First, all the monitors I tested featured auto-calibrate. Some, including the Ararnet 4, can also be manually calibrated. I recommend avoiding monitors that cannot be manually calibrated unless the monitor is regularly exposed outside air. Then the question is how regularly? Good luck with that! If the monitor will be used primarily in one location, auto-calibrate should be avoided as it will periodically adjust its baseline to the lowest reading over some typically undefined time period. Some companies actually warn not to use the auto-calibrate feature, presumably because they’ve received so many complaints.

Second, atmospheric pressure has a proportional impact on NDIR sensor readings. This requires some explanation… The impact pressure is significant if you use a CO2 monitor at higher elevations (or in an airplane where cabin pressure is typically set to about 8000 ft). Temperature also affects readings but much less so than pressure. Here’s a primer about that.

Based on my testing, the Aranet 4 is the only CO2 monitor in the $100 to $250 price range that auto-compensates for temperature and pressure. I live at 4,400′ MSL so this was easy for me to test. One product has a user-defined altitude setting that helps, but it baffles me that none of the other monitors I tested implement the compensations described in the above link, even though most have both temperature and pressure sensors!

If a monitor can be manually calibrated, that will adjust the baseline to the current elevation, but this just applies an offset. However, the pressure relationship is not linear… the error at higher CO2 levels can be substantial. I posted about this in the Aranet forum, and confirmed the results of my test of the Aranet 4.

John, I am sure you’ll never look back, and it wasn’t just CO2 that went down!

Going for the 15A Rheem HPWH is somewhat similar to choosing a right-sized heat pump for your home without the resistance emergency heat strips. The difference is, when you replace a WH, you replace the whole system (well, still got hot water delivery issues that Allison talked about in previous posts). When you replace a heating/cooling unit for your home, you’re only replacing the heat exchanger part of the system. The rest of this system, as John noticed, is the building envelope, just like the insulation surrounding the tank on a water heater.

Absolutely. That’s my preference as well.

My preference is to assume it might be harmful, even at levels not immediately perceivable, and to avoid/mitigate the conditions that lead to them. Research from the last decade makes it quite clear:

https://www.nature.com/articles/s41893-019-0323-1.epdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548274/

https://pubmed.ncbi.nlm.nih.gov/30172928/

https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037#r39

https://www.mdpi.com/2076-3298/8/11/125

https://newscenter.lbl.gov/2012/10/17/elevated-indoor-carbon-dioxide-impairs-decision-making-performance/

The impact of CO2 on individuals when the levels climb above 1000ppm has insufficient data to prove immediate harm. For individuals the ability to perceive an impact to their ability to think or perform is often confused by the suggestion that the IAQ is impacting them.

It’s not a hard red line that if you cross over it – you can feel or detect harm. In fact most people can’t detect it themselves – observers and researchers testing may be able to measure, then analyze the data and report the results days or weeks afterwards.

The reason that the impact is seldom noticeable – or even an impact at all, is because our bodies over the 1000’s of years has adapted to regulate the CO2 balance that impacts our bodies blood chemistry. We are actually very good at responding to slow changes that return back to a normal healthy level. And that response depends a lot on the individual their current health (age, gender, prior existing health conditions, e.g. a person with COPD may respond quite adversely to higher CO2, just as a person with congestive heart failure).

More importantly at this time is the education of the public that we want to manage the indoor CO2 levels to remain below 1000ppm if possible – in that the individuals at risk – are not harmed.

Keeping it below 1000ppm also means that we’ve a better means to address the body odors — which was in part — some of the original research done by Pettenkofer back in the 1850 -1860’s (German public health).

Interesting. According to the WHO:
“Most modern aircraft have recirculation systems, which recycle up to 50% of cabin air. Ventilation provides a total change of air 20–30 times per hour.”

https://www.who.int/news-room/questions-and-answers/item/air-travel-advice

Bravo!