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What Size Gas Line for Tankless Water Heater? (Answered)

Water heaters play a vital role upon the appliances we regularly use in various chores of our everyday life. Especially in colder states or during the winter, this appliance becomes crucial when we are bathing, washing hands or even drinking water. 

Water heaters provide us with warm comfort, so it is essential to know what size of gas line would be appropriate for it.

What size gas line for tankless water heater? 

The size of the gas line for a tankless water heater would be a ¾ inch gas line. However, if the gas pressure is high and the length of the gas line is short enough – a ½ inch gas line would suffice. While installing, you must make sure your gas line size meets the local gas line rules and codes.

Tankless water heaters are water heaters that heat the water on demand and unlike traditional water heaters – they do not rely on storage tanks. Traditional water heaters take water from the supply line, heat it up and store it in the tank for you to use. 

But in case of tankless water heaters, the water from the supplies directly passes through the heating elements and out through the water outlet – hence, removing the need of a container.

Due to this mechanism, tankless water heaters are much more compact compared to regular water heaters while providing you with a constant supply of water. 

But this is not the only advantage tankless water heaters have over regular water heaters – tankless ones are known to be more durable as they last longer and are safer to use. 

Apart from its issues with scale build up where it triggers the heater to automatically shut off, tankless water heaters are very efficient and effective for supplying hot water.

The working mechanism of tankless water heaters are similar to regular water heaters – they use gas to heat the water. 

However, tankless water heaters have greater demand and usage of gas compared to the regular water heaters since hot water has to be supplied within a short span of time. 

This requires your gas line to have sufficient gas pressure to accommodate the needs of a tankless water heater.

In order to do so, the gas line should be appropriately sized to enable enough gas to be supplied to the water heater. On average, a tankless water heater needs a ¾ inch gas line, however, if your gas supply has high pressure and runs a short line – a ½ inch line would suffice. 

What size gas line for rinnai tankless water heater?

The size of gas lines for a Rinnai Tankless water heater are usually ¾ inch gas lines which are used to supply sufficient amounts of gas to the heater unit’s burner.

But you can settle for a gas line of size ½ inch if the gas pressure from your gas supply is high and if the gas line is short – which would make up for the gas supply rate.

Many places in the United States and Canada commonly use ½ inch gas lines due to better supply lines – which makes it easy to shift from regular water heaters to tankless water heaters while saving time, effort and expenses. 

However, before making installations of your gas line – you should check the local gas line laws as well as the user manual for your Rinnai Tankless water heater.

What is the tankless water heater gas line code?

There are specific gas line codes that you must follow when installing gas lines to your tankless water heater. Maintaining these codes are necessary since they ensure proper safety as well as provide an efficient way to supply gas.

According to CPC 508.4, the water heater should not be installed in an enclosed space such as the attic where leakages could result in damages

And the combustion air vents also need to be well ventilated and unobstructed such that they have 12 inches of open space both at the top and bottom. 

CPC 505.6, 505.4, 508.5 & 608.5 states that the T&P drain must be a minimum of ¾ inch to terminate outside of the building within a range of 6-24 inches from grade. 

CPC 1214.3 states the gas piping test in pressure should not be less than 15 lbs pressure and have a 30 lbs gauge minimum for new or altered gas delivery systems.

What happens if gas line is too small for water heater?

If the gas line is too small, it could have many adverse effects on the water heater and prevent it from functioning as intended. Below discussed are some of the consequences of using an undersized gas line:

Poor performance issues in appliances:

If the gas line is too small, it could lower the efficiency and performance of the water heater and not only will it heat the water at a much slower rate, but also end up producing soot, causing the burner to be exhausted and form condensate in the furnace.

Corrosion of gas lines:

As discussed in the previous point, a small gas line would result in the furnace forming condensate. This condensate along with the air would result in corrosion in metallic gas pipes as they are vulnerable to rusting. 

If that happens, not only will the water heater stop functioning properly but also expose you to fire hazard.

Form toxic fumes:

Soot formed at the furnace could result in clogging the burners or flues which would prevent proper combustion and therefore produce toxic exhaust fumes which have high health risk if they enter your living space.

Cause gas shortages:

A small gas line would form a negative pressure in the gas line which would cause the water heater to work at a lower gas pressure. 

Thus they would try to intake a greater volume of gas to compensate for the pressure – which in turn could create gas shortages in other gas-reliant appliances. 

A large sized gas line is not only necessary to maintain gas pressure but also is necessary to ensure a good volume of gas passage.

How many BTU can a 3/4 gas line carry for tankless water heater?

Typically, residential gas supplies have a gas pressure of 7 w.c. which is considered to be a low pressure system. 

However, the gas pipe lines must be sized appropriately in order to ensure that the maximum drop in gas pressure is only ½ w.c when all the gas appliances around the home are turned on.

This is even more crucial for tankless water heaters because compared to regular tank-type water heaters, the tankless ones require a constant gas supply in high volume when in use – so any drop in supply rate would compromise its performance.  

Therefore, you need to change the gas lines as well if you are replacing an old water heater to a new tankless type. Almost all residential gas lines have a ¾ inch gas supply line which can carry gas of 200,000 BTU.

How to size gas line for tankless water heater?

In order to properly size your gas line for your tankless water heater, follow the steps below:

Determine the LMR of the system:

In order to properly size the gas line, you must first map out a diagram for the gas line from the supply to the heater – taking in account all the obstacles or bends the pipes must take. 

Once the diagram of the gas piping has been sketched, calculate the LMR (Longest Measure Run) of the tubing system required for the tankless water heater.

Determine the gas input requirement:

Next, you need to determine the total gas requirement for your tankless water heater. If you have one water heater, the BTU requirement would be only for that; In case of multiple water heaters – calculate the total BTU requirement for all of them.

Determine fuel, pressure and material:

Afterwards, check which type of fuel is being supplied to your gas lines – whether it is natural gas or propane. Then check the system pressure of the gas supply line – it should be within 7 – 14 inches of w.c. and lastly the type of material being used for the gas line. 

Many households commonly have scheduled 40 pipes and plastic pipes with pressures between 7 – 14 inches of w.c. with a pressure drop of 1 w.c.

Refer to Gas Code Table:

After all the measurements, use the values you have recorded and cross-refer to your local Gas Code Table to determine which size of gas line is most appropriate for use with your gas line system.

Final Thoughts

A ¾ inch gas line would be required for a tankless water heater. However, if the gas pressure is high enough and the length of the gas line is short, a ½  inch gas line will be sufficient. You must ensure that the gas line size fulfills the local gas line laws and codes before installing.