Hot Water Heating Systems 030203e Flashcards
What does each heating zone in a building have?
A heat transfer unit (HTU). Transfers heat from the hot water in a pipe to the cooler air of the zone.
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What are the two basic types of hydronic heating systems?
Gravity systems.
Forced flow systems.
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What do gravity systems rely on?
Convection currents within the piping system and the density of cold water vs hot water to promote movement of water.
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Why do forced flow systems provide greater flexibility over gravity systems?
They utilize a pump to move the heating water.
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What are the disadvantages of gravity hydronic systems?
Slow response time to changing heating demands.
More costly because of the larger diameter piping required.
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What is the difference between an open and closed system?
Open system is open to the atmosphere. (Can exchange both energy and matter with the atmosphere)
Closed system is not exposed to the atmosphere. (Only heat is exchanged to the air)
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How many zone valves can a single 40 VA transformer supply?
4 zone valves. Should not be loaded beyond 32VA (80%)
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What is considered the heart of every hydronic heating system?
The boiler.
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What are two styles of heat exchangers currently in use in hydronic heating boilers?
Cast-iron sectional heat exchanger.
Direct vent continuous flow heat exchanger.
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Describe cast-iron sectional heat exchangers?
Are heavy and hold a large volume of water compared to other boilers.
Classified as high-mass boilers and may be used with gravity heating systems.
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Describe direct vent continuous flow heat exchangers?
Made of stainless steel, steel, cast iron, or copper.
Water tube design that requires continuous water flow to prevent tube failure.
Compact design so typically are wall mounted.
AFUE ratings are in the 90% range.
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What are some of the flame-proving devices used in boiler control systems?
Thermocouples.
Flame rods.
Pyrostats.
Cad cells.
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What voltage does a zone valve operate at?
24 V.
It is a 24 V hot water valve supplied by a transformer and controlled by a 24 V thermostat.
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What resistance levels does a cad cell have when it sees light from a flame?
300 to 1000 ohms.
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What are some of the zone valve varieties available?
2, 3, or 4 wire options.
2 wire - spring return and. no travel switch.
3 wire without a travel switch - requires a special thermostat to power the open/close.
3 wire with a travel switch - spring return and uses coil voltage.
4 wire - spring return and has a travel switch.
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What is the purpose of the circulation pump?
Moves water through the system.
Typically have a power rating of 1 hp or less and operate at speeds between 1750 r/min and 3450 r/min.
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What are some of the safety and control devices on hot water boilers?
Low water cut-outs (LWCO)
Temperature control devices.
Aquastats.
Flow switches.
High-limit control devices.
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What is the purpose of a low water cut-out (LWCO)?
Shuts off the fuel supply to the burner if the water in the boiler falls below acceptable levels.
Usually a float-operated device installed in the boiler above the lowest permissible water level.
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What is an aquastats?
A switching device that reacts to water temperature. Can be either an immersion or surface mount type.
Can also be used as a high limit protection that will switch the burner mechanism and ignition sequence off if the temperature of the boiler exceeds a preset maximum.
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What is the purpose of reverse acting control with aquastats?
Prevents the start up of the circulating pump until the boiler water reaches a minimum temperature.
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What is the purpose of the safety flow switch?
Senses the velocity of the water flowing in the hydronic system and interrupts the heating circuit if the flow of water is too slow.
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What is the purpose of line air scoops?
Removes air bubbles from the hydronic system.
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What is the purpose of expansion tanks?
Allows for liquids to expand and contract within a closed hydronic system.
Compression tanks - Larger systems.
Diaphragm expansion tanks - smaller commercial and residential systems.
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What are low-pressure hydronic heating systems restricted to operating at?
No more than 30 psig (pounds per square inch)
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What is the test called to test a pressure relief valve?
Pop-test or try-test.
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What is the purpose of a pressure reducing valve?
Reduces city water supply to the pressure necessary for the hydronic system. City water supply pressure is between 50 psig and 70 psig.
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