imho, not normal. kasi once na mag expand yung water papuntang reservoir, nakapag release na ng pressure yun and nag decrease na yung temp dahil na dilute na sya ng reserve coolant mo in the reservoir. malamang masyado ng mainit yung lumalabas sa radiator kaya pag punta nya sa reservoir nag boboil pa rin cya. especially 20 mins lang yung period na nag run ang engine. it's too short... with our cars, we only use distilled water pero never pa nangyari yan even on long drives. check for leaks and yung fan for any abnormalities.