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Brian, Architect
Category: Plumbing
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Experience:  Licensed Architect- 17 years
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How to clear the air in pipes of the hot water system?

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My water heater is putting air in the hot water system when faucet is first turned on. Letting the water run the air stops but returns if the hot water is not used for a while.

Hello Customer,

Welcome to Just Answer.

Sorry to hear about your problem.

There are only two ways I know of to get the air out; push it out by increasing the water velocity, or open the pipe and release the air. To push the air out you need to temporarily increase the water velocity to the point the water "sweeps" out the air bubbles. To increase the velocity you need to turn on as many water outlets as possible. That creates a high water demand and the water velocity goes way up. As the water rushes through the pipe the trapped air is swept along with it and out of the pipe. Turn on all the faucets in the house and then flush all the toilets. Again, give it a few minutes to push that air out. If you know where the water supply comes into your house turn off the faucets starting with the one closest to the water supply entry point, then close them one at a time moving away from the entry point. As you come to a toilet when you are moving through the house turning off faucets, flush it again, then wait two minutes before closing the next faucet. Don't forget the faucets on the outside of the house. If this does not help, try the next option.

If the air can't be pushed out, you will need to find where the air is trapped in the pipe and "open the pipe" to release it. Air rises above water, so the air is likely trapped in a high spot in the piping. If you can identify a likely high point turn off the main water shut-off valve and open a faucet or valve to release the water pressure. Then cut the pipe at the high point and install a tee on it with a small valve on the tee outlet. A compression type tee may be easier to install. A 1/2" valve, or even a smaller one if you can find one, will work fine for the valve. Do not use a gate valve, since they tend to leak easily. Ball valves work good. See the drawing below. The valve needs to be on a short nipple, a few inches above the pipe as shown. Close the faucet and turn the water back on. The air will rise to the highest point which is the short upright nipple under the valve. You can then open the valve just a little bit to let the air escape. Some water is going to come out too, so be prepared for it to squirt! After releasing the air put a plug in the outlet of the valve for safety.


If you need further help or clarification on this answer, please do not hesitate to reply to this post.

Thanks for asking your question at Just Answer.

Best of Luck, Brian

Please leave feedback at your convenience.

Customer: replied 7 years ago.
Thanks for the answer, however, the problem is only with the hot water. I have bled the system for as much as ten minutes using every hot water faucet in the house. After, bleeding the system until there appears to be no more air, I turn the faucets off starting with the ones closest to the water heater. When I turn on any hot water faucet after as little as 20 minutes later the problem has returned. My cold water shows not signs of air at any faucet. Is it possible for something in the hot water heater that could cause air to enter or build up inside without coming from the cold water source?
Do you have well water or city?
Customer: replied 7 years ago.
Well water with a green sand filter tank and a water softner.

Do you have an expansion tank on the cold side, before the heater?

Is the water heater sitting directly on concrete?

Customer: replied 7 years ago.
If you mean a pressure tank, yes there is a 40 gallon pressure tank just after the pump, tank is less than a year old and the pressure bladder is holding at the original air charge of 32 psi. The water is sitting on the concrete floor of the garage. There is also a heat recovery system connected to the AC condenser that has the feed and return attached at the bottom (drain port) of the tank.


This will not be that easy to find. There are many things that can be causing this and you will need to try each thing to see if it does the trick. I will give you a few things to try, but there may be some items that I miss, simply because I cannot see the entire setup. Air can get into a plumbing system in any number of ways: a water main break, a repair process in your own home, dissolved air within water, etc.

You may need to replace the anode rod, since they have been known to add air into the water.

You may need to verify that the well is working properly, since the pump and/ or pressure tank could be allowing air into the system. You could have air in the cold pipes as well; it is just not as noticeable, since heated water expands and can release the air much easier. To test this, you could fill a cup with cold water and see if there are any water bubbles; if there are this would be a sign that the air is entering from the well pump or somewhere else in the system, prior to the heater. The heater is just exaggerating the air in the water, as I discussed earlier.

Another possibility, if this is an electric hot water heater, is that the high wattage elements could possibly be electrolyzing the water into hydrogen/ oxygen. If you can separate the water heater from the concrete floor with a piece of plywood or something similar, you can help to electrically insulate the heater from the concrete floor. Electricity could be leaking somewhere to ground, causing current to pass through the water and electrolyzing the water.

If you need further help or clarification on this answer, please do not hesitate to reply to this post.

Thanks for asking your question at Just Answer.

Best of Luck, Brian

Please leave feedback at your convenience.

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