National Oilheat Research Alliance 7 to rise from 32°F to 212°F, but it takes more than five times the heat (970.3 Btus) to get it to move from 212°F water to 212°F steam. There is no change in temperature, but there is a big change in the energy content of that steam. Figure 4. The boiling point of water will vary depending upon atmospheric pressure. Water boils at a higher temperature in a hydronic boiler that is pressurized to 12 pounds than in a steam boiler open to atmospheric pressure. Boiling is a reversible process, like freezing and melting. When heat is removed from the steam at boiling temperature, the steam returns to a liquid phase. This is called condensing. When water condenses, the same amount of heat that was needed to turn the water into steam will be released or transferred. When heat is added slowly to water, the water will either change temperature or change state. For example, once the water reaches 212°F, it will stay at that temperature until all of the water turns to steam. At that point, the temperature can begin to rise again. As a result, since there is always some liquid in a steam system, the temperature of the steam in a residential steam boiler is always the boiling temperature of water (212°F at sea level) when it is making steam. Steam heating systems use the boiling-condensing process to transfer heat from the boiler to the radiators. Since each pound of water that is turned into steam has absorbed 970 Btus of heat, the steam transfers that latent heat (970 Btus) to the radiators as it cools and moves through the system. Lost Heat—The Steam in the Combustion Gases When oil is burned, the hydrogen in the oil mixes with the oxygen in the air to create water. There is enough hydrogen in heating oil to create about one gallon of water for every gallon of oil burned. This water is created in the form of steam and it carries the latent heat with it as it exits the building with the rest of the combustion gases. Approximately seven gallons (59 pounds) of water are created per million Btus, which means 6.5% of the energy in the oil goes up the chimney as latent heat in the water vapor. Figure 5
NORA Advanced Efficiency
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