When I turn on the hot water in the guest room shower, after about 5 seconds, it creates a loud knocking sound. This lasts for about another 5 seconds and then the shower rod fixture drops back down - the hot water continues to run from the bathtub faucet with no issue. Interestingly, the water coming out of the faucet/shower head is not as hot as it is in the rest of the house.

-House built in 1963, all one story.
-Temperature outside 45-60 degrees.
-This is the only place in the house it does this.
-Only happens for hot water.
-Hot water heater turned up to 130 degrees

If I turn down the pressure by 40-50% at the faucet, the shower will work with luke-warm water coming out. I can get full shower pressure with cold water. Any idea on what might be causing this? Should I drain the pipes and try it again? Can't figure out why it only does this for hot water, one faucet, only when it's turned on, and water temperature is less than all other faucets. Confused...

Thanks in advance for the help!

"the water coming out of the faucet/shower head " how does the hot water compare between the sigot and the shower head? I am thinking your mixing valve is the issue, one knob or two?

Both the shower faucet and shower head produce luke warm water. The weird thing is, the sink right next to it produces hot water. The shower is adjusted using a small "arm" to adjust between hot and cold.

I think we have hard water in our house - so is it possible a valve could be clogged with sediment?

It seems like it could be a pressure issue of some sort, but the luke warm water thing is really frusturating.

"valve could be clogged with sediment" Gets my vote. Shut off the main and remove your cartrdige or more likely mixing ball. Clog is primarily on the hot supply line. Do you periodically flush your water heater, do you know how to?

Bob said:


Shut off the main and remove your cartridge or more likely mixing ball.

I think he forgot to say, "Turn on you water briefly to flush out the valve."

LOL, didn't forget, was waiting to see if RangerFan was game for the project. Can't give you a greenie for it but it made me smile

That did it! You guys rock! I took apart the faucet and cleaned the cartridge and manifold - there was some sediment caked on there. Also, discovered the hot limit stop was turned turned too far counterclockwise, so I adjusted that as well. I think the cleaning fixed the water pressure problem and adjusting the limit stop fixed the luke warm water issue. Super excited it works as it should now. Thanks again!

OK, great but answer Bob"s question.
How long since you flushed your water heater?

Every time my kids take a shower! No, actually, about once every 8 weeks.

I still get the occasional water hammer when I crank the hot water in the shower though - again, seems like a pressure issue - almost like the pipe's capacity can't hold that much water. Do you think this is an issue with the pipe or some type of valve problem?