Ever Size a Gas Pipe?

Part of my job is being the expert, when a real expert isn’t available. Generally, the request will be prefaced with the standard apology, “Sorry to bother, can you help me out?” Recently I got asked to size a high pressure natural gas line, which I’m not an expert at, but I wanted to help these guys out. 

One because I like them, and two because it would be a good sale for the kid to write up. Upper management keeps track of his income generation, or what used to be called “sales.” The kid puts his initials on all my sales so it looks like he’s selling lots of stuff. Big numbers look good. I get judged on total sales, so it doesn’t look bad for me.

But enough about the kid and I, you want to learn how to size a gas line or make sure it was done right. The right size of anything is better than the wrong size. Just like you don’t want to give your best girl the wrong size rock at engagement time, you don’t want to find out the gas pipe is too small after the job is done. 

A gas line is sized based on the type of gas used, so that is the first thing to know. The most common type of gas I size for is natural gas. The propane dealers in our area usually provide the sizing and the pipe for hooking up their tanks, so I haven’t been too involved with LP gas pipe sizing, but it’s the same procedure.

The pressure of the gas is the next thing to know. When I said high pressure earlier, it is two and a half pounds of pressure. To me that is high pressure. To a guy at the gas company, it’s probably low pressure. Ever talk to a Navy veteran about steam pressure? The guys that worked on the ships think low pressure steam is anything under a couple hundred pounds. I call low pressure steam two pounds.

When you know what type and pressure it is, then you find the right gas sizing chart. Most of the gas sizing charts are separated by the type of gas, the maximum gas pressure, and the gas pressure drop allowed. As gas moves through a pipe, friction reduces the capacity. Just like the friction loss of water as it moves through the pipes or air as it moves through the ducts or electrons as they move along the wires, it must be accounted for. 

The typical chart in a boiler installation guide will be for gas pressures after the house meter/regulator, which means less than a pound. They are based on a pressure drop of .3 inches of water column as the gas moves through the pipes, which isn’t very much, and shouldn’t be. Natural gas boilers are designed for a certain gas to air mix to achieve good and consistent combustion, so we want to have as little pressure drop as economically possible. 

We can always oversize a gas line to achieve almost no pressure drop, but the right size is going to be cheaper to install. Who has enough extra money on the job to throw away on oversized pipe?

Now you have to know how many btus per hour are going to be hooked up to the end of that pipe. My oldest son is installing an outdoor kitchen and wants to hook up his gas appliances to natural gas. Yes that is plural. One is a giant grill and the other is a high capacity burner for a wok. He thought he could hook it up to a ½” gas line in a furnace room nearby. It calculates out to a ¾” gas line and that has to come from the other furnace room that is far away and through three foundation walls. We are looking for a different route. As of this writing, he isn’t grilling with natural gas.

The request for the high pressure line came from two different guys. The guy that would be installing it wanted to know how many btus per hour the 3” line he had estimated could handle. He’s the son of one of the true gentlemen that was in the trade when I started doing outside sales over thirty years ago. He would smoke his pipe and listen to all the great new things I had to sell at the time like he was interested. The son took over years ago and didn’t fall too far from the tree. Always a pleasure to talk to and only wants to do the right thing. He was checking the sizing before proceeding. I will always help with that.

The guy that wanted the gas line installed was a neighbor of my wife as they were growing up. His father was a German emigrant that built up a very successful woodworking business. Thirty years ago he purchased a sawdust burning hot water boiler in Germany and had it installed in his new shop, complete with European-style panel radiators and European-inspired water distribution ideas, which at the time preceded primary/secondary techniques. He had plenty of sawdust, so why not burn it?

That system worked very well until the EPA started to become involved. So much so, that now the son is preparing for the day that he can’t economically burn sawdust. The current boiler seems oversized to all three of us, so his idea was to put in a couple of high efficiency boilers now, but be prepared to add two more if necessary. Now we had our btus per hour, our gas pressure, the pipe size, and the pressure drop allowed.

The final piece of info for their request was the length of pipe, since the further the gas travels, the less capacity is available. The chart showed that the 3” pipe could handle up to the four boilers that may be installed and then some, but 2-1/2” pipe would be too small for the distance from the new gas tie in to the boiler room. 

To review, to size a gas pipe you need to know the type of gas, the pressure of the gas, the gas pressure drop allowed, the btus per hour of the installed appliances and the distance travelled. Most charts read in cubic feet per hour. To translate to btus per hour, multiply by 1000.

Undersized gas pipe rears its ugly head as rough starts, poor combustion and inadequate performance. Look up the gas sizing chart in the install manual if there is any question. Just because there is a ½” gas connection on the boiler, it doesn’t mean that the ½” line can run clear across a basement. Generally the maximum length for ½” gas pipe is less than 20 feet for a residential size boiler. But you know that. Right?

Patrick Linhardt is a thirty-five-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.