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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeMay 7th 2017 edited
    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?
    • CommentAuthorRobL
    • CommentTimeMay 7th 2017
    We have one of these(older unit with no logging though), with the audible alarm used to increase the mvhr rate by 25%. (By 0-10v input). It took a bit of fiddling, and sadly it's just an on/off response. I'd like to improve it, but it does just work!

    • CommentAuthorEd Davies
    • CommentTimeMay 7th 2017 edited
    Personally I'd use one of these:


    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.
    Posted By: RobLWe have one of these(older unit with no logging though), with the audible alarm used to increase the mvhr rate by 25%. (By 0-10v input). It took a bit of fiddling, and sadly it's just an on/off response. I'd like to improve it, but it does just work!

    Is it just an on/off response because that's what the sensor delivers, or is it a limitation on the Vent-axia end?

    A bit frustrating not to be able to set up a proportional response on a unit that already has a variable fan speed control.
    • CommentAuthorRobL
    • CommentTimeMay 7th 2017
    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.
    • CommentAuthorlineweight
    • CommentTimeMay 7th 2017 edited
    Posted By: RobLIt'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.

    Ah, ok. Good to know. Thanks.

    It seems that CO2 sensors are just expensive. I guess that's why most MVHR units with built-in monitoring controls seem to rely on RH only.
    • CommentAuthorEd Davies
    • CommentTimeMay 7th 2017
    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
    • CommentAuthorcjard
    • CommentTimeMay 8th 2017
    I've connected a wired controller to my BMS socket on mine. Due to problems I had when setting it up, I've come to know that on the 4 pin connector the far left and far right cores are power for the remote, and the two inner receive and transmit. Which way round this is depends on which way you're looking at the connector of course. In the default colours of the supplied cable:
    Black/yellow: positive and negative (unknown which way round)
    Green: signals from the unit to the wired controller, to drive the display
    Red: signals from the controller buttons to the unit, to change the mvhr behaviour

    If nothing else, could the signalling pattern in use by this device be worked out so the display data could be parsed, and signals be sent down the red wire to change mvhr behaviour?

    There are a set of diagnostic screens with various unknown info on (0,1 are fan speeds, 2,3 are vent temps for example) that could be parsed if needed, but for the most part I expect that the control would really only need to be one way; work out the set of signals required just to control it as if someone is pressing buttons on the wired remote. Heck that might be even easier by just getting a wired remote (or attacking the main control board in the unit) and soldering the switches up to a set of relays/some kind of electronic switch controlled by a computer. Then the program can alter boost by sequences of "button presses" as though a human would. It doesn't need to operate particularly quickly
    • CommentAuthorsnyggapa
    • CommentTimeMay 9th 2017
    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 :)

    • CommentAuthorEd Davies
    • CommentTimeMay 9th 2017
    Agree - just “pressing” the buttons would likely be very fragile, reading and checking the output would be much better. The only exception would be if there was an easy way to get back to a known state. E.g, press two buttons together to get to the main menu or something then do that at the start of each sequence. Still, I'd expect the protocol to be reasonably easy to reverse engineer.

    The question would be, what would Ventaxia's attitude be? If you just do it for yourself then I doubt there'd be any problems; them wanting it disconnected for any service/warranty issues is likely the only thing. If you start making wall mounted displays to replace theirs for £2 less they'd probably sue you. Publishing the protocol or open-source software to talk it falls in the grey area between; it's the sort of thing companies could get upset about but might consider to be OK so long as you make it clear they don't support/approve it.
    • CommentAuthorcjard
    • CommentTimeMay 10th 2017 edited
    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..
    • CommentAuthorcjard
    • CommentTimeMay 10th 2017 edited
    So what is needed to reverse engineer the protocol that drives the display? Oscilloscope?

    Heck, thinking about it, it's probably a reasonable candidate for OCR if you want to get really crude/sophisticated(depending on your perspective) - standard webcam should be able to "see" the display clearly enough
    • CommentAuthorEd Davies
    • CommentTimeMay 10th 2017
    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.
    • CommentAuthorEd Davies
    • CommentTimeMay 10th 2017
    Huh, re-reading page 1 of the thread:

    Posted By: john_connett9600 baud, 8 data bits, 1 stop bit, no parity.
    • CommentAuthorRobL
    • CommentTimeMay 20th 2017
    Ok, I went ahead and bought one of these CO2 sensors for £118:

    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.
    • CommentAuthorcjard
    • CommentTimeJun 2nd 2017
    Quote the op:
    Let's say I think the effect could be better. I think loads of air is bypassing the bypass, but we'll overlook that and continue ... what I'd like ideally is more manual control of the bypass. In hot summer weather I'd like the bypass open all night to get maximum cooling, instead of closing again when inlet temp drops to 21C.

    Yes, I'm tempted to agree if what I thought was the bypass when I had the covers off, truly is. It looks like opening the bypass will allow air past the hx as well as through it. Perhaps there was another valve I missed somewhere, but I've just had 4 digital Chinese thermometers arrive so I'll start monitoring the vent temps and see..
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