Topics: Heat transfer, HVAC, Heat Pages: 20 (4731 words) Published: May 29, 2013
MECH 8250 – Building Systems Module 1 - Heating Load Calculations Module 1 Heating Load Calculations The amount of heat is required for the building heating system to maintain a balance of the building heat loss. The amount of heat and moisture are gained from the building internal and external heat sources that is required to be removed by the building cooling system. Both heating and cooling loads form the basis in designing the HVAC systems for the building and occupied spaces to achieve both thermal comfort and good indoor air quality in the building. In addition, the calculated heating and cooling load can be used to provide estimate of energy consumption of building. Page 1 of 33

Applications

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Note: Heating load relates to the building heat loss Cooling load relates to the building heat gain

Methods for Estimating Loads Accepted methods to calculate the heating and cooling loads are documented in the ASHRAE Handbook of Fundamentals. The earlier methods (before 1993) are simple and can be performed manually. The later methods after 1993 have become more complicated that it is only practical with the use of computer programs. However, improvement of the earlier method can be made using the Excel spreadsheet. Accuracy and precision of the heating and cooling loads calculations do not depend exactly on the manual or computational methods. This is due to the fact that building elements and contents are complex assemblies and it is difficult to model the actual heat flow paths through 100 percents of all the building assembly. So, it is more important for designers and engineers to appreciate the relative accuracy of the calculated building heating and cooling loads when designing the HVAC systems.

Critical Conditions for Designs Critical design conditions for the heating system will occur in winter and at a time when the building has minimal heat gains from radiant solar energy, the lights, appliance or people (i.e. around early morning). The selection of an appropriate cold outside air temperature for design is an important decision. Critical design conditions for cooling system will be the peak coincident occurrence of heat, Copyright of J. Cheung _2009

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humidity, solar effects, and the internal heat gains from equipment, lights and people. Since the position of the sun varies by season and throughout the day, it may require several estimates of the cooling loads at different times of the day to establish the highest combination of individual loads.

Inside Temperature Criteria The inside design temperature and humidity are determined for different occupancies with different expectation of comfort. It is intent to meet the temperature ranges within the comfort zone as shown in the ASHRAE Standard 55. However, some of the inside design temperatures are dictated by the local building codes. In British Columbia, BC Building Code has a set of inside temperature requirements for single family dwelling unit in establishing the heating load estimate. These inside temperatures are as follows: Indoor Design Temperatures 22 C (72 F) 18 C (64 F) 15 C (59 F) Type of Space in Dwelling Unit All living spaces Unfinished basements Heated crawl space

The temperatures are also adopted by designers and engineers for all buildings with residential occupancies. Inside design temperatures for commercial buildings are in the ranges between 68 F and 74F and they should be within the comfort zone as shown in ASHRAE Standard 55. The selected design temperatures depend on the type of occupancies and activities in the occupied space. Average temperatures typically selected by designers and engineers are: Indoor Design Temperatures 21 C (70 F) 22 C (72 F) Type of Occupancy Commercial spaces including offices and commercial retail units Critical spaces including residential, hospital, care...