Water heater pressure relief valve necessary ?

[Hyprdrv]

I'll qualify this by stating that my background is plumbing design for over 15 years. I'll further qualify this by saying I'm not real familiar with the boating systems but from what I've read above the systems are very similar with the addition of the heat exchanger which is dependent on the temperature of the engine water.
So first question is what is the temperature of the coolant water from the engine to the HWH? Since I don't see any type of automatic bypass valve at the heat exchanger to bypass the water heated by the engine, pressure above the rated blow off valve (yellow tagged) could be exceeded (normally set at around 180F) if the engine was run for an extended time and the water temperature from the engine was excessive to the point it raised the water temperature at the HWH above it's safety temperature. The electric element in the HWH has it's own t'stat to limit it's heating temperature normally set at or about 120F.

Anyway back to the pressure relief or Blow off valve.
Back in the good old days Hot Water Tanks were blowing up killing people left and right. The fix was the addition of the pressure relief valve in the tank which on pressure reached above 60% of the tanks (pressure vessel)rated max pressure a devise shall relieve the pressure in the tank. The National Plumbing Code goes on to say that this relieved pressure shall "spill" over the floor. The reason for this is so if there is an issue with over pressurization (water in the bilge) it will be seen and the problem fixed. The other issue most people don't realize is that these inexpensive valves are suppose to be replaced every 2 or 3 years. The springs normally fail (lower pressure to open)and the valves start to leak. These valves have also been known to fail closed in highly calcified water conditions where calcium build up and seals them shut. That's why they are suppose to be exercised once a year. There is a tab on the top you lift which opens the valve. Problem is probably 90% of the time after doing this they leak so I just replace them every 3 years.

Under no circumstances should you pipe this safety valve back into the system.

The Accumulator Tank mentioned above is used for reducing the run time of the water pump and that only. It has nothing to do with water temperature but does effect the water pressure. It allows the water pump to build up water pressure in the system and shorten the run time of the pump. Typically adjustable and set between 35 to 65lbs on house systems. It has a rubber bladder in it and doesn't like hot water. It is installed as close as possible to the pump. I'm not sure what the boat pressure systems are but would like to know if someone does.

The temperature at the "tap" can depend on run lengths and initial temperature at the HWH tank. I'm not a fan of the whole house system mixing valves since I like my kitchen sink water temp potentially higher than my shower temp. The temperature drop after a whole system balancing valve limits this to whatever it's set at and can drop the temp at the kitchen sink by quite a bit depending on the pipe run.

So can someone enlighten me to the typical engine water temperature at the heat exchanger in the HWH and the typical pressure at the kitchen sink? This would go a long way in determining the set points of devices.

Steve

[Northeaster]

Any idea why the HWH is installed on a secondary coolant loop, and not on the primary?

[Hyprdrv]

I just looked at this site Gord posted.

Goto: http://www.kuumaproducts.com/Eng0092...s%20Manual.pdf

Page 5 shows the plumbing of the HWH and indicates a bypass valve at the engine discharge "bypassing" the HWH loop. This appears to be a manual ball valve the owner is to adjust to balance the water temperature at the HWH and indicated to "not be completely closed". It probably should read "not completely open" indicating it should be bypassing the majority of the engine water to the heat exchanger. I'm surprised there is no temp Gage at the return engine water temperature but I'm a Gage happy kind of guy. This could tell you a lot and balance the temperature to the HWH system maximizing the free heat without over doing it.

Steve

[GordMay]

According to ABYC Section H-23, “INSTALLATION OF POTABLE WATER SYSTEMS FOR USE ON BOATS”:

Nominal working pressure - The greater pressure established by either
(a) the system pump cut-off pressure,
or
(b) the pressure established on the vessel side of the city water pressure regulator not to exceed 80 psi.

Shore Connection - If a shoreside connection is provided;
Automatic pressure pumps shall be controlled by a device that shuts off the operation of the pump motor on attainment of a predetermined system pressure.
A pressure reducing valve and check valve shall be installed at the system inlet connection to reduce the pressure to the nominal working pressure of the boat's potable water system.

Water Heaters - Water heater tanks shall be capable of withstanding a test pressure based on their capability as follows:
Capacity - Test Pressure
15 gallons or less - 125 psi
Over 15 gallons through 120 gallons - 300 psi
Over 120 gallons Refer to ASME Boiler And Pressure Vessel Code

The storage tank and the vent system shall be designed so that the tank is not subjected to pressure greater than 3 psi (21kPa), or the pressure specified by the tank manufacturer, whichever is greater.

[Hyprdrv]

Northeaster,
Looking at the diagram posted above (page 5, fig 1) shows that it kinda is the primary loop with a bypass for the majority of the water back to the engine. I'm not sure what the temperature of the water is or how efficient the heat exchanger is. The Heat Exchanger I saw on the Yanmar(sp) engine looked pretty small to me, I was surprised. But I'm not sure of the temperatures a diesel motor can generate. A gas engine gets close to the boiling point of water (with a thermostat) which would not be a good thing in a domestic heating system.

Lets say you get up in the AM, take a shower do the dishes and then kick on the engines and motor for 5 hours without using any hot water. How long will it take to raise the 6 gal HWH tank's water temperature to the temperature of the engine coolant? What is the discharge temperature at the engine and the discharge temperature at the return from the Heat Exchanger? In other words how efficient is the Engine Heat Exchanger? IF the discharge temperature at the engine is 140d all day long there isn't a problem and I would adjust the bypass toward the HWH. If the temperature leans to the 180d mark I would think that the bypass would be wide open and concerned about the water temperature exceeding the blow off valve so I'm guessing the engine coolant temp. must be on the lower side.

Steve

[Hyprdrv]

Oops,

The fix was the addition of the pressure relief valve in the tank which on pressure reached above 60% of the tanks (pressure vessel)rated max pressure a devise shall relieve the pressure in the tank.

This should have read:
"The fix was the addition of the pressure relief valve in the tank which on pressure reached AT (not above)60% of the tanks (pressure vessel)rated max pressure a devise shall relieve the pressure in the tank.

The 60% was some what of an arbitrary number, the idea was not to get close to the tank's max pressure numbers taking into account the age and water conditions the tank was subjected to.

Thanks Gord

Steve

[Therapy]

Originally Posted by GordMay

Whereas a pressure-balanced valve will typically cost from a low of $100 to $250, a thermostatic valve with a volume control (a separate feature with most), will run from $400 to $1,000 depending on the features and trim.

I'll just take my chances and try to stay alert.

If you visit it is a signed waiver or a cold shower.

[Jim St.Pierre]

You could do the same thing with a 1/2" mixing valve (Beacon or a Watts) if you have some plumbing skills. This valve will set the temp at the water heater for all your faucets and shower valve(s). The cost of this valve is about 30.00 USD

[MidLandOne]

Originally Posted by Hyprdrv

...Under no circumstances should you pipe this safety valve back into the system...

...The Accumulator Tank mentioned above is used for reducing the run time of the water pump and that only. It has nothing to do with water temperature but does effect the water pressure. It allows the water pump to build up water pressure in the system and shorten the run time of the pump...

I think you will find that you are making too much of a leap relating what is fine in a domestic or larger installation on land to what are appropriate systems in a small boat with limited accumulation in its own reticulation for expansion and no big pipe of replacement potable water running past the front gate should a significant amount of water end up in the bilge.