Using Energy Balances to Meet
Energy Efficiency

Fig.1: It is convenient, to choose the envelope for energy balances of a building at the external surface of the insulation.

Fig. 2: Heat losses (transmisson and ventilation losses) exit the building through the envelope. Heat gains enter the building through the same envelope. Using the law of energy conservation, the sume of the gains equals the sum of the losses as long as the Inner Energy does not change.

Fig. 3: Energy balance starting with an average contemporary house and ending with a Passive House: If losses are reduced consistently, passive solar gains (yellow) and internal gains (gray-yellow) can compensate for a major part of the losses. A separate heating will be dispensible, if the maximum heat load is not higher but 10 W/m²; in this case the fresh air system can be utilized for heating.

(red: active heating; yellow-grey: internal gains; yellow: solar gains (top southern, bottom northern windows); losses: light green: windows (top southern, bottom northern windows); magenta striped: roof; dark blue: external walls; brown: slab on ground; light teal: ventilation losses.)

The balances shown here have been calculated by PHPP. These balances belong to a passive house settlement really built: The Passive Houses in Honnover built by Rasch&Partner /faktor 10/. Realized is the standard of Passive Houese, that is the balance with the lowest energy losses in the animation. The eenrgy consumption monitored is in a quite good relation compared to the balance calcuated here.

Read more about the 11th International Conference on Passive Houses
2007-April-13 to 15th:

Is it the task of the heating system, to keep the temperature constant? Of course - this is a simplification. But it is a quite good approximation and it is getting even better, the better the thermal quality of the building envelope is. There are different topics in this discussion:

  1. Why heating parts inside of the building envelope, if there are no comfort conditions to be maintained?
    In fact, very often building costs as well as energy costs get lower, if "unheated parts" of a building are included inside of the thermal envelope. This happens in all cases, where the new envelope has a reduced surface.
  2. Isn't it better for health, if the temperature changes during the day and between different rooms?
    We cite Ole Fanger (taken from "Thermal Comfort"):
    "It is a well known phenomenon that the internal body temperature has a daily rhythm with a maximum occuring some time before sleeping and a minimum occuring some time before the person awakes. ...
    However, Nevins et al. found no difference in optimal temperature between experiments performed in the afternoon and those preformed in the evening, an observation which is confirmed in the present study <Fanger>. ....
    If any influence of circadian rhythm on comfort conditions exists it is probably so small as to be of no practical significance. "
  3. Does not a night or weekend set back save energy?
    Of course, it does. But the savings are quite small in well insulated buildings.

All these questions about different temperatures are discussed in depth in the preceedings of the 25th session of the working group on cost efficient passive houses (German).

PHPP: The Passive House design (planning) package is a tool for design of Passive Houses. The energy balance is a major part of this tool. You find additional informations here: PHPP-page.

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. 1).

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 are delivered.

Under the simplified conditions given here, it is simple to give the energy balance of the building:

The sum of the heat losses


the 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 Package (PHPP)".

Using 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 insulation, windows for Passive Houses and a highly efficient heat recovery from exhaust air.

Author: Dr. Wolfgang Feist

(updated: 2006-09-23 WF
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