Gas Line Improvement required for tankless water heater??

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Old 08-01-13, 01:26 PM
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Gas Line Improvement required for tankless water heater??

Hi,

I had a Navien tankless water heater put in about 3 years ago. It started to have problems and flashing an error code. I called out a plumber to take a look at it after I tried a few things. Long story short, it was a bad flow sensor (second one) and problem was solved after I replaced the flow sensor.

However, when the plumber came out he said I needed to increase the gas pressure to my house from 1/2 psi to 2 psi because the tankless unit was not receiving enough gas because the pipe was too small. The original installer is no longer in business and did not mention any of this. So I started to do some research and ask some questions.

So after measuring all of gas lines (length of pipe and width) I'm still not sure if I need to increase the pressure in my house. He said that they increase the pressure for every tankless unit they install because of the increase in the BTU required by a tankless unit.

Based on my calculations and understanding of the pipe codes, I'm fine up to a point (A in drawing). It's a 1-1/4 line that runs 64'. It's after it splits that it gets confusing. The line to the water heater is a 1" line that runs 30' from the 'T'. So am I suppose to assume that the whole run is 1" pipe for 100'? That seems counter-intuitive since over half of the run is on 1-1/4" pipe. There's also another appliance (furnace) that splits off of that 1" pipe that complicates things too. It's about 40' from the 1-1/4" 'T'. So part of me thinks that I can use the capacity of a 40-50' 1" pipe instead of assuming 110' (total run) of a 1" pipe. Assuming of course, i don't exceed the capacity of the 1-1/4" line.

Sorry for such a long winded post. Pictures say a thousand words. So here's my drawing. Thanks for any help or suggestions!
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Old 08-01-13, 03:28 PM
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Your 1 1/4 line supports 380k btu at 109 ft...This is bases on a standard meter and table 402.3(2) of the IFGC.

You must stay in the 100 ft column when doing the calculation in the chart...

If anything you would be starving the furnace 2 for gas when the water heater fires up... Or the heater if furnace 1 is on... This could cause sooting of the heat exchanger and CO issues/drafting issues... in either the furnace 2 or water heater...

So yes they should not of tapped the heater there or anywhere but the meter like the PH... You dont have enough capacity...

Your best bet is to have them increase the pressure...

Or increase the 1 1/4 to 1 1/2 some distance... I have not calculated that but you increase the btu to 590 K...
 
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Old 08-01-13, 07:46 PM
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Thanks for the help. It makes sense why I have to stay in the 100ft column of table 402.4(2). So basically the whole system won't support more than 380k btu? So moving the heater line before furnace 1 won't make a bit of difference?

Unfortunately, everything before point A is either buried in the ground or inside a wall. All of the furnaces and heaters are in a second floor attic. So the first possible tie-in is the 3' line right before point A. So it sounds like increasing the pressure is my only option. Good thing it's warm in Texas so Furnace 1, Furnace 2, and the water heater rarely run at the same time. I guess if only 2 of 3 of those things are running i shouldn't be causing in damage? If so, I guess the kiddo doesn't get heat this winter...

Thanks again.
 
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Old 08-01-13, 09:49 PM
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you could run a homerun from the meter to the heater I believe... Not sure... I never ran more that one extra appliance from the meter... It would be 1" at 100 ft supports 200k btu...

I would have to look up statistics on running multiples from meter directly...
 
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Old 08-02-13, 01:18 PM
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Okay, dumb question. This is probably due to having way too much time to think about it.

I was reading the tables and the pipe sizing section (402.4) and 402.4.1 and 402.4.2 describe the method for calculating pipe size and load. It states (402.4.1) that "The pipe size of each section of gas piping shall be determined using the longest length of piping from the point of delivery to the most remote outlet and the load of the section." It's the "load of the section" that has me puzzled. I only need 100cfh and not 390cfh at the end of the run (110') and 400cfh and the first branch and 300cfh at the second branch. So then would I use the 70' column for the section to A (64') and then subtract out the load of each appliance as I pass it?

Also, it's 3/4" pipe at the end of the run. If I use the 110' calculation wouldn't that pipe be too small for the furnace?

Thanks again for the help.
 
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Old 08-02-13, 02:13 PM
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does not mater what size pipe is at the end of the run... Its the total developed length of the longest run...

So take the total BTU of all appliances and the longest run and follow the chart... I am looking at 402.3 (2). At 110 ft you would round off between 100 and 125 ft... Thats how I got the 390 btu... 100 ft is actually 400 k btu...

I have old charts and I know the new charts are a little higher btu wise... ( Mine are all in books and not on line)

Once in the 100ft colum you must stay in that column. Then you run your pipe in the home and when you branch off then you start deducting as you go top each appliance...

By the time you get to the end and deduct the last appliance you should have close to 0 btu left from when you started with the original 390k btu...
 

Last edited by lawrosa; 08-02-13 at 04:13 PM.
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Old 08-02-13, 04:07 PM
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Got it. Yep, I definitely know my house is under-sized. I guess i was just trying to figure out why I couldn't jump around in the distance columns for my own curiosity. Thanks again for your patience and help.
 
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