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Ventilating through the door is not very helpful

by 5m Editor
12 February 2007, at 12:00am

By Nick Bird, FarmEx - One interesting point to emerge from our energy monitoring project on British pig farms is the effect of leaving the door open.

Sometimes it’s accidental, other times it’s intentional – leaving the door open and the fan switched off for young pigs. Where intentional, it’s not clear whether it’s to save money (ie. electricity for the fan) or whether it’s believed that ventilation is in some way ‘better’ with the door open than with the fan running.

Whichever, the result tends to be higher costs rather than lower, since leaving the door open loses a lot of heat. This is illustrated in Chart 1. Heat loss depends on the size of door and inside and outside temperature. The chart shows the calculated heat loss based on stack effect alone (thermal buoyancy) for a range of outside temperatures, at two inside temperatures, for a regular sized door (1.98m high by 0.79m wide aperture).

The amount of heat loss may seem surprising – 6kW of heat lost through the door at a relatively mild 10ºC outside. At 0ºC outside, it’s a massive 13kW! This huge heat loss is, of course, useful when power fails and the only ventilation is by leaving the door open. But somewhat less handy when it’s from fragile weaners, or electrical heating. It’s worth noting that – though calculated on an overall inside temperature of 25ºC – temperatures will be far lower at floor level where the pigs are.

Cold air flows through the bottom of the doorway, and over the pigs as it gets heated and rises to the ceiling. Airflow then pushes the hottest air down and out through the top of the doorway. To relate this ventilation rate as such, Chart 2 shows it as ‘ventilation equivalent’ compared to a regular 450mm (18’) fan.

For example, at 10ºC outside and 20ºC inside, ventilation is equal to that given by a 450mm fan running at 20% speed. Heat Loss is illustrated in a real time setting in Chart 3, showing electricity use and temperature in a 180 place flat deck. The top chart shows inside and outside temperatures (red and black respectively) and set temperature (grey).

The bottom chart shows electricity use in kW. Typically electricity use is low – around 100 watts – for lighting and fans, since pig heat maintains room temperature reasonably well. However, the door tends to fall open, as the latch is faulty. The first peak in electricity is due to door falling open, but it was closed by early afternoon.

But not fixed quite properly as it opened again during the night, when outside temperature was even lower. The heating came on to maintain room temperature, and needed an extra 4.5kW (close to the calculated value for those temperature conditions). That particular ‘slip of the latch’ cost around £5 in extra heating costs. The calculations – and the real time data shown here – only confirm that Larry Grayson’s advice was sound: ‘Shut that door!’

The ventilation rate and consequent heat loss through an open door are far higher than one might expect. This can be useful – during a power failure, for example. In fact, I’m often surprised by the fact that people put in drop out flaps but ignore the door. Drop out flaps held with magnets are typically small, and desperately unreliable as they get stuck with muck, and are poorly maintained. Putting a magnet and a spring or counter weight on the door(s) makes more sense and works better, but many don’t think of it.

But most of the time, ventilation through the door is not very useful. You can’t really control it, and – especially when colder outside – gives far more ventilation than you want. Plus, the cold air is at floor level, which is very undesirable for small pigs. Generally, it’s better to use fan ventilation – especially if heating is involved – since this is more controllable generally and provides air mixing so that heat is spread throughout a room and doesn’t just rise to the ceiling.
Courtesy of Pig world, November 2006