What is the cost of humidity control in a greenhouse?

First published August 2009

A controlled study of two humidity control strategies and their costs to show the effect of each strategy in the greenhouse.

Quick links:

Can you afford to control humidity?
A comparison of humidity strategies
What does humidity control cost?

Can you afford to control humidity? 

When energy is cheap you don’t often worry about the cost of using heat for humidity control. This is because the advantages of a good yielding and disease free crop far outweigh the cost of controlling humidity. However, when the cost of energy increases, you are left asking “Can I afford to control humidity?”

In 2008 high energy prices led Coletta & Tyson to take the view that active humidity control should be minimised that year. This may have left them exposed to potential disease problems; so was it the right decision?

A comparison of humidity strategies

The size and layout of Coletta & Tyson’s Millbeck Nursery gives us the unique opportunity to compare the performance of two identical greenhouses operating under different humidity control strategies. We therefore did side by side comparisons of the following two approaches.

Optimised humidity control – with this approach we used the heating and ventilation set-points discussed in How to achieve good humidity control with minimum energy use.
No active humidity control.

The graphs in Figures 1 and 2 show the heating pipe temperatures and the climate in the two greenhouses on the same day.

Line graph showing heating, greenhouse and pipe temperatures and relative humidity in a greenhouse with humidity control across one day where the humidity level stays more consistent throughout the day — Figure 1: Greenhouse with humidity control

Line graph showing heating, greenhouse and pipe temperatures and relative humidity in a greenhouse without humidity control across one day where the humidity level varies from below 80 to above 90% — Figure 2: Greenhouse without humidity control

If we look at the night period we can see that the greenhouse with humidity control had an average RH of 85% compared to 90% in the ‘no control’ greenhouse. However, to achieve this, the pipe temperature in the humidity control house was 10^oC hotter.

What does humidity control cost?

Over the period from 16:00hrs to 08:00hrs:

Active humidity control required heating for the entire 14 hours at an average pipe temperature of 35^oC.
No humidity control used heat for 12½ hours at an average pipe temperature of 21^oC.

We can use this information with data about the greenhouse temperature, heating pipe length and diameter and boiler efficiency to calculate the heat used in each scenario.

On this night the active humidity control used 0.7kWh/m² more energy, costing an additional 1.75p/m². Based on the plant density of 9 plants/m², the cost of humidity control on this night was therefore 0.2p/plant.

Scenario comparison

Scenario	Energy consumption	Cost based on gas @ 2.5p/kWh (73p/therm)
With humidity control	0.9kWh/m2	2.25p/m2
No humidity control	0.2kWh/m2	0.5p/m2

Based on 2008 costs

However, don’t assume that humidity control costs 0.2p/plant every night. This was a worst case night with mild temperatures; so humidity control was particularly difficult. On a colder night more heat would have been needed to maintain the temperature in the ‘no humidity control’ block and the differences between the treatments would have been equalled out.

When assessing the value of humidity control we have to remember the potential advantages including:

Reduced grade outs/rejects.
Labour saving due to less picking over.
Less fungicide applications.
Better shelf life.

So, before you rush out and change your humidity control settings to save costs, remember the additional work and costs that you might incur if you go too far.