HI 1999: New Guide Offers Help Calculating Heat Loss
The Hydronics Institute recently updated one of its longtime I=B=R guides, the Heat Loss Calculation Guide. The heat loss guide is only one of almost 50 publications HI provides especially for heating contractors.
The following is excerpted from the new heat loss guide. The guide also includes how to calculate heat loss for "special cases," such as fireplaces, cathedral ceilings, log cabins and green houses.
To order a complete copy of the 80-page guide, call 908/464-8200. Keep in mind, HI is currently updating its Installation Guide for Residential Hydronic Heating.
Heat is a form of energy that has both intensity and quantity. The intensity of heat is its temperature as measured by a thermometer. The awareness of temperature is a relative value since everyone does not react in the same manner to any specified temperature. For example, in a room heated to 72 degrees F, some occupants are too cool, some are too warm, but most are comfortable.
The quantity, or amount, of heat is measured in terms of British Thermal Units, commonly referred to as a Btu. This is a definite value, and is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.
The designer of a hydronic system is concerned with both intensity and quantity of heat. The system he designs must supply the quantity of heat necessary to maintain the desired intensity or temperature. The quantity of heat required for this purpose is calculated in terms of the number of Btu per hour.
The purpose of installing a heating system is to provide comfort. This is best defined as "the absence of discomfort." In their normal activity at home, occupants should have no awareness of temperature or drafts. The standard Indoor Design Temperature is 70 degrees F, at which most people feel comfortable. However, older people may want 75 degrees F, and many people want cooler temperatures while sleeping. In bathrooms, it is desirable to provide for warmer temperatures by adding 20 percent because the evaporation of water on the skin after a shower can cause a chill.
One of the characteristics of heat is that it constantly seeks to move from warmer to cooler spaces. This is referred to as heat flow. Since the outdoor temperature during the heating season is cooler than indoor, this characteristic of heat causes it to flow from the warmer indoor spaces to the colder outdoors. The amount of heat flowing to the outdoors is called transmission heat loss and is expressed in Btus.
The other means of heat flow is movement of air through cracks and openings in the building. Wind pressure outside and chimney effects inside the building cause the building to be at a lower pressure than the outside. So cold air from outside flows into the building. This air must be heated. The process is called infiltration heat loss.
Why Do A Heat Loss?You need to do a heat loss calculation because your customer deserves it - and because, when you have finished installing a new, high efficiency boiler, your customer expects his fuel bills will be lower. If you size the boiler to the heat loss, you will probably accomplish this.
Every time a boiler cycles on and off some heat is lost. The more often the appliance cycles, the greater the amount of heat wasted. Appliances cycle because the heat demand is smaller than the appliance output. If the heat demand were higher than or equal to the appliance output, the appliance would stay on indefinitely.
The effective efficiency of the boiler as the demand is reduced is referred to as part load efficiency. The graph below shows what happens as the demand becomes smaller than the boiler output. The fuel economy (part load efficiency) drops off rapidly as the demand vs. boiler capacity falls.
The key to lower heating costs for your customers is to supply them with heating equipment sized to meet the demand. Putting in a boiler the same size as the old one or using "rule of thumb" in selecting replacement boilers will cost your customer money excess fuel bills. Oversized boilers cycle too often, so they waste heat.
Size the hot water boiler to meet the heat demand of the building - Determine the building heat loss. Then select a boiler large enough to handle the demand.
You can determine the heat loss for a building based on the building construction and geographic location and the customer's desired indoor temperature during the winter. This guide provides you with the means to do heat loss calculations, with data taken from the ASHRAE Handbooks and other reference sources. Pay close attention to the construction details of the building to make the best decision when selecting heat loss calculation methods.
Local weather conditions can vary substantially in just a few miles distance, particularly in mountain locales. So contact your local weather bureau to assure the most accurate outdoor design conditions.