100 years old 3" steel always unscrews with 60's and a scaffold pole if heated enough.
I know what I would do !! For every 10 one pipe systems I have worked on unless large bore commercial, maybe one would give...
A few applications of Penetrating oil and some heat (LOTS)with a large spanner and extension bar....It should fly out .
Bspt male iron.
ONCE AGAIN just try it with a NON TRV to confirm it will heat up. Straight swap 10 minute job !!!
He has already confirmed it is a one pipe. Normal TRV's are too restrictive for gravity circulation due to the orifice size.
Try a Non thermostatic valve to see if it heats before spending more on a TRV.
If it don't work without one it certainly wont with one ....
Try telling customers that a warm room with cold rads is good.
As soon as the rad starts to cool 90 % of people try to turn things UP!
The TRV will severely restrict flow on a 1 pipe.
Special full flow valves were used.
In my experience very few systems on small bore...
I used to convert to a Satronic TF 801 B and FZ cell which was same diameter as original.
To OP I still have A static disc and Fan...
Which boiler and pump are fitted ?
Have you looked at pipe work under the floor. You may have a "One pipe" system.
In your first clip the motor was stalled.
In the second clip it was running, What did you do .
Rusty water by the looks of it.
That's why I said to check. Your fault is typical of this.
I presume the tank is steel.
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