How to Size a Tankless Water Heater

Tankless water heaters are mechanical household appliances that make heated water available to your home at all times. They help to readily supply your need for heated water at home without stress. 

These water heaters help to save space in your home and still provide you with hot water, unlike the tank-water heaters that take up space. They are placed on a small portion of your wall.

Many people prefer this tankless was heater due to many reasons, some of which are longer lifespan, more extended warranties, low maintenance cost, among others. To install the water heater, you need to know how to size a tankless water heater. And that is why this article is worth your time.

How to Size a Tankless Water Heater

• Determine the Mounting Space 

Tankless water heaters come in various sizes. The amount of space you have in your home largely determines what size of the water heater should come in. Most innovative water heaters can provide heated water for your home at once. So, going for a small-sized heater can be ineffective.

The water heater size you choose should not be too small that it cannot provide water to all areas needed in the home.

• Measure Your Cold-Water Temperature 

You have to determine the amount of heat your water heater needs to produce the required heat in the water outlets. This is determined by the temperature of the water that flows into your home and the temperature of the water you need to flow out of the outlets.

Measure Your Cold-Water Temperature

For instance, if the water flowing into your home is 45°F, and the showerhead is supposed to produce water at 105°F, the needed temperature rise is 60°F.

• Determine Your Water Flow Rates 

Tankless water heaters are different from the traditional tank heaters, in that the former steam water as it enters your house. Thus, the rate of water from your tap is determined by the flow rate that your heater supplies.

This is a crucial factor in knowing the size of the water heater for your home. This factor alone makes you be able to run more than one water outlet simultaneously without running slowly.

To calculate the water flow rate,

Assuming the rate of a shower head is 1.8 gallons per minute (GPM), you should be able to run another showerhead of the same GPM conveniently. The two shower heads give 3.6 GPM, and that gives you the required water heater flow rate.

However, other openings might also be running in other areas, like bathroom and kitchen, this then raises the required flow rate. The following are typical flow rates for most of the faucets in a home.

  • Shower head: 1.5 - 2.0 GPM
  • Kitchen tap: 2.0 - 6.0 GPM
  • Dishwasher: 1.5 - 2.5 GPM
  • Laundry washer: 2.5 - 3.5 GPM
  • Bathroom faucet: 0.5 - 1.0 GPM

Consider the Ground Water Temperature

Consider the Ground Water Temperature

The groundwater temperature of different states differs from each other since some states are colder or warmer than others. Some states have a temperature of 75°, while others can be as low as 35°. The groundwater temperature helps to calculate how hard your heater needs to work to produce hot water.

However, if you do not know the ground temperature of your state, you can assume it is 40° and work with that.

Factors That Affect Tankless Water Tank Performance

Factors That Affect Tankless Water Tank Performance

Tankless water heaters have become popular over the years, based on their functionality and benefits. These appliances are portable and are installed in the home or the office.

They provide instant hot water as they enter the home, and can expand to meet the hot water requirements of your house.

However, despite all these benefits, some factors affect the performance of the tankless water heaters.

• Mineral Deposits: 

The minerals found in tap water can get stuck in the water heater. Minerals like magnesium and calcium in hard water can affect the performance of the tankless water heater. This problem is solved by cleaning the heaters frequently. 

If the minerals deposits are left too long in the heater, they'll decrease the capacity of the heater to heat water. Most modern heaters come with a digital meter that works as an alarm to give you a signal when the mineral deposits are much in the heater.

• Corrosion: 

If the tankless water heater heated gets corroded, it can affect the quality performance of the heater. The metal parts of the heater get corroded when condensation drops on them and can cause the heater to stop functioning. This problem is challenging to manage and will need the services of a professional to fix it. 

• Cost: 

The modern tankless water heater is more complicated than the traditional one. It helps to save energy costs, but the installation and maintenance are quite expensive. It requires professionals to install and maintain per time. So, we can conclude that modern tankless water heaters are costlier in the long run. 

• Improper Venting: 

Tankless water heaters can stop functioning if the ventilation system is not well installed. The heater needs gas supply or electricity to supply hot water, which is not possible without proper ventilation. If the water heater gets too hot, it is essential to restore it immediately. Ensure to get professionals to work on the ventilation system, so you get a proper installation. 

• Sensor: 

The sensor in a water heater helps to ensure that water is provided at the same temperature as required. If the sensor fails, it affects the performance of the water heater to provide hot water. You must have experienced professional check on the sensor every few months. 

• Heating Overload: 

If the water heater produces hot water than the capacity it is supposed to, it can affect the performance and stop working. This can happen when you want your heater to supply hot water at different places all at the same time. The system can break down if the tasks are too much for their ability. Make sure that your water heaters can perform the tasks you're giving them. You can install more than one heater in your home to reduce the load on one. 

• Flame Failure: 

This can occur due to low gas pressure or electrical issues. The system gives you a warning when this happens and tells you that the flame can no longer generate heat. This issue requires the service of a professional. Trying to solve it yourself might result in more problems for the heater.

Types of Tankless Hot Water Heaters

Gas Heaters 

Gas Heaters

Gas tankless heaters are the best for large families and commercial use. These heaters operate on gas or a condensing unit. The condensing unit is more energy efficient because it runs on the heat from exhaust fumes. Gas heaters are suitable for a house that wasn't wired for a water heater from the start. 

