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24 Cards in this Set
- Front
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There are two main types of Gasvalves ------------&------------. Automatic valves that turn the gas supply to the main burner on or off inresponse to signals from the operating control or other devices controlling thesystem. |
Diaphragm and Solenoid gas valves BOTH gas valve must be downsteam of 100%shut-off valve and the appliance regulator.
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The main gas valve or relay isdefined------------on an electrical circuit orgas-fired appliances |
Primary Control |
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On a warm air furnace conversion,the code states that the high limit setting must not be higher then -----F fora gravity furnace and ----- for a forced air. All other applications, it willbefactory set |
Gravity Furnace not, not > then350 F (175 C) Forced Air, not > then 250 F(120 C). |
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Diaphram gas valve closed position |
- Above diaphram is filled with gas at standing/manifold pressure. - Force above is greater then below do to surface area. - This pressure differance hold the valve closed. |
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Diaphram gas valve open position The gas pressure causes the valve to open & close via the bleed line. |
- Electromagnet coil is energized, drawing in the armature. - The opposite end of the armature moves away from the vent port & closes off the pressure port. - Gas above the diaphram is vented through the bleed tube, which can be burned off at the pilot. *The gas pressure causes the valve to open & close via the bleed line. |
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Diaphram gas valve bleed tube. Single-valve installation the tube may vent to an adjacent continuous pilot burner: |
- provided the bleed flame won't interfere with safety control system (thermocouple). - 0.5 psig (3.5)kPa or less and the fuel is < air & - Melting pt of tubing material is > 1450F (790 C) |
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Diaphram gas valve bleed tube. Single-valve installation, the tube may vent to the flue or stack only where the appliance is fitted with a completely Automatic firing control system that? |
- Incorperats a pre-purge in the ingnitioin cycle. - This pre-purg blows fresh air throught the appliance & flue before establishing an ignition flame and thus frees the flue of any unburned gas prior to ignition. |
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Diaphram gas valve bleed tube. Multiple-valve installation, |
The tubes maybe manifold to a single vent to the outdoors, provided that the installation complies with Clauses 5.5.4.(a) and (b) and 5.5.7 of the CAN/CS B 149.1. |
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When the Solenoid Valves coil is energized |
It pulls the plunger up into the housing, lifting the valve disc off the valve seating and allowing gas to pass through to the main burner. |
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When the Solenoid Valve coil is de-energized |
The plunger slides out of the coil housing & returns the valve disc to the seating & the shut-off position. |
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Solenoid valves used on a gas burner manifold to control gas flow must be of the type? |
Normally closed NC. The valve is closed until an electric current is supplied. |
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Newer commercial appliances come equipped with electronic flame failure devices. If so, the 2 valves are piped in series, with 1 for main safety and the other an operator/modulating. Together on a valve train downstream of the regulator. |
The modulating gas valve can be electrically (AC/DC) activated or thermostatic expansion operated. |
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Thermostatic Expansion Gas Valve, doesn't need an electrical path? |
-T-Expansioin element is a liqid buld that reacts to temp chg (directly) without an electrical path. - The fluid travels through the capillary tube to a bellows as the liquid in the buld expands & contract with temperature change. |
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Rod and tube thermostatic contol valve |
- Operating element, controls the volume of gas flowing to the burner. - Sensing element that detects & reacts to the chg in temp. * Most popular is the Variable Temp, which is snap acting. Common smaller scale storage type HW heaters. |
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Aquatstat, automatic switching device that is operated by temperature chg. |
- On a HW boiler there a 2 Aquastats. - One acts on high limit protector & The second as an on-off control for the burners to maintain boiler temperature. To act in the mannor they are wired in series.
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When a aquastat is used as the controller on a large storage HW heater, the HWT can be equipped with an ECO as the high limit. If so how are they wired? |
- The aquastat and ECO are wired in Parallel. - The ECO is wired in series with the Safety Device & the Aquastat is wired in series with the Control Valve. |
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A Solid-State Control (relay) is: |
An electronic device constructed from semiconductor material and used to control electrical loads. |
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Solid-State Control: Advantage / Disadvantage |
Advantage: - Quicker response time, less space servicing. - Just a group of circuits controling an electrical system. Disadvantage - Cannot withstand a voltage surge. Must be protected from surges at all times. - They generage heat & need to be kept cool. |
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Solid-state Amplifier regulates the burner firing on an appliance. |
It takes an input signal from the temp sensor & compares it to the set point on the temperature selector. The amplifier then reacts to the changes & makes corrections by sending out an output single to the modulating valve. |
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What does the solid-state rectifier do? |
Changes the current from AC to DC and can be used to regulater the speed of a blower motor. |
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Semiconductors: The term solid-state refers to the physical desc of the device & the term semiconductor refers to the electrical conductivity of the materials used in S-S devices |
Semiconductors are half conductors, between copper and glass, and are known to be very versitile offering a wide range of electrical characteristics. Semiconductors cannot withstand power surges. |
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Thermistors: Are solid-state devices used in heating systems. They react to temperature change & are variable resisters that change resistance with temperature. |
They can have a positive or negative temperature coefficient. On a positive coefficent thermistor, the resistance will increase with an increase in termperature. On a neg coefficent, the resistance will increase with a decrease in temperature. Note neg thermistor is highly non-constant but may be used where a few % is acceptable. |
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Resistance Temperature Detector or RTD Another solid-state device used in heating systems |
- React to temperature chane and have the same physical appearance as a thermistor. RTD is a positive temp coeffienet. An increase in temp results in an increase in resistance. - RDT is linear more accurate within specified range. - Used widely on heating applications because of their accuracy. |
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What makes Resistance Temp Detectors RDT more popular then Thermistors? |
A thermistor is not linear and therefore is only accurate withing a range specified by the manufaturer. |
