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What Size Water Line for Water Heater? (Answered)

Water heaters are a must-have appliance, especially in colder states or during the winter. This piece of appliance provides us with warm comfort, so it is essential to know what size of water line would be appropriate for it.

What size water line for water heater? 

There are generally two sizes of water lines: ¾ inch and ½ inch in diameter with a length of 16 to 24 inches. ½ inch supply lines were used in older water heaters and they supply water at a lower pressure. ¾ inch lines have become the standard for most lines and are user-friendly as well.

During winter, water heaters become one of the most important and frequently used appliances. 

They are used to heat water and keep the temperature of the water high which provides us with a continuous supply of hot water through our showerheads, baths and other places as well. 

Water heaters are popularly used because they can store hot water in their containers for a long time without losing significant amounts of heat – which means you can have a steady supply of water for a while without increasing your utility bills.

In order to accommodate the proper supply of hot water from your water heater – you must use water lines of appropriate diameter. 

In older water heaters which were set up in the 90s, they used water lines of ½ inches diameter – however they had low water supply pressure and thus they were less efficient. Modern day water heater lines, however, use a much wider water line of ¾ inch. 

They can efficiently supply water at a higher pressure. 

A ¾ inch supply line is most recommended since hot water demand across homes has increased over time as there are multiple showerheads, body sprayers and tubs across homes – which means the water line must meet all the demands effectively. 

But for homes with low water supply demand, ½ inch supply lines would work well.

Before you consider picking a supply line, you need to check the supply line requirements of your water heater as well. 

Tankless water heater: 

A Tankless water heater generally requires a ¾ inch or 1 inch supply line. 

Hot water heater: 

In case of Hot water heaters, they require ¾ inch lines if it is installed after the 90s – but if it’s older than you would need a ½ inch line.

Does a water heater need 3/4 pipe water line?

There are two types of pipe water lines commonly used for water heaters: ½ inch wide and ¾ inch wide. However, the ½ inch lines have the disadvantage of slow water flow 

and cannot meet the demands of houses where there are more water outlets. 

Therefore, water heaters need to have ¾ inch connections to enable water to flow at an increased rate and to have a higher pressure as they are wider than ½ inch pipes. 

However, if your water heater was installed before the 90s then a ¾ inch line may not be appropriate for you since those water heaters are compatible with ½ inch pipe lines.

Another reason for you to use ½ inch wide pipes is if your total water usage is low – since ½ inch water lines have a lower supply rate, it should be suitable for your needs.

What diameter water pipe should I use for water heater?

Before you go on to choose a water pipe for your water heater, there are several factors that you need to consider which will determine that water pipe diameter size that is most suitable for you. 

The first thing to keep in mind is the model of your water heater and the time of installation – if your water heater has been installed long ago back in the 90s or is an older model that was manufactured with older designs. 

Then you would need to choose a pipe of diameter ½ inches wide. But if your water heater was installed after the 90s or is newer, then you would need a ¾ inch wide pipe.

Another factor is your total number of water outlets – ½ inch wide pipes are known to have limited water supply rate. So if your house has many water outlets for hot water, you should consider getting the ¾ inch pipe, otherwise the ½ inch pipe would work well.

Are all water line pipe the same for water heaters?

Our homes have several water lines that are each meant to serve purposes and thus have different water requirements. For this reason, not all water lines are the same and are made of different materials of varying dimensions

Cold water supplies are readily available to you as they reach to you directly from the water supply lines, but hot water has to be heated up in the water heater first before it reaches you.

To achieve this, one pipe feeds the cold water into the heater from where the water gets heated by the heater and is sent out through a separate heat-resistant pipe to be sent to your water outlets.

What type of pipe is used for water heaters?

There are several types of pipes that are suitable for water heaters, the types of pipes are detailed below:


Copper has been one of the most used materials in piping due to its strength and durability. It is also resistant to chemical strikes and corrosion while reducing heat loss when delivering hot water. 

Despite being durable and reliable, it has high set up cost as well as requires more compression fittings and joints – making it a less favorable option for people choosing to cut on plumbing costs.


CPVC has the same chemical composition as PVC but with an addition of chlorine which makes the plastic more durable and resistant to any form of corrosion. Its effectiveness at keeping away contaminants allows it to supply clean water as well. 

CPVC also has excellent insulating capabilities and is also much cheaper than copper or galvanized steel – making them more appealing to plumbers. However, it lacks durability in contrast to copper or galvanized steel – so choose according to your needs.


PVC is the most commonly used plastic pipe for water lines since they are very durable and versatile. They are also known to be the least expensive of all the options for water lines out there such as steel, copper or PEX while excellently insulating the water.

But the problem with PVC is that they are not much resistant to hot water and tend to be vulnerable to contamination. PVC can also start deforming if the temperature exceeds 140 degrees Fahrenheit. If you plan to use PVC, it would work best with steel or copper piping.

Galvanized steel:

Galvanized steel is another material that is commonly used in plumbing systems. They are great for water heaters but would rust in the long term but their durability is greater than copper. But it is less preferred for drinking water lines since they slightly change the water.

How do I calculate water line size for a water heater?

Choosing the correct water size may seem like a confusing process, but if you follow the steps below – you can easily determine which water line is most suited to your needs:

Determine your fixture count:

The water line size is heavily dependent on the total water supply demand of your house. If you have many water outlets that would require hot water – you would need a ¾ to 1 inch wide water line.

Determine your house size:

If your family size is bigger, the overall hot water demand would be greater as well. Depending on your family size, you need to increase the pipe diameter by a quarter inch. A typical family would need a line of ¾ inch to 1¼ inches wide in diameter.

Factor in your water heater’s age:

Older water heater’s are not designed to supply high volumes of water since there were fewer water outlets in the past. 

Therefore, if your water heater is older, it may not be able to accommodate your hot water supply demands since they are compatible with ½ inch wide water lines.

Once you have determined all these factors – you can estimate what water line size you need.

Final Thoughts

Water lines are typically available in two diameters: ¾ inch and ½ inch, with lengths ranging from 16 to 24 inches. ½ inch supply pipes were used in older water heaters, resulting in reduced pressure. ¾ inch lines have been the standard for most lines and are also user-friendly.