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    • CommentAuthorJohn Walsh
    • CommentTimeJan 29th 2017 edited
     
    Has anyone any experience with using a Raspberry Pi to control an mvhr unit?

    I'm installing a Sentinel Kinetic shortly and will then look at talking to it using python via the unit's RJ11 socket. As per the vent-axia supplied 'BMS Kinetic Current.docx' the unit simply acts as a serial device on a network and will act on/ reply to commands such as 'Sentinel Stop'.

    The above should be fairly straightforward. What I'm also hoping - and this is the fingers crossed part / has anyone done this? - is that I can also communicate using the Control Unit screen examples shown in the Installation manual. For example, will sending 'Normal Extract 30%' set the Normal airflow speed? In a sense, I'm hoping to be able to 'emulate' the unit's Wired Remote Control (which is designed to connect to the BMS RJ11 socket).

    One reason for doing this is that the house harvests warm air in the shoulder months, all controlled by a Raspberry Pi which monitors temps etc. When the Pi turns a fan on I'd like it to also reduce the mvhr unit's Supply fan speed and increase the Extract speed - thereby compensating for the air being pumped into the house from the solar gain spaces. According to the manual, this is similar to an 'antifrost' mode the unit adopts if the supply air temp falls below freezing. As such, the unit will happily run with unbalanced fans.

    I suspect the tricky bit could be getting the unit to accept commissioning mode commands. Any thoughts or pointers on this are very welcome.
    • CommentAuthorEd Davies
    • CommentTimeJan 29th 2017
     
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10862

    Don't know for sure but I really doubt the protocol would be as simple as sending the text shown on the control unit screen.
    • CommentAuthorJohn Walsh
    • CommentTimeJan 29th 2017
     
    Yes, you may well be correct Ed but I feel the need to try it out and see. I have looked at the postings you linked to and thought there was little evidence of people actually using the serial interface. The thing that encourages me to try is that the vent-axia docs say the unit "can be controlled and monitored by a BMS system". They then go on to provide an example (the 'Stop'/ 'Start' command), which looks to me to have been (mis)interpreted as the only thing you can do. For example, I'd be a little surprised if 'monitoring' doesn't include being able to send 'Indoor Temp' and getting back what the supply air temp sensor is reading.

    When I get round to it (within the next week or two) I'll report back. But in the meantime, if anyone has had a try at this or similar I'd be pleased to hear.
    • CommentAuthorwookey
    • CommentTimeFeb 1st 2017
     
    I (and Tim Small) considered doing our own control software for MVHR (including making a DIY box and stock motors), but in the end got a cheap deal on ITHO HRU4's instead. But that would have been rather grubbier than just using the handy serial interface (PWMing the motors). These machines don't have a serial interface, although there is a plug for an external 'boost controller'. One day I may get round to dicking with the controller, as it doesn't do in/out differential speed, and my pipework is such that that would be a good idea. And I'd like to have it wired up to the house server along with everything else.
    • CommentAuthorEd Davies
    • CommentTimeFeb 1st 2017
     
    Posted By: John WalshThe thing that encourages me to try is that the vent-axia docs say the unit "can be controlled and monitored by a BMS system".
    My guess would be that you'd need to talk to VentAxia. You might be lucky and they have a PDF specification of the protocol they can send you but it seems more likely to me that the BMS systems are done under NDA with lots of chatting backwards and forwards between the BMS's programmers and VentAxia's and that VentAxia would be reluctant to get into such a support situation with you for a one-off system.

    That's probably why there's little evidence of anybody using actually using the serial interface.

    The other approach would be to get the controller (if you haven't anyway) and watch its communications to reverse engineer the protocol.
    • CommentAuthorringi
    • CommentTimeFeb 1st 2017
     
    Offer to buy a 1000 of them and you may find vent-axia becomes VERY helpful.....
  1.  
    Good suggestion Ed - reverse engineer the Wired Remote Controller. This is my plan B, partly as it will be 50 quid wasted if I can figure things out without it.

    Re 'protocols', note that vent-axia state that the unit "does not use any BMS networking software but is simply a node on any network used". Communication with unit is by "sending the Kinetic a command string in ASCII Characters". As such, I'm hoping that it's simply a matter of getting the commands right.

