Design Heat Losses
From Swikipedia
Introduction
Design Heat Loss is the total heat loss from a house per hour when the outside temperature is at the Design Outdoor Temperature that one inputs.
Brief Description
The assumptions behind design heat loss are as follows:
1. Weather conditions are selected from a long-term statistical database. The conditions will not necessary represent any actual year, but are representative of the location of the building. ASHRAE has tabulated such data, as have others. The designer may select a "severity" of weather that seems appropriate for the building type in question - although energy codes often specify what data shall be used (to minimize the installation of over-sized, and inefficient, systems).
2. Design heat loss is assumed to occur at night, when there are no solar loads on the building.
3. The building is assumed to be unoccupied (as in "no one home" - not as in "abandoned").
4. All building equipment and appliances are considered to be turned off (not in operation).
5. Lights are assumed to be turned off.
6. Latent (moisture) loads are generally ignored (or are treated separately).
7. Heat flow is analyzed assuming static conditions (stable temperatures over a several-hour period), which means that heat storage in building envelope materials is ignored.
The total building heating load will involve heat transferred - from interior to exterior - through all elements of the building envelope (including infiltration and ventilation air flows). Heat gains from building occupants, lights, and/or equipment are assumed to not be available as an offset for envelope losses - a conservative, but generally reasonable, assumption.
Design heating loads consist of the following:
• sensible loads through envelope assemblies exposed to the outside air (roofs, walls, doors, windows);
• sensible loads through slab-on-grade floor constructions (when applicable);
• sensible loads through below-ground walls and floors (basements - when applicable);
• sensible loads caused by the leakage of outdoor air through the building envelope (called infiltration) and/or ventilation;
• latent loads through envelope components and due to infiltration/ventilation (often ignored if humidity is not controlled).
References:
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
