- posted on July 15, 2010, 7:08 pm

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?

- posted on July 15, 2010, 7:55 pm

To be on the safe side I would up it to 3/4 to the T where the range comes off. Just a swag though.

- posted on July 15, 2010, 7:56 pm

No. Run a new line from the source to the range.... the source being at least a 3/4" line, but preferrably a 1" line.

You don't want your water heater to have a drop in volume (and pressure, but volume is important), causing a low flame or for the pilot to go out. The smaller the line, the less volume the same pressure will move vs the larger the line, the more volume the same pressure will move.

You said it right, "capacity", meaning volume, as opposed to pressure. The source (your meter manifold) has a regulator to automatically maintain constant pressure.

Sonny

- posted on July 15, 2010, 8:03 pm

You don't say how long the existing half inch line to the water heater is. That will be important.

Having said that, I think this would be very marginal. It might work. But it might not.

Is that where you want to be? Guessing and hoping?

In your shoes, I would run some new pipe.

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- posted on July 15, 2010, 8:52 pm

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

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

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

- posted on July 16, 2010, 3:15 am

Wow, I've never seen these calculations before, not that I've needed them, but its good to know that someone here knows how to do them.

- posted on July 17, 2010, 1:05 am

Wow, Wayne, that was the long answer. :) And the absolutely "correct: way to calc oneself to the answer but there is a way to get there with a lot less work and typically fittings dont matter all that much. http://www.ci.pleasanton.ca.us/pdf/bldg-gaspipe.pdf

A while back I think Ed posted a great on natural gas piping system design / analysis link from the city of Newark, CA. It showed a pretty simple means to size a gas delivery system but they have since moved or removed the page. :(

Looks like the city of Pleasanton has come to the rescue. http://www.ci.pleasanton.ca.us/pdf/bldg-gaspipe.pdf

Their example looks a bit complicated at first but basically you just map out the pipe lengths & sizes making sure at each branching that the piping upstream can deliver the gas required at that point.

Here's a gas supply capacity table (if the format holds)

Capacity of Pipe (MBH ≈ CFH) Pipe Size (in) Pipe Length (ft) Nominal Inside diameter 10 20 40 80 150 300 1/2 0.622 120 85 60 42 31 22 3/4 0.824 272 192 136 96 70 50 1 1.049 547 387 273 193 141 100 1 1/4 1.380 1200 849 600 424 310 219 1 1/2 1.610 1860 1316 930 658 480 340

- posted on July 17, 2010, 4:31 pm

Yeah, I guess I was feeling verbose. Happens sometimes when I'm procrastinating a little. :-)

I disagree about the fittings. The chart in the table above is still based on equivalent length, so you need to include the fittings. And the equivalent length for a 3/4" 90 degree bend is 2'. So if you use 4 of them to jog around an obstacle, you need to count those fittings as 8 extra feet.

As for the method in the above PDF, it is a good method. It actually addresses a different question: given the lengths and demands, how should I choose the pipe sizes? Whereas the method I was using is more appropriate for the OP's question: will a particular choice of pipe sizes be adequate?

BTW, the sizing method in the PDF is quite conservative. In the example given, it comes up with 1 1/4" pipe for the main branch. But I'm sure that if you changed that to 1" and checked the pressure drops, it would still be adequate.

So what you can do is use the PDF sizing method, and then once you have a choice of pipe sizes, calculate all the pressure drops (which will be per force OK), and then see how much "safety factor" you have left at each outlet. Based on that you can try downsizing a few branches and then recheck the pressure drops to verify they are still OK.

Cheers, Wayne

- posted on July 19, 2010, 5:49 am

I think you hit on the real answer.....

yes, four elbows add to effective length but since the table steps are 10' at a time AND the delivered BTU's are conservative

Got a cite specifically for low pressure gas flow equivalent lengths?

Cutting the pipe size close (imo) is not worth the calc time, maybe as an academic exercise (which I'm not adverse to) but from a practical point of view the PDF is the way to go.

