Plumbing, Water Heaters

Can Tankless Water Heaters Run on Propane?

Tankless Wall Lg

If you’ve been wondering whether a tankless water heater can run on propane, it’s probably because your property doesn’t have a natural gas supply. So, perhaps you’re already using propane or thinking of switching from electric water heating and want to go tankless.

Tankless water heaters can run on propane (LP gas). Propane tankless water heaters provide instant hot water on demand, just like natural gas models. Propane can be used where natural gas utilities are not available. It’s important to note that natural gas models are not interchangeable with propane models.

Clearly, there’s a lot to look at when you’re thinking of buying a propane tankless water heater. So, we’ve set out below some of the things you’ll need to be aware of.

Cost To Use a Propane Tankless Water Heater

When estimating the cost to use a propane tankless water heater, it’s important to know how many gallons of propane you’ll use in a month. Also, keep in mind that every family is different, and some families will use more while others will use less.

  • A family of 2 will use about 15 gallons per month
  • A family of 3 or 4 will use about 25 to 30 gallons per month
  • A family of 5 or 6 will use about 40 to 50 gallons per month

As of this writing, the average cost per gallon of propane gas is $2.00 to $2.75 a gallon. For this illustration, we will assume an average cost of $2.40 per gallon.

  • A family of 2 will spend about $36.00 per month
  • A family of 3 or 4 will spend about $60.00 to $72.00 per month
  • A family of 5 or 6 will spend about $84.00 to $96.00 gallons per month

The price of propane per gallon in your area may be more or less depending on local market conditions. This also assumes no other appliances are using propane. If you use propane to heat your home or cook, you’ll likely use more propane than mentioned above.

How To Size a Propane Tankless Water Heater?

When you’re looking at a tankless water heater, the capacity you’ll need isn’t determined by your overall use of hot water. Remember, a tankless water heater differs from a tanked system. It doesn’t store hot water. Instead, it produces it on demand.

So, to find out which size of tankless water heater best suits your needs, you have to work out:

  • Your peak combined flow rate. This is the total flow rate of all the hot water appliances you want to use simultaneously. We measure the flow rate in gallons per minute or GPM.
  • The temperature rise. This is the difference between the temperature of your groundwater supply and your desired temperature.

Your Peak Combined Flow Rate

If your tankless system can’t supply all the appliances you want to use simultaneously, your water won’t be as hot as you want. The system might even shut down because the demand overloads it.

So, let’s say you want to be able to use two water-saving showers and a sink faucet at the same time. Each showerhead uses 2GPM, and the faucet uses 1GPM. That means your peak combined flow rate is 5GPM.

Now you need to establish your temperature rise.

Required Temperature Rise 

The colder your groundwater temperature, the longer it will take to heat up. That slows down the flow rate. 

So, a heater that specifies a flow rate of 9GPM might only operate at 3GPM if the incoming water is cold.

Groundwater temperatures vary on a regional basis. Although there are broad guidelines, you’ll need to check specifics with your local supplier. 

As temperatures fluctuate depending on the season, always use the lowest temperature if you’re given a range. That way, you’ll ensure your heater isn’t underpowered during colder months.

So, let’s assume you want them to run your showers and sink faucet at 105°F (40.5°C).

If your groundwater temperature is 62°F (16.7°C), your temperature rise is 43°F (or 23.3°C)

So, you’ll have to look for a tankless water heater that can give you a 43°F (or 23.8°C) increase in water temperature with the water flowing at 5GPM.

How Do You Use This Information?

Use this information to check the manufacturer’s specifications. 

For example, let’s look at the Rinnai RU Super High-Efficiency propane tankless water heater. You can see from the flow rate chart for this range that the RU130iP would meet your needs.

At a temperature rise of 43°F (23.3°C), you’d get a flow rate of about 5.8GPM.

What if I Live in a Colder Area?

If you live further north, say where the groundwater temperature is 52°F (11°C), you’d need a water heater that can increase the temperature by 53°F (11.7°C) at 5GPM.

In this situation, checking the flow rate chart for the Rinnai range above, you can see that the RU130iP has a flow rate of only 4.6GPM. 

So, you’d have to go for the RU160iP, which would give you a flow rate of about 5.8GPM.

What Will I Need to Install a Propane Tankless Water Heater?

If you’re considering a propane tankless water heater, there are some installation requirements that you’ll need to take into account. 

New Gas Lines

If you don’t already use propane, you’ll need to get a new gas line installed. 

Even if you already use propane appliances, you may still need to increase your gas lines’ size to run a propane tankless water heater.

It’s best to consult a professional installer as it depends on several factors. These include the inlet pressure, the line’s length, what other propane appliances you’re running, and the maximum load.

New Gas Tank

If you already use propane for some of your appliances, you might still want to consider increasing the size of your gas tank. That’s because your propane usage will increase if you start using it to heat your water. 

If you don’t already use propane, you’ll need to add the cost of a storage tank to the initial costs of your tankless system. 

A new 1,000-gallon (3,785 ltr) tank might set you back around $3,500. Renting a tank is an option, but you’d need to get quotes and work out how much it will cost you over the long term.

You’ll also need to ensure an adequate concrete base on which to place the tank.

You can avoid an unsightly above ground tank with a buried underground propane tank. Propane tanks of 250-gallons or more can be buried. The most common buried tanks are 500- to 1,000-gallon tanks.

Underground propane tanks cost between $1,125 and $5,150.

Venting for Propane

To use a propane tankless water heater, you’ll need to take providing venting for the unit.

The venting system provides the incoming air supply required for the combustion process. It also removes exhaust gases.

Venting can be expensive. Because of the high temperature of exhaust gases, you need stainless steel pipework for the exhaust vent.

This needn’t be two separate pipes. You can use a single concentric tube with the stainless steel exhaust pipe inside the PVC air supply pipe. It’s more aesthetic than having two separate pipes. Also, it means you’ll only need one hole on your roof or wall instead of two. 

The venting for most new indoor tankless heaters can be done horizontally, through a wall. This is often easier than going vertically through the roof. It also gives you some flexibility in where you can place the unit.

If you want to save on indoor space, you can choose an outdoor model. Outdoor units don’t need any venting.

For example, let’s again assume a groundwater temperature of 62°F (16.7°C).

Outdoor models like the Rinnai RUR160eP gives you a flow rate of 5.2GPM with a groundwater temperature of 62°F (16.7°C).

Or you could step up to the RUR199eP, which has a flow rate of 6.5GPM at that groundwater temperature. The RUR199eP would also be suitable if your groundwater temperature is the lower figure of 52°F (11°C) discussed above. Your flow rate would then be 5.6GPM.

But, it’s best to use outdoor tankless water heaters only in milder climates. Although most have freeze protection, it relies on a working electrical supply. So, if there’s a power outage, your freeze protection mechanism won’t kick in.


We’ve shown you that tankless water heaters can run on propane. If you don’t have access to natural gas, a propane option is just as good. Just remember there are some slight variations, and you need to be sure you purchase a propane model because a natural gas model is not interchangeable.

You’ll also need to select the right-sized heater and take account of the other installation requirements. But once you’ve sorted those, a propane tankless water system is more than capable of meeting your hot water needs.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.