So my Aircycle 1.2 has stopped going in to boost when the humidity is high. Manual boost works fine and over run is correct (frost function has always operated irrespective of the temp from new but I didn't do anything about it). Brookvent have finally decided that the new version of their PCB will work in my old machine and a new PCB is £40 plus as yet unspecified delivery to Italy.

The unit is still in warranty but then I am obviously not going to trash the airing cupboard where the MVHR unit is and then pay for delivery to NI in order to comply with the warranty T&Cs. So whilst they agree it is propbaly the PCB, it is at my risk and i have to pay for the PCB. C'est la vie.

However, it has always irritated me no end that the humidity control is a tiny 'pot' that requires gut instinct to set having no gauge/setting markings etc and that I have to climb a ladder, take off a load of sound insulation unscrew their not at accessible access plate (set between the 4 vertical 125mm pipes, which are obviously all lagged up) in order to fumble around with an utter guess as to what RH I am setting. I generally do this twice a year as with 3 bathrooms feeding the thing the RH setting seems to need to be reasonably close to the avg ambient to kick in (in the normal situation of only one bathroom being humid at a time), so close in fact that the summer winter RH are too far apart to work all year around. If I don't change the thing it will either not boost or stay on boost.

Sooooo. See below a dead cheap (£20!!) digital humidity sensor with inbuilt 5A relay and hysteresis function - it says temp corrected humidity, which I assume means %RH as opposed to absolute Humidity. If I am correct, I can parallel wire this in with my manual boost switch, set %RH accurately and, since it will be mounted on an accessible shelf in airing cupboard, change it really easily as per the weather. The only downsides seem to be that it is cheap Chinese electronics, I haven't yet found a solution for delivery to Italy yet and the instructions are so badly translated it takes a while to understand how it works. Here it is:
https://www.amazon.co.uk/gp/product/B008JLQXNA/ref=ox_sc_act_title_4?ie=UTF8&psc=1&smid=AG4A6CPO0BVL1

I would be really grateful for advice on my idea/alternatives. Especially as it is sooo cheap and MVHR manfs seem to charge a ridiculous (in comparison) amount for the Humidity function or stand alone humidity switches (like this one from Brookvent - note no details/spec on product at all, just pay them £70!!)
https://brookvent.co.uk/product/single-gang-humidistat-timer/

Am I missing something?

I don't see anything obviously wrong with it. Most humidity sensors also include a temperature sensor, so I'm not sure why it has a separate one? As Colin says, check the lead length.

In trying to find more details, I came across this rather remarkable page:

http://roundupreviews.com/uk/types-of-humidity-sensors

I have two units that look very similar if not the same. They're connected to in-line fans that ventilate shower areas. I did this as an upgrade to the analogue rh sensor that I built into a plenum above a shower area - always disappointed by the slow reaction time (on and off) of that set-up.
You can position the sensor remotely to the actual unit - I think the wire is a metre or so. The chinglish instructions were laughable but there's a website with a blogger reporting on his home-made curing cabinet which helped me with the wiring connections.
I'm not sure about a temp sensor on the same unit. There's a near identical unit that controls temperature - indeed some of the buttons on my units appear to be more relevant to it's use as a temp sensor.

Thanks red door. The different versions have been driving me a bit crazy and it is even worse for the temperature controllers as there are more versions of that - all in the same container, all with similar looking connections. Basically I can't get the one linked in Italy at less than 3x the price than in the UK but I can get the slightly cheaper version that doesn't have a temp sensor - it says...but then how can it know the RELATIVE humidity without knowing the temperature?????

Ahhh finally found the same one in Italy and it's Prime too - bargain!!https://www.amazon.co.uk/gp/product/B00GNT1NJY/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A2I0KD4PFXY56D

RedDoor, Did you look for a chassis for the thing; they must exist because there are about a zillion choices out there using the same box?

http://www.engineersgarage.com/articles/humidity-sensor seems to be quite helpful.

Ta everso - that helps!........and I love the idea of curing my own meat, which a lot of my nieghbours do the old fashioned way

Another GBF problem solved.

So it cost me about £20 incl delivery and it is amazing - tiny thing under door frame below MVHR unit - displays current humidity and so you can see what's going on and set the limits and hysteresis very quickly if the ambient conditions change. ALL MVHRs should have one!!!! Only shame was that it is parallel wired up to the boost switch (one of the switches on the 2 gang light switch to the right of the cupboard) and the boost switch has a 15 minute over-run time as standard but a trivial point (the controller has a user adjustable min run time). I just don't understand why for such an incredibly cheap item all MVHRs don't have a controller on a fly lead - completely changes the usability and frustration factor of boosting via humidity.

Actually the average humidity across the 3 extract rooms (all bathrooms) does change enough for this to work - but no way you could get the required level of accuracy with a 'pot'. Those colored wires going up the front of the MVHR are the sensor wires and I just drilled a 12mm hole to pass the sensors through (once I had opened up all the sound insulation!). The built in humidity sensor operation never worked properly (even when it functioned) so this is mildly revelational and I am offended that this level of sophistication is unavailable as standard in MVHRs when it is patently so inexpensive.

If you are sensing the humidity in the extract side of the MVHR does this mean that you are running the unit 24/7, or do you have it on a timer?

The simplest timer is one that goes between the socket and the plug. (plug the timer into the socket and the unit into the timer) this allows fixed times through the 24 hours which perhaps could set the unit to come on for some hours around the time(s) of high demand with the humidity sensor managing the boost for higher loads.

BTW any news on replacing the fan bearings (other thread)

So you would need a humidity sensor in each steamy place (at £20 a pop) wired back to the MVHR unit.
Hmm... lots of wiring and a cost / benefit calc. first

Easily done.
It is always 100% RH here