What is an air gap and why do i need my air gapped anyway? - DIY Chatroom Home Improvement Forum

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What is an air gap and why do i need my air gapped anyway?

Posted 10-09-2009 at 05:20 AM by faucetman886

Well itís happened. I learned something today that I had never known before. Actually, if the truth be known, I learn something everyday even when Iím not trying. Frankly I had wondered many times what that little silver air vent thingy on my sink was but never had to fix or replace it. I guess I really never understood its function. Itís called an ďair gapĒ and I always thought it was a form of device which allowed you to add vent capacity to an area or appliance where there hadnít been one or maybe where you had added something new like a dishwasher. The air gap has no moving parts, and they never go bad, ever. It's always what connects to them that causes issues.
So I ran across a forum discussion yesterday that not only gave me the answer but actually put me into sensory overload with WAY TOO MUCH INFO. So I thought I would clear up the confusion for those of you out there that might have been with me in the same boat of ignorance but am going to try to give you the shortest most concise answer but unfortunately there is no short answer. There were 16 different answers in the forum as an example. No wonder I never understood what it was and my head is still spinning.
So letís start with the simplistic example of why the air gap is needed:
ďYou now live on the first floor of a high rise. You have just started to clean supper dishes and decide to run some potato peelings down the drain (Remind me to tell you my potato peelings story on a Christmas morning, sometime). You fill your dishwasher and start the cycle and walk away. MEANWHILE, every one of your neighbors are doing the same thing above you. AT THE SAME TIME, there is a water break in the 12 inch water main in front of your building. You do not know that the drain line is backing up into your kitchen sink. You do not know the water supply has dropped in pressure so that the lines are pulling air where they can from the negative pressure.(even hot water supplies with recirculation pumps will do this when water pressure drops to the negative) You do not know that your dishwasher just jammed the pump. The pump being stuck with the solenoid valve now open so the dishwasher is trying to pump water out, but in reality, dirty water from the sink is flowing into the dishwasher. It does this several cycles, until, you guessed it, dirty water is being sucked into the side inlet and into the water supply lines. Thus what sits under that ugly cap is what saves lives, protects that potable water being used inside that dishwasher that sanitizes those dishes when it runs its cycle".
So why would one need an "air-gap" for a dishwasher? Because some dishwashers fill at the bottom, and contaminated water from the dishwasher could, in the scenario above, backflow into the water supply. The reality is that most new dishwashers fill from the side, above the door line. There would be no way for the water from the dishwasher to get up to the fill level it would leak out the door if the water got that high. Why would an extra air gap be required then? Just because of the same kind of crazy improbable situations as above.
A simple definition of the air gap is the space between a wall mounted faucet and the sink rim (this space is the air gap). Water can easily flow from the faucet into the sink, but there is no way that water can flow from the sink into the faucet without modifying the system. This arrangement will prevent any contaminants in the sink from back flowing into the potable water system thus protecting your life and those of your neighbors by keeping contaminated water from being siphoned into the fresh water lines. To further illustrate the air gap, consider what could happen if the air gap were eliminated by attaching a hose to the faucet and lowering the hose into a sink full of contaminated water. Under the right conditions (if the water supply loses pressure and the sink is higher than the point at which the water supply enters the house, for instance), the dirty water in the sink will be siphoned into the water pipes through the hose and faucet. The dirty water then will be dispersed throughout the fresh water system. Because of this, although improbable occurrence, all plumbing codes require backflow prevention and in most cases, especially if the code inspector doesnít like your plumbers solution, is the little air gap. If you look around your house you may even see one somewhere near your washing machine too.
Still confused? Yep me too, but know that codes are strange things and have been developed over many years of improbable situations. I think of the warning on the Preparation H box ďfor external use onlyĒ because somewhere along the line some poor guy has eaten several of them and got no relief from his hemorrhoids.
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