    As you say, they wouldn't be bothered with supporting a one-off.

    I'm waiting for some duct fittings but hope to finish installation and get the unit running over the weekend and then try the serial interface next week.
    • CommentAuthorSprocket
    • CommentTimeFeb 2nd 2017 edited
     
    The "...simply a node on any network used" suggests the wires are carrying RS485 (+/- power and two differential signal wires A & B) rather than 'normal' rs232 serial.
    Connecting an 2-channel oscilloscope would tell you for sure but it's worth a look at the circuit boards in the remote to see if the wires are labelled, and a check with a mater to see if there is a line voltage on one pair.
    The "multiple node" aspect also suggests that the protocol used is something like modbus (modbus RTU or ascii), which would be more typical for RS485 use anyway, rather than simple character strings.

    Not hard to reverse engineer with the right kit and a little bit of time.
    IF it IS modbus RTU or ASCII that needn't be a problem - there are plenty of python libraries available to do the fiddly bits.
    • CommentAuthorSprocket
    • CommentTimeFeb 2nd 2017 edited
     
    Ah - have just properly read the older thread Ed linked above.

    ie. http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10862

    Sounds a bit more limiting than I was expecting.

    I also only just found the installation manual? (anyone just google "Kinetic-installation.pdf" if not seen it)... It has details of all the sensor and switch connections on the MVHR.

    Emulating the wired remote does sound like a good way to go... but you'll probably have to buy one and monitor the signals to see how to make something that will fool the MVHR into thinking you are the wired remote.

    Anyway, do report back when you start playing with that BMS interface (or the "NET" interface)... it sounds interesting.
    • CommentAuthorJohn Walsh
    • CommentTimeFeb 16th 2017
     
    An update on attempting to use a RPi to talk to an MVHR unit - the installation is now complete (using Lindab manifolds and semi-rigid duct - so no urgent need to bother with commissioning), all working fine but the unit isn't willing to talk - yet.

    According to vent-axia, the unit has an RJ11 socket, standard 5v pin 1 etc. wiring and uses a bog standard serial comms set-up - 9600 8N1. How difficult can this be? Well, first off the RJ11 is (on the unit I have) an old-fashioned telephone handset 4P4C connector which is wired ground on pin 1. Also, and perhaps not surprisingly, the RS232 set-up (if that is what it is) isn't as advised.

    Using a level shifter and some basic pyserial I can get a response of sorts. According to the manual, after receiving a command, the BMS screen displays the first 16 bytes received and a byte count (by default, the screen displays "BMS 00"). Using 9600 8N1, the screen goes blank. 4800 7N1 gives a mangled output (I'm presuming the screen will only display ASCII characters and the mangled characters are a result of receiving an input the unit can't understand). All of the other 'regular' baud rates result in no change on the screen, so I'm hoping that 4800 or 9600 will work.

    I'm no expert on serial comms but am hoping that it's a matter of finding the correct baud/ parity etc. configuration. I don't have an oscilloscope or the wired remote or a lot of time at the moment, but will try multiple configurations over the weekend (i.e. write some python to do the work).

    Any suggests, ideas or tips would be most welcome.
    • CommentAuthorringi
    • CommentTimeFeb 16th 2017 edited
     
    To quote form a computer science professor I had at university:
    "Connecting two systems with RS232 (OR RSwhatever) can be a lot harder then designing a CPU, I know because I have done both…….."
    • CommentAuthorJohn Walsh
    • CommentTimeFeb 16th 2017
     
    <blockquote><cite>Posted By: ringi</cite>To quote form a computer science professor I had at university:
    "Connecting two systems with RS232 (OR RSwhatever) can be a lot harder then designing a CPU, I know because I have done both…….."</blockquote>

    ... wise words indeed - did he by any chance have a beard?
    • CommentAuthorEd Davies
    • CommentTimeFeb 16th 2017
     
    Do you know for sure if it's strictly RS-232 signal levels (idle/mark/1 < -3 volts, active/space/0 > +3 volts) or some sort of TTL serial +5 or +3.3 volts = idle/mark/1, < 0.2 or so volts = active/space/0? If you're playing to the wrong scheme here you could theoretically blow things up but, more likely, will result in sending inverted data which might account for your mangled characters.

