Intertherm Furnace Manual Model Mgh

Working With an Intertherm MG/H Furnace: What You Actually Need to Know

The Intertherm MG/H series is one of those mid-tier residential furnaces that shows up in a lot of older installations. It's basically a rebranded Bryant/Weil-McLain design, and the manual covers gas valve wiring, control sequences, and troubleshooting for both natural gas and LP configurations. If you're pulling up the Intertherm Furnace Manual Model Mgh for the first time, you'll notice it's dense with wiring diagrams and less forgiving than modern condensing units. That's by design—these machines are simple enough that a manual written in 1998 still applies. Don't bother searching for a PDF link on some random forum. The original manual lives in the blower compartment behind the upper access panel. It's a folded sheet or a small stapled booklet, usually fastened to the inside of the panel with a plastic tie. If yours is missing, the closest replacement is the equivalent Bryant 593A02 series service manual. They share the same control board layout and gas valve specifications. HVAC direct and a few archive sites have scanned copies floating around, but honestly, buying a used unit with the manual still inside is faster than hunting. The sections most people skip are the combustion analysis specs and the rollout switch testing procedure. That's where you get burned. The ignition sequence on the MG/H uses a hot surface igniter, not a standing pilot, and the control board monitors current flow through the igniter during the warm-up phase. If the current draw is outside spec, the board locks out before the gas valve even opens. Technicians who jump straight to "replace the igniter" without checking voltage at the board terminals are wasting half their visits.

I once spent forty minutes chasing a lockout code on an MG/H in a basement in Ohio. The owner had replaced the igniter twice. Turns out the problem was a cracked solder joint on the control board where the thermostat low-voltage wire connects to terminal R. The board was cycling power randomly during the ignition sequence, causing the current monitor to trip. I didn't find it until I tapped the board with the handle of a screwdriver while watching the status LED blink. One joint, some flux, and a reflow iron fixed it permanently. New board would have been two hundred dollars and still might not have held up.

Control Board Operation and Common Failure Points

The MG/H uses a single-board control system with a five-position terminal strip. Terminal R gets 24 volts from the transformer, G is the fan relay, W is the heat call, Y is the cooling call if your system has a heat pump configuration, and C is the common return. The board has a diagnostic LED that flashes error codes. Two short flashes means flame failure. Three flashes is a limit trip. Four is a pressure switch open condition. These are standard, but the timing matters. A two-flash code during the pre-purge indicates the pressure switch failed to close. A two-flash code after ignition means the flame sensor isn't reading current. The flame sensor on this model is a single-point UV rod, not the modern silicon nitride type. It's mounted in the burner sight hole and grounded through the frame. Over time, burner carbon deposits insulate the sensor tip. Cleaning it with emery cloth sounds obvious, but the real issue is the grounding path. I've seen two instances where the sensor was clean and the board was fine, but the grounding strap between the sensor bracket and the cabinet was corroded. Continuity checked out open between the bracket and the terminal block. A wire jumper across that gap resolved it immediately. Another thing the manual doesn't emphasize enough: the transformer on these units is undersized for anything beyond the basic heating circuit. If you've added a humidistat, a zone panel, or an air purifier to the low-voltage side, you're probably running the transformer at or near its limit. That causes the 24-volt rail to sag during the ignition cycle, which triggers false flame failures or intermittent lockouts. The fix is either a dedicated transformer or reducing the load on the circuit. Check voltage under load during a call for heat. If it drops below twenty-one volts, you've found your problem.

Burner Assembly and Combustion Considerations

The MG/H uses a conventional atmospheric burner with a cast iron heat exchanger. The burner orifice size varies by model suffix—some are nat gas rated at three-inch water column supply pressure, others are LP at ten inches. Mixing up the orifice between gas types is a safety issue, not a performance one. The manifold pressure is set with the regulator screw on the gas valve. Factory setting for natural gas is three to four inches w.c. with the burner running. You need a manometer to check it. Guessing by adjusting the screw without measurement is how you get either incomplete combustion or a cracked heat exchanger from overheating. The draft control damper is located on the draft divertor assembly at the top of the heat exchanger outlet. Some installers remove it entirely, thinking it improves efficiency. It doesn't. The damper's job is to maintain a slight negative draft in the heat exchanger passages so that combustion gases don't spill into the conditioned space. Removing it on a gravity-draft installation in a tight house can cause backdrafting when the kitchen exhaust fan or dryer runs. I've seen CO alarms trigger on houses where the damper was removed and the furnace was pulled into negative pressure from a bathroom fan. Reinstalling the damper and checking draft with a manometer at the draft hood outlet is the correct fix. Heat exchanger cracking is the eventual outcome for these units, and it's not always predictable. The primary failure point is the front baffle where the burners fire directly into the first passage. Thermal cycling creates stress at the weld points. The manual gives you a visual inspection checklist, but the practical test is checking for soot trails along the baffle seams. Soot at the baffle means combustion gases are leaking past the seal before reaching the flue. That's a heat exchanger replacement, not a repair. No sealant or welding trick is going to make that safe again.

