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low inlet gas pressure

zepfanzepfan Posts: 247Member
i have a problem on a 110,000 btu/in furnace while checking it i found only 2.4" manifold pressure.adjustng the regulator in the gas valve did nothing.i checked the inlet pressure with the burner off,and found it to be 5.5".as soon as the burner fired it dropped to 2.4".so obviously there is not a problem with the gas valve.i always thought that the min. inlet pressure should be held when the burner fired,and not drop that low. this was the only gas fired appliance firing in the house when this test was being done.the system has csst,running from a manifold in the basement with a main pressre regulator attached to it.i have not adjusted that yet,and did not want to for fear of harming other appliances connected to it.any suggestions before i call the gas company to correct this low gas pressure issue would be appricated.it is about a 30' run from the furnace to the manifold,and it is ran in 1/2 csst piping.thank you

Comments

  • JStarJStar Posts: 2,668Member
    CSST

    1/2" of CSST gas pipe is only good for 55,000 BTUH. You need at least a 3/4" line, but I would use 1".
  • icesailoricesailor Posts: 7,265Member
    edited October 2012
    1/2" CSST:

    Did the boiler installation manual say to connect "Full Size" from the source to the boiler where it may have reduced to 1/2" NPT going into the gas valve?

    The fact that the pressure at the valve dropped when the gas valve opened usually means that the line has too much restriction. And CSST is LOADED with restriction.
  • dondon Posts: 395Member
    edited October 2012
    2lb distribution system.

    I would check pressure before the regulator before calling the gas company.Also lots of time you would have problem with the other appliance if they to are off the same regulator and manifold.I had a house not to long ago on 2lb distribution system that was tied into a low pressure regulator and gas meter.The house was 9 years old and no one ever caught it until we went and replace the furnace that was eaten alive by condensation.Funny how gas equipment will fire and run when they have a hot surface igniter and inducer on them.This house had five other gas appliance on them and he claimed he never had a problem with any of them.After we got it all square away he called months later to say his gas bills are lower and now he get great hot water and dried cloths like never before.Just go to show that there is something to be said about firing at max.
  • zepfanzepfan Posts: 247Member
    a couple of more things

    thanks to everyone's help with this issue,i have had no internet access the last few days and therefore could not respond.what was the solution before you replaced the furnace? did you just have to adjust the regulator to get the inlet gas pressure up? also what table in nfpa 54 do you use to size the csst tubing that runs from the manifold to the furnace,i am looking at table 12.14( .5 psi system at .5 pressre drop) and it shows at thirty feet with an ehd of 19, 1/2 csst would be good for 55 cubic feet.this would mean that the pipe is too small for a 100,000 btu furnace.please let me know if i am using the wrong table,i do mostly service,and have not had to size csst tubing for a brand new installation.also the ehd number is that always printed on the csst? thanks to everyone again.
  • zepfanzepfan Posts: 247Member
    a couple of more things

    thanks to everyone's help with this issue,i have had no internet access the last few days and therefore could not respond.what was the solution before you replaced the furnace? did you just have to adjust the regulator to get the inlet gas pressure up? also what table in nfpa 54 do you use to size the csst tubing that runs from the manifold to the furnace,i am looking at table 12.14( .5 psi system at .5 pressre drop) and it shows at thirty feet with an ehd of 19, 1/2 csst would be good for 55 cubic feet.this would mean that the pipe is too small for a 100,000 btu furnace.please let me know if i am using the wrong table,i do mostly service,and have not had to size csst tubing for a brand new installation.also the ehd number is that always printed on the csst? thanks to everyone again.
  • zepfanzepfan Posts: 247Member
    more about the low inlet pressure

    thanks to everyone's help with this issue,i have had no internet access the last few days and therefore could not respond.what was the solution before you replaced the furnace? did you just have to adjust the regulator to get the inlet gas pressure up? also what table in nfpa 54 do you use to size the csst tubing that runs from the manifold to the furnace,i am looking at table 12.14( .5 psi system at .5 pressre drop) and it shows at thirty feet with an ehd of 19, 1/2 csst would be good for 55 cubic feet.this would mean that the pipe is too small for a 100,000 btu furnace.please let me know if i am using the wrong table,i do mostly service,and have not had to size csst tubing for a brand new installation.also the ehd number is that always printed on the csst? thanks to everyone again.
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