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Old 04-22-2019, 02:08 PM   #1
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New H2O heater, still get hot-cold-hot in shower

Just replaced my 20 year old Raritan 12 gal. unit that was clearly failing (water not hot, recovery slow.) It also was producing, when water was hot, a short blast of cool water, then hot water when taking a on/off/on navy shower. What is troubling is that the new water heater does the same thing. How does cool water momentarily get into the water lines after the lines are full of warm water? It is just a momentary blast of cold water (after hot) when the valves haven't been touched. I can't figure it out but it happens the same in all the boat's sink's as well. Nice warm water, then a second of cold water then back to warm. No air in the lines, no faulty water delivery throughout the boat except this, both before and after replacing the water heater. Any thoughts would be appreciated.

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Old 04-22-2019, 02:23 PM   #2
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Do you have a temperature attenuation valve that mixes hot and cold water? Might be there to manage water temp., especially if you have a heating loop from your engine to heat tank passively while underway without electricity, because then water can get extremely hot. So, if you do have a valve, it may be defective. Look for a valve junction where hot and cold lines come together.

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Old 04-22-2019, 02:37 PM   #3
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can you feel the hot/cold/hot in the actual hot water line, or is this happening just at the shower head? Is it happening also at any other sink/shower?

As you state it is happening only a the shower, I'd look at the shower faucet itself as the source of the problem.
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Old 04-22-2019, 02:42 PM   #4
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Is the temp cycling in sync with the pump cycling?
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Old 04-22-2019, 03:04 PM   #5
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Maybe the line runs through a hot equipment space between the heater and shower?
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Old 04-22-2019, 03:16 PM   #6
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You probably have unequal pressure in the pipes and when you shut off the shower wand or the shower head the cold pushes into the hot. It can go the other way as well.
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Old 04-22-2019, 03:33 PM   #7
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Thanks all for getting back. To answer a few replies: I have standard mixing valves in the shower and both sinks. it is 30+ years old but works as it should (I think). I don't want to replace it because access where it is mounted is almost nil and I don't want to damage the plastic tubing in the effort. The h/c water lines don't run close enough together to cause the problem. The problem does not cycle with the house water pump. Bayview's comment is intriguing about unequal pressure in the pipes. I think I see what he is saying but how would pressure be different in the h/c lines, especially with a new water heater and the fact that the problem developed over time in the old heater before replacement.
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Old 04-22-2019, 03:45 PM   #8
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Is there a check valve on the inlet to the water heater? If so, is it functioning? You should be able to hear it snap shut with a mechanical stethoscope or similar aid right at the valve. Without the check, odd things can happen to flow. If none, add one. Another thing to look for is a mixing valve that's bypassing. An example would be a 2 handle shower valve with a shower head with spring acting shutoff valve- cockpit shower. If the hot/cold valves are open and the shower head is closed, there's a bypass set up where hot/cold is free to flow in the wrong direction. It sounds like that's what your problem is. It's due to an imbalance in flow that allows the water to backflow momentarily. They can be difficult to locate because the flow only happens for a short duration. I'd suspect a missing/stuck check on the cold inlet on the water heater, followed by a mixing valve that somehow is bypassing. For cold to come out of hot requires flow in the wrong direction somewhere in the piping. Sometimes you can feel check piping for odd temperature variations when the anomaly occurs. Your hand is as good as any IR gun for detecting small, fast changes.
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Old 04-22-2019, 03:46 PM   #9
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Sounds like a tempering valve issue as already mentioned. Follow the output line of the water heater to the nearest sink. As mentioned, it will have a line from the water heater, a cold water line, and then the line going to all sinks and showers.

This is the one on my water heater. Left blue pipe is cold water. Right side is connected to the water heater outlet. Bottom is feed to all faucets. Green cap on top houses the adjustment of the output temperature.

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Old 04-22-2019, 04:31 PM   #10
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Do you have a pressure accumulator in the system? I added one and it greatly reduced this problem.
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Old 04-24-2019, 04:10 PM   #11
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Bingo Maerin! I'm almost positive that Maerin has identified the problem. I think it's in the spring-loaded shower head valve, as it relates to the shower. Now, how does this "reverse flow" problem affect the the sinks, many feet away from the shower head? It is my wife that complains about the hot-momentary cold-hot in the galley where she daily washes her hair. If it's the shower head, I'm guessing that another company's shower head might solve the problem? I'll be playing with all this over the weekend. Thanks all!

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