How Big of a Tankless Hot Water Heater Do I Need?

You might be wondering how to go about replacing the tank-top water heater you currently use with a newer model. You might be asking yourself, “What size tankless hot water heater do I need?”

That is a question that is understandable from someone who is overwhelmed with the number of options from this appliance. However, immediately giving you a specific choice will be inaccurate since some things need to be verified first.

How Big of a Tankless Hot Water Heater Do I Need?

Knowing the size of the tankless water heater suited for your home is the first factor to consider. Follow the steps below to know the right size for your homes.

Step 1: Calculate the GPM

Estimate the maximum GPM (gallons per minute) by getting the total amount of each fixture to utilize hot water at the same time.

You can use the information below to calculate your consumption:

  • Shower – 3 GPM 
  • Bathroom Sink – 0.5 to 1 GPM
  • Washing Machine – 2 to 3 GPM
  • Kitchen Faucet – 2 to 3 GPM
  • Dishwasher – 1.5 to 3 GPM
  • Bathtub – 4 to 6 GPM

Step 2: Find Out the Needed Temperature Rise

The temperature rise is the amount of heat required to achieve the desired outgoing water temperature. To calculate the temperature rise, first determine the incoming water temperature from the ground. Subtract this number from your desired water temperature to get your needed temperature rise. You can find out what the groundwater temperature is in your area online.

Step 3: Decide on Your Power Source

The next step is to select the power source for your tankless water heater. You need to determine what kind of power is available in your home. If you have access to both gas and electricity, choose the less expensive one for long-term use.

Choosing a Tankless Water Heater

Determining the size of the tankless water heater is not enough. You need to choose the type of tankless water heater appropriate to your needs.

1. Gas vs. Electric

It’s easier to decide on this factor if you already know what your power source is.


Electric tankless water heaters are suitable for small homes that need minimal water demands and with warm groundwater temperatures.

Additionally, the initial cost of electric tankless water heaters is lower as they are more affordable and easier to install.

There are also different types of electric tankless water heaters.


In a typical day, several members of the family will most likely use hot water at the same time. This kind of scenario requires a water heater that can cater to the family’s needs. The whole-house water heater is the perfect appliance to provide the hot water needs of everyone in the household.

Point of Use

This small single-use electric water heater is ideal in providing hot water for one faucet. These are easy-to-install units that are efficient and altogether remove the standby heat loss.


These portable and lightweight units have small GPM but can be installed and carried anywhere. This water heater type is perfect for camping and traveling or installed in your RV.


This outdoor water heater is typically installed on the wall outside of your bathroom. It is also equipped with a freeze protection system so that it can still function during winter.

Some models may even come with enclosures to protect it from the elements. This heater type is ideal for houses with small spaces.

Gas Tankless Water Heater

Gas tankless water heaters, however, can produce more hot water quickly. Thus,  these are recommended in areas with colder groundwater temperatures.

They are also suitable and economical for bigger homes that may require more hot water.

2. BTU

BTU or British Thermal Unit is the amount of heat you need to increase the water temperature by one Fahrenheit for every pound.

Hence, tankless water heaters with high BTUs can heat more water. If you wish to have hot water for the whole house, look for a water heater with high BTU.

3. Point of Use vs. the Whole House

Another factor worth considering is how you intend to use your hot water supply.

A sizeable tankless water heater unit will be perfect for the whole house to accommodate its higher hot water demand.

Nevertheless, you can opt to use smaller tankless water heaters for each point of use. As a result, it offers lower GPM, which will be sufficient for your on-demand needs.

4. Installation

Installing a tankless water heater is not as easy as buying the unit and placing it. The installation prices for a gas-powered tankless water heater supposed to provide the whole household is more expensive. You can forecast the price based on its location from the water and gas sources to the fitting and the duration of its venting system.

Electric-powered tankless water heater installation costs less as it does not require venting. Although, you will still need the service of an electrician to ensure safe and correct installation.

how big of a tankless hot water heater do i need

Tankless Water Heater Types

Currently, you can choose from these three tankless water heater types for your home.

1. Non-Condensing

The non-condensing type is the first generation of tankless water heaters. It uses heat exchangers in heating the water on-demand, giving it an efficiency rating level of 82% to 85%.

Japan and Europe have been using this technology even before reaching the United States. This appliance has proven its reliability with minimal manufacturing issues.

2. Condensing

The second iteration of this machine is the condensing type. Instead of just one heat exchanger, it uses two. The second heat exchanger takes the heat from the exhaust and uses it to help increase the water temperature.

As a result, the efficiency factor for the second generation increases to 92% to 94%. The cooler exhaust allows you to use an affordable PVC material for venting purposes. However, second-generation units are more expensive and less efficient for small amounts of water.

3. Condensing Hybrid

This third-generation type of tankless water heater was developed in the United States. This appliance includes a small holding tank, around one liter to two gallons capacity.

