One of the main problems of Mediterranean climates is the large diurnal amplitude of temperature, with too low temperature during winter nights and too high temperatures during summer days. This is particularly felt in the north of Portugal, where the low temperature during winter nights can be compensated by the introduction of a heat source. The objective of this is work is to simulate the effects in the temperature and velocity fields by the introduction of hot water tubes along a greenhouse in night conditions. Three different situations are simulated: natural convective heating (case A), artificial heating tubes (case B), artificial heating tubes, and natural ventilation (case C). The commercial CFD package $ANSYS^{(R)}$ (FLOTRAN module) is used for this propose. The turbulence is modelled by the RNG turbulence model. The numerical results are compared with experimental values, the procedure for which is also presented. The average increase in air temperature for cases A, B and C was $2.2^{circ}C,;6.7^{circ}C;and;3.5^{circ}C$, respectively. Turbulence is lower in case A, increases slightly when the heating system is introduced (case B), and increases significantly in case C due to the effect of natural ventilation. A very good agreement between experimental and numerical temperature values was verified. This allows validating the RNG turbulence model as suitable to simulate arch-shaped greenhouse microclimates. Some improvements can be done to this work: introduction of night-time crop transpiration, 3D simulations, or optimizing the size of the element mesh in order to reduce the computation time.