Electric Heaters 

Electric Heaters

These are energy-efficient water heaters. It is installed in a home that is wired for an electric water heater. Since you already have the supply in place, it is only wise that you install an electric tankless water heater. It is cheaper than the gas heater and takes up a little space in the home. The right size of an electric water heater provides you with an unlimited supply of hot water. 

Commercial Heaters 

The heaters available for commercial use are mostly gas heaters; this is because they must have a high flow rate. An infrared electric heater has also been introduced to function in this capacity. Most commercial heaters have a re-circulation system, which helps to reduce wastage of water. 

Small Heaters

These are small but powerful water heaters. They suit homes with a small budget or office that doesn't require much water. This water heater capacity is low and cannot handle the job of large heaters.

Whole-house Heaters 

Whole-house Heaters

These heaters can serve the whole house hot water at once without running slowly. They can draw water from different outlets in the home at once. 

Portable Heaters

These heaters are best in low-temperature areas. They are portable, and you can install them anywhere. They are useful in camping, garage, or garden shed. 

Hybrid Heaters 

Hybrid heaters have an electrical system and a heat pump that uses the electricity to transfer the heat, instead of creating it. These are innovative water heaters that mix the tank water system with a tankless system. They are expensive but mighty. 

Fuel Type: Electric or Gas? 

Gas tankless water heaters can heat water to five gallons at 70°F per minute. Electric tankless water heaters can heat up to two gallons of water at 70°F per minute. This tells us that gas tankless water heaters are more powerful than the electric ones.

However, the type you go for depends on the situation of your area and where you need it. If the house is wired for a water heating system, it is best to install an electric water heater.

Whole House or Point-of-use?

The whole house water heater is a single water heating system that serves the entire house. This water heater supplies hot water to every water openings in the house and found in the garage or basement.  

Whole House or Point-of-use?

Point-of-use (POU) water heater does not serve the whole house. They are specific tankless water heaters that are located in different locations in the house. It takes multiple point-of-use water heaters to provide hot water to the whole house. 

Whole house water heaters help to save costs as you don't have to buy multiple heaters for different outlets. One single whole house heater can serve hot water for the entire house at once. However, if you want instant water at various locations in the house, the POU is for you. This heater helps to reduce the workload on one single water heater. They tend to last long since it is only one faucet they supply hot water to per time. 

Choosing either of the two water heating systems is dependent on how vital instant water is to your home. It also depends on your budget and what appliances are available to you. Most houses with many water outlets would go for the whole house water heater, while buildings with not too many faucets go for POU water heaters. 

Size Chart

A sizing chart helps you to know the required water heater sizing needed in your home, on the average. It takes into consideration some factors and several possible outlets. This chart might not be accurate or fit into your state due to changes in groundwater level or temperature rise, but it does serve as a guide. 

Example of Size Chart 

Household Occupants


Capacity Required 

(Measured in gallon)






1-2 people








2-3 people








3-4 people








5+ people







How Long Does a Tankless Water Heater Last?

Tankless water heaters do not experience heat loss that the standard tank heaters have. They last longer than tank water heaters. Most tankless water heaters can last up to 20 years or more. The parts are easily replaceable, and this also expands their lifespan. 

However, maintenance is costly and should only be carried out by professionals. But in all, a tankless water heater has a longer lifespan than tank water heaters. 

Can You Run Out of Hot Water With a Tankless Water Heater?

A tankless water heater heats water as it flows in, in contrast to a stored tank. Tankless water heater only heats and provides water when you need it. You can not run out of hot water with a tankless water heater.

This is why the tankless water heater lasts longer than the stored tank. The former only works when it is needed, and this helps in saving energy costs.

Tankless Electric Water Heater Sizes

How to Size a Tankless Water Heater FAQs

Why is My Tankless Water Heater Not Working?

If you have an electric water heater, make sure the breaker box is in perfect condition. Your heater will not work if you have a burnt fuse in the outlet. If it is a gas water heater, be sure the gas supply is on. 

Can You Run a Tankless Water Heater on Propane? 

Yes, there are water heaters that run on propane. They have almost the same features as natural gas water heaters. The propane heaters do not suffer heat loss and heat water only when needed. 

Why is My Tankless Water Heater Not Getting Hot?

This is usually caused by a blocked vent or plugged heat exchanger. The ventilation system might be blocked due to too much heat and can not provide the necessary air for the heater. The heater won't get hot with this. It requires that you call an experienced professional to look into the problem. 

Can a Homeowner Install a Tankless Water Heater? 

Water heaters come in different types and have different installation processes. It is not advisable to install your water heater if you have no experience in it. An experienced professional is the best person to install your water heater, so you don't have issues afterward. 

What is the Downside of a Tankless Water Heater? 

The commonest disadvantage of a tankless water heater is the cost. To get a size that produces hot water effectively in the home, it costs a lot.


A tankless water heater is an appliance that makes getting hot water comfortable in the home. The water gets heated at the point of entry into the house by the tankless water heater.

The water heaters come in different sizes and for various purposes. You must know how to size a tankless water heater, to know the right size for your needs. The size of the water heater you need for your home or office depends on certain factors. The water flow rate, mounting space, and groundwater level are some of the factors that determine the size of the water heater you need.

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