Intro to Heat

Control of indoor environment has long been one of man’s primary goals. It involves much more than making a warm indoor climate. Humidity control, indoor air quality, cooling and efficiency are all parts of the equation. All energy we use comes from the sun, with the exception of nuclear energy. The burning of fossil fuels such as fuel oil, natural gas, propane and even hydroelectric all originate from the sun’s heat.

Temperature Temperature is a measurement of the intensity of heat. It does not tell us how much volume of heat there is. If the temperature is very high, we assume there is a lot of heat. This is not necessarily true. For measurements of volume of heat, we use scales such as the BTU.

British Thermal Unit (BTU) By definition, The BTU is the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit. From this we can measure the capacity of heating and cooling equipment. Generally, equipment is rated at the number of BTUs it will consume (or absorb in the case of cooling) per hour. Also, structures can be rated in BTUs per hour that they lose (or gain) to (from) the outside environment. By this method, we can match equipment to structures.

Heat Transfer There are 3 ways to transfer heat from one place to another. Radiation is the heat we get from the sun. This is also part of the heat we get from heating stoves. Radiation will travel great distances virtually instantaneously. It travels through space or less effectively through atmosphere. However, if an object is placed between the source of heat and the destination, it will heat the object and none will penetrate to the destination. Also, the amount of heat that is transferred will vary with the distance to the heated object. If I double the distance from a radiant heat source, I will only receive ¼ the heat. It is often said that radiant heat heats the person and not the room. It is often used in spot heating applications where a person is working in a warehouse. A small radiant heater is used nearby the person giving him warmth without heating all the air in the warehouse. Some homes have used radiant heat panels in the ceiling or floor to supply heat for homes or businesses.

Convection Convection is the movement of heat from one place to another by the density of the medium. Convection can only work in a liquid or gas. When a fluid medium is heated, its density is lowered. When its density is lowered, it weighs less and tends to rise. An example of this is the hot gasses from a campfire. Initially the hot gasses rise very quickly due to the large difference between their temperature and the temperature of the surrounding air. As the gasses rise, they lose heat to the cooler air around them. Thus the smoke from the fire may rise then level off or even drop back down. Early heating systems moved the heat through the structure by convection. The efficiency of these systems was lower than with fan forced systems, so most of these systems are no longer in use.

Conduction Conduction is the movement of heat through a medium. An example of this movement is a wood stove releases heat from the wood inside burning. The heat warms the metal of the stove. Metals are usually the fastest conductors of heat. You can tell how fast a conductor a material is by touching the material during cold weather. A fast conductor will feel very cold to the touch. A slower conductor such as wood will feel warmer. There is a relationship between fast conductors of heat and good conductors of electricity. This relationship does not always hold true. Touch glass in very cold weather and it feels very cold. However, glass is a very poor conductor of electricity.

When heating structures, we use all 3 forms of heat transfer. In the example of the wood stove the heat released during the combustion of the wood warms the steel sides of the stove. The then warmed steel sides radiate heat to articles in the room (chairs, people, walls etc). The warmed steel also conducts heat to the air next to the stove, warming the air. As the air warms, it becomes lighter than the cooler air around it and rises toward the ceiling in a convective current. When the air rises to the ceiling, heat is conducted to the material of the ceiling and cooler air. As this air gives up its heat it becomes cooler and thus denser and moves back down toward the floor and is drawn toward the stove and begins the cycle again.

Radiant Heating Systems In some applications, systems are designed to maximize radiant heat. These may be spot heaters located above checkout registers in home supply warehouses. The temperature in the warehouse is kept low but near the checkout stations the need for more heat is necessary for the comfort of the customers and employees at the checkout stand. These stands are normally near doors that are opening and closing numerous times, bringing in cooler outside air. The value of these systems is the radiant heat heats the objects and people in the room more so than the air in the room. This gives a feeling of warmth when the air is cooler. Considerable savings in energy usage can be obtained by using these systems in under the proper conditions.

