Posted By: RobLWe have a va sentinel, with diy co2 sensor. If I was to do it again, I'd give this unit a go:

https://www.digikey.com/product-detail/en/Amphenol-Advanced-Sensors/T8031/235-1413-ND/5774483" rel="nofollow" >https://www.digikey.com/product-detail/en/Amphenol-Advanced-Sensors/T8031/235-1413-ND/5774483

What did you use for yours? Do you have it on the 0-10V inputs?

Personally I'd use one of these:

http://shop.vair-monitor.com/index.php?rt=product/product&product_id=133

I already have one with the temperature/humidity/pressure option. A few others on here have them, too. I think it was Borpin who first mentioned them.

(At first they were a bit sensitive to noise or over voltage or something from the USB supply but the combination of an extra capacitor and firmware fixes has sorted that. My one of these plus two temperature/humidity sensors from the same source have been running quite happily for a few months now.)

It sends its output to an MQTT server (Mosquitto) running on a Raspberry Pi. Hence to control a Ventaxia Sentinel with it would be trivial if there was a simple way to drive a 0-10V output from the Pi. Of course it's not difficult anyway, DAC and op-amp plus a few other odds and ends on a bit of board but it'd need some work to get right and would finish up less reliable than a manufactured solution.

E.g., while looking into this I read that some 0-10V inputs sink current whereas other source it. That might not be dealt with very easily by a relatively high impedance op amp so you might want an emitter follower or something on the output to reduce the effective impedance but which way round you have a trivial circuit it might depend on the behaviour of the particular thing you're driving.

The advantage of using a Pi rather than just wiring up 0-10V sensors to 0-10V actuators is that you can be a lot more flexible in how it operates. E.g., target a lower CO₂ level in the evening then let it get a bit higher and avoid the fan running above a certain speed during the night. Or deal with humidity as well in a sensible manner. Or send yourself an text if a burglar increases the CO₂ while you're out… And it can log that sensor and others.

It's the co2 sensor that's the limitation, the va is fine. As I say, 'one day' I'll improve it, £100 and a cable would fix it.

Adding to my list of reasons to have a computer between the sensor and the MVHR: you can take account of the rate of change of CO₂ level either with a software PID controller [¹] or, equivalently but more explicitly, by estimating the rate of production and controlling based on that. That ought to reduce the maximum rates at which the fans are run by anticipating peaks meaning the MVHR is run more efficiently and with less noise.

[¹] https://en.wikipedia.org/wiki/PID_controller

ha. simulating user input by use of the buttons - crude but in theory should work to a degree.

We used to do a lot of that in the "bad old days" - called it screen scraping where you were reading the input from a foreign system , then getting another computer program to simulate keystrokes to send, and read the response back , hoping the correct result had happened

Your problem here is that you are doing it blind, so pushing buttons without knowing what the response is - so pushing up 5 times, then select, then down twice and hoping that you are changing the inlet fan speed :)

so if someone manually presses a button or the unit restarts or unexpectedly ends up on a menu item to get it out of sync or whatever, you have a right pickle on your hands!

I would trust it with the "boost" button but not the up and down selecting menu items. And if you just want a binary boost/not it already has a direct connection for that on the board, I think

It is an interesting academic exercise however, but not one that I would use myself :)

-Steve

Posted By: snyggapa
so if someone manually presses a button or the unit restarts or unexpectedly ends up on a menu item to get it out of sync or whatever, you have a right pickle on your hands!

On this particular unit, holding UP for 2 seconds (to scroll to the top of the current menu options) followed by UP for another 2 seconds gets you back to the base "normal airflow" screen, i.e. the root starting point. If youre already at the top, the first UP will root you, the second is no-op.
Probably a sensible precaution to take

I would trust it with the "boost" button but not the up and down selecting menu items.

Well, it's not like anyone's gonna die..

Yes, ideally a digital storage scope initially. Cheap USB one should do for a job like this as the data rate won't be high. Still, it's most likely one of 3.3V, 5V or RS-232 signal levels which should be identifiable with a multimeter. Almost certainly standard asynchronous serial (start bit, 7 or 8 data bits, maybe a parity bit, 1, 1.5 or 2 stop bits). Maybe (extended-?) ASCII or maybe some odd binary data format. Once that's all identified just watch the data going each way with a computer and scratch your head.

Ok, I went ahead and bought one of these CO2 sensors for £118:
https://www.digikey.co.uk/products/en?keywords=235-1413-ND

It was pretty easy to install - I used a 10m length of telephone cable and a 3 way terminal strip. No soldering required, just a screwdriver & cutters. Originally I planned on putting it into the exhaust duct, as it says to do - but that would mean putting it very close to the shower, and I worried that it would suffer from sporadic high humidity or even condensation. In the end I put it in the main bedroom upstairs.
I plugged it into the 24v supply of the VA kinetic plus and 0-10v input, and set the channel to "CO2" mode. That mode is already set up by VA so that 10V=2000ppm CO2, 0v=0ppm CO2, which is exactly how the unit above works out of the box. They've obviously targeted the VA unit, or maybe 24v supply with 0-10V 0-2000ppm is standard somehow?
All I had to do was set two CO2 ppm levels on the VA, between which it proportionally varies from min to max power. For now I set it to 400ppm=normal, 1000ppm=boost.
It's early days with the sensor, but so far so good. If there's anything interesting, I'll post it.