Linear stability analysis for study of the connective instability of a horizontal liquid layer in a porous medium subjected to a temperature gradient is approached by both finite-difference and finite-element methods for the case with uniform vertical throughflow and maximum density effects. Numerical results for the critical Rayleigh number are presented for the thermal condition parameters − 3.5 < λ1 < − 0.5 and − 1.4 < λ2 < 1.4 with Peclet number Pe = 0, 0.5, 1, 5, and 10. The convergence and accuracy of the numerical solutions are assured by the excellent agreement of the critical Rayleigh numbers for the limiting cases with the known values.