Heat Controller MGD Series Service Manual
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Heat Controller MGD Series Service Manual

Heat Controller MGD Series Service Manual

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SERVICE MANUAL
M
MGD Series
Gas Furnaces

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Summary of Contents for Heat Controller MGD Series

  • Page 1 SERVICE MANUAL MGD Series Gas Furnaces...
  • Page 2 MGD Gas Furnace Service Manual SEALED COMBUSTION GAS FURNACE SERVICE MANUAL INTRODUCTION This manual is designed to help understand the basic fundamentals, operation, and troubleshooting of the MGD line of gas furnaces. It was designed to be used in conjunction with the installation manual and/or technical manuals provided with each gas furnace.

  • Page 3: Electrical Supply

    MGD Gas Furnace Service Manual Fundamentals of the Gas Furnace All gas furnaces consist of four main components: a burner, a heat exchanger, a blower and a vent pipe or flue. The burner is where the gas is delivered and burned. The gas furnace uses an electronic ignition system and hot surface igniter instead of a constantly burning pilot light to ignite the gas.

  • Page 4: Supply Voltage

    MGD Gas Furnace Service Manual SUPPLY VOLTAGE Supply voltage to the unit should be a nominal 115 volts. It MUST be between 104 volts and 127 volts. Supply voltage to the unit should be checked WITH THE UNIT IN OPERATION. Voltage readings outside the specified range can be expected to cause operating problems.
  • Page 5 MGD Gas Furnace Service Manual If the high temperature limit or auxiliary limit opens while the furnace is in heating mode and the thermostat is calling for heat (W), and the switch remains open for a continuous duration of 5 minutes the control enters a fan failure lockout mode.

  • Page 6: Gas Piping

    MGD Gas Furnace Service Manual RETRIES A retry occurs when an attempt to achieve ignition fails and no flame is detected. The main valve is turned off and the ignition cycle re-started. If three tries occur in a given operation sequence, the furnace is shut down for one hour.

  • Page 7: Natural Gas

    MGD Gas Furnace Service Manual NATURAL GAS Inlet pressure to the unit should be checked (at the gas valve) with ALL OTHER GAS FIRED APPLIANCES OPERATING. Inlet pressure to the unit under these conditions MUST be a minimum of 4.5 in. W.C. If the inlet pressure is less, it may be an indication of undersized piping or regulator problems.
  • Page 8 MGD Gas Furnace Service Manual Connect a U-tube manometer or Magnehelic gauge (0-12 in. W.C. range) to the pressure tap on the OUTLET side of the gas valve. 2. Turn gas ON and fire unit. 3. Manifold pressure should be set to 3.5 in. W.C. for Natural Gas, and to 10 in. W.C. for L.P. Gas.

  • Page 9: Checking Temperature Rise

    MGD Gas Furnace Service Manual CHECKING TEMPERATURE RISE The unit is designed to operate within a certain specified range of temperature rise. See unit rating plate for allowable range. Operating the unit outside the specified range may result in lower efficiency and/or comfort levels, as well as premature combustion component failures.
  • Page 10 MGD Gas Furnace Service Manual An improperly operating, or poorly located room thermostat can be the source of perceived equipment problems. A careful check of the thermostat and wiring must be made then to insure that it is not the source of problems. LOCATION The thermostat should not be mounted where it may be affected by drafts, discharge air from registers (hot or cold), or heat radiated from the sun or appliances.

  • Page 11: Control Wiring

    MGD Gas Furnace Service Manual Measuring Current Draw The best method to obtain the required setting for the heat anticipator is to measure the actual current draw in the control circuit (“W”) using a low range (0—2.0 Amps) Ammeter. (See Illustration) After measuring the current draw, simply set the heat anticipator to match that value.

  • Page 12: Diagnostic Features

    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.
  • Page 13 MGD Gas Furnace Service Manual WARNING Fire hazard. Limit controls are factory preset and MUST NOT be adjusted. Use ONLY manufacturer’s authorized replacement parts. Failure to do so can result in personal injury and/or death. MAIN LIMIT SWITCH - 180°F MANUAL RESET A Normally Closed switch located on the blower deck of the unit.

  • Page 14: Pressure Switches

    MGD Gas Furnace Service Manual If, however, the switch is found to be opening prematurely, then it should be replaced. When replacing ANY limit switch, use ONLY a switch of EXACTLY the same temperature setting. Use of a different temperature limit switch can create a dangerous situation. Be certain you have the correct control part when replacing the limit switch.

  • Page 15: Blower Assembly

    MGD Gas Furnace Service Manual A 115-volt motor drives the exhaust blower. Basically there are only two problems that can be experienced with this component. It can have a motor failure, or it can become noisy due to bad bearings, loose blades, etc. When servicing the blower be certain to insure that gaskets and sealing material are replaced.
  • Page 16 MGD Gas Furnace Service Manual Blower Assembly removal and replacement 1. Disconnect electrical power to the furnace. 2. Remove the blower access panel. 3. Unplug blower-wiring harness from plug in top of control box. Remove the four screws securing the blower housing 5.

  • Page 17: Selecting Blower Speeds

    MGD Gas Furnace Service Manual Checking the blower performance data for this model, (see chart below) indicates that at 0.3” W.C., low speed will deliver 910 CFM to a system that has no coil installed. Accordingly, low speed should be used in this example for the HEATING speed. If a coil is installed however, the chart indicates that 865 CFM will be delivered on low speed and 1030 on MED speed.
  • Page 18 MGD Gas Furnace Service Manual BURNER/HEAT EXCHANGER INSPECTION & CLEANING Check the exterior and the interior flue gas passages of the heat exchanger for any evidence of deterioration due to corrosion, cracking or other causes. If signs of scaling or sooting exist, remove the burners and clean the heat exchanger as required.

  • Page 19: Heat Exchanger Removal/Replacement

    MGD Gas Furnace Service Manual HEAT EXCHANGER REMOVAL/REPLACEMENT Removal of the heat exchanger from the unit is only required if the heat exchanger has failed. The causes of heat exchanger failures are normally NOT due to defects in material and workmanship in the heat exchanger itself.
  • Page 20 MGD Gas Furnace Service Manual PRESSURE CONTROL LOCKOUT Pressure control lockout happens when the differential pressure switch fails to operate properly during a call for heat. If the pressure switch is continuously closed, even with the draft inducer off, no further operation of the furnace will occur, except if flame is detected, main valve is detected, or if the limit opens.
  • Page 21 MGD Series Gas Furnace Service Manual WIRING DIAGRAM, MGD 21 of 22...
  • Page 22 Design, material, performance data and components subject to change without notice. 1900 Wellworth Ave., Jackson, Michigan 49203 • Ph. 517-787-2100 • Fax 517-787-9341 THE QUALITY LEADER IN AIR CONDITIONING www.heatcontroller.com 08/06...

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