need help natural gas black pipe

[deanstewart]

I have natural gas residential use meter . I want to install tankless water heater ,the futherest appliance is a cook top stove about 60-70 feet from meter and max use is 90,000 btu .The other two fixtures are a gas dryer and 50 gal water heater which i want to change to a outside Rheem 74 series tankless water heater .Dryer and water heater are 15 feet from meter 1 1/4 feed to 3/4 x 15 feed. Question what size pipe is needed for tankless and dryer ? Is 3/4 enough or do i need more ? thanks for your help, Dean ,

[pushkins]

The pipe size is going to be dictated by your tank less system, how many BTU's is it and how many was your old system is a good place to start.
Your tank less system should have come with the details your looking for.

[HayZee518]

I'm sure lazypup with have the information you seek. He's out of Sharon PA so direct your questions at him.

[LazyPup]

Sorry for the delay, i have been working offline to write the answer and create an illustration.

I can see from the information in the post that Dean is in the ballpark on how to size gas lines, but I am afraid we would need considerably more information before we find our seat, so to speak.

Regardless of whether we are examining a main gas line or a branch line, the first step is to determine the total BTU load on the line in question.

If I understand Dean’s explanation correctly he currently has two branch lines from the meter. One branch line runs approximately 60 or 70ft and to the kitchen area and supplies 90,000btu/hr to a freestanding gas range (FYI- the term “cook top” usually refers to a built-in cook top with a separate oven, whereas we use the term Range to identify a free standing appliance which contains both the cook top & oven).

For the moment let us assume the branch line to the kitchen was sized correctly when they originally installed the gas system, therefore the line in question is a second branch line from the meter which runs approximately 15 feet to the dryer and water heater.

There is no mention of the Btu/hr load of the dryer however the code gives us a table of appliances and a typical Btu/hr load that we may use for estimating. The table lists a laundry dryer at 35,000Btu/hr.

Due to the extremely high gas consumption and broad differences from one model to another the code table does not list a tank less water heaters therefore we must find the manufacturers rating for the exact appliance in question. I went to the Rheem Mfg Co home page and looked up the Rheem 74 series Natural gas fired tank less water heater and found it is rated at 199,900btu/hr therefore the combined load on this branch line is 199,900btu/hr + 35,000btu/hr = 234,900btu/hr.

Properly, at this point we should contact the natural gas supplier and find out how many Btu/hr is produced by one cubic foot of the gas they are supplying, however in instances where we do not have the exact number the code will allow us to use the industry average of 1,100btu/hr per cubic foot of gas. We then divide the total gas load by the Btu/hr of a cubic foot of gas to determine Cubic foot/hr required.
234,900 / 1,100= 213.54cubic feet/hr.

Now here is where it gets a bit tricky. The gas line sizing tables in the code give us a maximum number of cubic feet of gas that may be conveyed for a specified distance, however that distance IS NOT the physical distance from the meter to the load, but rather it is the “Total Developed Length of Pipe (TDLP)” from the meter to the furthest fixture on the load.

TDLP is the exact length of all pipes, including rises, drops, offset’s and fitting allowances from the source to the load. Because fittings introduce a high margin of friction fittings are given a “Fitting Insertion Loss” value, which is expressed as the equivalent length of straight pipe that would create the same frictional loss. By example, a ¾” 90degree elbow would create the same amount of pipe wall friction loss as 2.2 feet of pipe, therefore the insertion loss value for a ¾” 90el is 2.2.

In order to compute the actual TDLP we would need either a scaled floor plan or a scaled isometric drawing of the actual proposed pipe run. As an example I have prepared a rather simplified illustration of Dean’s description. From the illustration you can see that even though the physical distance from the meter to the water heater is only 15’ when we compute the run of pipe including drops, rises and fitting allowances the TDLP is nearly 30’feet.

Based upon my example, if I were to then take the total cu.ft/hr and the TDLP to the line sizing chart it would show that we need a 1” line from the meter up to and including the Tee at the dryer.

If you could post a floor plan showing the pipe layout I could give you a much better answer to your question.

[596]

..pup has provided a very good analysis of how this is determined. To give you just some perspective on the calculation, from the charts in the National Fuel Gas Code....3/4" pipe will only carry 230,000 BTU about 25 feet. The complete analysys would look at the btu's carried in each length and size of the pipe run. Off hand, I think you are looking at a new pipe run to support a tankless.

[deanstewart]

tankless

Thank you all for your help , Iam going 1" pipe 12 feet to tankless direct instead of using 3/4 pipe from original water heater. Dean