Improvements to the control of the temperature in greenhouses heated by hot water pipes were developed in order to enhance both energy conservation and crop protection. Acceptable control can be obtained in some greenhouses by feedback of internal air temperature samples only, but much more robust control is obtained if heating pipe temperatures are included in the feedback. The heating system has long time constants, and with digital control the use of a relatively coarse sampling interval leads to better control. Gain coefficients for the feedback can be estimated from analysis of the uncontrolled response to heating if the pole positions for the controlled response are chosen. Suitable pole positions give slightly soft control, and the heating and cooling rates demanded can be achieved with limited adjustments of the heating valve.