On 2010-07-15, Ca****cher wrote:

Installation of a gas range requires running a new gas line from the
basement utility room to the kitchen, a total of about ten feet.
The installation instructions indicate either a 1/2 or 3/4-inch line
can be used. The easiest approach would be to tee black pipe into
an existing 1/2-inch gas line feeding a 40 gal/40,000 BTU gas water
heater. Will the 1/2-inch line now feeding the water heater have
enough capacity to support both the water heater and a gas range?
The combined BTU of all the range burners is about 60,000 BTU.
Opinions?

Short answer:

You need to count the fittings and the lengths of pipe from the gas
meter to the water heater tee and then from the water heater tee to
your range. Then you need to do some math to answer your question.

Long answer (the math):

I'm going assume you are talking about natural gas and that your
utility provides gas with a specific gravity of 0.60, and that the
heat content of your natural gas is 1000 BTU per cubic foot. These
are common values. By convention, the natural gas pressure after the
meter is 7.0" w.c., and the natural gas piping system needs to be
designed for a total pressure drop of 0.5" w.c. or less from the meter
to each appliance.

Now in measuring length of iron pipe for natural gas distribution, you
need to include each fitting via its equivalent length. When I google
"natural gas equivalent length 90", the first hit gives me a chart
with the appropriate values. For example, a 1/2" 90 degree bend is
equivalent to 1.55 feet.

So what you need to do next is to follow the iron pipe running from
the meter to the tee by your water heater. Add up the fitting
equivalent lengths to the lengths of the straight runs to get a total
equivalent length, call it X1. Here I am assuming the water heater
run is all 1/2" pipe and has no other outlets.

Now figure out the proposed run from the tee by your water heater to
the gas range location, including all fittings, and determine its
equivalent length, call it X2. With the fittings counted, X2 will be
greater than the 10' you specify.

The last piece of info you need is the pressure drop per unit length
for 1/2" iron pipe at flow rates of 100 cubic feet/hour (from the
meter to the tee for both appliances) and at 60 cubic feet/hour (from
the tee to the range). Those are 0.0183 and and 0.0071, respectively.
I got these from the page 77 of the design guide for the CSST I use,
http://www.wardflex.com/images/WARDFLEX_D&I_GUIDE_Eng.pdf

Then your pressure drop will OK if 0.0183 * X1 + 0.0071 * X2 0.5.

For example, if X2 = 20' after you include the fittings, then the
pressure drop on that leg would be 0.142" w.c., and so to be OK you
would need 0.0183 * X1 (0.5 - 0.142), or X1 20'. For other
cases, you can do the math.

Note that if your pressure drop calculation is close to but above 0.5,
you could try 3/4" pipe from the water heater tee to the range
location. You'll need to refigure X2 using the new equivalent lengths
for 3/4" fittings. And you'll need the pressure drop per foot for
3/4" pipe at 60 CFH, which is 0.0018.

So if the new run is 3/4" you need 0.0183 * X1 + 0.0018 * X2 0.5.

Also, note that if X1 0.5/0.0183 = 27' then you're out of luck
tieing into that tee by your water heater, the line to the water
heater simply can't handle 100 cubic feet/hour. You'll have to run a
new line all the way from the meter. As long as its equivalent length
is less than 0.5/0.0071 = 70', you can use 1/2" pipe for it; with 3/4"
pipe, the equivalent length can be up to 0.5/0.0018 = 280'.

Cheers, Wayne