And the typical new install (usually what's being modified) doesn't have that many elbows unless the installer was a hack.

Sometimes its just not worth the time to do a close calc when the quick & dirty method will give you an adequate design, its not like steel pipe is all that expensive. And most people can follow the PDF method way easier than the calc's method.

I disagree with respect which method suits the OP. It the OP checks his current piping system and proposed mods against the PDF method & the "checks ok"....he's done. If it doesn't check ok....then it's time to "sharpen" the pencil.

Plus it all depends on what is really the easiest way to physically do the job.

cheers Bob

- posted on July 19, 2010, 3:10 pm

From my first post, just google "natural gas 90 equivalent length". The first hit should do it.

My original post was very long because as a mathematician I want to explain why we are doing the calculations. Certainly it can be reduced to using a chart like the PDF method. All you would need to do is this:

Change the chart in the PDF to be pipe size versus flow rate, giving the maximum allowed length. That's just presenting the same information in a different way, you can still use the PDF method for your initial design.

If you want to sharpen it, label each segment with the ratio of "segment length/chart maximum allowed length". A solution is OK if for each appliance, the sum of the ratios from the meter to the outlet is less than 1. So if you see that somewhere the sum ends up quite a bit less than 1, you can try downsizing an appropriate pipe segment, update the segment ratios, and see if the sum is still less than 1.

Cheers, Wayne

- posted on July 20, 2010, 3:45 pm

Wayne-

Yes, your modification would allow very rapid & simple itteration (which the current PDF does but not nearly as easily)

But Wayne, you'd be asking people to create ratios and add a few numbers! j/k

Did you ever here the one about how an engineer, a physicist and a mathematician each respond to a small house fire in the middle of the night? :)

cheers Bob

- posted on July 17, 2010, 10:21 pm

Wayne Whitney wrote:

But the pressure may not drop. The gas meter is, among other things, a regulator - it tries to keep the pressure constant.

In the instant case, the OP can experiment. Get a 1/2" garden hose and duct tape it to both the source and target of the planned layout. Turn on everything.

Does everything work?

But the pressure may not drop. The gas meter is, among other things, a regulator - it tries to keep the pressure constant.

In the instant case, the OP can experiment. Get a 1/2" garden hose and duct tape it to both the source and target of the planned layout. Turn on everything.

Does everything work?

- posted on July 18, 2010, 12:41 am

Right, the regulator at the gas meter tries to keep the pressure constant at its output. So if no gas is flowing, the pressure throughout the system is a constant 7" w.c.

But whenever gas flows, the pressure drops as it travels, due to the friction with the side of the pipe. So turn on all your range burners and draw 60,000 BTUs/hr (60 CFH), and while the pressure will stay at 7" w.c. at the regulator output, 10 feet away down a 1/2" iron pipe, the pressure will have dropped by 0.071" w.c. to 6.929" w.c.

And the system is to be designed so that the pressure at each outlet is at least 6.5" w.c. So you need to size your pipes large enough to ensure at most 0.5" w.c. pressure drop between the regulator and the appliance shut off valve.

Cheers, Wayne

- posted on July 16, 2010, 7:16 pm

Thanks to everyone for the feedback. Clearly, installing a tee from the
existing 1/2-inch line has the potential to starve either the gas range or
water heater if both are operating. So, I'll extend the 1" supply line to
the point where both appliances can be individually attached. Much more
complicated but probably necessary.

Catwatcher

Catwatcher

- posted on July 17, 2010, 4:47 pm

Going back to the 1" supply line is the more robust solution for sure. Then if you want a surefire solution that won't require any calcs, you would need to run a 3/4" line to your gas range.

But if it that is alot of extra work, you can get an exact answer to the question of "will teeing off the existing 1/2" line be OK?" What you would need to provide is a diagram showing all your gas distribution piping (and the new proposed 1/2" piping) like that in the PDF Bob found:

With that information I can show you how to calculate the pressure drops and answer your original question.

Cheers, Wayne

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