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People have been attempting to control heat and ventilation since prehistoric times. Over the many centuries, the technology of heating has advanced from simple attempts to keep the body warm to very sophisticated systems. Ventilation has been used for a very long time as well, dating back to the time when royalty was cooled by servants and slaves fanning them using large palm fronds and feathers. Ventilation became important during the Industrial Revolution to protect workers and increase efficiency. Air conditioning is a relatively recent development, and involves many aspects including the control of temperature, humidity and air cleanliness. It wasn't until after 1945 that the use of air conditioning or
simple cooling of the air became widespread. Modern systems of air conditioning have greatly evolved
from the times of simply hanging wet towels across an open window.
Today, air-conditioning systems do not simply cool the air, but they actually condition it by controlling the air's temperature, moisture content, movement and cleanliness.
Heat Fundamentals
There are essentially three ways that heat moves from one area to another. When bodies of unequal temperatures are near each other, heat leaves one body and goes to the other. Heat moves from the hotter body, and the colder body absorbs it. The greater the difference in temperature, the greater the rate of flow of the heat. Heat moves from one body to another by the following ways: radiation; conduction; and convection.
Radiation is the transfer of heat energy by electromagnetic wave motion. Heat is transferred in direct rays. It travels in a straight line from the source to the body. The closer you are to the hot object, the warmer you feel. The intensity of the heat radiated from the object decreases as the distance from the object increases.
You feel cool in a room that has a cold floor, walls and ceiling. The amount of heat loss from your body in that room depends on the relative temperature of the objects in that room. The colder the floor is (relative to the temperature of your feet), the greater the heat loss from your body will be just standing there. If the floor, walls and ceiling of that room are relatively warmer than your body temperature, then heat will be radiated to your body from those objects and surfaces.
Radiant heating in residential buildings includes piping and electrical wiring in floors, walls and ceilings. Radiant heat emits in all directions. Reflective materials are commonly used in a radiant heat-emitting system in order to direct and control where the heat is emitted.
Conduction is the transfer of heat from one molecule to another, or through one substance to another. It is heat that moves from one body to another by direct contact. For example, heat is transferred by conduction from a boiler heat exchanger to the water passing through it. When you touch a suction line of an air conditioner and it feels warm, that's heat energy moving from the warm copper pipe to your cooler hand -- by conduction.
Convection is known by most people from the phrase "heat rises." Convection is the transfer of heat by warming the air next to a hot surface, and then moving that warm air. It's the transfer of heat by the motion of the heated matter itself. The air moves from one place to another, carrying heat along
with it. Since warm air is lighter than the cool air around it, the warm air (or heat) rises.
Warm fluids tend to rise while the surrounding cool fluids fall. This rising and falling tends to form loops -- convective loops -- where warm air rises and cool air falls. Early warm-air gravity furnaces used the principles of convective loops. In a gravity system, the warm air rises and cool air falls, and this is how the gravity warm-air heating system circulated air.
Forced-air furnaces function primarily by convection. Heat is transferred to the air, and the air is circulated throughout the house. Systems that heat water and use radiators and baseboards as their heat-emitting devices use convection, and radiation, to a lesser extent.
A radiator needs air freely moving around it in order for it to be effective. Covers over radiators might reduce the airflow around and through the radiator unit.
There are four heat-conveying mediums that can carry heat:
* air
* water
* steam
* electricity
Four Types of Heating Systems;
* warm-air heating system;
* hydronic heating system;
* steam heating system; and
* electric heating system.
Heating Fuels that are being used today by most heating systems
fuel oil (No. 2) { Up north}
natural gas;
propane;
coal{up north}
electricity
Natural Gas Teflon tape is not recommended.Pipe dope is preferred. Most jurisdictions do not allow the use of gas piping as a way to ground the electrical service. We do not want to rely on the gas piping as the primary means of grounding the electrical service. Bonding the gas pipes to the electrical grounding system is a requirement in most jurisdictions. This bonding is usually done by connecting the gas piping to the water supply piping that is near the water heater. This is assuming that the water pipes are grounded.
Natural gas has no color, no odor, and it's not toxic. It is highly combustible. It only smells because we put a scent in it. Natural gas has a specific gravity of about 0.6. Air has a specific gravity of 1. Natural gas is lighter than air. Propane has a specific gravity of 1.5, and a propane leak tends to pool on the floor surface and creates a dangerous situation.
To ignite natural gas, you need a mixture of gas and air that is conducive to ignition. If you have too little air in the mix, the gas will not ignite.If you have too much air, the gas will not ignite. You have to have between about 86% air to 94% of air mixed with a certain gas volume to get the gas to ignite. Once ignited, the ignition temperature of natural gas is about 1,200° F. That's hot.
In a conventional gas furnace with a natural draft, air is mixed with the gas initially for combustion. This air is called the primary air.Primary air is controlled by the air shutters at the front of the burner assembly.
