My SEN66 Indoor Air Quality (IAQ) Monitor Project

ESPHome
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Indoor air quality (IAQ) is an important aspect of a comfortable home and is often unmeasured and overlooked. After installing a mechanical heat recovery ventilator (MHRV) and controlling the unit in Home Assistant, I wanted a way to measure the impact of it and also have it ‘kick in’ when air quality was compromised.

After reading through tons of posts about air quality monitors and Home Assistant, including the Aura Project and the AirGradient models, I came to the realisation that I needed to build one myself based on the Sensiron SEN66 sensor (£55), which measures temp, humidity, CO2, VOC and particulates (1, 2.5 and 10).

I wanted to have the sensor itself split from any screens and basically work as a satellite. The main reasons behind this were to not be tied into putting the screen somewhere convenient and to also not have the heat of the screen affect the readings and to have to offset temp and humidity to compensate for it.

I also wanted to have a plug-and-play system without having to build cables, solder or use pins on an ESP board, so the choices became quite narrow. I ended up with an Adafruit QT Py ESP32-S3 (£12) and the Adafruit SEN6x Breakout for Sensirion SEN66 (£5), which are both tiny. They connect with a 4 pin Stemma QT and the sensor connects with a 6 pin JST GH cable. All nice and tidy and can fit in a small matchbox vented enclosure (still waiting for the 3D prints).

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After some trial and error YAML coding, I managed to get the unit up and running in ESPHome and got all of the sensors reporting in HA. So far the sensor setup has been solid and not dropping out.

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I also wanted to build a small bedside monitor that would give me the key statistics, as well as the time and weather, so bought a small ESP32-S3 Development Board with 1.28" IPS Round LCD which comes in a metal case (£20) and sits on a small USB phone charger (£7). It displays CO2 and the green dot in the middle changes colour based on the IAQ reading (good, fair or bad). This is enough info for a quick glance.

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So for about £100 I was able to build a high quality IAQ monitor, which is a straight ‘plug and play’ setup that will work with any HA compatible display and can drive my MHRV unit based on time of day, humidity and CO2 readings. All in all I am glad I went the DIY route and am very happy so far with the results!

1 Like
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Was also looking into this then i notice the good sensors that measure real co2 and oxygen are quite expensive you did a great job :slight_smile: for a budget all in one

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Very nice. I’ve built something similar with the same Adafruit breakout and the SEN66 module.
I do like your display and case :slight_smile:

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Great project. Have you managed to track any improvements since setting up the MVHR?

We have a modern house thats pretty airtight (SIP panel build), consequently it suffers with poor air quality which I have been monitoring with Airthings devices (2 of them), plus a Apollo MSR-2.
The advantage of the Airthings, is that they also track Radon gas, which is quite high here in Devon.
I’m already planning to retro-fit a MVHR system to improve our internal air quality, not only is the Radon a concern, but CO2 levels in bedrooms get high overnight.

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Early days yet, but I am tweaking as we go. CO2 is also really high at night in our bedroom so I am trying to find a balance of keeping the CO2 below 700 or so and running the MHRV all night. Boosting during high humidity works great to bring it right back down, but I’m surprised at how much fresh air a stuffy room takes to bring down the CO2.

It is great to be able to see realtime results and also monitor the exhaust and supply temps as well to see how efficiently it is working. Currently with 12C outdoor and 20C indoor I am getting a supply of about 18C which I think is pretty good.

With no MHRV we were at about 1400PPM at night and we are now down to 800 with the fan on 30% for about 4 hours. So potentially running for 6hrs at 25% may do the trick. Still only a few pennies to run at cheaper electric rates so well worth the fresh air at night!

If you are interested in controlling your MVHR with HA you can read my post here how to use a £20 Shelly and a VentAxia Sentinel: