Gas Fireplace Shuts Off

I asked this question a couple of months ago but I'd like to restate my problem and make sure I get it right. Installed unvented gas logs two years ago. The line for it originates between the gas furnace and gas water heater, about a 25 foot run with I think 5/8" copper, but maybe 1/2". I'm not at that house right now. Shut-off valve to the right of the fireplace. The OEM gas line running directly into the unit is small, so it seems 1/2 to 5/8 should be enough to feed it. But when the furnace kicks in, or the water heater, the fire gets sucked away and we have to relight the pilot also. It seems the easiest solution would be some sort of inline valve that prevents that backflow when another appliance kicks in, but I'm not aware of such a thing? Do I simply need still larger tube to the fireplace? Anybody experienced this? Thanks for any suggestions.
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What's the gas line pressure?
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I think I was told it was about 4 lbs. That sound in the ballpark?
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I haven's seen a fireplace in a ballpark.
It sounds to me like you need a professional technician to diagnose and resolve your problem(s).
You shouldn't be messing with stuff you don't understand. You might blow yourself up.
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So memory fails regarding pressure. The pressure was tested and deemed adequate before the install.
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Real simple, pressure has little to do with it, since the appliance will have, or BETTER have a regulator in it that drops it way below 4lbs.
You cant just go slapping gas lines in on existing, since most of the time, the builder, or installer will size the line for the volume needed for what is on the line, period. Gas lines must be sized for the demand, and it sure sounds like yours are not. Volume...not pressure is what matters most. Pressure is handled by the regulators, volume is handled by the size of the lines. Most, are undersized in the futile attempt to save a few bux.
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Again, I have no experience other than cooking with a gas stove, 20 years ago, but I'm trying to think like a stream of gas. Indeed, i AM a stream of gas. (jokes expected, but please, no really vulgar ones.)
Although it looks like the flame is being sucked in, I suspect that it would look that way even if the gas pressure dropped a lot. it wouldn't be necessary for gas to flow the opposite direction, so I don't think that a backflow preventer (if there is such a thing) would help.
Think about it. If one stops the gas pressure, the flame which was burning gas an inch from the gas jet will burn that gas up, and then it will start burning the gas that is closer to the jet until it burns all the way down to the jet (where iiuc it won't burn what hasn't come out yet because there is no air to burn it with, or the heat is dissipated by the pipe, and it isn't hot enough, or something.)
This is going to look like the flame and the gas it is burning is sucked in, but until someone says that is possible, I'm thinking my description of what happens would look like what you see.
Remove NOPSAM to email me. Please let me know if you have posted also.
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Houston wrote:

That sounds like a vent problem. I suspect you don't have enough makeup air coming into your home to allow venting of two or more appliances at a time. Has anyone in your home complained of head aches?
If you don't already have one or more working CO DETECTORS, get a couple at once! Have someone who knows what they are doing check out your venting. Don't use the fireplace or at least open a window close to it, anytime you use it.
--
Joseph Meehan

Dia duit
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There are several CO detectors installed in nearby locations and we haven't had a problem there, but the vent suggestion may make sense. I couldn't guarantee that my folks open a window (or open it enough) each time they use the fireplace. My concerns was -and is - that by tapping into the gas line between the furnace and the water heater, I've lowered the usable pressure on the long run to the fireplace. It's got just enough to burn normally until there is the slightest draw from another source. I assume the furnace is the biggest draw, and by tapping after it, I lose the pressure when it comes on for that initial flare burn. At least that's been my working hypothesis. But I see that a lack of vent (oxygen) might have the same effect. I guess "sucked" was the wrong terminology. It just goes out. Once the furnace has ignited I can relight the pilot and logs. I'm being very cautious indeed, would like to be able to finish what I started and getting working consistently right. Thanks
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