There are two distinctly different kinds of small incubator, those with a fan and those without
a fan; ‘forced draught’ and ‘still air’.
Users are sometimes confused by the choice. The difference in the way eggs are warmed is important but often not fully appreciated. Here are some of the considerations.
To achieve good lateral temperature consistency, ‘Still air’ incubators (those without fans) are
generally heated from above the level of the eggs and exhibit a marked temperature difference
between the upper and lower levels so that the tops of the eggs are up to 4°C (7°F) warmer
than the bottoms.
Introducing a fan into the incubator dramatically changes the situation and eliminates the
temperature gradient for all practical purposes. It follows that if eggs are to be set on different
levels in the same machine it is essential to circulate the air mechanically so that all the eggs
are exposed to the same temperature.
However since many of us are concerned with relatively small numbers of eggs which can all
be set on one level there is a real choice to be made.
As well as eliminating the temperature gradient a fan also eliminates variations in the Relative
Humidity. R.H. is closely related to temperature (see Brinsea® information sheet Humidity in
Incubation) in such a way that (for a given volume of moist air) the relative humidity reduces
rapidly as temperature rises.
The diagram illustrates the kind of temperature and humidity variations that may be expected
within a still air incubator.
The importance of this illustration is to show that definitive measurement of the RH as well as
temperature become very tricky in still air conditions. In practice it is rather inconvenient to
measure (or control) temperature or humidity at ‘mean’ egg temperature that presumably is
half way up the egg.
Sensors mixed up with eggs are exposed to damage and contamination and are liable to
mis-measurement due to direct contact with eggs or chicks. Remember that the metabolic heat of
the embryo will raise the egg temperature above that of the air you are trying to control. So,
control and measurement in still air incubators is normally done above the eggs and some
correction is necessary to allow for the temperature gradient.
The temperature gradient is itself variable, dependent upon outside temperature. In cold
conditions it is necessary to raise the temperature slightly at the top of the eggs to achieve the
same mean temperature because the egg bottoms are now colder.
Even the thermometer may be affected by the temperature gradient. A slight increase in
reading will result from the stem being in a warmer zone than the bulb. This is due to thermal
conduction taking place down the stem and raising the bulb temperature slightly above its
All these difficulties can be eliminated at a stroke by incorporating a fan. Almost all
experiments carried out by research workers studying the process of incubation are conducted
in forced draught conditions. This reduces the number of variables and makes for a more
So why bother with still air? Here we get into an area fraught with subjective views and
prejudice. Obviously a fan adds to the cost of a machine. Cheaper machines are therefore
usually still air.
One trade-off on the cost is that still air incubators need to be better insulated than those with
fans to keep the temperature gradient within limits. Insulated cabinets tend to be more
expensive unless they are made of polystyrene.
More important than cost is the relatively un-researched question of how eggs fair with a
temperature gradient compared with the forced draught condition. Experimental work using
dummy eggs filled with sensors and placed in wild nests indicate that a significant
temperature gradient is normal in the natural environment.
Also quite wide fluctuations occur which would be considered highly suspect in an incubator.
Our experience over many years of manufacturing incubators is that significantly more users
of still air incubators are likely to achieve satisfactory hatches. We don’t know why. We can
achieve virtually identical performance with either type of machine in our own laboratory
with a range of different species’ eggs.
Our suspicion is that eggs incubated in conditions with a temperature gradient similar to the
natural environment are more tolerant of ‘less than ideal’ incubator settings.
It is also probable that the eggs of wild birds are better suited to the more natural conditions
of still air incubators. After all, they have not been subjected to the process of artificial
selection over countless generations to dispose them favorably to forced draught conditions.
So much for theories. Now for some practical suggestions.
Most avian species incubate best at a mean temperature of 37 to 38°C (98.6 to 100.4°F)
Waterfowl are best kept at 37 to 37.5°C (98.6 to 99.5°F). In forced draught incubators this
would be the temperature indicated on the thermometer.
In still air machines the temperature indicated would depend on the position of the
thermometer, which is quite critical - so follow the instructions carefully about adjusting the
height. It will also depend on the construction and temperature gradient in the incubator, so
again, follow the instructions.
In the absence of instructions set the thermometer just clear of the top of the eggs and run the
incubator at an indicated temperature of 39 to 39.5°C (102.2 to 103°F).
It is important to appreciate that no incubator has perfect temperature distribution. Heat losses
from the cabinet must be balanced by heat provided by the heater.
The process of transferring heat from one to the other necessarily involves a temperature drop
- even with a fan- and this drop will mean that some eggs are warmer than others.
To keep these differences small, operate the incubator in warm, steady conditions.
Ideally use a thermostatic electric convector heater to maintain a steady room temperature of
20 to 25°C, day and night.
Low air speed and high humidity give the best hatching results. In forced draught incubators
the RH needs to be high (70% or more) to prevent excessive drying of exposed membranes.
In still air incubators this problem is much less severe and a dramatic rise in humidity
accompanies the first birds out which no doubt helps those that follow. Temperature in
hatchers is usually run about 0.5°C (1°F) lower than during incubation to compensate for the
high metabolic rate of the emerging chicks.
Improvements in the design of incubators together with reducing costs of reliable electronic
temperature controls is making it possible to control the incubation regime more precisely
than before. However, for non-domesticated species much work needs to be done to
determine the ideal conditions.
For example, why is it that eggs incubated under natural parents for the first few days and
then transferred to a machine hatch appreciably better than those set in an incubator from the
Why do we find that eggs incubated in still air incubators tend to be more tolerant of
deviations in temperature and humidity than those in forced draft incubators?
It may be premature to dismiss still air incubators as out of date until incubator design can
more closely mimic natural nest conditions.