The remainder of the air mixture comes from the air that actually surrounds the flames inside the combustion chamber.This air is called the secondary air. The secondary air (the air around the flames) and the primary air (the air drawn into the burners) combine to make up the total combustion air.
Combustion Fundamentals
Combustion involves the burning of a fuel that produces heat energy.Combustion requires adequate supply of air called combustion air. To have a successful combustion process, there has to be a fuel, oxygen, and an ignition source.
Burning a natural gas can be explained by the general equation:CH4 + 2O2 = CO2 + 2H2O + heat.
Natural gas is about 85 to 90% methane (CH4). Burning natural gas (CH4) with oxygen yields carbon dioxide (CO2) and water vapor (2H2O) and heat. This is referred to as complete combustion.
In reality, air is the source of oxygen (O2), and in the air, oxygen is mixed with some nitrogen.The resultant flue gas from the combustion will contain some nitrogen.
Combustion Air
Combustion is never complete (or perfect).In combustion exhaust gases, both unburned carbon (as soot) and carbon compounds (CO and others) will be present.Also, because air is the oxidant, some nitrogen will be oxidized into various nitrogen oxides (NOX).
Roughly 15 cubic feet of air is needed to burn 1 cubic foot of natural gas.Gas furnaces need also draft air (or dilution air) to maintain a draft of the combustion gases.Another 15 cubic feet of air is needed for every cubic foot of natural gas.This air helps with a chimney draft.Therefore, a conventional low-efficiency, standing-pilot gas furnace requires about 30 cubic feet of air (15 dilution plus 15 combustion) for every cubic foot of gas burned. If combustion air is inadequately supplied to a gas furnace, carbon monoxide will likely be produced. Carbon monoxide can be lethal.
Draft Types There are three types of burners relative to the draft.They are:
A) Natural-draft burners
Natural draft refers to the burners of a conventional low-efficiency gas furnace.This type of burner is also called an atmospheric burner.
With natural draft, we need to keep the chimney hot enough to get those combustion gases out of the chimney.Natural draft burners have no draft fan.
B) forced-draft burners
A forced draft is when the furnace has a fan that blows air into the combustion chamber through the heat exchanger and out through the venting system.
All oil burners and some gas furnaces use forced draft.Forced draft has the fan before the burner.
C) Induced-draft burners
An induced draft uses a blower fan to pull air into the burner through the combustion chamber and exchanger.The fan is located on the exhaust-side of the exchanger. It also blows the flue gases out through the vent connector pipe. When the induced fan is operating, there is a negative pressure inside the heat exchanger. Induced-draft fans are also called exhaust blowers or power vents.Induced draft has a fan after the exchanger and before the vent pipe. Induced draft fans are common on mid-efficiency and high-efficiency furnaces.
Backdraft
The lack of dilution air (the air used for draft) may cause a condition of backdraft at the furnace.Backdraft occurs when the combustion gases are not drafting or rising up through the chimney but instead are coming backward into the living area of the building.This is a hazardous situation since carbon monoxide could be entering the dwelling under this condition.
Backdraft could be caused by various conditions,including:inadequate dilution air; flue restriction or blockage; chimney downdraft; exhaust fans causing draft and pressure problems with in the building; and improper chimney or flue connector size.
Confined Space and Combustion Air
If the volume of space in which the appliance is located is less than 50 cubic feet of space per 1,000 BTUs per hour of aggregate input of the appliance, then it is a confined space: 50 cubic feet = 2.5 ft. x 2.5 ft. x 8 ft.
In unconfined spaces in buildings, infiltration may be adequate to provide air for combustion, ventilation and dilution of flue gases.However, in buildings of tight construction (for example, doors and windows that have weatherstripping, walls that are heavily insulated, openings that are caulked, floors and walls with vapor barriers, etc.), additional air may need to be provided.
Solution
Two permanent openings to adjacent spaces could be provided so that the combined volume of all spaces meets the requirements. If the building is sealed so tightly that infiltration air is not adequate for combustion, combustion air should then be obtained from outdoors.
All Air from Inside the Dwelling
If all combustion air is taken from the inside of the dwelling, then two permanent openings should be installed. One opening should be within 12 inches (305 mm) of the top and one within 12 inches (305 mm) of the bottom of the space. Each opening shall have a free area equal to a minimum of 1 square inchper 1,000 BTU/h (2,201 mm2/kW) input rating of all appliances installed within the space, but not less than 100 square inches.
All Air from Outdoors
If all combustion air is taken from the outdoor air, then one opening should be within 12 inches of the top and one within 12 inches of the bottom of the space.The openings are permitted to connect to spaces directly communicating with the outdoor air, such as a ventilated crawlspace or ventilated attic space.Each opening should have a free area of at least 1 square inch per 4,000 BTU/per hour (550 mm2/kW) of total input rating of all appliances in the space when using vertical ducts (2,000 BTU/per hour if using horizontal ducts).
Louvers
In calculating the free area of combustion air openings fitted with louvers, metal louvers obstruct about 25% of the opening. Wooden louvers obstruct 75%.
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