I recently bought a house and have a problem with the hot water in my
upstairs guest bathroom. When I turn on the hot water, it will flow
normally for a second or two, then the flow will drastically reduce
and a knocking sound occurs. When I turn off the water, the noise
stops. I can then turn the hot water back on and the same think will
occurr again. I was told it could be air in the pipes, but I'm not
sure this is the case. Either way, I'm not sure how to remedy the
problem. My main concern is that there is a leak and that there is no
hot water for my guests in the sink. I also thought it was strange
that it was only this sink and only the hot water. Any suggestions?
The knocking could be a water hammer effect. Do a google search. These
usually happen when flow stops abruptly. The flow-stoppage could be
occuring due to a bad washer. I would replace the washer in that hot
water valve and see if that fixes it.
Some older houses had "air trap" pipe extensions that were supposed to
hold a buble of air in a capped off pipe segment, and the air is
relatively compressible so would damp out any sudden flow stoppages.
These days it's considered better to put in water-hammer suppression
fittings. Again, google will help.
It is more likely to be caused by no air in the pipes than air in the
The knock could be water hammer and you may need to turn off the water
at the main then open all the water valves (make sure the hots are open) and
let them drain. You may take this time to drain a couple of gallons of
water from your hot water heater as well (do this last and turn off the
heater's thermostate before turning off the water)
Now when that does not do it (it may and it is cheap and easy to do so
it is a good place to start) I am guessing it is an older fixture. If so I
suspect a loose washer.
Check the valves on that hot water line.. If you get a noise when the
water is running the chances are high that you have some FOD (Foreign
Object Debris) in your line. It could be in the shut off valve under the
sink or other.
Water hammer will occur after the water is shut off.
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