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3/4" gas line on LP boiler required?

GW
GW Member Posts: 4,542
I heard years ago an LP high efficiency boiler needs full 3/4", not quite sure if it was a tale or not. if the pressure drop is under an inch, does one still need to bump it up to 3/4" ? It's a hypothetical question, so if you ask mt 'how long', lets go with '10 feet of gas pipe', and, lets go with '80,000 BTU boiler'. Thanks for your comments

Gary Wilson
Wilson Services, Inc
Northampton, MA
[email protected]

Comments

  • DZoro
    DZoro Member Posts: 1,048
    Todays boilers are relaxing on the minimum 3/4", if you can meet normal requirements. Enough btu for your max load. Lochinvar wants a minimum of 10' from your regulator. Absolutely no more than a 1" drop at anytime. I still prefer a minimum 3/4" but that's just me.
  • Ironman
    Ironman Member Posts: 6,869
    Well...it depends. The line supplying the appliance cannot be any smaller than the connection on the unit. So, if the manufacturer provides a 3/4" connection (like HTP does), then that's the minimum size line you can use.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • GW
    GW Member Posts: 4,542
    Bob, thanks, but that’s my question. Is it “Because someone told me and that’s what I’m going to do”, or is there a reason? The LP tables can move a lot of fuel with 1/2”. Just hoping to get into the why and not the because. Thanks.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • GW
    GW Member Posts: 4,542
    Dennis, thanks, I’ve never heard of the 10 foot min so that’s good to have in my back pocket.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Ironman
    Ironman Member Posts: 6,869
    That's code about not reducing the line size to be less than the connection size.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • ratio
    ratio Member Posts: 3,183
    Most of my experience is with nat gas, so take this with a grain of salt, but regulators can only do so much. They are more sensitive to inlet pressure than people seem to think. If your inlet pressure doesn't stay within an inch or so of what it was when you dialed in the outlet pressure, you're going to drift away from the setting. If there's only one appliance on the gas line, you're sorta ok as you can adjust for the sag, but if there are other appliances on the line your inlet pressure can vary, sometimes greatly; especially if the lines are sized for maximum pressure drop i.e. smallest size we can get away with. </rant>

    Do yourself a favor & size via the ½ tables.

  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221
    I have been told that the negative-pressure gas valves on the appliances don't function correctly with more than 1" of drop in gas pressure. Could cause issues with ignition.
  • GW
    GW Member Posts: 4,542
    Ratio, yes, this job---the high and low o2 numbers have walked more than normal on us, that's why we suspect the LP service. But my main question is 1/2" on an LP boiler that has a 3/4" connection. I have a boiler kicking an occasional miscellaneous code. there is a 5' drop of 1/2" black, then of course back up to 3/4 at the boiler. pressure drop from no fire to high fire is .3". So, if it's a meager .3 drop, would you think, "it can't be the 1/2'" or, would you think, "well you should have put 3/4' in, becasue you're supposed to".

    Thanks- gw
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • GW
    GW Member Posts: 4,542

    I have been told that the negative-pressure gas valves on the appliances don't function correctly with more than 1" of drop in gas pressure. Could cause issues with ignition.

    Thanks Harvey, yes, I thought 1" max was the golden rule too.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • ratio
    ratio Member Posts: 3,183
    Any other appliances served from this line? Maybe you occasionally get a perfect storm when everything tries to light off at once.
  • GW
    GW Member Posts: 4,542
    I guess it's possible but I'm quite positive there are jillions of homes where the gas piping is undersized and it all works well.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    It has been my experience from feedback from a lot of the LP suppliers and some of the Mod?Con boiler folks that having the second stage regulator to close to the equipment can cause a reaction with the negative pressure gas control. That and the first stage regulator real close to the second stage and then the second stage very close to the gas control has caused problems. The problem being the negative pressure regulator in the ga
    s valve reacting off the diaphragm on the second stage regulator. We have also had this happen with gas generators on LP.

    Solution is at least a minimum 3/4" gas line at least 10 feet long placed ahead of the gas valve.

    My communication with some folks and two reps from NH and Vermont had these problems way back when Mod/Con first came out.
    DZoro
  • GW
    GW Member Posts: 4,542
    Tim, Ok will do, it's simple enough for us to re-nipple the thing. Just trying to get my head around the dynamics.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • lchmb
    lchmb Member Posts: 2,997
    it almost sounds like your regulator isn't reacting fast enough. I just repiped a modcon (530K). It was piped 15 ft with 1" pipe. Reading were all over the place. The problem wasn't the length it was the street 90's used. Restricted to much flow and reading were all over. Sized to 1 1/4 and solved all the issues.. Another problem I found. 2nd stage indicated it was sized properly but... that was with 10lbs in. In this case the first stage was never set and was only at 7lbs..