I would like to chime in here...
I also have a towel rack that is connected to my underfloor heating. In winter it only gets lukewarm and in summer it remains completely cold (because the heating is in summer mode)... can I simply retrofit an electric heating rod like the ones you can get for 80-100 euros on a large sales platform with "A"? ... there is a power outlet nearby that could be used.
As it is now, it is useless for me... it is 1.20m x 60cm... how many watts would I need? Do you have any experience?
Edit: I am just wondering if you would then turn it on or not? Since the water from the heating runs through the heating circuit distributor and then through the radiator and back through the return flow to the heating...?... then the heated water would be sent back to the heating...?... and the cooler water flows back in... so that would be a closed system... I hope someone understands me... ;-)