Plastic Cap On Top Of Water Heater [Explained]

The water heater’s functionalities are tricky to understand. So, when discovering new features it may confuse you even more.

So, what is the use of this plastic cap on top of water heater?

The plastic caps on top of the water heater work as an insulator. It prevents the heat from seeping out of the tank. It also prevents the water from flowing out of the tank. There are two distinct color caps that are used for different inlet and outlet pipes.

Care to learn more about them? Then stick around until the end. 

Types Of Plastic Caps On Water Heaters

If you own a water heater you must have seen the caps on the top. Basically, two types of plastic caps sit on top of a water heater. One is red and the other is blue.

Source: The Spruce

The red cap on top is generally called the hot outlet. That’s because it restricts the hot water movement in the outlet. It also prevents the heat from seeping out of the tank.

The blue cap sits on top of the cold inlet. It acts as a doorway for the water to insert the right path. It also keeps the cold water restricted in the system.

For water heater caps, the responsibilities are the same. They have a contribution to controlling the water flow into the system. It also helps to maintain the desired temperature of the inside contents.

As for the dissimilarities, the blue plastic cap is usually connected to a long tube. The deep tube reaches almost the end of the tank. But the red one doesn’t come with a tube similar to it.

Rather it is affixed with a combo anode rod. Dissimilarities aren’t really needed to identify them as the caps are already of distinct colors.

What a Plastic Cap Does In Water Heater

As we mentioned earlier, plastic caps are used to trap the water flow from going out. But it also traps the temperature of the water in the system. In other words, they contain the heat trap mechanism and work as its part.

Water heaters either work with heat traps or check valve mechanisms. But in both cases, plastic caps on the tops have the same responsibilities. 

The heat we’re talking about here is the heat gained from the heat trap mechanism. This mechanism is what gives us our desired water temperature.

For example, if we’re talking about a gas water heater it needs electricity. The electric energy transforms to heat. This heat gets utilized by the thermal convection with the heat trap loops.

In the process, the hot water inside the tank goes upward. On the contrary, the cold water goes down. A U-shaped deep tube is used in the pipeline to limit the hot water.

Meanwhile, the cold water gathers at the bottom by flowing through the inlet tube. The blue cap sits on the top of this tube and keeps the gateway shut. 

Similarly, the red cap restricts the hot water from seeping out of the top. They both restrict the water flow and at the same time work as insulators. So outside temperature doesn’t swarm into the system.

Water Heater Plastic Cap Problems

A damaged water heater plastic cap doesn’t cause huge problems in the system. But it sure does bring inconveniences in the heater system. For example, problems like the water heater leaking from the top plastic plug.

The main problem that you’ll face is the loss of temperature in the water. A damaged or broken cap can’t function properly as an insulator. The heat trap loop mechanism saves about 60% of the heat energy. 

With a damaged cap that effort will be lost. Heat will seep out during the process. Therefore the stored water might not be as hot as before. 

Not only that it can cause water leakage problems. Another possible issue is the damage to internal components. Because the plastic caps are affixed above the heat nipples.

The caps keep them covered from exposure and external damage. These nipples are dielectric which means they can keep corrosion down.

Losing them will cause you many inconveniences in the future. If you don’t want corrosion to take over the entire system, it shouldn’t be exposed.

This pretty much sums up the issues of water heater plastic caps & their consequences. The solution is to replace the cap as they are not fixable. 

But there are a few scenarios where just replacing the cap isn’t the solution. These problems are related to plastic caps. So, let’s see the details of these issues and how to deal with them.

Scenario-1: Water Heater Leaking From Top Cap

Earlier we mentioned the water leakage issue. Hot water may leak from the top of the cap. But it may not be the cap’s fault. There could be other reasons for it. 

For example, a broken deep tube or loose fitting in the water inlet valves. A malfunctioning temperature pressure relief valve can also be the cause.

Especially if it’s a tankless water heater.

Solution:

First, pinpoint the cause of the leakage. If the long deep tube is broken it needs to be replaced. Not all water-deep tubes are suitable for the task. 

Make sure it’s compatible with your water heater model before buying. If the issue is loose connections, then tighten all the joints with a wrench. Do not overtighten them. That may further damage the connection.

There are two signs that can indicate issues with temperature and pressure valves. One is the water in the tank is not reaching the right temperature. And there’s a leakage too.

You’d also have to check the pressure inside the tank to be sure. Unlike a boiler pressure range, the water heater has higher psi. It should range between 45 to 80 psi.

If it’s higher than 80 psi then it needs to be checked. In case it seems damaged, get a professional to look at it. If the damage isn’t serious, just repair might fix everything.

Scenario-2: Heat Trap Washer Problem

Heat trap washer sits under the cap and just above the deep tube. It helps keep the cap airtight. But it’s not as effective as it should be when there’s a water pressure problem. It fails to trap the heat properly. 

Solution:

Here the main problem is the pressure. To fix this issue you can cut the centre of the washer. So that it sits properly under the cap. If it still doesn’t work, you’ll have to change the heat trap washer.

You may not find the washers without the caps. Thus it’s better to replace the water heater cap altogether. Saves a lot of trouble.

Frequently Asked Questions (FAQs):

We’re leaving a few more popular questions and their answers for you.

How to replace the t & p valve on the water heater?

First, drain all the water from the tank until the water goes below the valve. Now, open a tap of hot water and let air seep into the tank. Unscrew the t&p valve and inspect its condition. If it’s corroded then change it. If it’s not rust-affected then seal the valve with Teflon and readjust.

Can high water pressure cause a water heater to leak?

Yes, it can. If the tank is too full with high pressure it may break the seam. This not only causes water leaks in your heater but also damages internal components. The pressure should not be over 80 psi. Otherwise, the heater may lose space. This problem can lead to destroying drywall or flooring.

How to control the pressure on the water heater?

You need to open the pressure relief valve for this. Lift the valve from its position until the brass stem goes up. This will make space for the hot water to drain by the pipe from the bottom. Now, put the lever back in its position to stop the water flow.

End Note

That’s it for now regarding the plastic cap on top of water heater. We tried to explain all the details thoroughly. Hopefully, that aided you in getting the answers you were looking for.

Stay happy, and stay safe!