Hot water pressure relief valve

All U Get

All U Get

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Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Hi y'all,

Had a little water over the last two weeks in the aft bilge and been tracing the drips. Today as we were running the main engine the pressure relief valve on the hot water tank spewed out some water then closed. I worked it a few more times hoping to get it to seat better for the night before I start looking for a replacement. Would those be a standard hardware item?
 
D

dlochner

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Jan 11, 2014
11,422
Sabre 362 113 Fair Haven, NY
All U Get said:
Hi y'all,

Had a little water over the last two weeks in the aft bilge and been tracing the drips. Today as we were running the main engine the pressure relief valve on the hot water tank spewed out some water then closed. I worked it a few more times hoping to get it to seat better for the night before I start looking for a replacement. Would those be a standard hardware item?
Click to expand
Yes.
 
SG

SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
IT COULD ALSO BE A WARNING, that the water is too hot?
Have you checked the temp on the panel.
I had this happen once when running at higher RPM's when our heat exchanger was "coated" and not transferring BTU's to the raw water. The engine was fine up to (say 2575 RPM -- then, magically, it was overcome and the hot water heated-up.

The pressure relief valve should be respected unless you know otherwise when the engine is the source of heat.
 
D

dlochner

.
Jan 11, 2014
11,422
Sabre 362 113 Fair Haven, NY
SG said:
IT COULD ALSO BE A WARNING, that the water is too hot?
Have you checked the temp on the panel.
I had this happen once when running at higher RPM's when our heat exchanger was "coated" and not transferring BTU's to the raw water. The engine was fine up to (say 2575 RPM -- then, magically, it was overcome and the hot water heated-up.

The pressure relief valve should be respected unless you know otherwise when the engine is the source of heat.
Click to expand
Good point. :plus:
 
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jssailem

jssailem

.
Oct 22, 2014
21,102
CAL 35 Cruiser #21 moored EVERETT WA
As an additional thought you might check your engine temperature thermostat. If it gets stuck or weak in the spring it might not be opening fully restricting water flow when you need it.
 
All U Get

All U Get

.
Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Engine gauge didn't show any overheating at that time and we had only run about an hour but its worth checking. The water heater is 6 years old and many hours of operation so I'm thinking the T&P valve is tired.
 
capta

capta

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Jun 4, 2009
4,773
Pearson 530 Admiralty Bay, Bequia SVG
Most home water heaters are set to about 49 °C (120 °F). Your engine runs at closer to 180. I think you must find a relief valve that works to the higher temperature or is adjustable to it.
 
Sandy Stone

Sandy Stone

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Jun 2, 2007
403
Beneteau First 375 Slidell, LA
Just so you know, the relief valves you can pick up at the hardware store are set at 125 or 150 psi, but you probably need one set at 75 psi. I had to get mine from an online marine supply store.
 
D

dlochner

.
Jan 11, 2014
11,422
Sabre 362 113 Fair Haven, NY
Sandy Stone said:
Just so you know, the relief valves you can pick up at the hardware store are set at 125 or 150 psi, but you probably need one set at 75 psi. I had to get mine from an online marine supply store.
Click to expand
This may vary from brand to brand. The Raritan and Seward HW heaters rate at 150 psi. If the valve is releasing too often adding an expansion tank will help to absorb some of the pressure.

If you change to a 75 psi, it will blow more often, unless @Sandy Stone meant to write 175 psi.
 
RoyS

RoyS

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Jun 3, 2012
1,742
Hunter 33 Steamboat Wharf, Hull, MA
These devices vary in settings. As Sandy pointed out they have both pressure and temperature settings to consider. A typical home water heater model may be set for 210F and 150 psi. My home boiler model is set at 30 psi and has no temperature setting on the label. They may all fit but the settings are probably not correct for your application.
 
SG

SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
The general domestic HOME hot water heater is set for 150 Psi and 210 degrees F.

