Here in the UK it’s fairly normal practice to adjust radiators so that the temperature of the circulating water entering the radiator (Flow) is 11 deg C higher than the circulating water leaving it (Return) It is usual to adjust the temperature difference by adjusting a valve on the return side of the radiator that needs to be adjusted with a tool and has a plastic cap fitted once the adjustment is made (We term these Lock Shield Valves). Although here it is common to fit TRV onto the Flow side of the radiator there’s no reason why they can’t be fitted to the return side. The measurement process involves clamping thermometers to both pipes connected to the radiator. I have some radiators that have the TRVZBs fitted to the Return side and have successfully used this technique to balance those radiators from the iHost home screen using 2x switches that have temperature sensors ( I actually used a Sonoff TH10 and a Smart Home TH16) synced to iHost. The TRVZBs will need to have the latest firmware update that allows the % opening or closing of the TRV to be set. Method is to attach the temperature probes to the radiator pipes (inside a small length of foam insulation is ideal), set the TRVZB to open say 10%, make sure that it’s in ‘Heating’ status and then run the heating system. The iHost cards for the TH switches will show the Flow and Return Temperatures, it’s then just a simple job to adjust the TRVZB opening % to achieve the required temperature difference.
It is fairly normal (and justified) practice everywhere I believe 
It is fairly normal (and justified) practice everywhere I believe
I can only write about the territory that I am familiar with. It’s normal Professional Engineering practice to provide detailed background information when publishing a White Paper. Your comment doesn’t really bring anything worthwhile to the table.
Nothing excuses my highly inappropriate behaviour. Any penance is provided for such hopeless cases? 
Here in Spain we try the same system as you, and a couple of months ago I finished installing my TRVZB thermostatic heads and when I emptied the radiators I had to rebalance them. I had to put parts for the heads on the radiators, until I got the 11 degrees difference between the inlet and outlet of the radiator and I had also thought about putting heads on the outlet of the radiators but I didn’t know anyone who had done it before.
This is where the doubts come in, in the way of regulating the inlet and outlet temperature, you use Sonoff brand sensors but the balancing process has to be done one by one.
How do you carry out this process with the TH10 and TH16 but you have two of these devices installed on each radiator or do you put them on one, regulate the temperature and once regulated you put them on the next radiator?
We should also change the operation of the thermostat heads so that their opening or closing does not depend on the internal thermostat that they themselves have and that the opening or closing is in % that this can be done if you use Home Assistant, and MQTT once integrated from there you can regulate the % of closing or opening and achieve the 11 degrees that have to be in the radiators.
Once this operation is done, a lock of the TRVZB head should be carried out so that it does not open or close accidentally again. The first thing would be to activate the child lock so that if we turn the wheel the temperature does not vary and unlink the internal thermometer from those valves so that they do not make any movement, I do not know how to do it, unless if we regulate the temperature of the radiator, you remove the batteries from the head and it stays in that position in which it was left can be used, because the outlet piece once regulated should only be acted on if we are going to manipulate the radiator, for example remove it to paint it or manually clean it inside by injecting clean water.
Yes I use aTH10 and TH16 to monitor the temperature of the inlet and outlet pipes for 1 radiator, carry out the balance process for the 1 radiator and then repeat for the next radiator and so on. To control the system I’m using an iHost running the eWeLink cube using scenes. My system runs a scene at 59 minutes past each hour for every TRVZB that
- Switches the TRVZB to OFF (Frost protection)
- Switches the TRVZB to Manual 16deg C
- Switched the TRVZB to Auto
That is done so that if the TRVZB is set to a high temperature manually it will be reset at the next 59 minutes past the hour. I do retain the use of the TRVZB internal thermostat but corrected by calibration, but for a couple of radiators I open and close them with 240v ac Thermo- Electric Actuators, switched by Sonoff Basic Switches controlled by a Zigbee Thermostat. One thing to be aware of it that Sonoff TRVZBs self calibrate every Monday at 12:00 (local) time and open and close during the calibration process. You could achieve a fixed valve opening by using a Lock Shield Valve on the outlet pipe of the radiator and adjust it manually to give the desired % open.
I’m curious why you’re balancing using the TRV valve open %. Correct procedure would be to have the TRV open fully and balance using the lockshield. By balancing using the TRV open % your TRV will always be open in some capacity thus defeating the point of having a TRV.
There are instances where a standard TRV was fitted to the return side of the radiator (this works just as well as them being on the flow side). There’s no point in changing the valve bodies around and using the replacement TRVZB makes a convenient way to balance those radiators. Attach iHost compatible temperature sensors to the radiator in and out pipes. Monitor the in and out temperatures at the PC while adjusting the valve opening % also at the PC. Much more convenient and accurate than messing around with pipe thermometers and the LSV at the radiator. I also have TRVZBs fitted on the flow side for some of my radiators (the heating system was already here when we got the house) but I think the arrangement with the TRVZBs on the return side works better. The % open figure for the TRVZBs is how much the valve opens by from fully closed when the valve goes into heating status- it’s not how much the valve is permanently open.