Working on a Coleman Presidential II Furnace? Here's What You Actually Need to Know
The Coleman Presidential II is one of those older gas furnaces you still run into at job sites, usually in houses built sometime between the late 1980s and mid-1990s. Coleman was buying Amana branding at the time, and these units carry that pedigree. They're not the most complicated things out there, but they do have a few quirks that will bite you if you treat them like a modern high-efficiency unit.The furnace uses a standing pilot ignition system on most models. That's different from the hot surface igniter or intermittent spark systems you find on newer equipment. You've got a thermocouple or thermal sensor keeping the gas valve open, a gas control valve, and a main burner assembly. The blower is usually a split-phase or PSC motor with a fan control module mounted on the discharge plenum. If you're pulling a manual for reference, searching for the Coleman Presidential Ii Furnace Manual will give you the wiring diagram, gas valve specs, and component layout for your specific model number. Model numbers on these units are typically on a data plate inside the blower compartment door. It'll look something like FGR-XXXX or similar depending on the configuration. Column numbers and serial numbers are stamped right there too. Match that to the manual. The vertical and horizontal versions have different clearances, duct connections, and sometimes different gas manifolds. Using the wrong diagram for your configuration will get you confused real quick. I once had a unit come up on a service call where the previous tech had swapped in a universal gas valve that didn't match the orifice sizing on the manifold. The flame was lazy and yellow, rolling back into the heat exchanger. The manual's combustion analysis specs called for about 350-400 FT across the heat exchanger at low fire. We measured 620. Replaced the valve with the correct Coleman part, resized the orifice per the manual's chart, and brought it back to spec. Took about twenty minutes once we realized what was going on.
Troubleshooting the Standing Pilot System
These furnaces don't have the fancy diagnostics modern boards throw at you. No flashing error codes. No pressure switches telling you the draft is off. You've got a thermocouple generating millivoltage, a gas valve, and a manual gas control. If the pilot won't stay lit, the first place I look is the thermocouple connection at the gas valve. Corroded terminals are incredibly common on units this age. The green copper wire can oxidize and drop your mV reading below the holding threshold, which is usually around 18-25 millivolts. A multimeter set to microamps or millivolts DC will tell you what the thermocouple is producing. Hold a magnet near the thermocouple tip while the pilot is lit, then measure. Good thermocouples put out 25-35 mV when hot. Anything under 18 and you're replacing it. I keep spares in my truck because they cost about eight dollars and are the most common failure point on these systems. Sometimes the issue isn't the thermocouple at all. The pilot orifice can get partially blocked with lint or debris. I've had customers who'd never changed their return filter in twelve years and the pilot screen was clogged enough to starve the flame. Solution was a gentle cleaning with compressed air and a bristle brush. Takes three minutes. The furnace started right after.
Blower Motor and Fan Control
The fan control on the Presidential II is typically a basic temperature-activated switch mounted on the supply plenum. It has two circuits: one for the blower and one for the limit circuit that cuts gas if the heat exchanger gets too hot. The blower runs off line voltage from the transformer secondary, usually 24 volts stepped up through the fan control. If the blower isn't coming on, check that you're getting 24VAC between the G and C terminals on the thermostat side, then verify the fan control is closing at the right temperature. The factory setting is usually around 130-140 degrees F for fan-on. Use an infrared thermometer or a probe to check the plenum temperature when the burner is firing. If the control isn't switching at the rated temperature, replace it. They're inexpensive and wear out from thermal cycling. Here's something beginners miss: the fan control also has a delay function. Some models include a built-in blower delay so the motor doesn't kick on immediately when the burner fires. Cold air blasting into the heat exchanger on startup causes thermal stress. If your fan is turning on too early, check the fan control setting. Adjusting the differential usually fixes it without replacing the component.
