Hello Yasin,
after thinking about the transport delay in type 399 I had the idea that maybe you could use Type 31 (pipe) with the heat loss coefficient set to zero. Have a look at the description of type 31 in the mathematical references.
I am not familiar with type 399. I know about it from several publications. But I think there might be another issue: If your heated floor is so large that the transport delay is not negligable, that also means that in the different parts of the floor the floor materials and the liquid in the pipes react to each other also with a transport delay. By using a type 31 behind the type 399 (outlet of type 399 flows through a type 31) you cannot really prevent this problem. You could mitigate this problem by dividing your floor into several parts in series, simulating each part with a combination of type 399 and type 31.
Kind regards
Doris
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Dipl.-Phys. Doris Buck