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Logging indicates that in flatdeck rooms where room temperature is achieved rapidly after the introduction of pigs, supplementary heating is needed for a far shorter period.

One of the reasons is that the quicker young pigs warm up, the sooner their bodies start transmitting heat and contributing to room warmth.

If pigs go into an insufficiently heated room, they will spend more time huddling together; they will miss meal opportunities and consequently will release little heat.

But many producers are reluctant to take professional advice about the amount of heating they need for a flatdeck, taking the view that "a couple of kilowatts should be more than enough".

Producers tend to arrive at heating decisions based on experience in their own homes. But there is no similarity, says Nick Bird, of Farmex in a new report on heating and ventilation considerations.

"Human buildings have virtually no ventilation and a high thermal mass. Thermal mass - the walls, floor and ceiling - stores heat. So even if it gets cold outside, it stays relatively warm inside because the walls and floor release heat. Thus to heat a 15ft by 10ft room from 10ºC to 20ºC with a 2kW fan heater takes only about five minutes."

Pig buildings on the other hand have a high ventilation rate, and relatively little thermal mass. What little heat they do hold in the structure is quickly ventilated away.

At zero fan speed, "a couple of kilowatts" might well be sufficient to heat a room, but once ventilation is increased to ten percent the heating capacity required to keep pace more than doubles.

Lowest outside temperatures are usually around 4am-5am and this is also the time when the pigs' own heat output is lowest.

"If you check temperatures during the day, when outside temperatures are highest and the pigs are more active, you may well get the false impression that you have enough heating capacity," says Nick Bird.