Ventilation - Cost of Treatment?

By Nick Bird for FarmEx - Nowadays, it seems that producers are often advised to give "plenty of fresh air" to prevent ill health due to respiratory problems and contaminated air, which is interpreted as increasing the minimum ventilation rates. As with any "treatment", advisers and producers should consider the cost as well as the benefit.
calendar icon 18 January 2004
clock icon 15 minute read
Ventilation - Cost of Treatment? - By Nick Bird for FarmEx - Nowadays, it seems that producers are often advised to give "plenty of fresh air" to prevent ill health due to respiratory problems and contaminated air, which is interpreted as increasing the minimum ventilation rates. As with any "treatment", advisers and producers should consider the cost as well as the benefit.

This article looks at the potential costs using a simple thermodynamic model of the pig environment with sample data.

Simple Thermodynamic Model

Qt = Qp+ Qs= (Change in)T (Ks + 0.333V)

  • Q = sensible heat loss (Watts); p = pigs; s = supplementary; t = total
  • (change in)T = Temperature different (Inside - Outside) °C
  • V = Ventilation rate m3h-1
  • Ks = structural heat loss W°C-1
Whilst approximate, the model is adequate for normal calculations of sensible heat.

In the usage here, it is assumed that ventilation is increased (by thermostatic control) when room temperature rises above target (Set) temperature, and supplementary heating is switched on when room temperature falls below set.

The result of this thermodynamic model is that (leaving minimum ventilation aside for the moment), if pig heat output is greater than structural heat loss at set temperature, a level of ventilation will automatically occur.

For example, if there is 2 kW left over after structural heat loss at target temperature, there will automatically be enough ventilation to remove 2kW of heat. From the above model, it can be seen that with a (change in)T of 10°C (e.g. Inside 20°C, Outside 10°C), this would be 600 cubic metres an hour - roughly a 400mm fan running at 17% speed.

If the ventilation rate is greater than the amount afforded by the heat surplus (because of a higher minimum setting) then heat must be added to make up the shortfall, or else temperature will fall below target.

It should be noted that minimum ventilation rate is only relevant when there is a shortfall of pig heat output. For example, if there is enough surplus for a 17% ventilation rate, there will be a ventilation rate of 17% because of the thermostatic effect. It wouldn't matter if the minimum was set to 0% or 10%, the ventilation rate would be 17%.

The minimum ventilation setting only matters when it is higher than the thermostatic effect. In this case it is removing more heat than the pigs produce, so heating costs are greater, or temperature falls, or both.

Example building

The example situation is a well-insulated first stage flat deck for 120 pigs to 15kg. The calculations are based around a weight of 10kg with standard recommended ventilation capacity and a heating capacity of 4kW.

The model and example building have been run against the following profile of late autumn.

Ambient Temperature

temperatures (Suffolk 19th Nov to 19th Dec 2003).
Heating and ventilation calculated every 15 minutes.

The effect of changing minimum ventilation rate

For example, if Min Vent is set to 10%, the average ventilation rate is 10% (it is at or close to heat deficit all the time), but the amount of supplementary heat varies according to outside temperature. The main cost is then electric heating.

However, if the minimum ventilation rate is set to only 2%, the room is in heat surplus all the time (no supplementary heat is needed), and on average the ventilation rate 5.3%. The only cost then is the running cost of the fan.

Once minimum ventilation rate is set to a value above the amount of heat the pigs produce, supplementary heating cost increases in direct proportion to the amount of extra air to be heated as it passes through the room. 1% extra ventilation costs - on average (during the period studied) an extra £8.91.

To put it another way - treating the pigs by increasing the flow of "fresh air" by 8cc per second (5 litres per minute) costs an extra 7.4 pence per animal (in the circumstances given above). An extra 5% of minimum ventilation (from 8% minimum to 13%, say) means an extra cost of 37 pence per animal.

It's worth bearing in mind that the change in concentration of contaminants for marginal changes in ventilation rate is, of course, marginal. For example, if the concentration of ammonia at a ventilation rate of 8% were 20ppm, then increasing it to 13% would be expected to reduce it to about 12ppm.

Whether this is sufficient to significantly affect health status is the question for veterinarians, rather than ventilation engineers. However, it should be pointed out that progressive increases in minimum ventilation rate give a more or less linear marginal increase in running cost, but a diminishing marginal effect on improvement in air quality. Hence, a change from 8% minimum to 9% should be expected to reduce concentration by about 11%, at a cost of £8.69 a month, while increasing it from 14% to 15% costs almost £9, but reduces concentration by less than 7%.

When it comes to water vapour, there is - potentially - a sharp distinction between one rate of removal and another, since there is a critical point at which condensation may or not occur. One might expect a dramatic rise in ill health if humidity levels are close to 100% RH, but whether there are corresponding health benefits at other RH levels is uncertain.

