Water Heater T&P Relief Valve

[dkinzer]

One of the items on our pre-purchase survey referred to the water heater pressure relief valve. According to the report, ABYC standards require the maximum relief pressure to be no more than 150% of the working pressure of the system. It noted that the current T&P valve is 150# and it should be replaced with a 75# valve based on a working pressure of about 50#.

On the Kuuma website, the T&P valve listed for the water heater that I have is rated 150#. In fact, it is difficult to find a valve with a 75# rating. So, my question is - what is the relief pressure of the T&P valve in your boat?

 

[kloudie1]

I don't have the requirements here, but I suspect that the requirement is for the T&P is that it be set at no more than the 150% of the design pressure (or max allowable working pressure) of the hot water tank rather than the working pressure.. There should be a tag or sticker on the tank somewhere that states the design pressure.. probably on the order of 100 PSI or more.
I don't know what my valve is set for.

 

[jssailem]

KUUMA reports that the valve for their Hot water heater "All Systems" is:

10473 Water Heater Temperature & Pressure Safety Relief Valve, 3/4" Male NPT
150 PSI - 210 Degree with 4" Probe​

Kuuma Products by Camco​

My understanding is that the "system" referred to in ABYC is the Hot Water unit in which the relief valve is installed. Not the water system of the boat. If Temperature or pressure inside the tank builds to the limits of the relief valve it will open and relieve the pressure.

That may be why it is difficult to find the 75# TPR valves.

 

[dkinzer]

It may be that the pressure relief valve is to protect the tank, not the system. I checked our RV water heater and it has a 150# relief valve on it.

 

[JamesG161]

It may be that the pressure relief valve is to protect the tank []

Relief Valves are always to protect Tanks!
Why?

What would cause the biggest bursting problems, lines or tanks?

Jim...

 

[dkinzer]

That makes sense. Thanks.

 

[TimFromLI]

Wouldn't a buildup of pressure in the tank cause either the tank OR one of the hot water lines to burst? Why would a pressure buildup be limited to just the tank, how would hot water flow if it were restricted by some type of check valve? Maybe I'm wrong but that's what I always assumed. My boat uses polypropylene lines, I doubt they can withstand 150# of pressure. Sure a burst tank would be a bigger problem than a burst line but a burst line is still a PITA, especially if it's in an inaccessible spot. My tank and the tank on my previous boat used a 75# T&P valve, 3/4 in thread. I have had to replace them, they are pretty standard - at marine supply sites. You won't find them in a plumbing supply house.

 

[Hello Below]

If you go that way, here are a couple of alternative 75 psi/210 deg T&P relief valves. Sure Marine Service shows stock. The Watts should be able to be ordered in from any Ma and Pa plumbing shop.
Sure Marine Services
Watts

 

[dkinzer]

Thanks for the information. As an aside, I have noticed in the past that my RV would weep water from the T&P valve on the water heater (rated at 150#) from time to time as the water temp rises and falls. For those that have 75# T&P valves, have you seen similar weepage?

 

[Hello Below]

Reference: Watts - Thermal Expansion

 

[JamesG161]

Your entire boat systems have pressure relief from things like...
1) Pumps
2) Compressors
3) Engines
4) Anything that as a Power Input.

How is the pressure prevented from exceeding the systems that are pressurized?
1) Relief valves
2) Pressure switches that cut off power sources
3) Solenoids valves that bypass pump discharges, back to pump suctions.
4) Pressure regulators
5) Level switches

This is for all fluids... Liquids and Gases.

Our sailboats are pressure powered by the Winds.
How do you relieve the Wind Pressure on the Sails?

This is true for Landlubber Tanks and Pumps!

Jim... [aka Chemical Engineer]

PS: Many tanks have Vacuum Relief Valves too.

 

[JamesG161]

Just for fun...


Jim...

 

[TimFromLI]

Ok, I get it too much pressure is no good but what will blow first, the tank or one of the plastic water lines? I always assumed the lines were weaker than the tank and would blow first.

 

[JamesG161]

the tank or one of the plastic water lines?

A similar idea applies to Electricity too.

We put in fuses to blow before the wire melts.

So pressure relief on the tank protects the lines too.
Jim...

 

[Ralph Johnstone]

My tank and the tank on my previous boat used a 75# T&P valve, 3/4 in thread.

Exactly. A correctly rated safety valve ( the correct name is a relief valve for liquid) will protect the weakest link in the entire system. Water heater, pressure tank, lines, fittings, or whatever else is in there.

For those that have 75# T&P valves, have you seen similar weepage?

Weepage will occur if the system doesn't have a pressure tank for the reason shown in @Hello Below 's diagram in post #10.

 

[Ralph Johnstone]

Our sailboats are pressure powered by the Winds.
How do you relieve the Wind Pressure on the Sails?

The mast automatically collapses ?

 

[TimFromLI]

roundup?

 

[Hello Below]

Weepage will occur if the system doesn't have a pressure tank for the reason shown in @Hello Below 's diagram in post #10.

Ralph, I have often wondered why all/most/many boaters don't complain about this weepage issue. In my (layman plumber) thoughts every time a water tank is heated by engine coolant, electricity, hydronic furnace coolant etc the water in say a 6 gallon tank should expand in or around 8 ounces. Where does this expanded water go if an accumulator or pressure tank is not in the system? In fact I chased water in my bilge for a number of years until I eventually I put a container under the relief valve and yup if the tank was up to pressure (40psi) and the heater turned on a cup or so of water would be in the container. I changed the relief valve with a new one and the same thing resulted. Finally I installed a hose off the heater relief valve directly into the shower sump box. Water in bilge gone. Later I also installed an accumulator in the system more to reduce the pulsing than anything.

 

[TimFromLI]

When we first got our boat a few years ago, I was showing my wife the two bilge pumps. The first pump takes care of any water in the bilge and the second pump, which was higher up in the bilge was a backup pump that triggers an ear piercing high pitched alarm on the panel. I called it the shit hits the fan alarm. I demonstrated it and said let's hope we never hear that sound again. We then headed out of the marina, It was a calm day and we had to motor for about 6 hours to Block Island, as we approached the island, the shit hits the fan alarm went off. I went below to see what was going on and saw the bilge filled with water. I was afraid we were sinking and I couldn't see where the leak was. Then the alarm stopped and the water level went down. Finally I noticed that the freshwater pump was going nonstop. I tasted the water and it was fresh. My hot water tank is engine and electric heated. After motoring all day, the T in the T&P valve sensed that the water was too hot and it opened up flooding the bilge until the tank was empty. Scared the hell out of both of us. Luckily it was an easy fix, new valve and no more problems.

 

[Stu Jackson]

until the tank was empty.

Good story, thx for sharing. Another good reason to NOT have your freshwater pump in the ON position unless you're using it or at anchor or moored. At least that's what I do.