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Heat Exchanger Inspection and Safety
The heat exchangers in these units are aluminum-coated steel. They hold up well if the furnace was properly maintained, but cracked heat exchangers are a real concern on furnaces over twenty years old. The common failure points are around the burner ports and the upper pass where the flue gases change direction. Use a flashlight and a mirror to inspect through the burner access panel while the unit is off and cool. Look for cracks, rust-through, or discoloration that indicates overheating. I found a hairline crack in a heat exchanger on a unit that was still technically operating within normal parameters. Carbon monoxide was present in the return air at 12 ppm, which is above the safe threshold but low enough that the homeowner wouldn't have noticed any symptoms. This is why I always run a CO test at the return grille before leaving any combustion appliance job. A combustion analyzer would catch this too, but not every tech carries one. Cracked heat exchangers mean the unit needs replacement. There's no reliable patch for this on the Presidential II series. The welding repair attempts I've seen from DIYers on forums don't hold under thermal cycling, and they create a liability if you certify them as safe. The gas valve and controls might be fine, but the core component is compromised.
Gas Valve and Burner Maintenance
The gas valves on these furnaces are either Honeywell or Cole-Mfg branded, depending on the production year. They're manual adjustable valves with a thermocouple holding circuit. Adjusting the gas valve requires a manometer. Connect it to the test ports on the valve, fire the burner, and adjust until you hit the manifold pressure specified in the manual. For natural gas, that's typically around 3.5 inches water column. Propane runs higher, usually 10 inches WC. The burner ports collect carbon and lint over time. Remove the burner assembly and clean each port with a wire brush or a drill bit sized to match the port diameter. Don't enlarge the holes. Reaming out the orifices changes the air-fuel mixture and will throw off your combustion readings. I've seen techs do this and then blame the furnace for poor performance when the real issue was their own work. After cleaning, reinstall the burner and light the pilot per the manual's sequence. The proper lighting procedure matters on standing pilot systems. Turn the gas control to pilot, depress it while holding the igniter button, then once the pilot is lit, continue holding for about thirty seconds to warm the thermocouple. Release and verify the pilot stays lit. If it goes out, the thermocouple isn't generating enough heat or the connection is loose.
Common Issues That Aren't Addressed in the Manual
The official documentation covers the basics, but real-world experience adds a few things worth noting. One is the pilot bracket positioning. The pilot flame needs to impinge on about three-eighths to half an inch of the thermocouple tip. If the bracket loosened or got bumped during service, the flame misses the sensor and the gas valve thinks there's no pilot. Trim the flame height with the pilot adjustment screw on the gas valve. It should be blue and firm, not yellow and lazy. Another issue is the millivolt system's sensitivity to interference. If you're running new thermostat wire parallel to live HVAC power conductors in the same conduit or j-box, you can introduce induced voltage that interferes with the thermocouple signal. Keep the low-voltage and line-voltage wiring separated. I've had situations where the furnace would work fine until another circuit in the same wall was energized, causing intermittent pilot shutdowns. Move the thermostat wire and the problem disappears. The draft hood is also worth inspecting. These units are natural draft appliances. If the chimney or vent connector is corroded, blocked, or improperly sized, the draft can be insufficient to clear combustion products. Check that the draft hood is intact and the vent pipe has a continuous upward slope toward the chimney. A negative pressure test in the house during high exhaust fan operation can reveal whether the furnace is being back-drafted under certain conditions. This is rare but dangerous.
When to Call a Professional
If you're comfortable working with gas lines and electrical systems, the routine maintenance on a Coleman Presidential II is straightforward. Replacing the thermocouple, cleaning the pilot and burner, checking the heat exchanger, and verifying combustion settings are all within the reach of a competent DIYer. But if you encounter cracks in the heat exchanger, signs of gas leaks, or persistent ignition problems that cleaning and adjustment don't resolve, it's worth calling a licensed HVAC technician. Combustion appliances can produce carbon monoxide, and the consequences of getting it wrong are severe. The manual will give you the specifications and procedures. Experience will tell you where the real problems hide. Those two things together will get most furnaces running properly.
