c=4186;
rce1=Qout==m1*c*(T3-T4);
rce2=Qout==Pel*ϵ;
rce3=m1*c*(T3-T5)==Pk;
rce4=m1*c*(T5-T4)==Qtop;
rce5=Qtop==kTop*(((T5+T4)/2)-Tin);
rce6=Qtop==kDum*(Tin-Tout);

rceStav={rce4,rce5,rce6};
Stav={Qtop->7000,Tout->-12,Tin->22,T5->40,T4->35};
dos=Solve[rceStav/.Stav][[1]]//N;

dosrce={rce1,rce2,rce3,rce4,rce5,rce6}/.Union[dos,{Pk->0,ϵ->0.5*(T3/(T3-Tout)),Tin->22}];
Cases[dosrce,_Symbol,∞]//Union

reseni[ToutMoje_]:=Solve[dosrce/.Tout->ToutMoje][[1]];
epsilon[ToutMoje_]:=(T3/(T3-Tout))/.reseni[ToutMoje];
vykon[ToutMoje_]:=Qout/.reseni[ToutMoje];
Plot[{epsilon[Tout],((vykon[Tout])/3500)},{Tout,-18,15},AxesLabel->{"Tout[°C]","ϵ[1],3500Qout"},AxesOrigin->{0,0}]