Hadn't seen this mentioned but perhaps I missed it, more plumbing hardware than electronic, but perhaps useful for others.
Having tried a variety of plates and heat-sink compounds and tie-wraps to hold these little chips in position on small sections of metal pipe (my rads are fed with fused plastic pipe), I came up with the solution of using pipe clips, the cheap plastic variety from DIY stores and hardware merchants.
The first problem hit was with attaching plated sensor the the boiler clamp nut, it was off by 10c and no way could I get a more accurate reading, presumably angle? Finally found a pipe clip which got hacked in half to fit in the tiny gap in the photo... Some hacking with a Dremel, some soldering to a pin connector, some fiddling around with epoxy and voici....

The gap
The tie wrap in the top of the photo is the original plate mounted version which was "off"..

Having scored on that front with more reliable readings I looked at the roving radiator test kit and realised the same solution might apply so epoxied the chip on the end of some telephone line in a 22mm clamp and it works a treat. Cable and chip fit neatly into the screw-hole, flood with epoxy and done. The pipe it fits is 21.5mm so tighter than a duck's rear end...

As said, a tight fit to a twinwall radiator feed pipe, and a complete swine to get back off even with pliers, but no broken wires or oopsies to contend with, and so much faster to fit.
For those looking to measure metal pipe temperatures, possibly an unusual if fiddly solution, but no need of tie-wraps in the latter case...

Not so much a MySensors build as an example of how even the most basic information can inform changes for the better, in this case space heating.

The system here is fairly basic, an array of DS18B20s, some ultrasonic tank probes and a gas reed sensor, temperature is updated every 5 minutes, the gas updates every 0.05m3...
With winters here down to -20, the first priority last year was insulation, and even though a modern house, the gas bills essentially halved over the year, effectively funding not only the insulation, but replacement axial radiator valves and thermostat heads (Heimeier) to replace the typical arrangement of unknown origin, with spare... But now the MySensors impact..
This autumn's attention turned to the central heating unit, a modern combi unit of good manufacture, installed by a 'certified' heating engineer, but aside what little I knew about condensation boilers and the steep learning curve that followed, I was bemused by the return from the radiator loop almost burning my finger within 10 minutes of the system being fired up. This did not make sense for what I understood of a condensing boilers, which compelled a look inside for the first time, the manual and some googling.

The boiler is a 25kW combi with minimum output 7.6kW, the radiators account for ca 13kW at Delta 60 set for 15c drop (previously set ca 20c drop), settings since day one were 65c and the pump was set at max output of 3, last year's -20 resulted in 13.5m3/day gas consumed, not crazy by historical records, but hmmm.
So now comes tinkering with data from MySensors via Domoticz to inform...
Currently the boiler is set at 55c, the pump is on Low (40 v 84w), but the results are surprising - Slower rising temperature when ON, 42 minutes v 25, but gas use dropped from 0.75 to 0.6m3, but here's the kicker from that longer heating time, not only less energy used per cycle, but longer and thereby fewer cycles per day. Current evaluations are between 15 and 20% savings, so thank you to all the MySensors community and contributors..

Surprised I'm not bald will all the hair pulling and failed attempts to get there but after a year it is finally working...
The task was simple enough, to measure fluid depth in two tanks, one a sewage holding tank, the other a bulk water tank. Just figuring out which method worked with which board was an exercise in frustration, but the JSN-SR04-2.0 turned out the only one of the two I could get to reliably work.

Despite the industrial look, these waterproof light switch enclosures proved ideal, as the batteries in holders fitted beneath the clear plastic. A little hacking out of the switch 'fingers' with a Dremel gives plenty of space in the base area.
Battery holders are held in plastic conduit which are hot glued to the back of the box poking through the front section aperture.

The top case is a 3v3 WhisperNode with onboard RTC and RFM69, and tucked under the spaghetti in the top left back corner is a level converter. The RTC wakens the Node every hour to initiate readings, and despite a few hiccups seems to be reliable. The WhisperNode is allegedly capable of draining the 2 AA cells down to 0.9v, time will tell...

The lower box holds the 5v ProMini, a small latching relay, and the ultrasonic board, the button type ultrasonic (like parking sensor) is mounted on the end of a 3/4" collar to 1/2" galvanised steel tube drilled centrally through the roof of the 1.34m x 1.67m tank area.

Connection between the cases is via Cat5e in 16mm conduit, 2 cores for I2C, 3 cores for relay control and 3.3v power, 2 cores for 5v and ground to the level converter.

Despite worries over echoes in the resulting space, the tank was emptied last night, and the reading is coming in with the occasional glitch at 1620mm depth from the head, which I believe has a cone angle of 45 degrees.
The Node control of a Relay by On/Off pins to Mosfets may seem wasteful, but I had them anyway. The 5ms on/off of this small latching signal relay boots up the ProMini and seems reliable.
Still to incorporate a WDT/Reset on the 5v as it periodically hangs, and a time limiter on the Node to complete the task, but glad it is all all working semi-reliably, even if some of the programming side I still don't understand...
The 5v tests for 2 consecutive identical readings, the 3.3v tests that it fits within acceptable range, and if not calls for a further reading.

