HEATING PROBLEMS OF 504 1994 Full

Hello ..... I have a problem with the heating of the Peugeot 504, Srx 1994 full, does not circulate hot water through the system, check everything and it is not covered ... it is anically circulating before the thermostat opens and then not, take it out and it does not circulate, it only does it when I go on the route and when the engine is demanded more ... connected but I don't know which one ... I recently bought it and now I discovered that because I hadn't needed heating ... I would appreciate if someone can give me an idea