    E.g., most receivers for RS-232 (mark/idle/1 < -3V, space/active/0 > +3V) will actually have a threshold between the two just above zero volts so if fed TTL serial (mark/idle/1 ~= 3.3 or 5 volts, space/active/0 < about 0.3 volts) they'll receive OK but with 1's and 0's swapped and weird effects detecting start bits and so on.
    • CommentAuthorringi
    • CommentTimeFeb 16th 2017 edited
     
    Posted By: John Walsh... wise words indeed - did he by any chance have a beard?


    Can’t remember, but he also said:

    "The nice thing about communication standards is that there are so many to choose from….. "


    And that was over 20 year ago, and we are still using RS-232, at least I have not had to solder up a RS-232 leads in the last 20 years as ethernet connected printers took over. I had my time with RS-232 before my comp sci degree.
    • CommentAuthorringi
    • CommentTimeFeb 16th 2017
     
    Should it be a RS432 link? Or some other RSNNN for a unknown value of NNN.

    Can you see what chip the socket connects to and the value of any pull up resisters etc?
    • CommentAuthorEd Davies
    • CommentTimeFeb 16th 2017
     
    Also worth checking the open-circuit voltage of the data pins. The input pin to the unit will likely be around 0.3 volts though perhaps pulled up to 3.3 or 5 volts pretty much whatever is used but the output data pin could be useful to know: if it's negative (likely around -5 or -6 volts) it's proper RS-232, if it's +3.3 or +5V then it's serial TTL for that voltage.
    •  
      CommentAuthordjh
    • CommentTimeFeb 16th 2017
     
    Quoted by: ringi "The nice thing about communication standards is that there are so many to choose from….. "

    https://xkcd.com/927/
    • CommentAuthorJohn Walsh
    • CommentTimeFeb 17th 2017 edited
     
    Ed - open circuit voltage on the data pins (as noted above, when RPi and SK are connected together it's via a level shifter):

    RPi Rxd/in +1v
    RPi Txd/out +3.3v
    Sentinel K Rxd/in +0.1v
    Sentinel K Txd/out -7v

    Is this what you'd expect for RS232 and should there be a discrepancy? Thanks for help.
    • CommentAuthorEd Davies
    • CommentTimeFeb 17th 2017
     
    Posted By: John WalshSentinel K Txd/out -7v
    Yes, that's what I'd expect for proper RS-232 signal levels (not TTL serial). So, assuming the “level shifter” is something like a MAX232 chip or a modern 3.3 V equivalent (3232? - been a while since I've done much of this stuff) then that should be right.
    • CommentAuthorborpin
    • CommentTimeFeb 21st 2017
     
    Posted By: SprocketIF it IS modbus RTU or ASCII that needn't be a problem - there are plenty of python libraries available to do the fiddly bits.
    The problem is that one of the manufacturers nodes is the master and MODBUS is rather fussy over having 2 masters!

    I've got my Helios sending data over RS422 to a Pi. Not tried controlling it yet.
    • CommentAuthorsnyggapa
    • CommentTimeFeb 21st 2017
     
    I looked at this briefly in the past (also have a sentinel kinetic) but decided the lac of info would kill it. I do have vague recollections if it being RS485 rather than 232 but that is from memory and I can't quote the source.

    My plan if I went for it was instead to use the proportional control sensors or the relay to trigger it rather than the BMS connection - but as it turns out the unit does OK for me without any help - all I adjust is the default humidity every spring and autumn otherwise it either doesn't boost when we use the shower, or it runs on permanent boost

    -Steve
    • CommentAuthorringi
    • CommentTimeFeb 21st 2017
     
    Posted By: snyggapaall I adjust is the default humidity every spring and autumn otherwise it either doesn't boost when we use the shower, or it runs on permanent boost


    Would a flow sensor on the pipe to the shower be an option? Or a humidity sensor near the shower that detects the rate of change of humidity.
  2.  
    Mine also has an in-built humidity sensor but it is a tiny 'pot' under a cover with diy insulation above and above 2m in a cupboard. So a right PITA to change, and so wildly inaccurate as to be practically random...and anyway I don't find 2 changes enough where I live and with my set up (3 bathroom extracts). This little baby is just £20

    https://www.amazon.co.uk/gp/product/B008JLQXNA/ref=ox_sc_act_title_4?ie=UTF8&psc=1&smid=AG4A6CPO0BVL1

    And I can't understand why all MVHRs don't have one - super easy to adjust, very accurate, easy to fit, displays actual measured-in-stream RH so you know what is going on, adjustable hysteresis et al.

