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Old 05-09-2019, 15:50   #1
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Hot water pressure valve blowing

I have a relatively new Isotemp marine hot water cylinder fitted to the boat. It has functioned perfectly but recently it has been repeatedly blowing the pressure relief valve when the engine is running. i.e. not when connected to mains power. The engine is not running hot. The hot water however seems to be very hot at the outlet. Diagnostically I am struggling to determine what the problem is. Any answers please?
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Old 05-09-2019, 15:54   #2
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Re: Hot water pressure valve blowing

If your pressure water is at room temp and you run the engine, water expands.
The expanded water has to go somewhere.
If you have an accumulator tank, make sure the air pressure is correctly set.
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Old 05-09-2019, 15:55   #3
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pirate Re: Hot water pressure valve blowing

Bleed some air out of the tank.. should be a valve or screw on the top to do this, sounds like not a lot of water in there.
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Old 05-09-2019, 15:58   #4
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Re: Hot water pressure valve blowing

The pressure relief blows when the water boils - it's steam that produces the pressure. I'm going to guess that your engine cooling system is pressurized - the cap seals - and that you don't have enough cooling capacity to keep the coolant temperature below 100 degrees centigrade/ 212 degrees Fahrenheit. The only other route for your water heater to boil/pressurize water would be the electrical coil also being hard at work. Somebody is putting heat above boiling temperature into your heater.
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Old 05-09-2019, 16:01   #5
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Re: Hot water pressure valve blowing

OK, I shouldn't answer posts after happy hour. Yes, you could have air in there that expands, but you should be rid of it by now, and yes, water does expand a little with temperature before it boils. But, if this persists, I'm inclined toward boiling.
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Old 05-09-2019, 16:42   #6
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Re: Hot water pressure valve blowing

I fought the same problem with our water heater. It turns out there was a check valve in the system between the hot water heater and the accumulator tank. Therefore the accumulator tank was not able to absorb the volume expansion of the hot water. I removed the check valve and the problem went away.

Cheers!

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Old 05-09-2019, 17:29   #7
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Re: Hot water pressure valve blowing

Thank you very much for this Steve. It fits exactly. I never twigged the relationship. I recently removed the accumulator. Well done!
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Old 05-09-2019, 20:00   #8
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Re: Hot water pressure valve blowing

A check valve etc should only cause it to pop or leak just a little, then seal back as the pressure drops, water isnít compressible so just a small amount of water should drop the pressure, Iím talking teaspoons, not quarts.
If itís leaking quite a lot, then itís not pressure related, most likely temp.
It could either be a bad T&P valve or one with too low a temp setting.
Our engine heat the water to a much higher temp than the electric does, I donít know how much, but itís certainly hotter.
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Old 05-09-2019, 20:13   #9
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Re: Hot water pressure valve blowing

My electric thermostat is set to 120 f. The engine heats it to 180 f.
When mine burps out water, it's because I forgot to turn off and relieve the pressure water system.
It is related to the too small accumulator I have, but no room for a bigger one.
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Old 05-09-2019, 21:48   #10
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Re: Hot water pressure valve blowing

Mine failed. Lost all 280 gallons of fresh in big seas between Bonaire and Puerto Rico. Replace the valve an have a spare. You can buy at a hardware store for about $15
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Old 06-09-2019, 05:33   #11
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Re: Hot water pressure valve blowing

Quote:
Originally Posted by Grant Ford View Post
I have a relatively new Isotemp marine hot water cylinder fitted to the boat.
Quote:
Originally Posted by a64pilot View Post
It could either be a bad T&P valve or one with too low a temp setting.
I think the newer T&P relief valves are set to a lower temp than they used to be. Maybe less than the 160F that many engine thermostats are set at. It doesn't take long for your water heater gets up to the temp of the engine coolant. The T&P valve has to allow for that.

To test this theory, try opening the hot water tap at the sink for a second or two after running the engine for 20 or more minutes. You'll probably notice a surge of water releasing that built-up pressure others mentioned.

If doing that every so often for the first hour or so of running resolves the problem, that's your answer - it's pressure. If not, it's temperature.

Edit: I try to remember to do this a couple of times, for the first couple of hours after getting underway. I feel like that extra pressure in the potable water system can't be doing any good.
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Old 06-09-2019, 07:03   #12
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Re: Hot water pressure valve blowing

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Originally Posted by Grant Ford View Post
Thank you very much for this Steve. It fits exactly. I never twigged the relationship. I recently removed the accumulator. Well done!
Hi Grant. The accumulator tank will normally absorb expansion of the water as it is heated, so this may solve your problem.

However, can you tell us what you mean when you say your relief valve is "blowing"? Do you mean it suddenly releases and a LOT of water comes out? or is it a dribble?

If it's a dribble then simply reinstalling the the accumulator should solve your problem. If it's a gush, then the concern from the other posters is that you have an additional problem with your water getting too from some additional source of heat. If I remember correctly, in my case I was leaking about a cup of water each heating cycle, or 1/4 liter for our metric readers.

So, can you describe a little better what you mean by the relief valve blowing? We want to make sure we help you correctly diagnose the problem.

As always, Cheers!

Steve
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Old 06-09-2019, 07:10   #13
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Re: Hot water pressure valve blowing

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Originally Posted by senormechanico View Post
If your pressure water is at room temp and you run the engine, water expands.
The expanded water has to go somewhere.
If you have an accumulator tank, make sure the air pressure is correctly set.

THIS ONE ^
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Old 07-09-2019, 04:01   #14
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Re: Hot water pressure valve blowing

Greetings and welcome aboard the CF, Grant.
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