All energy is conserved
- no energy gets lost. However, energy can escape out of the region,
where the energy service is utilized. This is, what we call an "energy
loss", although the energy only moved to another place
and may have changed its form.
Already this introduction
shows, that energy balances only make sense within a well defined
region with a well definded boundary. The boundary of the region
is called the envelope.
In the case of heating
or air conditioning the region of interest is the "heated or
conditioned space". More precise: It is the volume in the building,
which is conditioned to comfortable thermal conditions. In most
cases it is convenient, to include "passively heated"
parts, as long as the balance envelope will be simplified. Generally
the envelope should be chosen by pragmatic considerations: For a
building it is convenient, to choose the envelope at the external
surface of the insulated external building shell (Fig.
The task for heating
or air conditioning now is just to keep the temperature inside of
the envelope constant (Discussion).
Lets have a look at a
heat flow going inside out through the envelope, it may be hot air
moving out through a window. Such a "heat loss"
would reduce the "Inner Energy" inside the volume; and
that would cause the temperature drop inside of the building. Just
that should be avoided in order to keep the conditons comfortable.
Therefore the energy flow to the outside has to be compensated:
Another heat flow has to be created, going outside in, to keep the
level of the temperature.
That is in important
insight: The need for heating is always only a reaction on heat
losses. Because of the law of energy conservation a building will
stay well conditioned - as long as there are no heat losses. It
is a pit, that the physical mechanisms by which hotter systems transfer
heat to a cooler environment are quite numerous and efficient. If
we do not "isolate" the hotter system (by insulation),
in general a lot of heat will get lost by heat conduction, convective
heat transfer and radiation. "Heating" always is the subsitution
of energy losses - therefore "heating" can be reduced
to an arbitrary low amount by effectively avoiding losses.
There is some luck when
looking at the heating task: There are some free "heat
gains", too: For example the solar radiation through
the window panes (so called passive solar energy)
and the energy of the electricity supply, which is converted to
"internal heat sources" in the building.
This addes to the heat radiated from persons inside the building.
This energy is as well tranferred through the envelope into the
house - at any time, when the persons enter the building or nourishments
Under the simplified
conditions given here, it is simple to give the energy balance of
sum of the heat losses
sum of the heat gains.
It is quite simple to
calculate the heat losses (depending on the insulation).
The internal heat sources and the passive solar energy can be estimated
as well. Therefore, on the basis of an energy balance the heating
energy required can be calculated.
There is only one minor
problem: The amount of the solar gains which can not be utilised
has to be determind. But even for this there are well validated
simplified formula given e.g. in the European norm EN 832.
For practise, these methods have been integrated to the
"Passive House design
this knowledge, it is simple to understand how a passive house is
working. Our animation (left side) illustrades
the main principle. The passive house concept bases on the reduction
of the heat losses. Doing so, the free heat gains will be almost
sufficient to keep the temperature at a comfortable level. To reduce
heat losses - that means: good
for Passive Houses and a highly efficient heat
recovery from exhaust air.
Author: Dr. Wolfgang Feist
© Passive House Institute;
unchanged copy is permitted, please give reference to this page)