array because T1 is array
fTemp1 = @(t,T1) (Qin*Ain-Q1*A1-Q2*A2)/(Cair*V1*rho)
Uses the array Q1 and therefore computes an array
T1(i+1) = T1(i+1)+h*fTemp1(t(i),T1(i));
You are trying to store the array of results in a scalar
t = zeros(1,N);
Tout = -10*sin(2*pi*t/86400);
t is all 0 so that is sin(0). What is the point of that?