Why is pipe size important when running a line for a gas appliance? I understand P.S.I., but not volume.

Question

I am installing a 250,000 BTU natural gas heater for my in ground pool. The manuf. says if distance (from the meter) is 50-100 ft. use 1-1/4" pipe. Can someone explain what significance does increasing the pipe size have when considering the following information?

I have 3/4" pipe coming out of my meter for about 12", then is increased to 1" for my main run of 12 feet. Three appliances are tee'ed off of the 1" main- (3/4" to furnace; 3/4" reduced to 1/2" for hot water heater, and 3/4" reduced to 1/2" for a wall-mount space heater). If my pipe size starts out at 3/4" at the meter and increases to 1" for the three appliances, what is the purpose of increasing the pipe size to the heater for 48 feet? Making it more confusing to me, the manuf. requires the 1-1/4" pipe to be reduced down to 3/4" at the heater (no more than 10 ft. away from the heater). Can someone explain what the increase in pipe size does, when it is reduced down to 3/4" again? I see the limitation (or restriction) being at the meter, with the 3/4" pipe coming out of it.

Answer 1

Can your gas meter adequately handle gas flow the pool, furnace, water heater and space heater all at the same time and does the gas meter have a 3/4" outlet connection (or less) and is the gas meter inlet pressure enough to adequately  supply the pool, furnace, water heater and space heater, all at the same time?

If the answer is yes. Than the reason for the 1-1/4" line to the pool is pressure drop. You have the capacity at the 3/4" meter, you need to keep the pressure to have the full capacity at the pool. To accomplish this the bigger pipe has less pressure drop than the 3/4" pipe. If you run the 3/4" pipe to the pool the supply pressure to the pool burner will be significantly less and you will de-rate the burner capacity to less than 250,000btu/hr.

If you answered no to the first question you have other problems.

Answer 2

The easiest answer is flow rate. Q = V * A. Assuming the velocity of the gas stays constant the best way to influence the rate at which your appliances recieve gas is to manipulate the area (in this case of the pipe conveying the gas). A larger diameter pipe will have a slower flow rate then a smaller diameter pipe but will convery more gas. The manufacture has you constrict the pipe to 3/4" to regulate the flow the heater recieves. In other words, the large pipe gives a safety factor by allowing for more gas then is necessary. The heater only needs the 3/4 inch supply to operatee efficiently but over a long distance the supply of gas will be consumed as fast as it flows through the pipe (asuuming) therefore the manufacture recomends a larger pipe to increase the supply of gas to the heater while using the smaller pipe at the heater to control the rate of consumption. The big pipe = supply while the small pipe = rate of consumtion.

Answer 3

Basically, its the gas velocity that dictates the size of the line .  If the gas flows a speed to fast, then friction on the gas causes the pressure to drop.  If the appliance doesn't have enough pressure, then it could flame out or at a minimum, not supply enough fuel and put out less energy than it is rated for,

250,000 BTU/hr is a HUGE appliance.  It's 5 times bigger than your water heater, so it needs 5 times as much gas and 5 times bigger lines to keep the velocity correct.  In lines its the area of the hole in the line that counts to move the gas.  so a 3/4" line has an area of .75^2*Pi/4 = .44 square inches, so you need a line that is 2.2 inches^2 and that's about 1 1/2", but because its close, they can accept a 1 1/4 inch.

If you do not follow the manufacturers recommendations your 250,000 BTU/hr heat may only put out 100,000 BTU/hr so you paid extra money for nothing.

I'd recommend starting the 1 1/4 in lines as close to the outlet of the meter as possible, I'd also ask you utility company if your meter is big enough to handle all your appliances on at once, most likely is, but it would be a good thing to check up on.