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Any natural gas experts out there?

Any time I am away from home for two days or longer, my stove takes 30 seconds to light. After that it works fine day after day after day!

Another similar problem is with my high eff modulating / condensing boiler every year at first fire-up. The boiler fails with error for no ignition once or twice, then lights but woofs for 30 to 40 seconds. Then it ignites normally every time all winter.

I cannot find any reason posted anywhere but I theorize that the NG in the line goes stale rather quickly (two days) leaving useless gas in the pipe.

ANY other suggestions?

  • to HJ: I realize that ng pressure post meter is generally 7 inches H2O/hg. Though that is very low pressure, it is a positive pressure and therefore outside air/moisture cannot enter even if there were a small leak. – Larry Birt Dec 25 '14 at 3:50
  • I can hear the gas coming out of the burner but it does not light immediately the first time if not used for a few days. Just puzzling! Always, after that first delayed lighting all burners light in less than one second; like on the first or second spark of igniter. – Larry Birt Dec 25 '14 at 3:53
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    Is it possible that the actual ng is heavier than the odorants or other additives and settles to the lowest point of pipe run leaving only non-combustibles immediately available to the appliance? – Larry Birt Dec 25 '14 at 3:59
  • I still think the gas is getting out somehow and being replaced by air. Just because you hear something doesn't mean it's gas coming out. Have you had your gas lines pressure tested recently? I would call the gas company and see what they say about the situation. They may be willing to do a quick check for free. – Hank Dec 25 '14 at 18:23
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    HJ, are you for real? The gas lines are under constant positive pressure. In order for air to get into the pipes the outside atmospheric pressure would have to be consistantly greater than the pressure inside the pipe! – Larry Birt Dec 26 '14 at 20:56
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The short answer is no, natural gas does not go bad in any reasonable amount of time.

I wonder if you have a slow leak somewhere that is letting gas out, or air/moisture in.

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You haven't mentioned it, but are you closing the shut-off valve for the stove and/or the shut-off valve for your furnace while it's not being used for these few days or longer time periods?

If that's the case, then that could explain what you're experiencing. When you close one of the shut-off valves, the natural gas on the supply side is under pressure but is sealed off.

What might be happening is the gas in the appliance, and the gas in the line on the appliance side of the shut-off valve could be dissipating, or being consumed by the pilot. If neither of these appliances has a (traditional) gas pilot, then it also means there is likely a leak in those appliances or gas lines, although probably a very small leak (small or not, a gas leak should not be ignored).

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I have also experienced this occurrence, otherwise-quick-to-respond gas appliances (stoves, but especially water heaters) becoming slow to ignite after not being used for several days. It happens whether or not the gas shutoff valve is closed, though the effect is stronger/quicker when it is. Hank's suggestion above is actually correct, this occurs because a portion of gas inside the terminal end of the pipe (after the last branch into another house) gets slowly replaced with air. It is true that pipes are under positive pressure, but that does not preclude diffusion into and out of the pipes; it just means diffusion out of the pipes occurs at a higher rate than in the reverse direction. So, if you have an even very small leak from some valve of an appliance that doesn't close completely, this will allow for some gas in the pipe to be replaced with outside air. Since outside air has essentially no natural gas, this means that, over time, unflowing gas in the end of a pipe will be gradually depleted and replaced with air.


[Editors note: converting second answer to an edit]

Thanks Daniel, for your welcoming reply. Just to extend on my previous answer, and as i don't possess enough score to comment yet, i'll note here that you are right that a properly-sealed system will not allow for gas/air exchange; however, i assumed in my answer, as you can see, that there will be some imperfect sealing somewhere along the way (ie, after the shutoff valve), as it most often is the case, mostly in the appliances themselves rather than the actual pipes. In case anyone is curious, i'll add here that when you have two reservoirs of static gases (such as the atmosphere and the pipes), the ratio of diffusion rates of each gas into the other reservoir increases exponentially (roughly) with the pressure difference between the reservoirs. So if Rg is the seepage ratio of gas out into the atmosphere (in units such as molecules/second, or equivalent), Ra is the infiltration rate of air into the pipes, and Pg is the pressure inside the pipes while Pa is the atmospheric pressure, then:

Rg/Ra = K^(Pg-Pa)

Where K>1 is some constant that depends on the mechanical properties of the gases (their respective molecular weights, etc). This is of course a very rough approximation, as both natural gas and air are a mixture of different gases with different properties.

So when the inside and outside pressures are the same, you get equal rates of diffusion in both directions, as you'd expect, but diffusion into the pipes doesn't cease as soon as there is positive pressure in them; rather, it slows down dramatically, but it is still there. Since the gas coming out dissipates immediately in the atmosphere, everything that goes in is air, which slowly but inevitably replaces the gas inside.

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    Hello, and welcome to Home Improvement. I find it hard to believe that a properly-sealed piping system would let air and natural gas swap. But, I'm no expert... – Daniel Griscom May 11 at 21:05
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The only valve closed is the burner control; the burner tube itself sits directly over the orifice at the valve output.

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