It is worth bearing in mind rates of humidity production are closely linked to pig heat output. At higher rates of metabolism there are higher rates of CO2 production, leading to increased rates of respiration and correspondingly higher rates of humidity release from the lungs, but also a higher rate of pig heat output. That is, when there is more water vapour to remove, there is liable to be a higher production of pig heat, which allows a higher rate of ventilation for it to be removed.

One might say that the base line for comparison (of costs and benefits) should in this case be having no minimum ventilation setting at all. In this case, the average ventilation rate would actually be 5.3%, but there would be no heating cost during the period shown (as it is a wellstocked and well-insulated building and the weather is not severe).

On this basis, a minimum ventilation rate of 10% - less than double the resulting ventilation rate, and therefore reducing average contamination by under 50% - has a cost of over £45 per month (£1.50 a day). A cost of a little over a penny a day per pig is not big money, but it adds up. Projecting to an annual cost, it amounts to an extra £2000 pounds or so for a 250 sow producer.

This shows the results graphically - costs are very low when ventilation rates are at or about the level which is supported by pig heat output (in this case, a little over 5% of capacity).

Please note that - viewed in this way - the residual costs (being fan running costs) are virtually negligible. Hence, one is somewhat surprised that producers use ACNV ventilation in weaner accommodation.

However, as soon as minimum ventilation rates are pushed above this "self supported" region, costs rise rapidly.

Effect of Set Temperature

According to the model, reducing Set Temperature by 1ºC reduces running costs by 10 to 20%. However, it should be noted that this is, for the most part, smaller than the effect of minimum ventilation rate. For example, compared to Set Temp = 26ºC and 10% min vent, pushing up temperature by 1ºC costs an extra £6.13, whilst pushing up min vent by 1% pushes up costs by £8.48. Here, costs are projected over a wider range of set temperatures.

Effect of Heating Capacity

Above, we have considered the impact that changing minimum ventilation and set temperature has on costs, when there is (what is considered to be) an adequate heating capacity. This is illustrated below at a Set Temp of 27ºC (resulting room temperatures at the outside temperature profile given previously).

In this case, target room temperature is achieved nearly all the time, though you should note the couple of "out of the blue" temperature dips when the heating system "runs out of steam" at low outside temperatures. However, many buildings have less heating capacity.

If - as many farmers do - heating capacity is reduced (perhaps on the basis that what you don't have you can't use), systems run out of steam rather more quickly.

This table summarises the resulting temperatures for the foregoing graphs with a range of heating capacities (ignoring functional issues such as proportional control bands). When heating capacity is reduced, the average values may not change that much - for example, losing a kW of capacity (from 4 to 3kw) with a 15% minimum drops average temperature only half a degree, but the minimum drops by 7º.

Going down to only 2kW heating capacity means it drops to less than 16ºC - hardly a comfortable temperature for a 10kg pig and - as the graph shows - the room is in control of temperature for only a small part of the time.

It should, therefore, be a matter of debate as to whether this may be more damaging to the pigs than a higher level of aerial contamination.

In Practice

In the foregoing, we have looked at the thermodynamic and financial results on a simplified physical basis, using a constant value of pig heat output. How does this compare with real life?

In practice, recent studies of temperature and financial results suggest that the extremes are somewhat greater than those shown. "Real life" is better or worse than the theory (if it's reasonable to apply a judgmental term to a physical process). When there is generally insufficient heat, temperatures tend to be worse than those shown. When there is generally sufficient heat, they tend to be better.

The reasons are not entirely certain, but it appears that when things are good, and the pigs are generally comfortable, they tend to release more heat if room temperature has a slight tendency to drop. This stabilises the thermodynamic so that room temperature then tends not to drop.

On the other hand, in rooms which tend to go out of control and lose temperature, pig heat output (i.e. heat release into the air) appears to reduce, making the situation worse. There is a temptation in thermodynamic modelling to regard the pig as a passive part of the equation, but in practice it probably takes an active (though not necessarily conscious) role. For example, if a room has a tendency to be cold, it is likely pigs restrict their activities and exposure at these times (making the heat balance worse); they may be more active and release heat more readily when the room is warmer, when their heat output is less required.

Effect of under stocking

The largest (potential) electrical cost in weaner buildings is heating. Electrical heating represents the balance between how much heat the pigs produce, and how much heat is required to achieve a desired room temperature (at a given ventilation rate and outside temperature).

