I want to install a hot tub in my back yard. The heater for the hot
tub has an output of 100,000 BTU/hr. I have a 3/4-inch natural gas
pipe nearby that I would like to "T" off for the hot tub line. The
manufacturer's web site (the heater is a Minimax 100 made by Pentair)
says that a 3/4-inch line is OK, so long as it doesn't exceed 80 ft in
length from my gas meter. The length is OK, but the same gas line also
supplies an 80-gallon water heater, a stove, and a gas fireplace (the
"Everest" model made by HeatNGlo).

My question: For the hot tub heater to properly work, can I connect it
to the existing 3/4-inch natural gas line, or do I need to install a
new gas line from the gas meter and run this new gas line to the hot
tub, uninterrupted?

Hello oliop, once you find the proper information your question is
very simple. This web page:

http://www.propane-generators.com/natural-gas-chart.htm

Contains most of what we need to know. First we have to determine how
much gas is required by the appliances you now have installed on the
existing 3/4 inch line. The list on the provided link shows estimates
for all your appliances except for the HeatNGlo fireplace. I found
that info here:

http://www.thefiresideshop.com/s.nl/sc.2/category.5/it.A/id.493/.f

The Everest model is rated at 33,000 btu/hr.

Here is the gas required for your existing appliances:

80 gallon water heater =  80,000 btu/hr
gas range              =  70,000
fireplace              =  33,000
TOTAL                  = 183,000

Here is a copy of the chart from the web page. It is much clearer
there, but is still readable. Another piece of info we need is this:

"There is a more exact formula but you can round off the conversion of
CF to BTU by using 1=1000 (for example: 174 CF on the chart would be
equal to 174,000 BTU). This will give a more conservative total
estimate."

Pipe Size 10  20  30 40 50 60 70 80 90 100 Feet
1/2       174 119 96 82 73 66 61 56 53 50 
3/4       363 249 200 171 152 135 127 118 111 104 
1         684 470 377 323 286 259 239 222 206 197 
1-1/4     1404 955 775 663 588 532 490 456 428 404 
1-1/2     2103 1445 1161 993 880 795 734 688 641 605 
2         4050 2784 2235 1913 1696 1538 1413 1315 1234 1165 
2-1/2     6455 4437 3503 3049 2703 2449 2253 2096 1966 1857 
3         11412 7843 6299 5391 4778 4329 3983 3705 3476 3284 

You mention the length of 80'. From the chart we can see that a 3/4
inch line 80' in length can supply 118,000 btu/hr. Your existing 3/4
inch line is overloaded (or close to it) right now if two or more of
the appliances are used at the same time. Therefore, the answer to
your question is simple. As you probably suspected, you need to run a
dedicated 3/4 inch line to serve the new hot tub.
If this does not fully answer your question, please ask for a
clarification and I will try and comply.

Redhoss

Great answer that not only helped me evaluate my question, but a great
reference job for any future questions of the same subject. Thank you
redhoss!

problay

i mean its probly ok to use the the same line.