a broken toilet tank. top view.

Why Is the Water in My Toilet Tank Hot?

A toilet should have cold water running through it for a number of reasons. There can be a rare issue where instead of cold water, hot water runs into the toilet water tank. Here’s a summary of why this occurs, and how to fix it.

Overall, if it’s a new toilet the plumbing can have been installed incorrectly. The hot water inlet hose has been connected to the toilet by mistake. The other reason is if you have a plumbing set up with a single control valve. If the valve fails it causes hot water to flow into the toilet.

To diagnose the issue is a bit tricky. But, if the toilet has recently been installed, or the plumbing has been changed it’s virtually guaranteed that it has been installed incorrectly. Below, I will cover how to diagnose why it’s happening, and what you should do to fix it. As well as, if it’s bad to continue to run your toilet with hot water.

How To Diagnose Why Hot Water Is Going Into Your Toilet Tank

Ideally, a plumber will need to take a look at it, and the water supply lines to a toilet are typically located behind the wall. In plumbing hot water and cold water are separated by two different lines. In newer homes, red and blue hoses are used to be able to tell them apart. 

red and blue hoses installed in water supply line
Image from Capital Plumbing & Heating, Inc.

But, in recently built homes, or much older homes the plumbing pipes can be the same color. For this reason, it can be a bit of a mistake on the part of the plumber. Where they have connected the hot water line to the toilet by mistake (source).

If the toilet has recently been installed you should contact the plumber directly. They will need to disconnect the hot water line and connect the cold water line to your toilet instead. When this is the case, a small part of the wall will need to be removed to see whether the hot water line or cold water line is connected to the toilet.

A single valve has failed and needs to be replaced

If your toilet has been operating for some time with cold water, then all of sudden the water is now hot, it’s more likely that one of the valves has failed. In some plumbing setups, hot water is controlled by one valve rather than two separate water lines. 

This valve directs hot water to the nearest fixture – such as the shower head or vanity unit. If this valve fails it won’t direct water to the right fixture. And hot water can flow into the toilet tank. For this issue, it’s best to get in touch with a plumber to diagnose the issue.

Here’s a video from Twin Home Experts about how this occurs:

Another issue – condensation from cold water

It’s possible to mistake condensation for hot water. As you may know, condensation occurs when the water in the air partially freezes to a cold surface, then melts. This causes water to form on the exterior of surfaces such as the toilet water tank. It’s important before calling a plumber to lift up the water tank and see if the water in the tank is indeed hot. It’s possible the weather has been particularly cold, and condensation is forming on the outside of the tank due to condensation.

Is hot water in a toilet tank an urgent issue

If your toilet tank is filling with hot water it’s not an urgent issue. But, it shouldn’t be left for a number of reasons:

  1. It will degrade the parts in the water tank more quickly
  2. It uses more electricity or gas
  3. It causes more bacteria and mold to form

The components in a toilet water tank are made of plastic, and rubber types of materials. In hot water, they will break down more quickly. Especially, because the hot water typically sits on them for at least a few hours, in between flushes. It has been shown in experiments that plastic becomes weaker when soaked in warm to hot water compared to cold water. This will cause them to break apart.

The appliances that use the most electricity in a home are the hot water tank, heaters, dryers, and ovens. They also use a reasonable amount of natural gas, if this is what your home has. 

Therefore, it’s best to minimize the amount of hot water your home produces where you can. A toilet also uses a tonne of water with typical use. I explained how much water a toilet uses in this article about how much water a toilet tank holds.

molds in toilet bowl
Image from Osceola County SHIP Housing Rehab Program – Gone Wrong

Bacteria and mold grow best in a warm environment. As the hot water enters the bowl it will warm it up, and make it more hospitable for bacteria and mold. In some cases, if it’s hot enough it will kill mold, and bacteria but the areas around the bowl that don’t come in contact with water will get slightly warm.

Can Hot Water Damage a Toilet

As you may know, virtually all toilets are connected to the plumbing so that cold water fills the toilet water tank rather than hot water. However, hot water is good for cleaning, and your plumbing may be installed incorrectly so that hot water is going into your toilet water tank instead of cold water. But, does hot water damage a toilet?

Overall, hot water will not damage a toilet. It’s perfectly fine to pour hot water into the toilet bowl to clean or unclog it. As well as, use hot water to clean the exterior of the toilet. But, the water that enters the toilet tank should be cold, and avoid using boiling water. 

crack in toilet tank
Image from Paul Davis Restoration of Cleveland Metro West

Boiling water has the potential to crack the bowl. This occurs because a toilet bowl is typically cold. The sudden change in temperature can cause the porcelain to crack. It’s unlikely to occur with a toilet bowl because it’s very thick but is known to happen with porcelain cups. Therefore, it’s best not to risk it by pouring boiling water into a toilet bowl.

What Temperature Is Toilet Water

Toilet water makes its way into a toilet bowl from the water tank that empties and fills each time a toilet is flushed. The water is filled by cold water lines, rather than hot water lines. But, here’s what temperature toilet water is.

On average, toilet water is the same temperature as the air temperature in the room where it is. Room temperature is 68 °F (20°C), so toilet water in a room that is room temperature will be 68 °F (20°C). But, room temperature water feels colder than its actual temperature due to thermodynamics.

Cold water requires more heat to warm it up compared to the air. As a result, cold water draws more heat away from your skin, for example, when you dip your hand into cold water. And gives the sensation that it’s colder than it is. 

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