Good humidity control in glasshouses: initial climate control settings

A close look at temperatures and relative humidity resulting from our initial strategy.  

Back to: Achieving good humidity control in your glasshouse with minimum energy use

Figure 1 shows a graph of the greenhouse environment at Millbeck Nursery on a typical day in November 2008. These results were obtained after we implemented our climate control settings that were designed to achieve good humidity control with minimum energy use.

Graph showing three key times which were critical for developing a humidity strategy

Figure 1: The graph shows three key times which were critical when we developed our strategy.

Area 1, 5:00 am

To implement ‘Drop’ we needed to achieve the minimum temperature by sunrise. To do this in an energy efficient way we wanted the greenhouse temperature to fall naturally; rather than opening the vents at the last moment after we had heated the greenhouse all night. To do this we reduced the heating temperature before the ventilation temperature.

Area 2, 8:00 am

Here we needed to control the rate of temperature rise to avoid potential condensation problems. To do this we set the ventilation temperature 1oC above the heating temperature. These settings were used even when the humidity in the greenhouse was good.

Area 3, 10:00 am

Once the plants had warmed up and the condensation risk was low we wanted to trap as much ‘free heat’ from solar gain as we could; but only if the humidity was acceptable. To do this we used a humidity influence on the ventilation temperature. This meant that the vents only opened when humidity conditions were unacceptable.

Table 1. Key points to remember:

The reduction in ventilation temperature starts later than the reduction in heating temperature
Accurate temperature control and low humidity is required in the warm up period so the difference between the heating and ventilation set-points is small
Reduce ventilation to a minimum once the plant has had time to warm up and when the humidity is good
Increase the ventilation temperature to avoid unnecessary venting when the humidity is good

No humidity related minimum pipe or minimum vent set points were used.