Glasshouse minimum pipe settings in practice

Read about managing minimum pipe heat in your glasshouse and what can go wrong with an apparently good energy saving strategy.  

In an attempt to save energy, one nursery used a minimum pipe temperature setting of 35^oC which reduces when solar radiation increases. This setting remains unaffected by relative humidity changes.

At first glance this looks like a good energy saving strategy; however a closer examination of the climate graphs shows that things might not be quite as good as we think.

Figure 1: This graph shows that between midnight and 7am the pipe temperature averaged 60^oC, and the vents were also open for a significant proportion of the same period.

Why did this happen?

Humidity influences were being used which caused the heating and ventilation operational temperatures to converge when the relative humidity was increased.

As such, the minimum pipe temperature setting was effectively  overridden and had no bearing on the heating inputs or ventilator positions. As a consequence heating was being applied at the same time as the vents were open.

What did it cost?

Running an extra 25^oC of pipe temperature for 7 hours in a 1 Ha greenhouse led to the extra consumption of 7,650kWh of gas.

Even with gas prices at around 1p/kWh (assuming day-ahead buying), this still represents £76.50/Ha/night! Note: prices as at April 2009.

What are your options for different ventilation strategies to manage humidity alongside pipe temperature?

Ventilation temperature setting – make sure that the ventilation temperature is always at least 0.2^oC above the heating temperature. This setting should be maintained all the time, even if the relative humidity is high.

Minimum ventilation settings - use these sparingly and reduce the amount of ventilation when it is cold outside.

Suggested settings are:

Or, if you are controlling humidity based on humidity deficit (HD):

Also use outside temperature influence on the minimum ventilation so that the opening reduces by half (i.e. a maximum opening of 1%) as the outside temperature falls from 12^oC to 8^oC.