Converting a Goodman Furnace From Natural Gas to LP
The conversion kit for Goodman furnaces is usually pretty straightforward once you have the manual in front of you. Most Goodman packages are designed to be converted with a single kit part number, but they vary by model series. The kit includes new orifices, a manifold pressure check port gasket, and sometimes a replacement regulator spring. You need the specific kit for your furnace model number — it's not universal across the entire Goodman line. Here's how the actual process goes when you open the box. First, shut off the gas supply at the valve and turn off the electrical power to the furnace. Wait at least five minutes for any residual gas to clear before opening the cabinet. Pull the model and rating plate off the inside of the blower compartment door and write down the exact model number. Goodman model numbers like GMSS, MSG, or DCMSS series each have their own kit part number. A mismatched orifice size here will cause either a dangerously rich flame or a heater that won't reach temperature. Remove the existing natural gas orifices from the burner manifold. They're small brass pieces threaded into the manifold, usually sized between 35 and 85 depending on the burner station. Keep track of which orifice goes to which burner port. Some installers tape them to a piece of paper labeled by position, which saves time when reinstalling. Take each old orifice out with a properly sized wrench — a 7/16 socket works on most, but some are metric and require a deep-well socket. Forcing these with pliers will strip the brass and you'll be replacing the manifold too.
Install the new LP orifices from the kit. Hand-tighten first, then give them a quarter turn with the wrench. Do not overtighten. Brass threads strip easily and an overtightened orifice can crack the manifold fitting. Apply a tiny amount of pipe dope or Teflon tape rated for gas service to the threads if the instructions call for it, though most Goodman orifices are designed to seal against a copper or brass seat without additional sealant. The next step most people skip and shouldn't. You need to adjust the manifold pressure. Connect a manometer to the test port on the manifold. Light the furnace and run it on high fire. The manifold pressure for LP is typically around 10 to 11 inches of water column on most Goodman residential furnaces, but check the rating plate or the data sheet inside the control panel. Natural gas runs lower at around 3.5 inches WC. If your existing regulator doesn't have an adjustable range that covers LP pressure, the kit may include a heavier regulator spring or a replacement regulator. Goodman has included a dual-range regulator on newer models that simplifies this — you just swap the spring clip. After the orifices are installed and pressure is set, do a soap solution leak test on every connection. Spray the manifold fittings, valve connections, and the test port. Bubbles mean a leak. Tighten slightly and retest. Then run the furnace through a complete heating cycle and verify that each burner lights properly and the flame is a steady blue with minimal tip lift. Yellow flickering at the edges of the flame is common on LP and usually just means the air shutter needs a slight adjustment, not that the conversion failed.
I ran into a situation a couple years ago with a GMSS98 where the conversion kit orifices fit, but the manifold pressure wouldn't hold steady above 9 inches WC even with the correct spring installed. The furnace was a high-efficiency model with a sealed combustion system and an inducer motor that was pulling enough vacuum to destabilize the manifold reading. What fixed it was closing the combustion air intake damper slightly and rerunning the measurement — the induced draft was creating a slight backpressure on the manifold that made the gauge read artificially low. Once I accounted for that, the actual effective manifold pressure was fine. This isn't in the standard instructions because it's an edge case specific to that high-static-inducer setup. There are a few things the standard instructions don't always make clear. One is that some Goodman furnaces with variable-speed modules or modulating gas valves require a control board reconfiguration after the conversion. The board monitors gas pressure and airflow ratios and if it was calibrated for natural gas, it may fault out on LP without updating the configuration jumper or dip switch settings. Check whether your control board has a configuration table on the back of the panel — if it does, your conversion may not be complete until that's addressed. Failure to do this usually results in a lockout after one or two ignition attempts, and the unit will cycle until someone figures out why. Another thing people miss is the venting implications. When you convert to LP, the flame temperature changes and the CO levels in the flue gas shift. A furnace that passed a CO test on natural gas may not pass on LP without some tuning. It's worth running a flue gas analysis after conversion rather than assuming the kit alone makes it safe. Goodman doesn't require a professional to do this, but doing so will catch problems that a visual inspection will miss entirely.
Get the Full Details
The main limitation of the conversion kit approach is that it only works on single-stage and two-stage furnaces designed to be convertible. Some Goodman models, particularly certain high-efficiency condensing units, are manufactured differently enough that a simple orifice swap isn't sufficient and Goodman may not even offer a conversion kit for those models. In those cases, you're looking at a full gas train replacement or a new furnace, which is a much more expensive proposition. Always verify that Goodman actually publishes a conversion kit for your specific model before purchasing anything. The Goodman parts lookup tool on their website will tell you immediately whether a kit exists. The complete documentation and diagrams for your specific unit can usually be found in the installation and servicing sheet that Goodman provides for each model number. These sheets are separate from the homeowner's guide and include the orifice sizes, manifold pressure specifications, and any model-specific notes about conversion requirements. Look up your model number and pull the servicing document — it will have the exact kit part number and all the figures you need during the job. This is generally more reliable than generic third-party instructions because it's written by the engineering team that designed the unit.
