Absolutely right. You can try to estimate how much heat will escape through the surface of the pipes, and then through whatever screed and floor coverings you have. The calculation is complex although there are many rule of thumb tools out there to give you a rough idea.
Or
You can measure with complete accuracy the amount of heat being extracted from a system by measuring the temperature of the water in and out of the pipe and the flow rate. That directly gives you the amount of heat which is being lost from the water from its passage through the pipes.
Or
You can measure with complete accuracy the amount of heat being extracted from a system by measuring the temperature of the water in and out of the pipe and the flow rate. That directly gives you the amount of heat which is being lost from the water from its passage through the pipes.
