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Gas Pipe Question

Hi. I posted this in the gas section but hopefully, someone here might also provide help.

I had a tankless water heater installed.

However, I believe that there is a gas issue because when the furnace turns on while the shower it on, the water will turn cold. I am assuming that the furnace takes too much of the gas off the main gas line and there is nothing left for the water heater.

The size of the main gas line is 1.25''...then branches out to five different 1/2'' lines....one goes to the stove, one goes to the furnace, one goes to the tankless water heater, etc. The tankless water heater requires a 3/4'' line. This is achieved because the 1/2'' line turns back into a 3/4'' line.

Given this setup, is there enough gas pressure to the tankless water heater?


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,335
    I think...

    ... you answered your own question.  Your heater is supplied by a 1/2" line when it needs 3/4".  You have two ways to verify this.  One would be to start with all the gas usage info from the different appliances, plug that into pipe length and size and see what a sizing calculator tells you.  Another way would be to measure gas pressure at the tankless unit with different things running.

    I'd be thinking about running 3/4" pipe back to your main line if it isn't a long run.  Otherwise it might need to be stepped up a size.

    Yours,  Larry
  • TeachMeSteam
    TeachMeSteam Member Posts: 128
    Thank you

    Thanks, Larry.

    That's exactly what I needed to hear.

    Now, I know the contractor screwed up. I'll probably just fix it on my own dime.
  • lchmb
    lchmb Member Posts: 2,997
    make sure

    to either have a pressure test done or get some sherlock leak check to test any fitting's. It propbably would be a good idea to have a flow test done to make sure your getting the necessary pressures with all units running ...
  • TeachMeSteam
    TeachMeSteam Member Posts: 128

    Will do exactly what you say...
  • Jack
    Jack Member Posts: 1,047
    While 3/4" may be

    What you connect to the appliance be certain that it can handle the length of the gas line. I don't have a book in front of me, but I believe that 20' of 3/4" black pipe can handle 180k btu. Count fittings, etc and you can be in a jam. There is no substitute for a digital manometer here...but...if you know the units design output at a specific temp rise and your incoming wter temp, you should be able to read the water flow and see if it is within a couple tenths of gallon of the "design numbers". Once you are certain that the numbers are within spec you can begin operating other appliance and see if your system is supplying appropriately. The number one mistake in the installation of tankless water heater is inadequate gas lines
  • icesailor
    icesailor Member Posts: 7,265
    Gas piping:

    If you called thge manufacturer about your problem, the first thing they would ask is the heater connected full size from the incoming gas main, and is it properly sized. If you told then that it is connected to a 1/2" line and increased to 3/4" to the appliance, they would tell you to get that changed first and then call them back.

    Unless it is LP gas, and it is a short run, you can't get away with it.

    Does anyone read an installation manual any more?
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Does anyone read an installation manual any more?

    I do, but that is because I find them really interesting. I am more interested in figuring out why they say to do something than just how to do it. Since I am not a contractor, I do not really have to know how to do any of this stuff. Unfortunately I have already run into contractors who know it all (and maybe they did in 1950 and have not kept up) and do not have to read the manual. They also do not need digital combustion analyzers because they can tell if it is adjusted right by the color of the flame through the peephole inspection port. I have looked at color pictures of what my burner should look like at 20% firing rate and at 100% firing rate.

    This is for the Ultra-2 series. Picture on page 2. The controller described in there is obsolete; current controllers are much different from the one described.


     They are quite different. Not much seems to be happening at 20%. I could tell if it was firing or not, and if the flames were seriously yellow (gas burner), but how I would tell 60ppm or less of CO  (good) from 60.6 ppm or more (bad) I could not. It gets so sometimes I want to get a combustion analyzer and stop worring about technicians. Trouble is, perhaps I could do the measurements correctly. But if they were off, the only thing they tell in the book is how to adjust the mixture screw. And if that did not do it, I would have to get a pro anyway. Also If I test the burner twice a year, and the sensors go bad every year or two, the cost would be impractical. Those analyzers are meant to be used frequently, and I would not.

    I have figured out how to measure the pH of the water with a pH meter; I do not get accurate results with the test strips. Even with the meter it is tricky, I found out. If I calibrate it with buffer solutions at one temperature (say room temperature) and then test the water from a running boiler at another temperature (say 100F), I get seriously bad results. And If I let the water cool down to room temperature, that does not help much because the water absorbs CO2 from the air and becomes more acidic. I now test this in the summer when the boiler is not running much (indirect hot water only).
  • icesailor
    icesailor Member Posts: 7,265


    I wasn't referring to you. I was referring to the  same "technicians" that you were writing about. The ones who do installs and they (the installs) end up on HeatingHelp.com for all of us to see, They have no shame. Nor a sense or responsibility.

    You, on the other hand, should be hired out by some of the dubbers to read the manuals and advise them. But I fear that they would still not listen.

    When you take a water sample, use a test bottle with a tight cover. Take the sample and put the cover on. Let the sample cool. Then take the test. Covered, the sample can't absorb CO2. Pool filter kits will keep you in the ballpark though. I've owned a few PH pens and they all died from lack of consistent use. And you need the buffering solution to calibrate the instrument.

    Again, I don't think that anyone here would consider you a non manual reader. I always look forward to whatever you have to say.
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