FWIW…
Step 1:
30% open is when the valve has moved 30% of the way from closed (0%) to fully open (100%).
That is, you would expect water to flow through the valve at a reduce rate because of a smaller aperture. The radiator might ‘whistle/sing’, due to the turbulence that this gives (a phenomenon that you get with smart/motorized TRVs, but not so much with wax-TRVs).
However, whether the vale is ‘open’ or not (i.e. water can flow through it) would also depend upon other factors:
- is the valve ‘stuck’ (this is why the TRV cycles occasionally)
- is the TRV sitting on top of an adapter
- is the TRV configured for ‘full-stroke’, etc.
NOTE: How far a valve is open - or closed - is a separate issue to whether the boiler is heating the CV (the circulating volume of fluid) and/or whether the CV pump is on…
Step 2:
If the transform function (the graph in the link above) creates a heat demand > 0% for a zone (after considering zone temps, setpoints, system/zone modes, optimalizations, etc.), then the controller will make a ‘call for heat’ from the boiler (or similar). It is asking fro the CV to be heated & pumped.
Step 3:
Step 2 with either be TPI, or modulated, depending. With TPI, a 50% call for heat might me boiler on 5 mins, off mins.
If modulated, the water will effectively be heated to a lower temp (say 30C instead of 45C).
This depend on a lot of other parameters, and - in addition - the boiler may decide to it’s own thing if configured to do so.
Step 4:
Even though the boiler is pumping hot water around the CV, the TRVs may device to go to 0%.
I don’t know if you need to bother trying to understand all of that - the system is full of feedback loops, effectively tuning itself as it goes - it works.