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SERVICE MANUAL

M

MGD

Series

Gas Furnaces

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Summary of Contents for MGD Series

Page 1: ...SERVICE MANUALM MGD Series Gas Furnaces ...

Page 2: ... these products WARNING The information contained in this manual is intended for use by qualified individuals who are familiar with the safety procedures required in installation and repair and who is equipped with the proper tools and safety equipment Installation or repairs made by unqualified persons can result in hazards subjecting the unqualified person making such repairs to risk of injury o...

Page 3: ...ditioned air The circulation blower moves the conditioned air through ducts to the location in the home where it is needed This is a squirrel cage type blower powered by an electric motor both of which have been sized for their specific application The flue exhausts the gaseous byproducts of combustion to the outdoors Venting is also provided through the chimney The MGD furnaces use sealed combust...

Page 4: ...ntinues to be energized then the draft inducer is re energized Main valve failure with W being de energized will result in the main blower normal delay off being performed once the problem has been corrected Main valve failure during an ignition sequence will cause a recycle to occur During a normal ignition sequence if the NPC has been tested and has functioned properly an opening of the NPC will...

Page 5: ...onnected to L2 then a rapid flash code will be indicated on the LED Normal operation will still occur however it is highly improbable that flame sense will be detected This condition will persist until power is cycled to the board RECYCLES A recycle occurs when ignition has occurred and flame failure flame circuit failure or a main valve failure during a call for heat is detected If a recycle occu...

Page 6: ...ned in HUD Title 24 Section 3280 705 as well as A119 1 and A119 2 for recreational vehicles See chart in this manual for minimum sizes for NAT and LP pipe and tubing serving the furnace only NOTE The gas inlet on the gas control valve is NPT The gas line may be installed through the bottom casing or on left or right side of furnace An adequately sized gas supply to the unit is required for proper ...

Page 7: ...this cannot be obtained problems are indicated in either the regulator or pipe sizing Gas Pressures Natural Gas LP Gas Minimum Inlet 4 5 W C 1120 Pa 11 W C 2740 Pa Recommended Inlet 7 W C 1740 Pa 11 W C 2740 Pa Maximum Inlet 13 W C 3230 Pa 13 W C 3230 Pa Manifold Pressure 3 5 W C 870 Pa 10 W C 2490 Pa CHECKING INPUT FIRING RATE Once it has been determined that the gas supply inlet pressure is corr...

Page 8: ... explosion hazard Turn OFF gas at shut off before connecting U tube Manometer or Magnehelic Failure to turn OFF gas at shut OFF before connecting Manometer or Magnehelic can result in personal injury and or death CLOCKING GAS METER NATURAL GAS 1 Check with gas supplier to obtain ACTUAL BTU content of gas 2 Turn OFF gas supply to ALL other gas appliances 3 Time how many seconds it takes the smalles...

Page 9: ...ecessary to change the heating blower speed If the rise is too high it will be necessary to increase the blower speed If the rise is too low it will be necessary to reduce the blower speed HIGH ALTITUDE OPERATION This series of unit is designed to operate in the majority of the country without any modifications Beginning at altitudes of 2 000 Ft above sea level certain measures need to be taken to...

Page 10: ...eturn air grille is the best choice Mercury bulb type thermostats MUST be level to control temperature accurately to the desired set point Electronic digital type thermostats SHOULD be level for aesthetics THERMOSTAT LOCATION HEAT ANTICIPATORS Heat anticipators are small resistance heaters built into most electro mechanical thermostats Their purpose is to prevent wide swings in room temperature du...

Page 11: ...t the factory Cycle rates are normally field adjustable for different types of equipment The method of adjustment varies from one thermostat manufacturer to another Check with the thermostat manufacturer to find out the proper way of adjusting the cycle rate CONTROL WIRING Control wiring is an important part of the total equipment installation since it provides the vital communications link betwee...

Page 12: ...T 5 GAS VALVE ENERGIZED WITH W OFF GAS VALVE RAPID FLASH INCORRECT LINE VOLTAGE PHASING L1 AND L2 CONNECTIONS REVERSED LIMIT SWITCHES Two different kinds of limit switches are used on the MGD series of gas furnaces They are the main limit manual re set and the auxiliary limit automatic re set and are used on ALL models Typical Limit Switch It must be remembered that a limit switch is a safety devi...

Page 13: ... operation it is more than likely due to a high temperature rise through the furnace See checking temperature rise section in this manual High temperature rise can be caused by either OVER FIRING high manifold pressure incorrect orifices etc or LOW AIR FLOW dirty filter blower speed too low excessive static in duct system etc To verify this the cut out temperature of the switch should be checked u...

Page 14: ...re switch The particular setting is required to insure continued SAFE RELIABLE operation NEVER SUBSTITUTE a pressure switch with one that is similar in appearance ONLY FACTORY PROVIDED or AUTHORIZED SUBSTITUTES ARE ACCEPTABLE If you are servicing a unit whose pressure switch will not close or remain closed during operation the operating pressure of that unit should be checked and compared to the s...

Page 15: ...esistor before testing A 20 000 Ohm 2 Watt resistor can be used for this purpose The condition of the capacitor should be verified with a capacitor analyzer one that indicates the capacitor s value in microfarads rather than with an Ohmmeter The reason for this is that an Ohmmeter test can only indicate if a capacitor is OPEN or SHORTED It cannot verify if its value microfarads is within an accept...

Page 16: ...ntry makes it impossible to factory select blower speeds that will provide proper operation for all installations The blower speeds for both heating and cooling must be field selected for each particular installation to insure proper operation The criteria for selecting the proper blower speeds IS NOT High for cooling Low for heating That may be how it works out SOMETIMES but it can be exactly the...

Page 17: ...810 1765 1720 1680 1635 CFM WITH COIL LOW 950 905 865 835 790 MED 1145 1115 1080 1035 1000 MED HIGH 1375 1345 1315 1280 1245 HIGH 1700 1660 1620 1575 1540 SELECTING BLOWER SPEEDS After determining necessary CFM and speed tap data refer to the unit wiring diagram for the correct speed tap BURNERS Burners used in this series are of the INSHOT type Their operation can be compared to that of a torch s...

Page 18: ...power to unit 2 Turn gas OFF at manual shut off valve 3 Disconnect gas pipe at union 4 Disconnect wires from gas valve note connections 5 Remove screws securing the burner box cover and remove the cover gas valve and burner assembly as one unit 6 Clean burner assembly with a stiff brush Note The burner assembly is equipped with a hot surface igniter Use extreme caution not to damage it This ignite...

Page 19: ...g heat exchanger to never achieve design operating temperature Operating outside allowable temperature rise range can allow condensation to form inside the heat exchanger eventually causing corrosion to appear in the heat exchanger Blower speed may be too high return air duct may be open and leak ambient air or unit may be under fired as a result of manifold pressure being too low or incorrect ori...

Page 20: ...closes normal operation will return but the differential pressure switch failure will be counted as a recycle If the differential pressure switch does not re close the control will continue to flash the lockout code until it does close or the call for heat is removed FLAME SENSOR INPUTS The flame sensor input is capable of reliably detecting the presence of a flame through the use of the flame Rec...

Page 21: ...MGD Series Gas Furnace Service Manual WIRING DIAGRAM MGD 21 of 22 ...

Page 22: ...rth Ave Jackson Michigan 49203 Ph 517 787 2100 Fax 517 787 9341 THE QUALITY LEADER IN AIR CONDITIONING www heatcontroller com Design material performance data and components subject to change without notice 08 06 ...

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