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Old 16-12-2019, 17:57   #1
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Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Hi Guys,

Hoping that you can help. Since we are having some mild December days on the Chesapeake, I decided to de-winterize my freshwater system for a 2 day visit to the boat to tackle some projects.

I filled my freshwater tank and then switched on my fw pump and waited until my hot water heater (Atwood ehm6) filled and the motor turned off once properly pressurized. Everything seemed as normal.

I then turned on my water heater and waited about an hour. I turned on my kitchen faucet (to hot) and noticed something weird. There was plenty of hot water coming out but the freshwater pump was not coming on. It ran for a good 7-8 seconds and then the pump came on. Closing the faucet, the pump remained on for another good 7-8 seconds and then shut off. My FW system has never acted like this before in my 3 seasons on the boat. There is no expansion tank (that I know of).

I then turned off the circuit and drained the hot water heater, refilled it with cold tank water and tried again. Same thing. This made me very worried that my tank is somehow over-pressurized.

I checked my pressure relief valve and it seems to be working properly. Draining water into the bilge when pressed.

Any ideas of what could be causing this? Thanks in advance for any insight or diagnoses.

Joe
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Old 16-12-2019, 19:27   #2
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Air in the water system acting as an expansion tank. Possibly air trapped in the hot water tank or elsewhere. Did you make sure the air was out of all the lines?
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Old 17-12-2019, 08:02   #3
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

This is pretty normal. There can be a "bubble" of air in the top of the water heater, or as DeepFrz points out, in the lines.

With use, the bubble shrinks and the air works its way out of the lines, and you'll notice less time between opening the hot tap and hearing the pump.

My water heater is plumbed into the engine coolant, so when running the mains it gets really hot. I try to remember to open a hot water tap about an hour after getting underway to release the built-up pressure.

Water expands when heated, but it can't be compressed. That extra volume has to go somewhere. If there's no air above the water in the tank to compress, it'll force its way back through the cold water inlet. That's why many boats have expansion tanks and/or one-way valves.
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Old 17-12-2019, 08:25   #4
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Your pressure controlled pump is a wonderful gauge of what's happening in your freshwater system. It pressurizes the system, including any air, such as you just discovered. If pressure ramains up, nothing then happens, but if you have a leak or left something turned on, the pump will cycle, bumping along for a few revolutions, and then off as pressure is restored. With no air in the system, the pump will cut off just after you close a valve, because it is just pressurizing the expansion of your hoses and pipes. Being able to hear the pump lets you know what is going on.
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Old 17-12-2019, 10:36   #5
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

If you noticed that the water was too hot and the pressure was high, perhaps there is an issue with the thermostat and the water is boiling creating the pressure. I would check the temperature of the pipes at the exit of the tank they should be below 150 deg F
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Old 17-12-2019, 10:41   #6
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

PV = nRT

You increased the temperature of a closed system. Of course pressure increased as a result.
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Old 17-12-2019, 14:43   #7
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

probably not a problem. there is normally a lag between turning the hot tap and the pump sensing a pressure loss. a few seconds is no cause for concern unless your hot water is substantially hotter than it should be but that seems unlikely.
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Old 17-12-2019, 15:14   #8
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Adding an expansion tank might be a good idea.
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Old 17-12-2019, 16:05   #9
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Did you open the through-hull of the pressure relief valve? Every hot water tank has a pressure relief valve, to the outside or in the bilge. You were lucky that there was air in the system to compensate for the increase in water volume.
Without air the increase in water volume could have split your tank.
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Old 17-12-2019, 16:27   #10
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Quote:
Originally Posted by sailormed View Post
Did you open the through-hull of the pressure relief valve? Every hot water tank has a pressure relief valve, to the outside or in the bilge. You were lucky that there was air in the system to compensate for the increase in water volume.
Without air the increase in water volume could have split your tank.
Interesting, I'd never heard of plumbing the P/T relief valve to a through-hull.

But you make a good point. If there's too much pressure, that valve should release it. Likewise if it overheats, which can happen if you have an engine jacket water loop connected. Those valves are pretty reliable. The usual failure mode is to start leaking, not fail to open.
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Old 17-12-2019, 16:56   #11
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

There s assumed pressure with hot water. System is normal.
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Old 18-12-2019, 00:58   #12
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

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Interesting, I'd never heard of plumbing the P/T relief valve to a through-hull.

But you make a good point. If there's too much pressure, that valve should release it. Likewise if it overheats, which can happen if you have an engine jacket water loop connected. Those valves are pretty reliable. The usual failure mode is to start leaking, not fail to open.
My 2005 Hunter 38 has the pressure relief valve going to a mid-ships through hull. My neighbour has the same boat and had the pressure relief TH closed. He had to replace his hot water tank
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Old 18-12-2019, 04:44   #13
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

Quote:
Originally Posted by CaptTom View Post
... Water expands when heated, but it can't be compressed. That extra volume has to go somewhere. If there's no air above the water in the tank to compress, it'll force its way back through the cold water inlet. That's why many boats have expansion tanks and/or one-way valves.
And your tank has a pressure release valve.
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Old 18-12-2019, 16:19   #14
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Re: Water Heater suddenly over-pressurized? Hot water flowing w/o FW pump

You should see what happens to the psi of a tempered water system when the expansion tank's internal bladder fails and the crew, seeing the drop in loop psi, naturally add water to compensate....works great until winter when the whole thing blows up while running in heat mode...
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