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When turning the hot water on at the bathroom sink or tub on the second floor, as soon as the water comes out hot, it starts to spurt for a second or so. Is there air in the line and, if so, how do I clear.
What kind of hot water system do you have?
Electric, gas, or boiler?
Damaged or worn washers in the faucets can allow air to back feed when the water is turned on down stairs and the pressure drops at the up stairs faucets.
It would be very helpful to know what type of system you have. Would you mind posting photos?
Since the spurting doesn't happen until the water starts to get hot, I would suspect the pressure relief valve in the water heater, but I'm not sure how you would get enough Motomotive Force (MMF) to cause a vacuum at that point. Also, it would affect all hot water faucets.
If you have a on/off valve at the cold water inlet to the water heater, this could be drawing air into the system, and there would be enough MMF for this to happen, but again, it should affect all hot water faucets. But it might be possible that if this was the case, when you turned on a downstairs faucet, the air would bypass the faucet and head for the highest point in the line. It would collect around the upstairs faucet. If this were the case, you would get a hiss every time you opened the faucet upstairs before the water started coming out. You might not notice it because it might not spurt, but when the the hot water arrives, it has air mixed in it from the top of the water heater and you get some spurting.
If you are on a well, you might have a problem with a check valve in your well but this should also show up with the cold water and the spurting should occur as soon as you turn on the faucet but go away after a couple of seconds.
If you are on city water and their water treatment plant uses aeration, then the water supply will have dissolved air in it. It won't affect the cold water as much as the hot water because the air will tend to migrate out of the water and to the top of the hot water tank as it is heated. If this is the case, then there is nothing you can do about it. In fact that can be a little helpful as it would moderate the pressure in the hot water tank that occurs from the expansion of water as it heats up.
I'm with Jack on this one: start by checking all washers, connectors, shut off valves and all other valves.
Checking means taking apart, check, replace if needed and reassemble. Every faucet in the house, inside and outside, starting at the water meter.
Second option is to let it be.
Pipes expand when hot water passes through them. This does not happen with cold water. This causes noise which is very irritating.
OR the hot water heater HWH could be set too high and boiling the water causing steam to form. That would also put a gas into the line.
Check the temp on the HWH. While you're at it this may be a good time to flush out the tank. If too much crust forms at the bottom of a gas HWH, then there will be boiling water under the crusty layer.
Install an expansion tank on the cold side of the water heater if one is not already in place. Check if you have a circ. pump it could be causing it also could be softener etc.. lots of stuff not enough info to give you all the advice but if its associated with hot water it more than likely has something to do with the water heater and if it is creating pressure then a expansion tank would fix it make sure cold side not hot!
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