Chapter 3 Equipment Sizing The various components that are used in hot water heating systems were covered in Chapter 1. The proper sizing and installation of those components (boilers, circulators, piping and expansion tanks) is essential to ensure troublefree comfort for your customer. All hydronic system installations need to start with a heat loss calculation for the building. This establishes 20 National Oilheat Research Alliance a target for the heating system and provides some documentation for the customer. There are many heat loss calculation methods–the most recommended are the Hydronics Institute guide H-22 and the Air Conditioning Contractors of America’s Manual J, Figure 1. Boiler manufacturers often provide free load calculation software as well. Programs can also be obtained online: www. acca.org, www.wrightsoft.com and www. elitesoft.com Additionally, there are heat loss calculators available in software form. They may differ slightly, but will provide a reasonable estimate of the heating needs of the building. All heat loss formulas have a built-in safety margin, so designers/installers should not adjust these carefully derived estimates to add more heat! The ONLY way to properly size a heating system is to perform a heat loss calculation on the building. Many authorities having jurisdiction (AHJ) or building inspectors, require that a heat loss be performed when installing a new or replacement boiler. Heat is lost two ways—the first is air infiltration, or heat lost due to heated air leaking from the inside of the house to the outside, being replaced by cold air coming in. This replacement, or infiltration air, will need to be heated up to comfortable levels. Infiltration heat loss is, along with window loss, the largest single heat loss element. The second method of heat loss is conductive heat loss. Heat loss calculations also include conductive heat transmission through walls, ceiling and floors, doors, windows and skylights. A house is extremely variable and these heat load calculations will not be precise. Thus, some rounding and estimating will still provide for acceptable results. In addition to a whole house heat loss evaluation, it is also important to do a room by room heat loss analysis. This enables correct sizing of the terminal units—baseboard, radiators, radiant floor heat—for each room, ensuring that each room in the house is comfortable. Example A simple 10′ by 15′ room with 9′ ceilings and two outside walls with windows. When conducting a heat loss analysis, the indoor design temperature (the desired indoor temperature) needs to be known. In most cases, the number to use is 70° F. The outdoor design temperature, or the “coldest day of the year,” needs to be determined. A heating system should be able to maintain the structure at 70° F when the outdoor design temperature is reached. Figure 1
NORA Advanced Hydronics
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