We have a problem with the hot water back feeding the cold water. We have a Nautic boiler. The water starts off cold then comes out extremely hot. It even back feeds the cockpit shower and the cockpit shower doesn't have a hot water line going to it. Any suggestions on the fix for this problem.
Hot water
- Thread starter Southpaw
- Start date
Is there a check valve in the cold water inlet?
As the water heats up, it expands and will push back through the cold water inlet line unless there is a check valve.
As the water heats up, it expands and will push back through the cold water inlet line unless there is a check valve.
You're supposed to have a check valve in the cold water feed line into your hot water heater. Mine screws into the fitting in the water heater and the cold water line screws into that ( I have hard plastic plumbing). You can get check valves with barb fittings if your water lines are soft plastic hose. https://www.amazon.com/SHURflo-340-...92434348&sr=8-21&keywords=plastic+check+valve
Interesting, I am not sure I understand why there would be a back filling of hot water into my cold. In my system the hot and cold split at the water pump discharge. It would seem that since my hot water system is a closed system. If I turn on only the cold water, unless there is a hot water air leak or cold water back filling the hot through the water manifold. There should be no back filling of the pressurized system. Can some explain why I would have back filling. Also, I live in San Diego and my water tanks are part of the hull. When the water temperature starts warming up, my water tank take starts warming up. Not sure if you are experiencing the same think.
I found out about the pressure buildup in the water heater by turning off the water pump then opening the hot faucet. a pretty good amount of water will come out until the water system is "depressurize". Likewise, when the water pump is on and you open the cold water faucet, the heated water pressure will push the hot water into the cold lines and out the cold faucet.
I never heard of jibes' "on-the heater" solution, but it sounds reasonable. Also, when closing up the boat, shut off the WH breaker and open the faucet to depressurize the water system..
I never heard of jibes' "on-the heater" solution, but it sounds reasonable. Also, when closing up the boat, shut off the WH breaker and open the faucet to depressurize the water system..
I am going to have to try this test. Depending on the location of your hot water tank in relation to the system. The depressurizing you are seeing might be the weight of the water pushing through the system, similar to a siphon. I know when I turn off my water pump and turn on the hot, I get a quick decreasing pressure and about 6 oz of water. I am running at 40 psi system pressure.
An check valve that attaches to the HW tank can be purchased at any big box hardware store or plumbing supply store.
The normal variation of water temperature that BlueLagoon mentions won't generate enough pressure to backfill the system. However, if the HW heater thermostat is set too high, it can and when motoring the internal temperature will reach the operating temperature of the engine somewhere in the 160-180 degree range. At that temperature there will be a lot of expansion and pressure on the system. It may be enough to override the pressure from the water pump so that when the faucet is opened water is coming first from the HW tank until the pressure drops and then from the pressure system when the pump pressure is higher than the HW back pressure.
Take a look at your car's cooling system expansion tank and you can see the volume of coolant increase. Same for the boat.
The normal variation of water temperature that BlueLagoon mentions won't generate enough pressure to backfill the system. However, if the HW heater thermostat is set too high, it can and when motoring the internal temperature will reach the operating temperature of the engine somewhere in the 160-180 degree range. At that temperature there will be a lot of expansion and pressure on the system. It may be enough to override the pressure from the water pump so that when the faucet is opened water is coming first from the HW tank until the pressure drops and then from the pressure system when the pump pressure is higher than the HW back pressure.
Take a look at your car's cooling system expansion tank and you can see the volume of coolant increase. Same for the boat.
Yes, good call out. I need to do some maintenance on my water system. When I take on that task, I will investigate the the back filling of the hot water. It should be easy to do with a infrared temp gun and a couple of pressure gauges. Thanks for the information and education.
If you are going to dig into upgrading the HW system consider adding a tempering valve to the HW outlet. This will let you set the max temp that the water exits the tank and does so by mixing cold water to the hot water that is flowing out. The tempering valve accomplishes 2 things, it prevents scalding from excessively hot water and saves water because you don't fiddle with the temperature at the faucet.
The check valve is that the same as the safety valve at isotherms website model ITP-SFD00005AA or do I need a new mixing valve?
I couldn't find that part quickly and don't have time at the moment to look. There are 3 safety devices on the HW tank.
A pressure relief valve. This will look very much like the one on your home HW tank. It will be at the top of the tank and should have a hose or pipe running down to the floor or bilge. It's purpose is to keep the tank from blowing up from too much pressure.
A check valve. This will be on the cold water inlet either attached to the tank or in line. This keeps hot water from back feeding and pressurizing the cold water system.
A tempering valve. This will be on the HW outlet and will will have a connection to a cold water supply. This valve will mix cold water with hot to get to a desired temperature. It prevents scalding.
If the hot water outlet is not at the very highest point in the tank then you will have some air trapped in there. When the system is pressurized that air compresses up to the system pressure. With the pump off and without a check valve it can push hot water back through the system.
If you put a check valve on the input of the hot water heater, then when the water expands as it is heated, it will pressurize even more, since it can't back out into the rest of the system. The result, at least in my case, was that the pressure caused the pressure/temperature valve to open, allowing the pressure to vent hot water into the bilge. After a fairly short period, this eroded the seal from the valve, causing a steady leak. If you're going to add a check valve, you also need to add a pressure accumulator to contain the increased volume of water.
Turning off the water heater when not in use will also cause greater expansion of the water when it is heated, causing more backup of hot water into the cold system. If left on, the water will get to temp, and the hot water expelled into the cold system will cool. Water only expends when it is heated, not when it is kept hot.
Turning off the water heater when not in use will also cause greater expansion of the water when it is heated, causing more backup of hot water into the cold system. If left on, the water will get to temp, and the hot water expelled into the cold system will cool. Water only expends when it is heated, not when it is kept hot.
I've never had this happen and don't have an accumulator. Here's an idea, what if the lines were reversed going into the tank so the cold water was entering in the outlet fitting and vice versa? Wouldn't that push the heat right up the cold water pipe until eventually heating up the cold water supply?
Probably not. As the water cools it will sink and the hot water will rise. The incoming water will immediately drop to the bottom of the tank because it is colder and denser. It may work a little when heating the water with the heat exchanger, depends on how the heat exchanger is configured.
The point about keeping the water warm all the time and that will reduce expansion has some merit. raising the water from 72 degrees to 180 degrees will cause a lot more expansion than heating from 110 degrees to 180 degrees.
The pressure relief valve should be temperature and pressure sensitive, it if blew then the tank was seriously over pressurized. If the water was overheated by that much, then either the thermostat is faulty if it happens while on shore power or the engine is running excessively hot if it is heating on the heat exchanger. The water tank can not get hotter than the engine water temperature.
My Attwood owners manual says : to set the internal air "gap", open the hot faucet closest to the water heater. Have all water at its normal "cold" temperature(not heated). Open the pressure/temp valve handle to straight out. Let water drain as much as it will to be level with valve opening. Push valve handle so that it snaps shut. A few drips from the valve does not mean it is defective, and might be normal when the water is heated.
My temp/pressure valve did not Blow, it just dribbled, at first only when water was heating, but eventually all the time until I replaced it. The new one also dribbled when heating cold water until I removed the check valve from the heater input. Keeping air in the HW tank would probably be a solution, but my Raritan tank does a good job of expelling all air when I run the hot water. Note that many community's building codes now require check valves AND expansion tanks on home water heaters.