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Old 09-09-2012, 08:55   #1
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High Fresh Water Pressure When Running Motor

Have an ongoing problem with the fresh (drinking) water hoses popping off fixtures when the motor is running for a long period of time.

Apparently the hot water tank must get heated so much that it builds up pressure to the point where it maxes out the accumulator and blows at the weakest point in the system. At least it has blown at clamped connections, and not blown/ripped a hose open


I am sure I double-check all the clamps to make sure they are all tight..but that could possibly divert the pressure to blow the hose open at any point in the system.

Any idea what to install to relieve the pressure before blowing the hose off? I was thinking of a pressure valve for a plumbing fixture..but wasn't sure if that would blow soon enough. I didn't see anything in the West catalog that would fit the bill.
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Old 09-09-2012, 09:10   #2
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There is pressure relief valve on a water heater.
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Old 09-09-2012, 09:25   #3
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Re: High Fresh Water Pressure when Running Motor

As I understand your problem, you are running hot coolant from the engine through a heat exchanger in the water heater and on long runs the water gets too hot and raises the pressure in your water lines high enough to cause damage.

Your water heater should already have a temperature and pressure relief valve that will open if the temp or pressure exceeds a certain value. Raritan water heaters come with one set at 75 psi. Check to see that you have one and that it is functioning.

The best solution of course is to prevent the water from over heating in the first place.
What would happen if you put a ball valve at the coolant inlet and used it to control how much hot coolant is going through the heat exchanger?
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Old 09-09-2012, 09:30   #4
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Re: High Fresh Water Pressure when Running Motor

Get thee to the big box store and by a small thermal expansion tank in the plumbing department. (about 10" dia x 12" high Put a tee in the inlet with two hose barbs.. Install it anywhere between the pump and water heater.

Yes, It is thermal expansion and the weakest link will fail The relief valve probably will not pop off till 100-125 psi or higher. The hose clamps are good to about 80 psi on a good day.
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Old 09-09-2012, 09:32   #5
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Re: High Fresh Water Pressure when Running Motor

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Originally Posted by HopCar View Post
As I understand your problem, you are running hot coolant from the engine through a heat exchanger in the water heater and on long runs the water gets too hot and raises the pressure in your water lines high enough to cause damage.

Your water heater should already have a temperature and pressure relief valve that will open if the temp or pressure exceeds a certain value. Raritan water heaters come with one set at 75 psi. Check to see that you have one and that it is functioning.

The best solution of course is to prevent the water from over heating in the first place.
What would happen if you put a ball valve at the coolant inlet and used it to control how much hot coolant is going through the heat exchanger?
right, it is a heat exchanger.

here is the tank. I don't see a pressurer relief valve on it. I thought about putting a valve in line, but still want hot water of course.. I guess it could be partially closed to reduce flow

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Old 09-09-2012, 09:38   #6
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Re: High Fresh Water Pressure when Running Motor

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Originally Posted by sailorchic34 View Post
Get thee to the big box store and by a small thermal expansion tank in the plumbing department. (about 10" dia x 12" high Put a tee in the inlet with two hose barbs.. Install it anywhere between the pump and water heater.

Yes, It is thermal expansion and the weakest link will fail The relief valve probably will not pop off till 100-125 psi or higher. The hose clamps are good to about 80 psi on a good day.
They are all double clamped at each connection with top quality AWAB clamps, but I still prefer to have it fail as it is.. AT the clamps! There is a tank in line already (similar to this one West Marine - Sorry for the inconvenience)

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Old 09-09-2012, 09:44   #7
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Re: High Fresh Water Pressure When Running Motor

You will find if you take the Jabsco tank off that the diaphragm has ripped and the tank is water logged (That is filled with water). You could turn the pump off, open a faucet and put a tire pump on the valve at the top and pump the water out, but that will last for only a week or two. The Jabsco tanks are too small to handle thermal expansion. Which is why the diaphram rips in them...
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Old 09-09-2012, 09:47   #8
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Re: High Fresh Water Pressure When Running Motor

I think it's pretty dangerous to not have a temp / pressure relief valve. I think there are all kinds of regulations that require them.

My thinking on the valve was that you could use it to reduce the flow of coolant through the heat exchanger. What I don't know is if that would affect the cooling of your engine. We don't want to overheat the engine.

I've got to say that that is the strangest looking water heater I've ever seen.


1700 Series Water Heater




World-renowned water heater specially designed for the marine environment. Provides the comfort and convenience of reliable hot water.

Benefits:
  • The tank is covered by a five-year Limited Warranty.
  • Ignition-protected thermostat.
  • Attractive polymer jacket resists corrosion from dampness.
  • Glass-lined steel tank is more durable than stainless steel.
Options:
  • Available in 6, 12 and 20 gallon models with or without engine heat exchangers.
  • Available for 120V AC or 240V AC.
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Old 09-09-2012, 09:47   #9
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Re: High Fresh Water Pressure When Running Motor

Water pressure problems are not something to take lightly. T&P relief valves trip at a set pressure for a reason. Lines blowing indicates the T&P valve is missing or not installed properly/malfunctioning. I've seen them plugged with a pipe plug or cap, that negates their purpose.

