New range: whole house BTU line capacity calculation?

SparklingWaterJanuary 28, 2013

Is there a maximum NG BTU line capacity a house cannot exceed due to main gas line pressure dropping? We live several hundred feet above see level near our Capitol.

I'm about to switch out an older four burner 36" cooktop (estimate 48,000 BTU's tops) to a six burner 96,000 BTU with convection oven which requires the same 3/4" pipe.

From the main gas line to the house basement where it comes in, first off (3 feet away) is the approximate 60,000 BTU water heater, then off the main line (5 feet further) is the 120,000 BTU furnace. Off this furnace feed is the smaller 3/4" gas line to the current cooktop (2 feet to left wall and up 4 feet or so through kitchen floor). Finally coming off after the gas furnace is a smaller gas line to an outdoor NG grill with two burners, no idea how many BTUs (longest run, probably 15 feet and present when we bought the house). We've never had any problems with any suggestion of inadequate gas pressure.

Do you see any problems with changing out the gas cooktop (48,000) BTU to a higher BTU NG demanding 36" pro range w.r.t. minimums and maximum and maximum pressure to the regulator? Gosh, I tried the old blue tape method of wrapping the gas pipes to get their circumference so as to guesstimate their pipe diameter and got part way done but never finished my amateur calculation of whole house load. I don't think I have a caliper but may and I'd have to refresh myself on how to use it.

Thank you for general guidance and comments on this matter. Am I limited to how much gas I may draw or limited by pressure minimums? In short, do you see a major problem here I'm overlooking in getting this pro range from a gas pressure load point of view?

Thanks as always. Harriet Homeowner

This post was edited by SparklingWater on Mon, Jan 28, 13 at 23:14

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To get the diameter, wrap a cooord around the pipe and measure the length of the wrap. Take that value and divide by 3. That should get you close enough.
Without doing the math, I would say that increasing your BTU from about 220,000 by 48000, or 20% should have little affect on you system.
But get the diameter if you can.

    Bookmark   January 29, 2013 at 8:32AM
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I got hold of an old caliper. The OD of the current gas supply line rigid pipe is 3/4". The regulator rigid pipe OD is 3/4". There is a flexible metal connector connecting the cooktop to the gas supply line.

The new NG range calls for a gas supply line of 3/4" rigid pipe. I don't see any problem here, except for following specs on the back wall 3/4" rigid pipe location (height and depth) and ensuring a 1/2 to 3/4" ID flexible metal appliance connector is allowed for ease of connection.

I will be increasing from a 48,000 BTU cooktop to a 96,000 BTU range with a 30000 BTU oven burner.

Any one see any problem with pressure drop or whatever here? What are these whole house NG BTU calculations done for other than to ensure proper pipe size (and regulator control) feeding the gas appliance(s) and that the total amount maximally drawn doesn't exceed the city's main NG rigid pipe load capability?

I estimate my whole house BTU will be as follows (water heater (60,000 BTU max, may be it's less), furnace (120,000 BTU), new range (96,000 BTU), and out door 2 burner grill (guess 30,000 BTU) total 306,000 BTU. That's an increase of about 16 %, (258/306) in total BTU house draw.

Thanks joeplumb. Does this information help you further?

    Bookmark   January 29, 2013 at 5:54PM
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What is the diameter of the main piping and how long is the run from the meter until it gets to the water heater?

    Bookmark   January 30, 2013 at 8:21PM
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Many NG companies have the final regulator right at the user meter and run higher pressure in their distribution system.

Without knowing how you local company is set up there is no way to tell if your company line is large enough.

All you can do is make sure your lines after the meter are large enough.

Gas lines are not often run as branching, but as 'home run' lines back to the meter.

    Bookmark   January 31, 2013 at 10:12AM
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The first step in designing the gas supply system is to make a list of all gas fixtures & there load to determine the total load.

In this case they have:

Water Heater........ 60k/btu
Furnace............ 120k/btu
Kitchen range...... 48K/bt
BBQ(frm code table) 40K/btu
TOTAL existing load 268K/btu

Per code we are to contact the gas provider to find out the actual average btu per CF of the gas they provide. When that figure is not known code says we must use 11K/btu per Cubic Foot

268K/btu divided by 11K/btu equals 24.36cu/ft per hour

Your existing range consumes 48K/btu but you will be changing that to a range that uses 96K/btu for a total demand load of 316K/but divided by 11K/btu per Cu/ft = 28.7cu/ft per hour

We then need to know the YDL(total developed length) of the pipe from the meter to the furthest fixture.

Note-TDL equals the actual physical length of the pipe plus the fitting insertion loss lengths for all fittings on the run.

Per IRC table T2413.4(1)

A 1/2" line could handled 40cu/ft out to 150ft TDL
A 3/4" line could handle 84cu/ft out to 150'TDL

The bottom line, you have no problem.

And for the record, your gas company is in the business of selling gas. The only limit on how much you can have is how much you can pay for.

    Bookmark   January 31, 2013 at 1:36PM
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I've been off GW for a bit but not forgotten my post.

I did measure the main gas intake pipe as presents in the basement room (not from outdoor main). With calipers, it appears to be 1 1/2" OD. There is a fitting and 90 degree turn with down size to 1 inch OD soon after entry, which serves the gas water heater (3 feet away), furnace (7 feet away). From the 1" OD long pipe servicing the furnace, there is a fitting, and a 90 degree turn to service the pipe leading to the out door grill: first 1" OD then it transitions 3/4" OD. From the same long line to the furnace, a fitting and 90 degree turn brings the 1" OD pipe over to the back wall of the furnace room with kitchen above. There is a fitting a 90 degree vertical turn with downsizing to 5/8" rigid pipe for the current cooktop.

I haven't gone back down to measure/plot the TDL (I have our city form) as yet as I wish to do it one time, as accurately as I can.

Thank you all for your help. I'll be back if serious questions arise.


This post was edited by SparklingWater on Thu, Jan 31, 13 at 16:09

    Bookmark   January 31, 2013 at 3:34PM
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From your 306,000 BTUH, and using the standard heating value of NG as 1 cubic foot per 1000 BTU, you need about 306 cubic feet/hr in the main
Your main at 1.5 inch would give a pressure drop of only
0.004 psi per 100 feet of length equivalent and since you have very little main it would be a fraction of that.
Your local pressure drops for 3/4 inch piping would be at most for the furnace would be 0.2 psi per 100 feet and you have significantly less than 100 equivalent feet in that line
So, your total drop from the meter is much less than 0.2 psi (at the furnace)
I would agree that your house piping is adequate.

    Bookmark   February 1, 2013 at 1:56PM
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Thanks joeplumb. I truly appreciate your interest in my question from the beginning. Reading your post tonight is very reassuring and educating. I will work to complete the TDL to have it ready for the selected GC, as our city requires it as part of its permit process. I'm excited at the prospect of using my new range down the road. I'm very appreciative of your time and experience.

    Bookmark   February 1, 2013 at 10:40PM
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