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.

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.

Offer to buy a 1000 of them and you may find vent-axia becomes VERY helpful.....

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.

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.

<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?

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.

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.

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.

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.

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.

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.

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.

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.

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.