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Manifold pressure, orifices and checking gas input
Natural gas is supplied to the piping inside the structure at pressures from 8 inches water column up to 5 #. Propane is supplied at 11 in wc. This pressure is usually reduced to a lower pressure that is supplied to the burner(s) thru a manifold. The picture upper left is a manifold with 5 orifices supplying 5 burners. The gas valve is mounted to the end of manifold.
The gas valve contains a pressure regulator that reduces line pressure to the manifold pressure that is listed on the model plate. Note on the model plate below the natural gas manifold pressure is 3.5 in wc. Propane pressure is 10 in wc.
The gas valve contains a pressure regulator that reduces line pressure to the manifold pressure that is listed on the model plate. Note on the model plate below the natural gas manifold pressure is 3.5 in wc. Propane pressure is 10 in wc.
When this pressure is supplied to the manifold, it passes thru orifices mounted into the manifold. The amount of gas that passes into the burners is regulated by the pressure in the manifold and size and number of the orifices in the manifold. Below is a closer look at some orifices. Note on the right, the number stamped on the fitting. This number is the size of the hole expressed in number drill size. The number 42 is the drill size.
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You can change the gas flow by either changing the manifold pressure or changing the size of the orifices. Below are charts that give the gas flow thru different size orifices at different pressures. The top charts are for natural gas, the bottom ones are for propane.
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Below is a video about how the manifold and orifices are set up.
When the gas furnace is installed, when the gas valve is replaced or when servicing the unit the gas input is checked.
There are 2 methods to determine the gas flow into the appliance. The first method is to clock the meter. This is simply determining the amount of time it takes for one of the dials on the meter to make a complete rotation. If you know the time for a rotation, the amount of gas moved in one rotation and how many BTUs are in the gas moved, you know the input.
As an example, if you are using the one cu ft dial, one cubic foot will be moved for each revolution. So if it takes 60 seconds for a rotation, one cu ft will be moved per minute. So, in 60 minutes it will rotate 60 times. If one cu ft of gas equals 1000 BTUs, 1000 X 60 = 60,000 BTU. Appliance ratings are in BTUs per hour. The video below gives an illustration of how to do this.
There are 2 methods to determine the gas flow into the appliance. The first method is to clock the meter. This is simply determining the amount of time it takes for one of the dials on the meter to make a complete rotation. If you know the time for a rotation, the amount of gas moved in one rotation and how many BTUs are in the gas moved, you know the input.
As an example, if you are using the one cu ft dial, one cubic foot will be moved for each revolution. So if it takes 60 seconds for a rotation, one cu ft will be moved per minute. So, in 60 minutes it will rotate 60 times. If one cu ft of gas equals 1000 BTUs, 1000 X 60 = 60,000 BTU. Appliance ratings are in BTUs per hour. The video below gives an illustration of how to do this.
The second way to check input is to check the manifold pressure, determine the size of the orifices and use the charts above to determine the flow in BTUs thru the orifice. Then multiply by the number of burners. Compare to the model plate.
To do this, you will need a manometer or water column gauge.
To do this, you will need a manometer or water column gauge.
The gas regulator
Below are pics of a gas regulator. The gas enters on the left. There is a diaphragm in the center. When the gas enters, the pressure acts on the diaphragm and moves it up. When it moves up, the plunger in the center moves up and closes the valve. In the center top, there is a bias spring (center pic)that tries to open the valve. If the bias spring is loosened and does not bias the diaphragm, the gas will not flow because there is only pressure on the inlet side. As the bias spring is tightened, it will overcome the pressure on the diaphragm from the incoming gas. The gas then will flow out on the left side. If there is no outlet for the gas, the pressure will increase back thru the valve and onto the diaphragm. The valve will close. As pressure drops in the outlet, the bias spring overcomes the diaphragm pressure and the valve opens. The tighter the spring is, the higher the outlet pressure becomes. When a fixed amount of gas is being used, the regulator settles to a partially open position maintaining a steady outlet pressure.
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If this sounds complicated, it is. Below is a video explaining this part.
Adjusting the manifold pressure
To adjust manifold pressure first shut off all gas at the gas cock near the appliance. You will need to find the outlet tap on the gas valve. Below are several gas valves showing the location of the outlet tap. Most of these valves use a 1/8 inch pipe fitting that is removed with an allen wrench. The location varies but usually is close to the outlet to the gas manifold. Once the tap is found, the plug is removed and a 1/8 in MPT by 1/4 in barb fitting is installed which is connected by flexible hose to the manometer. Once this is done, the unit may be started. As the burners light, the manometer will register. A look at the model plate will give the pressure to reached.
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Below is a video of several gas valves with the pressure taps shown.
Regulator adjustment
Note the pics below. On the right, is a gas valve with the regulator on the lower right. It is covered by a brass cap. On the left, a gas valve is shown with the cap removed. The regulator is at the upper right.
Regulator adjustment is done by rotating the screw inside. Clockwise increases manifold pressure. Once the desired pressure is reached, the cap should be reinstalled. The pressure should be double checked after the cap is installed. If the pressure changes when the cap is reinstalled, readjustment is necessary. When the adjustment is finished, the tap that you placed on the valve needs to be removed and the plug reinstalled. Do not forget to put the plug back in! It is a very common mistake. When all is finished, leak check all fittings you have disturbed.
Regulator adjustment is done by rotating the screw inside. Clockwise increases manifold pressure. Once the desired pressure is reached, the cap should be reinstalled. The pressure should be double checked after the cap is installed. If the pressure changes when the cap is reinstalled, readjustment is necessary. When the adjustment is finished, the tap that you placed on the valve needs to be removed and the plug reinstalled. Do not forget to put the plug back in! It is a very common mistake. When all is finished, leak check all fittings you have disturbed.
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The video below covers how to make the manifold pressure adjustment.
The White Rodgers Gemini gas valve pressure taps
One of the types of gas valve that has a rather interesting way of reading manifold pressure is the Gemini valve. Below are 3 pics of this valve. In the center pic, note the Allen head fitting lower center of the valve. To test the pressure, the Allen is loosened, not removed. The manometer hose is then attached to the fitting. This will give manifold pressure. On the right is a pic of a similar fitting at the inlet of the valve. This fitting will sense gas line inlet pressure. Note the arrow on the casting. The brass cap shown in the center pic covers the adjustment screw.
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The video below shows how this is done.
When the pressure is adjusted, the primary air should be adjusted after the setting is done. Many newer furnaces do not have primary air adjustments so when the adjustment of the gas pressure is done the job is finished. The inshot burners below show, on the right, the non adjustable primary air.