On many pig units, the operation of the system for mechanical ventilation is less than satisfactory. Malfunctioning or poorly designed systems give poor temperature control and they waste heat while also contributing to poor pig performance.

Usually we expect the ventilation to keep the air temperature in the room or house from going too much above or below the temperature that has been set to keep the pigs comfortable. In fact the thermal comfort of a pig depends on much more than the temperature of the air alone. It also involves the temperature of the floor, as well as airspeed, bedding, dampness and radiation. Animal factors are equally influential, not least the pigs' feed intake and growth rate together with the stage of production, body condition and health, besides the difference between group and individual penning and whether the pigs are free to move around.

Nevertheless we can make a guess as to what is a reasonable target temperature for each category of pig. It may be more difficult to put this into practice. Most units now operate with computer-based controls which can work very well, but they can be intimidating if problems arise. As a result, few people are comfortable making adjustments. Consider, too, that temperature displays on ventilation systems are often wrongly calibrated. They need to be compared regularly with a calibrated thermometer. A digital thermometer with a read-out display can be bought cheaply for the purpose, although old-style maximum-minimum thermometers are similarly low in price and give valuable information.

In warm weather it is often impossible to keep day-time house temperatures consistent. The daily variation in outside temperature tends to be greater at the height of summer than in winter, but planning ventilation arrangements to cope with occasional summertime extremes would be too expensive. A more reasonable objective in sizing the system is to plan for the house temperature to rise about 4 degrees Celsius above the outside temperature when the ventilation is operated at maximum capacity. You would need to double the fan capacity if you opted instead for a 2 degrees C temperature ided the change is gradual.

For convenience and to avoid confusion, do not alter room temperature settings on a frequent basis unless necessary to cater for changing pig weight or age. The fan should be running at its minimum speed when room temperature is at or below the set temperature and increasing towards the maximum speed as the room becomes warmer. The difference between the set temperature and the point at which maximum ventilation begins is referred to as the bandwidth. Setting this between 1.5-2 degrees C provides a nice gradual increase in air movement that will be comfortable for stock, but you may want to make the bandwidth wider (3-4 degrees C) during the winter. Heating systems should be arranged to switch on at a temperature slightly below the set-point.

The minimum ventilation rate is that which you wish the system to deliver regardless of how cold the house has become. The minimum is often set at 5% or 10% of maximum. This is fine where the fan capacity is low, but it causes over-ventilation in cold weather especially where fan capacity is too great. Setting the minimum ventilation rate even higher (up to 15%) will cause severe chilling, poor growth, depressed feed efficiency and ill-health, not least in weaners.

There is no single correct fan capacity for a house. Having too big a fan capacity is a bigger problem than having too little capacity. The mechanical ventilation system should be sized on minimum and maximum design temperatures externally and internally that are exceeded only for a period of hours or days per year. The size also needs to take account of other factors: Minimum stocking rate (lowest number of pigs or lightest weight); Maximum stocking; Insulation standard; Pig performance and heat output. Using this approach we know that the system will keep the house within our target range for almost all the time and we can judge whether it is acceptable to deviate from this target range for a certain amount of time each year.

Fans should be compared on their output against a certain level of static pressure (back-pressure), usually 1/8 inch or 30 Pascals, and not at their rating in free air which would be a higher figure. Different models of fan should be compared on their output against the same static pressure. Fans vary in efficiency (power consumption per unit of air moved) and may be more or less efficient than the figures shown in Table 2. From those data one can calculate the number of pigs for which the ventilation requirements are covered by fans of various sizes.

Air moves through an opening from the high pressure side to the low pressure side. In pighouse ventilation, the biggest influence on air pressure is wind, followed by the difference between inside and outside air temperature. Wind causes a build-up of pressure on the windward side of the house and low pressure on the side that is sheltered.

This has a large influence on fan systems. Usually the wind pressure will increase the output of ceiling-mounted fans or of those on the sheltered wall, but will slow, stop or even reverse wall-mounted fans on the windward side. A high minimum rate or speed may be chosen to protect the fan.

Excessive air movement at pig level chills animals more than does a drop in temperature. Incoming air is usually colder than room air and tends to fall towards the floor, especially if the speed of entry is low. At high speeds of entry (through the inlet) the incoming air will be thrown further into the room and is likely to be mixed (and heated) before reaching pig level.

The location of inlets, shape of the inlet and presence of obstructions (beams, light fittings, pen divisions) will affect how air is perceived at pig level. Smoke pellets can be used to visualise air movement such as at inlets and to demonstrate draughts at pig level. Door ventilation delivers slow-moving, cold air that can cause chilling (and undesirable dunging behaviour) if it reaches pig level from openings around pen fronts, under feeders or under slats.

The position and size of inlets and not the fans dictate the pattern of incoming air. A small inlet area means air speed is higher and the amount of air will be reduced. Too much inlet area means that the pattern of incoming air is determined by the wind speed and direction rather than the inlets. If inlets are not opening, closing or adjusting properly this can greatly affect the room conditions (temperature, humidity, odour) and the conditions down at pig level especially air speed.

Fans, controllers and inlets should be maintained regularly to improve their efficiency and to extend their working life. Dust build-up reduces fan throughput which means it has to run faster and/or longer to achieve temperature control. Dust also interferes with inlet flaps, chimney flaps and sensors. Over 80% of the heat lost from a piggery is through the ventilation system, so its correct functioning is important to minimise fuel costs.