Natural Convection Systems When housing was small with one or two rooms a heater located near the center of the room was sufficient to give reasonable comfort to the entire structure. However, when housing expanded to multiple rooms with doors separating them, even heating was less possible. This gave rise to centrally located heating units with piping connecting the heating unit to each individual room. Using the natural convection of warm air rising, the pipes, if installed with a rise from the furnace to the outlet, heat could be transferred to all the rooms of the house. These systems offered even heat throughout a multiple room structure with no heating appliance located in the living space. They could be sometimes operated during power outages. Disadvantages were they had to be located in a basement and their size filled most of the space. They were less efficient than fan forced systems and in large structures the longest runs were cooler than the close runs. These systems are sometimes called “octopus systems” because of their large diameter piping emanating from a central source. These systems were sometimes equipped with a fan to assist air flow

Forced Air Systems Even though fans could be retrofitted to the octopus systems, the quest for efficiency led to heating systems designed to have a fan to move the air throughout the structure. This led to much smaller heating systems with ducting placed in less obtrusive places and the need for upward angled piping could be eliminated. With natural convection systems, steady state thermal efficiency was 50-60%. These systems increased steady state thermal efficiency to approximately 80%. This means that of all of the heat released by the fuel, 80% was distributed to the structure and 20% was lost through the chimney or metal vent. In modern systems, thermal efficiency has increased to as high as 96%.

Boiler Systems With a forced air system, air is circulated through the furnace to pick up heat. Then the heated air is moved to the conditioned space. In a like manner, water can be used to transfer heat. Advantages are that very small amount of water can be used to move a given amount of heat compared to air. This eliminates the need for large ductwork. Water also absorbs heat much more readily than air making the heating unit smaller than for forced air systems.

Steam and Hot Water Boiler systems are of 2 types. Hot water and steam. The hot water boiler pumps cool water through the boiler where it picks up heat. The water is then pumped throughout the structure and the heat is transferred to the conditioned space using baseboard convectors, pipes located in the floor or cast iron radiators.

Steam Boilers warm the water to the boiling point and convert it to steam. The steam then travels by natural convection through the piping to heat exchangers where the steam is condensed back to water and the latent heat of condensation releases large amounts of heat to the conditioned space. The water then travels back to the boiler to be converted back to steam and the cycle continues. The difference between a steam boiler and a hot water boiler may not be obvious at first glance, perhaps not even after looking for a long time. They do tend to look like plumber’s nightmares. Look at the 2 boilers shown. See if you can tell which is steam and which is hot water.

Heat Sources There are several sources of heat that are commonly used. oNatural gas oPropane oFuel Oil oCoal oElectric

With the exception of electric, all the above fuels require combustion to occur. Combustion is the mixing of fuel and oxygen at high temperature to release heat from the fuel. Products of complete combustion are usually CO2 and water with some trace compounds. Incomplete combustion will produce CO, a poisonous gas and other chemicals such as soot and aldehydes.

Natural Gas Natural gas is formed as a fossil fuel underground related to oil deposits. Under normal conditions, it is a gas and is metered in cubic feet. Natural gas in this area contains 1000 BTU per cubic foot. It is lighter than air, so it rises when released.

Propane Propane is sold as a liquid that boils into a gas prior to use as a fuel. It is also a product of fossil fuel origin. Its BTU content is approximately 2500 per cubic foot. It is heavier than air and can gather on the floor.

Fuel Oil Fuel oil is usually sold as #2 fuel oil similar to diesel. It is a liquid that must be vaporized or atomized prior to burning. Its BTU content is 140,000 per gallon.

Coal Coal is a solid fuel that looks like a rock. It is also a fossil fuel. It is usually fed into the furnace by a screw mechanism. It is similar to a wood fire in that it must have a continuous fire. It is used mostly in power generation although some is still used for space heating.

Electric Electric heat is not an energy source but a form of energy. Electric furnaces are simply a converter that converts electricity to heat. It is sold in kilowatts with one kilowatt equaling 3,417 BTUs.