Sizing natural gas systems the right way
Give ‘er the gas...
The goal of my last column was to begin to change the way most plumbers size natural gas distribution systems. In my experience most plumbers, and even Authorities Having Jurisdiction size natural gas systems based on the example in the Uniform Plumbing Code where the Btus/ft3 is shown as 1,100 (2015 UPC Figure 1216.1.1).
Before I get into the thick of things, here are a couple things to be aware of that others might question on any value used for Btus/ft3:
1. Why not use 1,000 Btus/ft3? It makes the math real simple; it’s an easy number to remember and gives a margin for error in the future.
While all that is accurate, it’s not the way the UPC says to size natural gas piping. Being stuck on 1,000, even when it seems to provide a margin for error, will typically mean the cubic foot/hour (CFH) of each section of pipe is knowingly designed to carry more Btus than required with the piping perhaps being larger than required.
2. Are there problems with sizing a natural gas system larger than required; will the appliances still work correctly?
I’ll answer the second half of the question first. It’s quite simple, the answer is yes. The appliances will still work as they are rated to. So what’s the big deal?
First, you’re not using all the details you’re supposed to use to size the system and if you decide that oversizing is a good design strategy, where are you going to stop? How oversized are you going to go and will it be limited to natural gas distribution systems? You’re not doing anyone a favor by oversizing any system, except whomever you’re buying your materials from.
One further thought. If you take an exam that includes sizing a gas distribution system and you use the wrong Btus/ft3, you will likely fail the exam.
Now let’s explore another issue while thinking 1,000 is a good number to use for the Btus/ft3. I wrote in the previous column that the Btus/ft3 range from 800 to 1,200. What if the gas you’re being supplied has 850 Btus/ft3?
If you insist on using 1,000 Btu/ft3 you’ll end up with an undersized distribution system. Fortunately there should be no danger and everything should still work but the FAU will take longer to heat a space and the water heater will take longer to give the same amount of hot water, etc.
No harm, no foul? Maybe, but if you design, size and install systems as they are supposed to be, you won’t need to make any excuses when challenged.
So let’s review a gas drawing and see how this plays out using different values for Btus/ft3. In the example we will be using Table1216.2(1) and the “Branch Length” method, which I believe is more accurate. Cubic feet per hour will be rounded off with typical numeric rounding.
As you can see there are some differences. Not huge, but they are there. Next time you size a gas system get the therm multiplier, billing factor or whatever your gas utility calls it and size the system properly for your location.
I’ve had people challenge me saying, “What happens if the Btus/ft3 goes up or down?” Generally my response is, “I can’t size systems based on what ifs.” I size the system with the most accurate information as of today, based on the most recent therm multiplier.
The idea of sizing a system for a future demand has some merit, but what is that future demand going to be?
On a related issue, have you ever been asked to add an appliance to an existing system? Say they have an existing gas fired water heater in the garage, and they went out and purchased (or someone gave them) a gas clothes dryer. They call and say: “All you need to do is cut in a tee at the water heater gas line and run a new gas line over the garage door to the new dryer location. How much will that cost?”
I hope you realize you can’t just arbitrarily add an additional load on the system. You have to determine, by doing the sizing, starting from scratch, if there is any capacity left over to add a 35,000 Btu load. In my experience that just doesn’t happen. (Yes, I’m sure it worked out for you once and you should go buy a lottery ticket!)
Then you have three choices:
1. Do it correctly by either redoing the entire system or perhaps just running a new line from the meter. And because that is so inexpensive they can’t wait to sign a contract with you.
2. Thanks, but no thanks. You’re not going to like what I have to say, and then you’ll get angry and accuse me of trying to ______________ (you fill in the blank).
3. Or do you say, knowing it is a code violation: “Look I’m your buddy, your new best friend and even though it’s wrong and I may be struck by lightning, I’m gonna do it just like you described. You know, looping a gas line from the water heater over the doorway to supply gas to your new dryer.
“The only problem is, if there’s a hot water demand and the water heater fires off at the same time as the dryer is in operation, neither the water heater nor the dryer can possibly operate at full capacity. There shouldn’t be any danger while both the appliances are starving for gas. You okay with that?
“And because we’re such good buddies I’m going to do it for free!”
"This article was originally posted on ww.reevesjournal.com."