The central heating unit comes on and heat for about 20 minutes then the breaker trips off if I reset the breaker the unit comes on but there is no heat

Could be a bad breaker, or something could be pulling more amps than it shoud be, when you reset the breaker be sure to turn completely off and back on, you may need to get someone to check out the amp draw,, you didn`t say if you had gas,oil or electric heat