Netherlands have a monitor in each room (bedrooms, kitchen, living room and bathroom/shower) that control the mechanical ventilation.

In fancier homes they can control the ventilation per room, though the standard is that they control the ventilation for the entire house.

They are part of the sensor suite in big buildings.[1] The usual sensor set is temperature, humidity, and CO2. It's mostly hotels with conference rooms that are serious about this, because their product is comfortable conferences.

As buildings approach airtightness, this becomes essential. I was expecting more in this area from the IoT crowd, but they never delivered. It's the old-school HVAC systems which do this.

Some years back I went to an IoT meeting in San Francisco, in a beautifully converted industrial building in Dogpatch on the waterfront. Big windows and skylights, with openable windows driven by manual chain falls on both. Ceiling fans. Plus a standard HVAC system. Was all of this coordinated in any way? No.

The place should have had motor drives on the windows. They were all on two long shafts, so it wouldn't be all that hard. As the place filled up with people, the skylights should be opened and the ceiling fans driven to blow upward and exhaust hot air and C02. Open the bay-facing windows slightly for intake air. As the evening went on and outside temperature dropped, close the bay-facing windows, continuing to monitor CO2 and temperature. If heating, open a damper to take air from the outside as necessary to keep the CO2 down. When everybody leaves, flush the air for a few minutes, then close everything up, turn off the fans, and let the temperature drop a few degrees.

The place should have been a showpiece for intelligent building control. But what were they pushing? Refrigerators with a tablet built into the door, for more than the cost of a refrigerator plus an iPad. The tablet had no useful interaction with the refrigerator; it was just a general use tablet.

Even home "smart thermostats" don't do much. They need this set of sensors, plus an external air damper so they can switch from recirculate to outside air as necessary. And the ability to run fans, without AC or heat as necessary. But that requires actual installation, which neither Amazon nor Apple can competently handle.

All this stuff saves enough energy to pay for itself.

[1] https://buildings.honeywell.com/gb/en/products/by-category/s...