dont' want to be a whooz but this is not sounding too good. When we switched out gas dryers at work, although a short run, they fired right up.

(I posted this in HVAC, but I see it could help here too)

I have seen posted before and again (gas grill post), the reference to natural gas igniting or exploding with out a spark or flame source when it reaches a certain concentration or level.

This is I guess can be added to the urban legend list.

An ignition source such as a pilot flame, spark from ignition, wall switch, CFL bulb and even static electricity is required, along with the correct mixture of gas/air.

The explosive limit of a gas or a vapour, is the limiting concentration (in air) that is needed for the gas to ignite and explode. There are two explosive limits for any gas or vapor, the lower explosive limit (LEL) and the upper explosive limit (UEL). At concentrations in air below the LEL there is not enough fuel to continue an explosion; at concentrations above the UEL the fuel has displaced so much air that there is not enough oxygen to begin a reaction. Concentrations of explosive gases are often given in terms of percent of lower explosive limit (%LEL).

In addition other conditions must be met to allow an ignition or explosive atmosphere. One of those and the most important ingrediant is Oxygen.
Oxygen (O2) must be between 19.5% (LEL) and 23.5% (UEL)

Without the correct mixture of the two above, 75% or more of natural gas and an O2 of 19.5% to 23.5% and there is no explosion. Too little oxygen just as well as too much, hinders the explosive properties of the ambient air.

That is why careful ventilation by fire departments or gas companies in some cases must ventilate with caution since adding fresh air to a structure without isolating and shutting off gas brings the two mixtures into harmony and with that only open flame or spark is required.

Most state and federal agencies have adopted the Gas Piping and Technology Committee (GPTC) standards for grading natural gas leaks.

A Grade 1 leak is a leak that represents an existing or probable hazard to persons or property, and requires immediate repair or continuous action until the conditions are no longer hazardous.
Examples of a Grade 1 Leak are:
1. Any leak which, in the judgment of operating personnel at the scene, is regarded as an immediate hazard.
2. Escaping gas that has ignited.
3. Any indication of gas which has migrated into or under a building, or into a tunnel.
4. Any reading at the outside wall of a building, or where gas would likely migrate to an out-side wall of a building.
5. Any reading of 80% LEL, or greater, in a confined space.
6. Any reading of 80% LEL, or greater in small substructures (other than gas associated sub structures) from which gas would likely migrate to the outside wall of a building.
7. Any leak that can be seen, heard, or felt, and which is in a location that may endanger the general public or property.

Caution:Do not light fires/cigarettes, and do not operate electrical appliances/light switches/phones, as these may act as points of ignition. Dont touch any thing, leave carefully from the area and call FD or 911 from a distant neighborÂs house.

Past Fire Chief.

Before making the final connection to the water heater (especially if you are using flex line) simply open the service valve and wait until you clearly smell the gas.
Close the valve, wait a few minutes, then make the final connection.

Some open windows will limit any chance of an explosive buildup.

The chances of creating a dangerous environment in a few seconds to minutes of an open gas line are VERY small.

This is handy, I'm just replacing a water heater. I hope to re-use the gas pipes if everything lines up, I was also told I can use one of the yellow corrogated stainless steel 'flex' lines to make a transition from the black pipe to the hw heater (not sure if I'll use it or not) - so we are talking about a very short connection. I should purge BEFORE connecting line to the hw heater, or after? There is a gas cock about a foot or two before the connection to the heater so theoretically, nothing'd change if I open it before it's connected...In such a short run I'd assume I'd connect it all up, with hw regulator valve closed, check for gas leaks, open up and hold the button down to allow gas flow, give it a short burst then wait for gas to dissipate, then light heater - sound right?

It might take more than a single try to get the pilot lit, but a few feet of flex are not worth purging.

Thanks, kind of what I'd figured. If the pilot doesn't light after a reasonable try and the gas is building up, I'll let it sit. I totally shave my head, but I like my eyebrows and eyelashes....you know, on gas hw heaters in Australia and NZ, they usually include a piezo ignitor, just push the button a few times. Kind of beats finding a long match or bbq lighter.

I replaced our water heater in April this year, using a flex pipe. It took f-o-r-e-v-e-r to bleed the damn thing. An interesting note. Being low income now (living in Michigan sucks, and it is going to get worse) the state did a weatherization to our house. As one guy ran an "electronic gas sniffer" down the line, everything was ok till the end of the run, my water heater. The thing found a leak at the center of the flex line. I bought the damn thing new, was VERY careful, and it was not rubbing on the floor or anything else. We hadn't fired up the furnace yet (gas circulating through house?) and never smelled a thing. SCARY!!

The gas sniffers can detect leaks that would never result in a dangerous condition, especially on an older house.

They are VERY sensitive.

I have used then on 50+ year old lines and every joint registered.