Hot water in shower slowly cooling off


Hello all, I have a quick plumbing question. I just bought an old house with a very new 40 gallon gas water heater (mfg 2006). The hot water seems very abundant at all the faucets. However, after running a hot shower for approximately 4 or 5 minutes, the water gradually gets cooler and cooler. Eventually, it gets uncomfortable and I can tap the knob (its a single knob for temperature control) slightly warmer and it suddenly is pretty much right back up to temperature for a short period of time. After getting out of the shower, the hot water coming out of the faucet at the sink is very warm but certainly not as hot as it usually gets. I replaced the shower value with a new pressure balancing Kohler unit, but it exhibits the same problem. So two valves are doing the same thing. I did some research and though that it might be a dip tube problem in the water heater. But I'm not so sure since I CAN get the shower back up to temp at least for a minute or so and its a new heater. Its seems very mysterious to me and I would appreciate any help you can provide. Thanks in advance.
Zach
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Zach,
I'd think that it is still the dip tube, that still makes sense even though you are getting the temp back up for a minute. When you initially turn on the tap, the water temp you feel is being cooled by the cold that is included in the stream. if you adjust the knob it only makes sense that with less cold now running, the water would feel warmer. Do you have any faucets with separate controls for hot and cold? try the test on those, run the hot only wide open for a while and see what happens.
good luck
Dave
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Yeah, the faucet at the sink next to the shower has separate hot and cold valves. With the hot valve wide open it still only gets very warm, but not hot after a shower. That does make sense though, the cool water flowing to the top of the water heater takes the hot water output temp down quite a bit. By turning the shower hotter and hotter we're just mixing in less cold water. But one interesting point here is that by the end of the shower, no hot water in that bathroom is ever as hot as it is in the beginning. So I suppose i'll investigate the dip tube a bit more. Thank you for your help, Dave.
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snipped-for-privacy@gmail.com wrote:

All modern shower valves had a safety built in to keep from scalding the bather. It sounds like yours needs replacement, repair or adjustment.
The instructions are specific for different kinds and it requires some disassembly. If you can fully identify the valve, someone might have the instructions. You might want to google the valve type and maybe you can identify it and find instructions. You can also check with the library or hardware store for a book that includes some general instructions on this along with some very helpful photos.
--
Joseph Meehan

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wrote:

Joseph, You are right. When I installed the new shower valve, I did not adjust the temperature safely limiter. I was under the assumption that after taking a temp reading at its "full hot" setting and it was at a safe level it was safe and I just left it alone. So if this is faulty or malfunctioning that might cause the temperature to deviate substantially from its initial temperature? I hope so, cause I can easily adjust that. Thanks.
Zach
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