You should have a thermostat of some sort to limit the water temperature. That is likely malfunctioning too causing the over pressure.

An expansion tank would be a good idea if one is not already in the system.

Here is what can happen when water heater/heat exchanger safety systems are plugged, by-passed or otherwise not present and functioning.

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Old 09-09-2012, 09:53   #10
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Re: High Fresh Water Pressure When Running Motor

If there is a relief valve on that tank, it may be on the back dish. Sort of looks like a frabricated unit and might not be up to code. The good news is as long as the engine coolant temp stays below 212 degrees, the tank will not explode.

T&P relief valves are designed to prevent water heaters from exploding btw, and will relief on either high water temp (180 ish F) or pressure.
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Old 09-09-2012, 09:58   #11
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Re: High Fresh Water Pressure When Running Motor

Phantomracer,

if you don't have a pressure relief valve you are simply operating a dangerous system. Most likely there is a port for one on your water heater, and someone removed it because it was leaking. This is simply an accident waiting to happen - a second component failure, for example of the high limit thermostat, could create a nice little bomb inside your boat.

I would not worry about the hose clamps - screw type clamps are at least good for 150 psi if installed right. As far as your expansion tank is concerned, it has primarily nothing to do with this problem - it is supposed to be a pressure storage, and if large enough, it can hide the absence of the pressure relief valve. To test it: When you open a tap and it takes a while for the pump to come on, the tank is working. If the pump comes on immediately every time, the tank is not working. If it's not working you can try to pressurize it with a bicycle pump, although to do it properly it should be filled with Nitrogen. If the diaphragm is still intact, it will start working again. If the diaphragm is defective, you will be back to square one the next morning and then you need to replace the tank.

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Old 09-09-2012, 10:00   #12
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Re: High Fresh Water Pressure When Running Motor

Two issues really. Your system should have a thermal/pressure relief valve. If you can't find one on it then install one. One day it could be something more damaging than a hose that fails because of the pressure. Doesn't have to be right on the tank (although that is best), can be someplace you can conveniently get into the system and where you can discharge any relief water. No valves between it and the tank though. You should be able to pick one up at any plumbing supply or big box place.

But, pressure relief valves are not designed to, and should not, operate daily/regularly. They are a safety device of last resort. Besides, when they open they blow down fresh water, and depending on where you are that can be precious. Your system needs more accumulation volume. If the Jabsco is failed then you might succeed with a new one (depending on how long it lasted before it failed). If the Jabsco is not failed then you simply need more volume in your accumulator(s).
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Old 09-09-2012, 10:04   #13
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Re: High Fresh Water Pressure When Running Motor

Quote:
Originally Posted by sailorchic34 View Post
You will find if you take the Jabsco tank off that the diaphragm has ripped and the tank is water logged (That is filled with water). You could turn the pump off, open a faucet and put a tire pump on the valve at the top and pump the water out, but that will last for only a week or two. The Jabsco tanks are too small to handle thermal expansion. Which is why the diaphram rips in them...
unsure which i have. Just wanted to show i have something similar to this one. unsure of the brand/size. just that there is a tank.

but will take a look. it does seem to work, the pump doesnt always cycle, and the flow is smooth
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Old 09-09-2012, 10:07   #14
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Re: High Fresh Water Pressure When Running Motor

Quote:
Originally Posted by Dsanduril View Post
Two issues really. Your system should have a thermal/pressure relief valve. If you can't find one on it then install one. One day it could be something more damaging than a hose that fails because of the pressure. Doesn't have to be right on the tank (although that is best), can be someplace you can conveniently get into the system and where you can discharge any relief water. No valves between it and the tank though. You should be able to pick one up at any plumbing supply or big box place.

But, pressure relief valves are not designed to, and should not, operate daily/regularly. They are a safety device of last resort. Besides, when they open they blow down fresh water, and depending on where you are that can be precious. Your system needs more accumulation volume. If the Jabsco is failed then you might succeed with a new one (depending on how long it lasted before it failed). If the Jabsco is not failed then you simply need more volume in your accumulator(s).
riiiight. THats what I said in my opening post. There IS no pressure relief valve. I just want to know WHAT to install to fix it. I understand what is wrong. Just not what to do to fix it.

what do i need, where do i get it?
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Old 09-09-2012, 10:07   #15
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Re: High Fresh Water Pressure When Running Motor

Hi Sailorchick, you're right of course about the tank not exploding as long as the water doesn't boil. I guess the danger is that thermostats can fail and electric elements can boil the water. Did you ever see the Myth Busters blow up a water heater. Wow!
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