Once delivery of a second identical board from China is done, the water tank should be a lot easier. This one is the more important, as ran out of water previously due to corrosion of the level probes in the tank (3 core cable and short bolts) which stopped the borehole pump topping up the tank, and the hidrofor hit it's cut-off electrode. The level monitoring will give early warning to go investigate long before the bulk tank is 'empty'...

Postscript 12/7/18 - Water tank worked flawlessly from first deployment, but led to concerns over dropping battery voltage.. The problem appeared to lie with the Master sinking power through the I2C lines, but a routine to null the SDA/SCL lines prior to sleeping seems to have halted the decay.
One of the unexpected results of the Sewage Tank deployment was detecting a dramatic rise in level during a heavy storm (85mm in 24 hours) initially thought to be ground water intrusion.
Excavation to the incoming pipe connection to the tank found a round pipe in a square hole (not exactly an unexpected building practice here, now packed out and sealed.
Losing 30% of storage capacity would otherwise never have been detected, so it the deployment has already paid for itself handsomely...

@pepson This reminds of that famous quotation "Any ship can be a Minesweeper..."
"Once...."

@neverdie Would this work ?

@sundberg84 Yeah, this brings back memories, particularly of quite a few hangovers.
I used 3 and four wide fireclay pipes for bottle storage, they were used for ducts back in the day before plastics took over, cable went optic fibre. I noticed recently here they are back in stock in stores as....yep...wine storage racks.... but wacko pricing relatively...
The beauty of the fireclay was it's slow temperature and humidity change, and this formed the bulk of the thermal mass in the cellar I made below the floor.
I used a 150mm glazed ceramic drainage pipe dug in under the garden as a loop (rope caulked joints - anybody remember them), from memory down about 1.5m, both avoiding frosts and baking sun, don't think the temperature varied over a degree all year round, the ground acting as a massive heatsink which maintained a constant temperature all year round.
Only when the hatch was opened did the temperature jump, a small fan kicked in when the hatch was closed and ran for 30 minutes, a second contact switched on the lights and shut them off, long before LEDs were so prevalent, old reliable (until you were depending on them) incandescent bulbs.
That was it really.
Biggest problem I found was humidity in the early days, probably the fresh construction...

@tbowmo Hah, you were lucky, I finally upgraded from the monochrome to colour...

@sundberg84 Easy, one is disconnected... At 90 degrees?

Finally the round-tuit UPS got built after the last power cut clobbered the Controller/Gateway system, lesson learned...
Meanwell AD-55A, 7.2Ah Acid gel battery, two usb 5v buck converters, a spare socket for raw volts, and a 15 euro IP66 box from the local shops.. Some drilling and filing to the lid, couple of brackets, spare bolts, banding, soldering and hot glue...
A bit bulky, but disappears in a void under the stairs, two tiny drill holes let the buck converter leds shine threw...
No monitoring as yet, but sailed through a power cut this morning and the Pi didn't skip a beat.. First up is the Pi's RTC then can put the cover back on the Controller...

First of the winter projects done, just the programming to finish and replacement temperature chips to get (broke the legs off both DS18B20s through clumsiness).
With mains/battery backup and RTC, it will record boiler start/stop and run times as well as feed/return temperatures, and as an aside the inevitable power cuts which plague this part of the world.
The MCU plugs into a socketed backplane hot-glued to the back of the box should removal prove necessary, but quite pleased it is sturdy and all fitted into a slim 25mm deep standard (cheap) box.

@tssk A basic pro-mini clone needs modified (VR and LED), but with a few mods can run direct on batteries to fairly low voltages.
I bought these modules in as it would have cost more to make them even I was skilled enough (I still ain't), but they are essentially 3.3v Pro-mini with an onboard booster, regulator, rfm69 and various other refinements which gave it the edge over a Moteino.
Sundberg84's board might be an alternative worth considering, plenty of knowledge on it and help always at hand.

As to sensors, have a DHT22 somewhere but never deployed it when I found DS18B20s did all I required on a single run of Cat5e throughout the house and the outside, a second line covers the boiler.
I went for the 433MHz version of the RFM from the start, it easily passes through multiple masonry and concrete with a quarter wave antenna, so your floors may not be the problem you think later. The higher the frequency, the more attenuation effects you encounter, if .433 is permitted in your country I'd recommend it.

@tssk said in Which radio / wireless module to choose? Please recommend:

I prefer to use same module for everything but from some forum post I got impression that the GW might better use different module?