Under-stocking - less pigs than a room is intended for - is damaging in two ways. Firstly, the amount of heat produced by the pigs is less, so more supplementary heat is needed. Secondly, the actual cost is divided by fewer pigs. This is illustrated below by reference to the (previously calculated) electrical cost of 120 pigs @ 27ºC and 10% ventilation

Pigs 120 100 -16.7%
Heating cost / month £45.52 £59.87 +31.5%
Heating cost / pig £0.38 £0.60 +57.8%

16.7% fewer pigs means 31.5% more heat required, which increases heat cost per pig by 57.8%. But note - this assumes that the ventilation rate is kept the same (the same number of air changes per hour at minimum ventilation. If minimum ventilation is reduced from 10% to 9% (on account of fewer pigs to ventilate) then -

Pigs 120 100 -16.7%
Heating cost / month £45.52 £50.81 +11.6%
Heating cost / pig £0.38 £0.51 +33.9%


Lower set temperatures and higher minimum ventilation rates are often recommended by specialists, with the aim of improving pig health. Pig health specialists may object to the suggestion that they have "prescribed a treatment", but it is certainly often reported that, say, "the vet told me that I needed to…". At the least, producers may be unclear as to what is best for the health of their pigs.

It may or not be an effective treatment - proper trials would seem sensible, rather than "general suggestions", or "it seems like a good idea". The thermodynamic model makes it clear, however, that minimum ventilation rate and either heating cost or ability to maintain temperature are not unrelated.

If enough pig heat is available, an increase in the minimum setting is not required. Higher levels of ventilation will result from the heat surplus, without increasing the setting. If the thermodynamic ventilation rate is higher than the minimum, then this setting is irrelevant, and changing it will have no effect whatever. (The treatment is either unnecessary, or ineffective.)

If increasing the minimum (above a previous setting) has an effect, it can only be of two kinds - to increase heating cost, or to worsen temperature control (i.e. to fail to reach target more often and for more of the time). Depending on the degree of change, there may be both. The settings employed by many users suggest that they may be unaware of the close interrelationship of ventilation rate and heat loss - such as setting high target temperatures, with extremely high levels of minimum ventilation rate.

It should be realised that - to achieve sufficient changes in contaminant concentration as to be noticed - from "smelly" to "reasonably fresh" - much larger changes to minimum ventilation settings may be needed than expected. A few percent change in the min vent setting is unlikely to have a noticeable effect on contamination (given the human nose's relative insensitivity), though whether it is sufficient to have an effect on pig health may be another matter.

This is not to say that losing room temperature or incurring more heating cost is not the correct policy, but one should be clear that one is doing it. In this study, we have looked at quantifying the costs of a higher ventilation regime against a risk of ill health. (A difficult calculation, perhaps, though one that should be familiar to drugs companies in respect of prophylactic treatment.)

Whatever the choice of regime, it is clearly very important to follow it, and to know that one is following it, particularly where heating is involved. For example, control and ventilation systems that cannot be set to any particular level of minimum ventilation, or rely on "user assessment" for adjusting the minimum ventilation at the time are questionable.

Repeatability is a key attribute of regulation systems. That is, if setting a particular value, having reasonable confidence that it is the same as last time and also that it is the same from room to room.

It's not suggested that airflow measuring systems - as employed in some other countries - are essential, but a reasonable consistency of performance is. This can only be achieved with modern controls such as Dicam.

Older control systems with un-scaled or analogue dials, while perhaps adequate in their day (and probably better than the rough and ready systems that preceded them) have certainly had their day. Monitoring shows that performance of such systems is extremely variable. To what extent this is due to variation in settings, or loss of calibration over the years is uncertain.

Much advice appears to be given in the absence of specific information or targets. For example, increasing minimum ventilation so as to reduce ammonia, without measuring the ammonia concentration, or reducing (or increasing) set temperatures without any data on the temperatures that are actually achieved.

Data logging shows that there are often considerable differences between what appears or is thought to be to be the case, and what actually is the case. This is not surprising when one realises that most advisory visits are made during the day, in "normal" (as opposed to extreme) weather. Damage to pig health due to poor ventilation - if there is any - is more likely to occur at 4 in the morning in very cold weather than at 4 in the afternoon on a mild day.

The level of stocking is of very great importance in both maintaining temperature in general, and generating a sufficient heat surplus so as to stimulate ventilation. A small shortfall in pig numbers through the building makes a large difference to the thermodynamic ventilation - or a large increase in the amount of heat required. Given that - with a general fall off in production throughput in recent years for a variety of causes on many farms - it may well be that low stocking rates are a particular factor in loss of ventilation rates.

It's not suggested that more ventilation per se is bad for pigs. However, trying to achieve higher ventilation ignoring the thermodynamic balance, or any targets in terms of what contamination levels should be changed from or to, is certainly questionable. A general instruction to "get more air through the place" without assessing the consequences is somewhat questionable, and little better than asking people to "drive more safely". Measurement is a crucial requirement in assessing any treatment.


Changing set temperatures and minimum ventilation rates are often "prescribed" in a rather loose way with the aim of improving health.

As with any treatment, it should be assessed in terms of cost-benefit, even if calculations are only approximate.

This article has tried to explain and discuss some of the underlying thermodynamic issues, and has illustrated the cost side of the equation using sample data in an example situation. In the example shown, there is a clear correspondence between minimum ventilation rates and production costs.

It is suggested that there is a need to quantify the corresponding benefit in terms of improved pig health.

Source: FarmEx - January 2004

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