How To Tell Which Water Line Is Hot And Cold On Water Heater

If you are wondering how to tell which water line is hot and cold on the water heater, you’ve come to the right place. Without identifying the two, you can’t properly install your water heater.

You’ll get a high gas bill but not high heating. But you don’t have to worry. We’ve got you covered. Just keep on reading!

What Happens If You Hook Up a Hot Water Tank Backwards?

You may end up hooking your heater’s hot water tank up backward without being able to tell apart the hot and cold water lines.

In that case, the water at the bottom of the tank will be taken up by the dip tube. This water is hot but will become cold as the cooler water from the top that enters the heater simultaneously fills up the tank once again. 

That is because the cold water descends to the bottom of the heater and cools down the hot water taken by the dip tube. This means you don’t get water as hot as your heater is capable of supplying. Installing a water heater backward makes your water heater inefficient and wastes a lot of energy.

So, you must make sure to hook up your water heater tank as the manufacturer suggested. 

How to Tell Which Water Line is Hot And Cold on Water Heater?

The pipe that is running to your heater is the cold water inlet. Usually, the water line runs into the tank at the top. 

The water line running from your heater to the rest of the house is the hot water outlet. The hot water outlet also starts at the top in the case of most water heaters. That is because water once heated automatically rises to the top. 

Your heater manufacturer will mark the inlet “Cold” and the outlet “Hot” so it will be easy to spot.

When it comes to identifying the hot and cold water lines in different areas of your house, the hot water is generally on the left side and the cold on the right. 

That is the standard in North America. The only time you’ll find a hot water line to the right side is when the installation was done incorrectly. This standard also applies to dual-lever faucets.

So the next time you are wondering which ones are the hot and cold water lines under the sink, the left one is hot and the right one is cold.

How to Fix Hot Cold Water Crossover?

If you find hot water coming out of the cold side of your faucets, you likely have a cartridge that’s failed in any one of the faucets in your house. One failed cartridge can cause this issue in your whole house.

These cartridges are used in single-handle faucets, like in the shower, or sometimes bathroom sink or kitchen sink. They mix and control the flow of the hot and cold water. 

How Do You Find Which Faucet Has Failed?

There are a few different ways to diagnose this water heater issue and find out which faucet in your house has failed exactly. You can go to each single-handle faucet in your house and turn it onto the cold side. The faucet which is the warmest is likely the one that has failed.

Sometimes, a failed faucet won’t have any cold water running out of it at all. 

Another way you can identify a bad faucet is to think if you have used a faucet recently that hadn’t been used for a while.

If you have used such a faucet and after using it you’ve been facing a hot-cold water crossover issue, then that faucet is most likely the one you need to fix.

How to Replace the Cartridge?

It’s a very simple, and easy task. All you’ll need is a cartridge, a cartridge puller to remove the old cartridge, a Philips screwdriver, and some needle-nose pliers. Here’s how you can do it:

Step 1: Make sure to turn off the water to your house, or if you can isolate and turn off the section you’ll be working on. Let the water in your pipes drain. So water doesn’t come gushing out when you replace the cartridge. 

Step 2: Remove the knob. Sometimes there’s a set screw that you can remove which then pulls off the handle, and sometimes there’s a cap that covers a Phillips head screw.

Step 3: It’s a good idea to take something like a paper towel or any kind of towel and just go ahead and plug the drain. So if anything falls out while you’re working on your faucet, you won’t lose it down the drain.

Step 4: After the screws are removed, you can just pull the knob right off.

Step 5: Next, You’ve got the plastic hot water limiter. And this device goes in here and allows you to set the maximum hot water mixture that you use with your faucet. If you like where it was. Make sure to leave the two pieces of plastic set together in the same configuration for when you replace it.

Step 6: And then you’ve got a washer to remove, and then the sleeve. 

Step 7: Use your needle nose pliers. There’s going to be a pin holding the cartridge in. We can pull that pin right out. 

Step 8: Next,  grab your special cartridge removal tool. Take that and unscrew the Philips screw from the back, and then make sure the teeth are exposed on the front part, which will wrap around the plastic sides of the cartridge.

Step 9: Once that’s up against the cartridge. Screw in Phillips screw into where the faucet handle was screwed into that screwed it. You can now crack down on the nut portion of the cartridge removal tool. And that’s going to pull it out a bit. You should be able to kind of hold and wiggle it out.

Step 10: With the cartridge removed, you can now take the lubrication package that came with the new cartridge. Open that up and lubricate the rubber seals, and the gasket on the new cartridge.

Step 11: Now, put the new cartridge in, you’ll see an ancient Icy Hot and cold engine aligned for the bottom. So put that in with hot on the left gold on the right and the line at the bottom. Push the cartridge back in.

Step 12 Reinstall the clip, and slide the sleeve back on. Put the washer and hot water limiter device back on. Take the handle and line that up here correctly, and put that back on.

Step 13: Give that a couple of turns, and make sure that it’s working correctly. And then we’ll take the screw that holds the handle in place. 

This should solve your hot cold water crossover issue


Should both pipes on the water heater be hot?

Ans: The cold water inlet can get hot by convection. So, both pipes are likely to be hot but not to the same degree. 

Should a water heater be hot to the touch?

Ans: Water heaters are made with insulating materials so they don’t get too hot. So, your water heater should be hot but not so hot that it can’t be touched.

What would cause a water heater to catch on fire?

Ans: Gas leaks and electric sparks can cause a water heater to catch on fire. But most heaters these days, have safety measures to detect safety hazards and shut down the unit during such occurrences.

Is it OK to turn off the gas water heater?

Ans: It is not only OK but good if you turn your gas water heater off if You’re not going to use it for a while. Doing that saves gas and prevents overheating.

Can you still use hot water if the water heater is leaking?

Ans: I’d say it’s safe but quite bothersome. You should get it fixed as soon as possible. 


Now you know why it is so important to know which one is the cold water inlet and which one is the hot water outlet in your water heater.

If you’ve got more questions, you can leave us a comment below. We love hearing from our readers!

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

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