(volume) of air within the building is replaced by the infiltration of cold air. The factors range from one-half air change per hour for few air leaks and heavy insulation to as much as two changes an hour for buildings with lots of air leaks and little insulation. The infiltration heat loss factor is one Btuh for every cubic foot of room volume for every degree difference between indoor and outdoor temperatures. Multiply the Infiltration Factor times the Volume (cubic feet) of air in the building times the Design Temperature Difference to find the Infiltration Loss. Infiltration Loss Factors • Very tight, very heavy insulation, ½ air change per hour—0.009 • Tight, heavy insulation, ¾ air change per hour—0.011 • Standard construction and insulation, one air change per hour—0.018 • Older house, good condition, 1½ air changes per hour—0.027 • Loose construction, air leaks, 2 air changes per hour—0.036 Total Heat Loss The total heat loss calculation is how many Btus per hour (Btuh) it will take to heat the building on the coldest day of the year. The total Btus per hour that leak through every square foot of building surface that is heated on one side and not heated on the other at the Design Temperature Difference–add the total Btus per hour it takes to heat every cubic foot of cold air pulled into the building to replace the warm air that is leaking out– this equals the Total Heat Loss of the Building. The total Heat Loss is also called the heating 40 National Oilheat Research Alliance load. It is the total Btus needed per hour to heat the house on a design temperature day. Boiler and Furnace Sizing The total heat loss calculation provides the information needed to size the boiler or furnace to heat the building. Heat for warming the domestic water does need to be considered in the overall boiler sizing unless hot water is a primary load (for example, a commercial laundry or kitchen application). Inspecting the hot water requirements however, is critical, especially for drain down tubs and high volume showers. The trick is to take care of the additional hot water demand by increasing the size of the storage tank. Most manufacturers recommend the same volume in storage as the largest demand. A 100-gallon bathtub requires a 100-gallon storage tank. Hot water can be addressed with either firing rate or storage, or a combination of both. Something else to consider: as building heat loss gets lower and lower, hot water may become the primary design criterion instead of the heat loss in the future. In the USA, boiler and furnace ratings are shown in Btus per hour. Manufacturers also often use the term MBH–1,000 Btus per hour. European equipment uses the Metric Rating— Kilowatts (kW). A kilowatt equals 1,000 joules per second, about 3,412 Btuh. Commercial and industrial boilers are rated in boiler horsepower. One boiler horsepower equals 33,446 Btuh (don’t confuse this with brake horsepower used to rate motors). To size a furnace, use the furnace output rating to select the furnace that is the closest higher number to the Total Heat Loss for the building. Use the Net Load Rating when selecting a boiler. The Net Load Rating includes a 15% pick-up factor that allows for heat loss from piping. Select a boiler with a Water Btuh Net Load Rating just higher than the heating load.
NORA Advanced Efficiency
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