Blower Motor and Limit Control Maintenance

The blower motor on the MG/H is a permanent split-capacitor type, usually running at around one hundred to one-fifty watts. The capacitor is located in the blower compartment access panel. If the blower is slow or humming without starting, the capacitor is the first suspect. Measure capacitance with a multimeter that has an FAR range. A thirty-five microfarad capacitor reading twenty-two microfarads is weak and will cause the motor to overheat and trip the internal thermal protector. Replacement capacitors are cheap. Running a furnace on a failing capacitor will cook the motor windings within a few months. The limit control is a manual-reset or automatic-reset device depending on the revision. Most MG/H units use an automatic-reset high-limit switch rated around two hundred degrees Fahrenheit on the heat exchanger surface. If the limit is tripping frequently, the problem is almost never the limit switch itself. It's restricted airflow. Check the return duct connection, the filter tray, and the blower wheel for dust buildup. I had a unit where the blower wheel was packed so full of lint that airflow was down to about four hundred CFM on a design requirement of eight hundred. Cleaning the wheel and balancing it cut the limit trips from once a week to never. There's a lesser-known issue with the limit switch wiring harness. The spade connectors on these switches oxidize over time, especially in humid basements. The resistance across the connector can be high enough to cause the control board to read an open limit circuit even when the switch is closed. Clean the terminals with contact cleaner and check continuity across the harness while the furnace is running. A fluctuating reading means corrosion in the connector, not a bad switch.

Gas Valve Specifications and Replacement Notes

The gas valve on the MG/H is typically a Honeywell V35 or B41 series valve. It's a standing pilot or intermittent pilot valve depending on the exact model number. The valve receives 24 volts on the PI terminal for pilot duty and the H terminal for the main burner. If you're replacing the valve, you need to match the inlet and outlet orifice sizes exactly. Using a valve with a higher BTU input rating than the original will increase combustion output beyond the heat exchanger's design limit. That's a cracking risk and a safety code violation in most jurisdictions. When testing the gas valve, measure voltage at the H terminal during a call for heat. You should see a steady twenty-four volts for at least ninety seconds before the igniter glows. If the voltage pulses or drops, the control board is the problem, not the valve. I've seen gas valves replaced three times on units where the board's triac was failing and couldn't maintain a solid lock on the valve circuit. The valve would close partially, cause incomplete ignition, and trigger a flame failure lockout. Each time, a new valve was installed before checking the board output waveform. That's expensive and unnecessary. The manifold pressure should always be verified after any gas valve replacement. Connect a manometer to the test port on the valve. For natural gas, stable pressure at three and a half inches w.c. with the burner running is the target. If pressure reads high, adjust the regulator screw clockwise. If low, counterclockwise. Do not exceed four inches w.c. on a unit of this age and size. Higher pressure increases flame velocity and can damage the heat exchanger baffles over time.

DIY Maintenance vs. Calling a Professional

There are things you can safely do yourself with an MG/H furnace. Change the filter every month during heating season. Visually inspect the burner flames through the sight port—they should be steady blue with minimal yellow tipping. Clean the flame sensor with fine sandpaper. Check that the area around the furnace is free of flammable materials and that the vent pipe connections are sealed. Those are routine maintenance tasks that prevent most common problems. What you should not do is open the gas valve, adjust the manifold pressure without a manometer, or attempt to repair a cracked heat exchanger. Combustion analysis requires a calibrated analyzer, and a misadjusted gas valve on an old furnace can produce carbon monoxide at dangerous levels. The MG/H doesn't have a inducer motor or sealed combustion, which means it pulls air from the room and vents directly to the atmosphere. Any crack in the heat exchanger or loose vent connection is a direct pathway for CO into your living space. If the furnace is more than fifteen years old and you're experiencing repeated issues, replacement may be more economical than continued repair. The efficiency gain from a modern condensing unit is significant, and parts for the MG/H are becoming harder to source. That said, these furnaces can run reliably for twenty-five years or more with proper maintenance. The key is addressing small problems before they compound—dirty filters, weak capacitors, and corroded connections are all early warnings that, if ignored, lead to major failures.

Download the actual manual from the HVAC manufacturers' archives or request a reprint from Intertherm's successor company documentation department. Reading through the troubleshooting flowcharts before something breaks will save you time and money. Most people don't touch the manual until the unit locks out, and by then, they're flipping pages blindly looking for an error code that should have been obvious from the start.