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The mystery around 0ºCelsius (32ºF).
In a seminar some years ago, we presented an investigation of the mystery
around 0ºCelsius (32ºF). It was shown how the room temperature was
affected by variations in the outside temperature when an outside sensor
operated the heating system. The reactions on changes were too fast and when
the outside temperature was falling, the room temperature was going up too
much. When the outside temperature was rising, the room temperature was
going down too much. This is because of the accumulation in the construction
and the delay that it causes.
One obvious conclusion was that the common simple system with an outside
sensor, that does not consider the dynamics of the building, is quite
useless and only works with very slow weather changes. It creates more
problems than it solves and among these is the coordinated call for energy
(peak demand for heating or cooling) caused by a weather change. Pulsating
or variable energy supply, based on the room temperature, works better.
The result is large energy waste and an uncomfortable internal
environment. The normal situation is that the outside temperature goes up in
the morning and the falling room temperatures naturally occurred when people
had to get out of bed and prepare for the upcoming day. The rise in
temperatures affected offices in the afternoon, when temperatures started to
fall and close to people preparing to leave the offices. After experiences
from warmer climates like Spain and its heavy constructions, it is even more
obvious.
A very common observation is that it often is cold inside when the outside
temperature is around 0ºCelsius (32F), when an outside sensor is
used. Outside sensors are in theory used to prepare the environment for
changes in weather and to make the heating (cooling) system more economical.
The outside temperature varies quickly all the time and the temperature in
the building has a delay because it is slower. The dynamic of the building
must be considered, but is not in the majority of systems. With a change
from warmer to colder weather, the construction is already warm and the
immediate call for energy by the outside sensor will raise the temperature
further until the construction adapted to the change. The opposite will
happen with a change from colder to warmer weather.
In a system with an outside sensor, a room thermostat was introduced which
complemented the system in order to try a simple operating policy, "do not
heat when it is already warm" in the calculations and avoid the excessive rise in
temperature. The following tables of outside and inside temperatures, show
the result during two weeks in January. The first 4 days the temperature
goes up, followed by 3 days of colder weather and then a rapid shift to
milder temperatures.
In the table of room temperatures is shown the effective temperature
(optemp), which is the perceived temperature by the body and considers both
the air temperature and surface temperatures. We will later describe this
criterion in a separate article. The effect by the limiting room thermostat
at 22ºCelsius can also be seen.
The tables show that the changes outside and inside are going in opposite
directions and explains the mystery of 0º Celsius (32ºF). Notice the
drop in room temperature when the outside temperature is rising above 0º.
The room thermostat only works like a security switch to avoid excessive
energy consumption.
A more sophisticated computer based system with outside sensor can learn how
the building reacts and can therefore improve the control of the building.
We will discuss the control philosophies and possibilities separately in a
later article.
The perception that there is special problem around 0ºC (32ºF), is
because the average value during the heating season in Sweden, often is around that
temperature and the changes often are in that range. These phenomena would
occur at a higher temperature for other average values or cooling systems
and, if outside sensors are used, make a considerable contribution to peak
demand problems. In Spain, the 0º problem for heating is more like an 8º problem.
One of the participants in the seminar, was a control engineer that later
used this knowledge to develop control equipment to adopt existing systems
to the buildings. It has been successfully sold and installed in a large
number of buildings in Sweden and shows remarkable energy savings, peak
demand reductions, and increased comfort levels.
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