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Plastic Caps On Top Of The Water Heater [All Doubts Cleared]

The functions and mechanisms of the water heater may sometimes seem tricky.

On top of that, discovering a whole new feature may riddle you more.

The plastic caps on top of the water heater are such features.

Most people fail to recognize it even though they are very important.

What are these plastic caps on top of the water heater?

They are heat-trap nipples. They keep the heat in the heater when it is not being used. This helps in storing the hot water for further use. There can be two caps on the heater. Both of them help to function the heating system of the heater.

This isn’t sufficient to clarify your confusion. We’ve discussed everything in detail. Read along to find out!

Types of Plastic Caps And How They Work

There are two types of caps on top of the water heater. Generally, One is blue and the other one is red. The red one’s called the hot outlet. The blue one’s called the cold inlet. 

They are responsible for maintaining water flow and heat transfer inside the tank. 

The blue cap is usually connected with a long deep tube. Which almost reaches the water heater tank’s bottom.

On the other hand, the red one does not have a tube like this. But it has a combo anode rod. Both of the caps contain a heat trap mechanism.

Plastic Caps On Top Of The Water Heater: Explained!

Plastic Caps On Top Of The Water Heater [All Doubts Cleared] 1

The plastic caps on the heater work as gateways for the water heater. They are partially responsible for the preferred temperature inside the heater. They also work as insulators so heat can’t get out or swarm in.

Why Plastic Caps are Used on Top of Water Heaters

The caps are part of the system of the heat trap mechanism of your water heater. The heat trap mechanism is responsible for keeping the correct temperature of your water.

If you have a gas water heater, it needs electricity to create heat. The heat trap loops make use of thermal convection and utilize the heat. That is hot water rises and cold water sinks. The system limits hot water at the top of the loop by utilizing a U-shaped dip tube in the pipeline. 

In the meantime, the cold water can pass through and gather at the bottom. The action is controlled by the inlet tube. This inlet tube contains a blue cap on top of it.

In the fitting for the hot water outflow, there is a Teflon ball. However, a polypropylene ball sits in the one for the cold water input.

The balls stay near the valves when your water heater is in standby mode. The balls, however, are lifted out of their location when a hot water tap is turned on. 

Always remember to turn the hot water tap off by the way. Otherwise, It may lead to shower cables overheating.

In both cases, the caps are a doorway for entering the water. They also play the role of insulators for keeping your preferred temperature.

Damaged Plastic Caps Problems And Their Solutions

Damaged plastic caps may not cause a big issue in most cases. You may lose some heat while the heating procedure is in progress. Even while the heater is off, the stored hot water may not stay as hot as before. 

The caps sit just above the heat nipples, so if damaged, they can keep them exposed to harm.

A heat trap loop can save up to 60% of the energy. That would otherwise be lost. When your electricity bills arrive, that adds up well.

Another thing about the nipples is they are dielectric. That means they reduce the amount of corrosion.

So, a damaged cap can cause inconveniences in the long run. This can hinder the overall hot water circulation system of the heater.

Damaged plastic caps will lead you to two greater specific problems. We’ve explained those along with the guidelines to fix them.

Damaged or Broken Deep Tubes

If the deep tube is broken, there will be less hot water. The stored hot water will also run out fast. The faucet area will be clogged with plastics. 


To solve the problem you first have to make sure if the tube is damaged. A broken one is easily recognized. You have to remove the deep tube and insert a new one then

Not all water tubes may be suitable for your heater. From our experience, we can recommend some you might find helpful:

Product 1
Product 2

To remove the broken tube, Insert 2 screwdrivers into it. Now, pull outward on screwdrivers while lifting upwards.

To insert a new tube repeat the same process as the reverse. It will be done with quite an ease.

Heat Trap Washer Problem

This mainly occurs if your house has water pressure regulator problems.

Heat trap washers usually are inserted under the caps and over the dip tube. So, damaged caps can also create heat trap problems. 


If the problem is the pressure, cut the center of the heat trap washer. That should solve your problem.

However, note one thing. These solutions are temporary. After these quick remedies, call the company to ensure they replace the caps soon. 

Try to remember all these things you learned. It should keep you from all the confusion about plastic caps on the water heaters.


Should I remove the water heater heat traps?

Sometimes your water heater heat trap is damaged and causes sound. Then you can remove and replace it.

What is the black thing on top of my water heater?

That could be rusted fragments in the water. Your water heater is likely corroding.

Is a leaking water heater an emergency?

It’s a very serious concern. Call the plumber and fix it immediately.