The gas piping system has a direct effect on appliance operation and performance. Many nuisance shutdowns can be attributed to gas-related piping errors. We will look at some gas piping basics and identify common issues that come up in the field.
GAS PIPE SIZING
Proper gas piping is essential for each appliance to function correctly and most efficiently. When planning a gas piping system, it is important to draw it on paper and plan it out. You can flow only so much gas (or water) through a given pipe size, so restrictions will manifest themselves as a lack of pressure or volume.
Sizing a gas line is not too difficult. It is easier to start at the last appliance and work your way back to the as meter and/or regulator. The main pipe size will increase as a branch is added and increase again as another branch is added. Each branch must be properly sized to carry the Btu load of the connected appliance. If a branch will serve more than one appliance , then it must be sized to carry the total Btu load. As you work your way back toward the meter or regulator, the main pipe will increase in size to accommodate the total Bty load of the system.
he gas sizing charts shown are from the National Fuel Gas Code/ANSI Z223.1/NFPA54. They indicate thousands of Btus that can flow through a given pipe size based on the length of piping. Fittings must be added to complete the calculation. Each fitting has an equivalent footage equal to straight pipe, and these must be counted and added to the total footage.
Pipe is too small to carry total Btu input
Size reductions or fitting restrictions can create a lack of gas volume or unacceptable gas pressure drops. This is common on piping branches. The length of the branch plus fittings must be considered and appropriately calculated.
Reduced-size flexible connectors
Flexible connectors are typically smaller than the connection itself. A 3⁄4″ flex connector will normally have 1⁄2″ or 5⁄8″ corrugated SS tubing. Flexible connectors have a rating label attached – Check length and Btu capacity!
Small or reduced port ball valves
Always use full-port valves to prevent reduced volume or pressure. Many valves I see in the field are small-port or reduced-port. These valves do create issues and they can cause flame failures.
Gas regulator too close to heater
A regulator cannot always respond when it is mounted too close to the heater. It can’t always open fully and regulate properly when installed too close.
First and second stage regulators too close to each other
The fist and second stage regulators can “fight” each other if they are located too close to each other. It takes a length of piping to allow the first stage regulator to open and operate properly, and the same applies to the second stage regulator.
Lock-up style regulators
Lock-up style regulators may require a pressure relief reset to operate properly. They can “lock up” and cease to function. Gas pressure must be relieved from the regulator inlet for it to reseat/reset.
Ball-check vent limiters must be installed with the regulator in the horizontal position and the ball-check vent limiter in the upright position. The ball-check can stick if installed in another position.
Tankless retrofits almost always require a gas pipe size up- grade. A typical 40-gallon gas tank-type heater uses approxi- mately 40,000 Btu per hour. The standard tankless heaters are rated at 199,000 Btu per hour! The branch and main lines must be considered if a tankless heater is being added. The branch must be able to carry the increased tankless Btus. The gas main line must be able to carry the additional and total Btus.
Modulation allows a heater or boiler to operate at a lower input. Tankless heaters are good examples of this technology. As flow increases, the burner will ramp up and require more gas to operate, all the way up to its maximum input. Tankless heaters can operate on low fire with an undersized gas line, but when input demand increases, the heater will experience flame failure issues.
Always verify gas pressures with a slack-tube manometer. You cannot effectively troubleshoot a gas issue without a slack-tube manometer. They are the only device approved for a pressure-drop test and they always give a true reading. A gauge-type manometer will also give accurate readings. Avoid using a digital manometer for troubleshooting gas volume or pressure issues. They can be erratic with jumping pressures and require calibration. A digital manometer is best applied to measuring minute pressures or differentials. Gas inlet pressures must be taken with the unit off and with the unit on to make sure there is not a substantial drop when the burner comes on. Gas manifold pressures must be measured with the unit on or as it comes on.
A 1⁄2″ WC drop is an acceptable pressure drop—any higher drop in pressure indicates a gas supply issue that must be found and corrected. Some regulators will have a higher pres- sure drop, but it drops when the regulator opens and then holds steady at the set pressure.
More than a 1″ WC drop can cause many fan-assisted heaters to go out on flame failure. Many newer appliances use a variable frequency drive (VFD), and dropping gas pres- sures can affect the ignition or burner operation resulting in flame failures. Fan-assisted appliances also require a mini- mum gas pressure that must be present at all times to oper- ate. If the pressure falls below the required minimum, the heater will experience nuisance flame failure shutdowns.
Atmospheric heaters will operate on low gas pressure or volume. They are more “forgiving” of low pressure or volume. They are dependent on gravity make-up air, and the open burner can still burn at reduced input. This can be a big issue if the gas supply is undersized. The heater will operate at a lower Btu input and have a reduced recovery rate. If a heater is marginally sized or in high demand, the reduced Btu input can cause premature failure and operational issues such as condensation. Reduced gas supply = lower Btu input = less hot water gph recovery.
A gas supply problem can indicate many potential issues. It can be tedious trying to figure out what the cause of the prob- lem is. It may require sketching out the piping and doing a cal- culation. Sometimes it can be easily overlooked like a fitting re- duction, or it can be hard to find like a reduced-port ball valve. I always say that the slack-tube manometer never lies, and it can clear up any confusion. We have listed a lot of the common issues here, but there are many more to be encountered!