Your Gateway has to use the same radio as your nodes to communicate. Generally speaking the easiest and simplest Gateway is identical to your nodes, connecting by serial to your Controller.
It is the Controller which has to be a more powerful device such as a Pi, it and the Gateway will live in the same box and will run 24/7 on mains power, usually communicating via a router.

Your Controller+Gateway should be accessible, so would not recommend putting it into a loft with no easy access.
If your reasons for the location are radio communications (to the hives?), a 433MHz RFM will penetrate walls with ease...

Further to the point made by @mfalkvidd on batteries, a 9v wastes 60% of it's capacity on a 3v3 Node, 2xAAs will actually have a higher mAh capacity.
All my Nodes have 2xAA and are still going after 2 years..

@ady_e_n Presumably a low current DPDT switch.
Since I have them anyway I'd be using a latching DPDT signal relay fired by two logic level mosfets controlled by two pins, hence emulating the switch itself with no running current. The relays I have are NEC EE2-3TNU around 7mm thick and 9mm tall, 5v version should be available if that is your logic voltage level.
If space permits you could hot glue the relay to the board on it's side with the mosfets/resistors/3 pin control mounted onto it's base, hard wired to the original switch pads.

@wiredfrank Not sure what you meant by 433MHz not being allowed, so far as I recall that band is permitted worldwide but has transmit duration restrictions as it's a shared band.
I started off with the 433MHz RFMs and never had the slightest issue, penetrating walls and floors with plenty of range outside, even at one stage buried in snow.
The range and penetration of 868 v 433 will be slightly reduced, but performance should still be better than 2.4GHz.
The point about the recommended wire length being close enough remains valid

@wiredfrank It's a bit of overkill to be honest for hobby purposes, a quarter wave wire works fine when cut to the recommended dimension, or if concerned over the groundplane, a dipole as outlined by @scalz above.

A VNA is typically used to determine the exact resonant frequency of an antenna, if it's out by a fraction it won't make much difference in performance.

@Yveaux Yes, but was hoping someone had already gone through similar experiments, and could advise what did/didn't work
Your post prompted a reminder there is a 1/2" valved drain cock for the water meter/PRV in the bathroom... This would simplify testing without having to make plumbing alterations for the final locations

Didn't find exactly what I was looking for in a forum search...

Bought two of these (Chinese?) pressure transducers and 1/2" adapters with the intent of monitoring pressure (initially at 2 points) in the pumped water supply which has various headlosses in the line.
These are listed as 5v supply, 0.5-4.5 output for 0-0.5MPa, 20mA consumption, beyond that only claims the response is "perfectly" linear.

The Node is 3.3v, 5v is available on-board from an external PSU, but the voltage output will have to be scaled down to suit the ADC pin, even more so if aiming for Vref 1.1.
Normally a high value resistance ladder would suffice, but concerned over effect on response (presumably a variable resistance device), checked output resistance to ground, at atmospheric ca 6.7 Mohm, purportedly corresponding to 0.5v.

It occurred to me that if it IS a variable resistance it should also function the same at 3v, so any here able to offer any insights, experience, or tips ?

@amkochaki You imply the sensor sends two different voltages, one for standby and one for active, perhaps you might establish what these voltages are for each condition.
All your ADC is doing is measure the voltage, your sketch takes that value and converts that to a logical 1 or 0, standby/active.
If the voltage swing is large, say 5v for stable and 1v for triggered, perhaps conversion via resistor divider can provide a logical 1 or 0 input simplifying the process ?

@Encrypt It sounds as if you are making progress, moving off breadboard will certainly make for greater reliability.

My reasoning for checking HIGH and LOW is that these are fixed states rather than in transition. FET type sensors are very fast, but the Elster Water Meter Node would send an intermittent false increment (No idea why/what), which is why the additional check on pin state was introduced.
That pin check was spurred by experience with the Gas Node, which gave rapidly escalating readings, found to be caused by the reed closing for ca 6 seconds. Checking in a Sleep loop that the pin had changed back from LOW to HIGH before going deep sleep resolved it, 3 years on it's still in complete sync with the register.

@Encrypt Sorry, meant to respond to this when you posted it but got distracted again...
If I recall correctly the calls to Gateway are limited to 7 or 8 attempts or thereabouts, but if your Gateway falls over every increment it will do the same. This can hammer Node batteries as I discovered after a power failure (regular feature) killed the Pi3, now solved with a DC/DC UPS.
Never had comms issues with the RFM69 on 433MHz but they do use proper whips.

Have a similar setup with an Elster sensor, but by the time the water meter was done I'd learned from similar "issues" from my first ever Node on the gas meter which uses a Reed...

Interrupt on FALLING was causing rapid cycles of the increment the trick was to ensure the increment only happened once on every wake from Sleep.
Set the interrupt to LOW, verify that the pin does indeed read LOW after wakening (in case of false triggers), increment the count, send the updated value, then verify in a SLEEP loop that the pin has indeed gone HIGH again before going to deep sleep.

Hope this helps