Marine and RV heaters are set for a pressure standard which is much lower for a variety of reasons:

Raritan Pressure Relief Valve 75 PSI - Help Maintain An Environmental Balance

  • Pressure Relief Valve - 75 PSI.
  • Raritan's Pressure Relief Valuce Has 75 PSI.
  • Raritan Manufactures Equipment For Pleasure Boats Throughout The World.
  • Products Include Marine Toilets, Waste Treatment Systems, Icemakers And Water Heaters.
  • Committed To The Environment And Preserving Sea Life, Raritan Designs Products To Help Maintain An Environmental Balance And Ensure The Quality Of Marine Waters.
DON'T INSTALL A 150 PSI relief valve in your system. Your water heater isn't designed as a pressure vessel your plumbing system isn't designed to take those pressures!!!!

Again: Since the relief valve closed back down, I'd FIRST suspect that you are running hot. The pressure will build-up rapidly as your reach toward the boiling temp in the water heater.

It might be the valve -- but that shouldn't be your first concern.
 
D

dlochner

.
Jan 11, 2014
11,422
Sabre 362 113 Fair Haven, NY
SG said:
The general domestic HOME hot water heater is set for 150 Psi and 210 degrees F.

Marine and RV heaters are set for a pressure standard which is much lower for a variety of reasons:

Raritan Pressure Relief Valve 75 PSI - Help Maintain An Environmental Balance

  • Pressure Relief Valve - 75 PSI.
  • Raritan's Pressure Relief Valuce Has 75 PSI.
  • Raritan Manufactures Equipment For Pleasure Boats Throughout The World.
  • Products Include Marine Toilets, Waste Treatment Systems, Icemakers And Water Heaters.
  • Committed To The Environment And Preserving Sea Life, Raritan Designs Products To Help Maintain An Environmental Balance And Ensure The Quality Of Marine Waters.
DON'T INSTALL A 150 PSI relief valve in your system. Your water heater isn't designed as a pressure vessel your plumbing system isn't designed to take those pressures!!!!

Again: Since the relief valve closed back down, I'd FIRST suspect that you are running hot. The pressure will build-up rapidly as your reach toward the boiling temp in the water heater.

It might be the valve -- but that shouldn't be your first concern.
Click to expand
Of course nothing is as simple as it seems. Raritan provides what could be considered conflicting information. The tanks are rated for 150 psi, however, they recommend a T&P valve rated at 75 psi and 210* F. If one only looks at the rating, it would be easy to misinterpret the specs and install a higher rated T&P valve.

Here are the installation instructions: https://www.defender.com/pdf/Raritan_Water_Heaters.pdf
 
SG

SG

.
Feb 11, 2017
1,670
J/Boat J/160 Annapolis
With a 150 PSI pressure rating, using Pascals law:
Pascal's Law states that the "Pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls. This is the basic principle for any hydraulic system."
​
While your "expansion tank" (if you have one) and your plastic hoses (for short while) and/or your fittings might take the 150 psi pressure, if you ever had that happen on MOST boats, it wouldn't be a good thing.

It would only be an issue if your system got hot enough -- but then, it wouldn't be a happy time.
 
Gunni

Gunni

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Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
The T-P relief valve is manufacturer specific, and All You Get hasn’t stated which HW heater he has right. If he has the HW heater I have his T-P valve is OEM 150psi/210 F deg...and I haven’t been blowed up once. :)
 
All U Get

All U Get

.
Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Gunni said:
The T-P relief valve is manufacturer specific, and All You Get hasn’t stated which HW heater he has right. If he has the HW heater I have his T-P valve is OEM 150psi/210 F deg...and I haven’t been blowed up once. :)
Click to expand
Superstor water heater. I talked with Raritan and they said take mine to a plumbing supply shop because Superstor was no longer available. Mine is stamped 150 psi and 210 F.

PPS I got a Watts just like the old one and its in.
 
Last edited:
D

dlochner

.
Jan 11, 2014
11,422
Sabre 362 113 Fair Haven, NY
All U Get said:
Superstor water heater. I talked with Raritan and they said take mine to a plumbing supply shop because Superstor was no longer available. Mine is stamped 150 psi and 210 F.
Click to expand
There should be a metal tag on the T&P valve that lists the rating.
 
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