What Size Tankless Water Heater do I Need? – The Size Guide

When switching from the traditional water heaters to tankless ones most people don’t know which size of tankless water heater unit they need and often ask What Size Tankless Water Heater do I Need? Hot water is an inevitable need of winters. To have a continuous supply of hot water coming from your taps you need to install a water heating unit in your home. The water heating units heat up the water up to the required temperature so it can be used by the whole family. The most common type of water heating unit used nowadays by most people is a tankless water heater. This water heater starts heating water as soon as the faucet is turned on.

One important feature when it comes to buying the perfect water heating unit for your family is the capacity of the water heating unit. It is one of the most important factors that you need to carefully decide while planning to buy a new unit. What if you buy a small unit? This will cause less hot water supply. This means the whole family won’t be able to enjoy a hot water bath at a time.

Less water heating capacity can result in an abrupt splash of cold water from the tap causing the hot water to be used up. The other scenario can be you end up buying a bigger size unit than you need. In such a case that buying cost will be high. Never try to save money by buying an undersized water heating unit cause it will cause a long list of issues later.

This article aims at guiding the common people about how to choose the right size tankless water heater for your home. A water heater should have the apt size that will fulfill all needs of the family. The important aspects to keep in mind regarding this and how to calculate the right size are all discussed in this article.

Factors Determining Tankless Water Heater Size

Following are the important factors that you need to be aware of when choosing the size of your water heating unit.

Flow Rate

It’s measured in gallons per minute. This is a measure of the amount of water that comes out of your tap per minute. It varies from faucet to faucet. Some taps have a high flow rate others have a lower. Choose your tankless water heater depending on this flow rate. If you have purchased an undersize water heating unit then you can use low water flow rate faucets to help reduce the problems caused by the underside of the water heating unit.

Temperature Difference

The temperature difference between the cold water in your tap and the water temperature you need for daily functioning is also important to consider. Make sure you make the groundwater temperature assumption right. If you assume your water temperature to be lower than actual then you will end up buying a lower size water heating unit. So it’s better to assume a safer water temperature limit. Wondering how an undersized water heating unit affects the hot water supply? A smaller unit will have less water heating capacity thus lower water flow rate. If you decide to open multiple faucets at a time you will get lukewarm water cause the unit is not efficient enough to heat up the water. You won’t be able to do multiple glasses of water requiring tasks at a time if your unit is not of a bigger size.

Size of Your Home

Do you live in a small apartment or a big-sized home? It matters when it comes to buying the right size water heating unit. The number of members in your home affects the size of the water heater. If you have few members in your home then install a small unit or it is bigger then opt for a big-sized unit. Most water heaters are efficient enough to heat up the water coming into your whole home. An undersized unit, however, will warm up water only for certain bathroom faucets.


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The storage water heater stores a certain amount of water over time and then heats it. The tankless heaters are different. They heat water as it flows through the pipes. Thus they will heat as much water as flows through the pipes. Thus don’t buy a unit that’s output won’t match the demand of your house. If you have a big-sized home with multiple bathrooms then you need to have a unit with a higher flow rate. If you don’t need a hot water supply in all your bathroom faucets then you can choose a smaller unit.

A water heating unit can be connected to many faucets at a time. It is up to decide how many faucets you want to be connected to your water heater. Add the flow rate of each tap together to get the total water flow rate of the unit. You can connect the water heating unit to a single faucet or multiple depending on your convenience.

The next feature to look for is the temperature difference between the tap water and the desired temperature. If the difference is high it means the unit will be able to supply hot water to a few fixtures only. It is cause high-temperature difference means the longer time needed for heating water.

Right Size Calculator

Calculate the needed flow rate of your water heater carefully. Considering you need dishwashing, a shower, and the standard tap running at the same time. The flow rate for the washing tap is 2.5 gallons per minute. For standard tap water, it’s 2.5 gallons per minute. The shower head water flow rate is 2 gallons per minute. This totals to 7 gallons per minute so you need to purchase a unit with this much water flow rate.

You can estimate the total flow rate by summing the typical flow rate for different fixtures. Some flow rates are mentioned below. This typical flow rate gives an idea about the flow rate of different faucets. As the value is not fixed so water flow rate of your fixture can vary from this.

  • Bathroom fixture flow rate
  • 5 to 1 GPM
  • Showerhead flow rate 1.5 to 2 GPM
  • Kitchen fixture flow rate 2 to 6 GPM
  • Dishwasher flow rate 5 to 2.5 GPM
  • Laundry fixture flow rate 2.5 to 3 GPM

To make the right tankless water heater choice you need to list out the number of water-using appliances you have at home. Now calculate the water flow rate of each device and sum them up. This total flow rate shows your family’s daily water needs and gives the idea of the right water heater size idea.

For example, if you plan to run the dishwashing tap with a flow rate of 1 GPM and the showerhead with a flow rate of 2 GPM simultaneously then the total flow rate needed is 3 GPM. Thus you need to purchase a unit that has a 3 GPM hot water flow rate. If you want to reduce this water flow rate try installing water flow rate lowering fixtures. They reduce the water flow rate through the faucet thus the water heating unit will get more time to heat the water.

Electric vs Gas Tankless Water Heater

The energy source of a water heater affects the rate of water heating. A gas tankless water heater can heat 5 Gallon of water up to a temperature of 70 degrees Fahrenheit in 5 minutes. An electric water heater on another hand can heat only 2-gallon water to the same temperature in 5 minutes. Thus it indicates a huge difference between the per minute water heating capacity of the two units. The choice of the right unit depends on your family’s water needs. At times if you have already electric wiring in your home then it will be more feasible to get an electric unit installed. 70 degree Fahrenheit temperature is used as a standard because the groundwater temperature is 40 degrees Fahrenheit. It needs to be raised by 70 degrees to reach the steamy hot water temperature of 110 degrees Fahrenheit.

If you live in a warm climate then your case is different. In hot climates, the groundwater level is higher so there is less temperature rise. The water temperature needs to be raised to 110 degrees Fahrenheit in each case. With higher groundwater temperature the temperature rise will be 55 degrees Fahrenheit only. A higher groundwater temperature reduces the need for prolonged water heating.

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