Why You Can’t Connect PEX Directly to Your Water Heater

A steamy, hot shower goes a long way in rinsing away the day’s stress and muscle aches. But to achieve such a result, you’ll need access to hot water. Enter your water heater. This appliance ensures you have plenty of hot water to wash dishes, take soothing soaks, and do anything else you need hot water for.

Key Points:

  • PEX is a flexible plastic tubing used in DIY plumbing projects due to its ease of use and affordability.
  • While PEX offers advantages such as freeze resistance and ease of installation, it also has drawbacks such as vulnerability to UV light and susceptibility to rodent damage.
  • PEX is not recommended for direct connections to a water heater due to numerous issues that can arise, and it is best to use other materials for this purpose.

The water heater is hooked up to your home’s plumbing, receiving cold water from the supply lines and spitting hot water out the other line. Installing these systems is simple enough with a bit of know-how and a few standard tools. But can you use PEX for direct connections to your water heater? It’s not recommended (and usually not allowed according to the plumbing code).

Here’s what you need to know about why you can’t connect PEX directly to a water heater.

What is PEX?

Cross-linked polyethylene, better known as PEX, is a flexible plastic tubing popular in home DIY plumbing projects. The flexible nature makes it easy to work with, so it’s a staple among DIYers, as you don’t need an excessive number of joints or special tools to install it.

The tubing is made of high-density polyethylene, which costs less than half the price of copper but is much easier to work with. It comes in three standard colors: white, blue, and red. Generally, blue PEX is used for cold water supply lines, red works for hot water supply lines, and white is used for either.

You might find other colors, like grey PEX, but white, blue, and red are the most common.

Pros and Cons

Like any plumbing material, PEX has a few pros and cons. When considering material for your next plumbing project, it doesn’t hurt to consider these aspects. Let’s start with a few advantages of PEX plumbing:

  • Easy to work with: The flexible nature of PEX makes it easy to work with, as it isn’t nearly as rigid as metal piping. This makes it much easier to plumb your home, as you won’t need nearly as many joints as you would with metal piping.
  • Freeze resistance: PEX can expand and contract somewhat, offering better freeze resistance than other materials. This means it is less likely to burst if the pipes get too cold and water freezes in them. Considering how expensive and messy burst pipes can be, this is a major plus!
  • Inexpensive: Compared to other popular plumbing materials, like copper, PEX tubing is shockingly affordable. In fact, it often costs less than half the price of copper piping!
  • DIY-friendly: The ease of installation and affordable cost of PEX tubing makes it an incredible DIY-friendly option, especially for less experienced folks and those on a budget.

Of course, PEX has a few drawbacks to consider, including the following:

  • Not freezeproof: Although PEX offers freeze resistance, it isn’t freezeproof. So, while the material has some flexibility that allows it to expand and contract, it can only do so much. This means there’s no guarantee that it’ll prevent burst pipes entirely.
  • Susceptible to sunlight: PEX is vulnerable to damage under UV light, like that of sunlight. Because of this, PEX is only suitable for installations where it’ll be completely isolated from UV light, like underground or indoors.
  • Rodent issues: Unfortunately, rodents enjoy the convenience of a close and accessible water source, like your PEX plumbing. While thick metal pipes can be enough to deter tiny pests, PEX plumbing doesn’t pose any difficulty to these creatures. Instead, the flexible material is surprisingly easy for them to chew through, potentially leading to leaks and water damage throughout your home.

Can You Run PEX Directly to a Water Heater?

While PEX tubing is a popular pick for various plumbing projects due to ease of use, it isn’t suitable for water heater applications. Numerous issues arise when running PEX directly to a water heater, so it’s best to steer clear of doing so.

However, this doesn’t mean you have to scrap PEX from your home’s plumbing altogether. You can still use PEX to outfit your home, but when you get to the water heater, you’ll need to stop 18 inches short. You’ll need to use 18 inches of metal piping for the direct connection, but beyond that, you can use PEX.

4 Reasons Why PEX Isn’t a Suitable Pick for Water Heaters

Although you might be tempted to skip the hassle of using an 18-inch length of metal pipe to connect your PEX tubing to your water heater, it’s best to think again. Here are four reasons why PEX isn’t a good fit for direct connections to water heaters:

1. Plumbing Code Requirements

When working on a plumbing project, it’s essential to work within plumbing code requirements, whether you DIY the project or hire a professional. Of course, hiring a professional eliminates the guesswork, as they should be familiar with any code specific to water heater installations.

However, if you’re DIYing the project, you’ll need to browse through pertinent information. In this case, we can find information in the Uniform Plumbing Code (UPC) 604.11.2, which explicitly states that PEX tubing cannot be installed within the first 18 inches of piping connected to a water heater.

While the specific plumbing code may vary based on your location, the consensus is that PEX is generally only suitable for use indirectly on a water heater. If you’re unsure what plumbing code your state or region abides by, do a quick Internet search or ask your local plumber.

2. Fire Hazard

When outfitting your water heater with plumbing, choosing a more fire-resistant material is imperative. Unfortunately, PEX doesn’t meet these requirements, and instead, its high-density polyethylene composition makes it highly flammable.

So, if it were to catch fire due to a complication with your water heater, it would burn fast and easily. If your entire home is outfitted with PEX tubing, the fire could move into other areas, potentially creating catastrophic results. Once PEX tubing starts on fire, it will stay lit. To remain on the safe side and err on the side of caution, it’s best to skip PEX tubing around your water heater.

3. Warranty Complications

Water heaters are expensive pieces of equipment, but luckily, most of them come with warranty coverage. The warranty comes in handy in situations when the system encounters issues or taps out sooner than it should, protecting your investment and relieving stress on your wallet.

However, many manufacturers outline specific terms and conditions surrounding their water heaters, including numerous exclusions. When you purchase a water heater, it’s essential to consider these exclusions because they could spell out trouble for you down the road if you don’t take heed of them.

One of these exclusions is the material you use for connecting the heater to your home’s plumbing system. Some manufacturers specifically state you cannot connect PEX directly to the unit. If you do, you will void the warranty. This means that if or when an issue with your water heater arises, you’re on your own.

Of course, the exclusions vary based on the manufacturer, so not all warranties may become void due to using PEX on the heater. However, it’s worth the extra few minutes it takes to glance through your warranty and determine whether connecting PEX directly to the heater voids the warranty, as it can be an expensive mistake.

4. Increased Chances of Leaks

PEX is renowned for its flexibility, allowing it to expand and contract under temperature fluctuations. While this is ideal to aid in preventing pipes from bursting in the cold, it’s not ideal for water heaters.

As the water heater functions, the PEX will expand and contract due to temperature fluctuations. This can lead to various issues with the tubing, potentially allowing it to loosen and leak. Of course, you could use heat-rated PEX, but in general, it isn’t a good idea to use this material on your water heater.

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