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Old 19-11-2007, 06:17   #1
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Hot Water Heater Installation

I installed my hot water heater and all is well, almost all ...

About once an hour, the fresh water system pump cycles for a second or two. It never did this before the hot water line was installed.

I double, triple checked all the connections to ensure no leaks, so that's not it ...

Could it be air in the line? But running it out doesn't seem to solve the problem.

How about an expansion tank in the cold water inlet. Would that help?

It's not a serious problem, as there are no leaks and the amount of battery the pump uses is minimal, but it's annoying (to me).

Any thoughts welcome.

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Old 19-11-2007, 09:10   #2
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Sometimes potable water pumps themselves start losing pressure as the valve seals get old, hence the cycling. It sounds like you did all the right things by checking for slow leaks. Air in the lines makes no difference. It will compress and stay at the system pressure. My pot water pump leaks pressure as well. I just live with it. I shut it off when I am not aboard or don't need it.

A real easy way to find slow leaks is to take one of those blue paper towels for shops and start wiping it around...any sort of moisture shows up well on those towels.


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Old 19-11-2007, 10:00   #3
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My pump will do that if the engine's been run recently. The tank gets hot from the engne, water contracts as it cools and the pump makes up the difference. If no water is used during the running of the engine, the expanding water goes out through the pressure relief valve into the bilge.

I have an expansion tank, but it's rather small.

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Old 19-11-2007, 13:12   #4
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Originally Posted by senormechanico View Post
... water contracts as it cools...
Most substances steadily contract as they cool, but water first contracts, and then expands, so that its density reaches a maximum near 4 degrees Celsius (its liquid-liquid critical point).
Water contracts as you cool it towards 4 C; then the water begins to expand (about 10%) as it cools further until freezing.
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Old 20-11-2007, 14:28   #5
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We had the same problem when our boat was first launched. Builder then put a small expansion tank in the pipework on the pressure side of the water pump and that fixed it instantly. The expansion tank is one of those with a rubber membrane separating the water and air sides so only rarely needs pumping up as no air is lost to the water.

Although the pump was one of the type that normally doesn't need an expansion tank for smooth operation, addition of the tank improved the smoothness of its cold water performance as well.

Another cause as well as the expansion already mentioned is that if the hot water tank has a relief valve then leakage through that will cause the pump to cycle. Leakage may be due to the valve being faulty or else the water pump pressure setting being too high. In many boats this is easily seen as the relief just drains into the bilge, but in some, like our own, the relief is returned to the fresh water tanks and so invisible and may be missed.
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