A storage water heater is a device that stores hot water in a tank and releases it in small amounts, thereby increasing efficiency by 94%. These water heaters have less pressure fluctuation and faster delivery of hot water than earlier models. Many storage water heaters are produced by small companies, so quality and warranty may be questionable.

Tankless Water Heater Maintenance Tips

Whether you call a plumber or you do it yourself, regular tankless water heater maintenance is essential. These appliances need regular cleaning if you want to maximize its lifespan.

Areas where the water is full of minerals can affect a water heater’s performance. A professional can clean the heat exchanger from scale buildup to improve the water heater’s performance. A yearly cleaning is generally sufficient, unless the temperature setting is too high or there is hard water in the area.

If you opt to clean it yourself, here’s a step-by-step guide on how to do it. Before starting, make sure you read the user’s manual on how to care for your water heater.

Step 1: Turn off the power before removing the unit’s cover. Then, close the water valves going to the heater.

Step 2: Remove the valve caps on the purge valves of both cold and hot water pipes to ease the pressure.

Step 3: Then, hook up the hose lines at the same angles as the hot and cold water valves.

Step 4: Remove mineral deposits from the heating elements using white vinegar.

Step 5: Drain and flush according to the manufacturer’s instructions. Afterward, close the valves of the purge port after flushing by twisting the T-shaped handles.

Step 6: Detach the hose lines, return and tighten the caps on the valves of the purge port.

Step 7: Search and remove the filter then clean its screen and housing before putting it back.

Step 8: Finally, check your user’s manual to ensure that you can safely restart the heater.

Tankless Water Heater Benefits

There are many reasons why several households are choosing tankless water heaters. Here are some of them:

1. Endless Hot Water

This water heater will not deplete its hot water supply as long as it is connected to the water source. Hot water is readily available whenever you need it but not instantaneously, as it may take several seconds for the water to heat up.

2. Space Saver

Tankless water heaters are space-savers because they are mounted on the wall. They don’t need large storage tanks that occupy a lot of space. There are even units that you can install outside of your house to free up indoor space.

3. Energy-Efficient

The amount of heat transferred measures the efficiency of a tankless water heater from the power source to the water. The current efficiency level of tankless water heaters is more than 80% since there is no need to reheat stored water in a tank. Therefore, you can lower your water heating expenses by 30% to 60%.

4. Longer-Lasting Product

These water heating appliances are made from premium quality materials, and components are not easily affected by leaks and rust. As a result, the tankless water heater can last up to 20 to 30 years.

5. Increase Home Resale Value

Having a tankless water heater installed in your home can add to its market value in case you decide to sell your home.

Tankless Water Heater Downsides

We have talked about the benefits of tankless water heaters. This time, we will check some of its disadvantages.

1. Retrofit Cost

If you decide to change your traditional water heater system to a tankless one, be prepared to spend a lot of money.

The cost of the initial installation project ranges typically from $2,000 to $4,500. The amount includes changes with your venting, gas, and water lines to adapt to the new system.

2. Wait Time at the Faucet

Some people believe that they will instantly get hot water after opening the faucet. However, tankless water heaters do not provide instant hot water, much like the traditional type.

Sometimes, it may even take a few more seconds longer before hot water comes out of the faucet.

3. Unsure Savings 

Some people are unable to reduce their water heating expenses after installing the tankless water heater.

Their consumption increased, knowing that they have an unlimited hot water supply.

4. Requires Electricity

Some homeowners decided to use an electric-powered tankless water heater. They will not have any hot water supply in case of a power outage.

5. Additional Equipment Needed

You may need to install a water softener for the water heater to work correctly.

Buying this additional device adds to the overall installation cost. This unit will also occupy additional square footage in the house.

Common Problems With Tankless Water Heater

Just like any other product, tankless water heaters experience issues, as well. One of the most common problems is the heater’s failure to produce sufficient hot water.

Here are some reasons why it is happening and some tips on how to resolve it.

1. Demand Is Too High

If you have numerous fixtures and appliances that necessitate hot water at the same time, you will have scarce supply. Some potential causes for this elevated demand may be an upgraded bathtub or shower that necessitates more hot water. Adding a new bathroom signifies additional demand for hot water. New appliances that utilize hot water can also contribute to the required supply. The most favorable solution to this problem is to replace your water heater with a larger capacity. You can also make a schedule so that hot water utilization is evenly distributed.

2. Cold Incoming Water

In areas where the weather is freezing, your incoming water may be too cold for the water heater to heat up quickly. You may feel that your unit is not producing enough hot water.

What you can do in this situation is to increase the temperature setting of your water heater.

3. Reduced Incoming Water Pressure

If the cold incoming water pressure is reduced, the outgoing hot water pressure will also decrease.

This problem may be a bit harder to resolve as there may be old and corroded pipes requiring replacement.


In the first part of this discussion, you asked, “How big of a tankless hot water heater do I need?”

We have provided information on how you can calculate the size of your water heater.Once you have determined the right size, you can check some of the best water heaters and buy the one that meets your needs.