    This is the thread if you want to know more:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14624&page=1#Item_23
    • CommentAuthorringi
    • CommentTimeFeb 22nd 2017
     
    I don't think it is possible to have a single humidity sensor control a MVHR well when you have more then one wet room. It is a shame that there is not a standard way to connect wireless humidity sensors to MVHR units.
    •  
      CommentAuthordjh
    • CommentTimeFeb 22nd 2017
     
    Does anybody who has an MVHR actually have a problem with condensation in wetrooms? I don't even bother to turn mine up from trickle (50 m³/hr) any longer.
    • CommentAuthorringi
    • CommentTimeFeb 22nd 2017
     
    We do if we both have long showers and don't press the 10 minute boost button. The button is just outside the bathroom so is not an issue. The condensation starts a few minutes before the hot water runs out.

    The best condensation I got with our MVHR was making some chutney that needed 3 pans boiling fast on the hob for over an hour, the drain pipe backed up and the unit leaked water! I have leaned to use the cooker hood as well the next time I do it, maybe even open a window....

    However the chutney was just after the MVHR unit went live, so it had not yet had time to dry out the property, these days I expect we have a lot more buffing.
    • CommentAuthorGotanewlife
    • CommentTimeFeb 22nd 2017 edited
     
    Posted By: ringiI don't think it is possible to have a single humidity sensor control a MVHR well when you have more then one wet room.
    Errrrr Ringi, what did i just say? To repeat, I have 3 wet rooms being controlled by one highly accurate, sophisticated, astonishingly cheap and easily adjusted humidity sensor - it works brilliantly!! Note that the RH sensor is on a cable from the control unit (that I link to above) and that the sensor was retrofitted into the extract duct just before it enters the MVHR unit. And yes this means the increased humidity air is mixed with 2 other normal air flows before passing the humidity sensor but I assure you the accuracy of the control unit and precision of the adjustments mean it works.

    Occasionally the MVHR is still in boost by the time I have finished breakfast and I just nudge the cut in RH up 2% and occasionally the MVHR unit hasn't gone into boost and I nudge the cut in RH down a couple of percent.

    We had until then also been using the 15min boost switch but it doesn't always happen given that people are forgetful and that there is only one switch in the hall, which is not near either of the en-suites.

    Posted By: djhDoes anybody who has an MVHR actually have a problem with condensation in wetrooms?
    Yes, me! We have solid stone walls some of which are uninsulated and our our en-suite is one of those rooms.
    •  
      CommentAuthordjh
    • CommentTimeFeb 22nd 2017
     
    Ah, interesting, so it's not just ventilation that matters. Some combination of the amount of insulation and the materials I used must be helping, I suppose.
    • CommentAuthorGotanewlife
    • CommentTimeFeb 22nd 2017 edited
     
    For sure DJH! In another bathroom I have IWI'd (with just 30mm and it doesn't have a window) and it appears no mould no matter what...
    • CommentAuthorringi
    • CommentTimeFeb 23rd 2017
     
    Posted By: GotanewlifeErrrrr Ringi, what did i just say?

    Occasionally the MVHR is still in boost by the time I have finished breakfast and I just nudge the cut in RH up 2% and occasionally the MVHR unit hasn't gone into boost and I nudge the cut in RH down a couple of percent.


    You have just admitted that your control system does not work, we can't expect "normal people" to nudge control settings.....

    Posted By: djhAh, interesting, so it's not just ventilation that matters. Some combination of the amount of insulation and the materials I used must be helping, I suppose.


    If all the walls and windows are close to the air temperature and you have some buffering of humidity (only the top 1mm or 2mm of buffer is useful for this), the MVHR often has hours rather than minutes to get the humidity under control before anyone notices.
   
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