In order to properly adjust and / or maintain a residential gas furnace, a manometer is needed. Digital manometers are prefers as opposed to U-Tube variations, due to the readout display. A manometer can read the incoming pressure of gas in a gas fired unit. Some combustion analyzers, such as the Wohler A 450, also offer the feature of measuring the gas pressure.
In Natural Gas lines, the gas pressure is typically 5" to 7" WC, but the furnace usually operates at pressures of 3.2" to 3.7" WC. The optimum setting can be found on the furnace rating plate. Depending on the altitude, a lower pressure may need to be set.
At the beginning of the pressure adjustment, the manometer is connected to the gas valve with the furnace switched off. This provides a reference point for the measurement. The manometer should indicate the pressure in inches of water column ("WC).
To connect the manometer to the gas valve, remove the 1/8" MPT plug located on the outlet portion of the valve. In its place, a 1/8" brass barb fitting is inserted. It is recommended to use thread sealant to ensure that the barb tap is tight and that no gas can escape at the sides and thus falsify the measurement. The barb fitting can then be used to attach the measuring hose of the digital manometer.
If no 1/8" MPT plug is available, there is an alternative connection possibility. In this case there is usually a nipple on the top of the valve where the measuring hose can be attached. At this small tower connection there is a screw, which has to be unscrewed slightly beforehand (one turn).
When the digital manometer is connected to the gas valve, the burner can be switched on. By turning the gas pressure adjustment screw the gas pressure can be changed (clockwise rotation -> increase pressure). The Wohler A 450 also offers a tuning guide to ensure optimized combustion and thus minimum costs. All this is graphically supported to facilitate the technician's work. For large burners, the free Wohler A 450 app can also be used if the gas valve and measuring point are far apart.
If it is a two-stage furnace, both low-fire and high-fire operation must be set correctly. The correct settings can also be found on the furnace's rating plate. There you will also find the desired temperature difference between supply and exhaust air.
At the end of the measurement, the entire connecting procedure must be reversed and it must be ensured that no gas escapes from the furnace. A gas sniffer helps here to play it safe.
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