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The Science of Tankless Water Heaters

eccotemp-l5-4Consider, for a moment, how many times in a day you use hot water. From showering in the morning before work to washing your hands in the bathroom to doing the dishes after a nice dinner, hot water is an important part of our day to day lives. Despite its importance, most people don't spend much time thinking about where their hot water comes from.

Water heaters are a central part of a home's water and plumbing system. Most hot water heaters store water in a large tank, keeping it warm for when it's needed. Although largely effective, standard water heaters can run out of water with prolonged use, leaving unhappy bathers shivering under freezing streams. When this problem becomes too much to handle, it's time to look for a better alternative. Tankless water heaters, also known as on demand water heaters, can provide simpler and more energy efficient alternatives.

What Is a Tankless Water Heater?

A tankless water heater is an alternative to standard water heaters that doesn't require the use of water within a tank. Instead of heating and storing water for later use, tankless water heaters heat water on demand without holding a supply of water. While users of traditional heaters may feel as though this is counter-intuitive, increasing wait times for hot water and prolonging exposure to a cold shower, tankless water heaters can prepare heated water very quickly, providing access whenever needed.

Popular in Europe for decades, the tankless water heater is slowly growing in recognition in the United States as well. Common for larger families and environmentally-conscious individuals, tankless water heaters are able to provide as much hot water as necessary without waiting for a tank to fill.

How Do Tankless Water Heaters Work?

Tankless water heaters supply water to a home through the pipes, much like a standard heater. Traditional water heaters work by holding water in a large tank until needed, continually heating it to ensure hot water is available. This constant use of energy is referred to as standby heat loss and it can cost property owners significant amounts of money over time.

Instead of keeping hot water in a tank, tankless water heaters heat water on demand. By eliminating the power it takes to keep water hot and prevent standby heat loss, tankless water heaters are energy efficient and highly effective, especially in situations that require more water than a standard tank can hold.


Tankless water heaters heat water quickly and effectively through the use of a heat exchanger powered by electric coils or gas. The heat exchanger is activated by the flow of water into the heater, causing the water to heat rapidly based on the unit's preset temperature. Within moments of turning the knob, hot water will run from the faucet, accomplishing the same end objective as a traditional system that has hot water in a tank to be accessed.

Rather than serving as a portable alternative for a short term fix, tankless water heaters are permanent replacements for traditional water heaters. Installation costs for whole house models are slightly higher on average than standard systems, and tankless units often come with a ten to twelve year warranty, guaranteeing longevity.

Despite the ability to heat water without a tank, tankless heaters are highly efficient, even in cold weather. The heat exchange feature can rapidly heat even the coldest water for year-round comfort.

Whole House vs. Point of Use

Like most home appliances, tankless water heaters come in numerous forms. Notably, on-demand water heaters are available in two main varieties:

  • Whole house
  • Point of use

Point of use systems are designed to serve only one outlet, like a kitchen sink or a bathroom shower. They are hooked up in close proximity to the water source and are designed to provide the proper amount of hot water for intended use. Point of use heaters are very effective and due to their location as there is very little lag time. Lag time refers to the time it takes to water to pass through the heat exchange and warm sufficiently and can be an issue with both tank and tankless water heaters.

Whole house system are designed to heat a home's full water supply. They exist in one central area in the house, usually in the basement or a bathroom, and are used to provide water to numerous outlets. While whole house systems are larger and more expensive than point of use systems, they are also more powerful. Lag times are not significant, but can be longer and more frequent than smaller options, especially for homes that have many outlets in use simultaneously.

Tankless water heaters can be powered by electricity, propane, and natural gas, depending on factors like size, scale, and frequency of use. Smaller systems, like most point of use water heaters, are electric, while whole house heaters are generally powered by propane or natural gas.

Is a Tankless Water Heater Right for You?

at-900-13-3Consider, for a moment, how many times in a day you use hot water. From showering

Tankless water heaters have many benefits, but they are not necessarily the right fit for every family. A standard water heater holds between 40 and 80 gallons of water on average, which is enough water to do a load of laundry, run the dishwasher, and take an average shower. For individuals or small families who don't routinely need this much water in a short amount of time, a traditional water heater is perfectly acceptable. For larger families, however, a traditional water heater may have some noticeable shortages, especially for units with small tanks.

For individuals concerned with the environment, a tankless water heater is a better alternative. Unlike standard water heaters that can waste significant amounts of water over the course of a year and require extensive energy on a regular basis to keep water hot, a tankless heater only uses the minimum amount of water and energy. Energy efficient water heaters can cut energy usage up to 50%, reducing the cost of utilities by thousands of dollars over the course of the life of the unit.

Tankless heaters are generally more expensive than traditional systems and installation prices for gas and propane models can also cost homeowners more. Most individuals find, however, that the initial purchase and installation expense is easily offset by reduced utility bills within the first several years of ownership. While electric systems are often less expensive and easier to install, gas and propane systems are eligible for an energy tax credit, providing even greater savings.

Choosing a Tankless Water Heater

Like standard water heaters, all tankless models are not the same. Individuals and families should consider all relevant criteria in order to choose the right system, including:

  • Flow rate
  • Temperature rise
  • Age of household fixtures
  • Number of household fixturesat-900-13

Flow rate refers to the amount of water heated at once and can be a significant determinant. For example, systems with low flow rates may not be as effective in larger homes with multiple outlets. Temperature rise is a component of flow rate that refers to the difference in temperature required between groundwater temperatures, which can be estimated based on average air temperature, and desired temperature in the home. Temperature rise varies based on season and location, with families in Florida requiring a much lower temperature rise than homes in Wisconsin. Manufacturers label their models based on temperature rise for flow rate, making an understanding of these concepts very important.

To choose the right model, individuals must calculate average flow rate for their home based on standard flow rates for home features. Flow rates for household water fixtures are limited by the government at 2.2 gallons per minute at 60 pounds per square inch as of 1992, but low flow and fixtures pre-dating 1992 may offer variation. For example, shower heads produced in 1987 may fall between 4 and 8 gallons per hour, while shower heads produced in 1999 are limited to 2.2. Most low flow fixtures range from 1 to 2 GPH.

If a home has three faucets and a shower, all installed after 1992, GPH requirements can be approximated at 8.8. If this home has an average groundwater temperature of 50 and a desired temperature of 100, a tankless water heater should, at minimum, be able to heat 8.8 GPH at a rise of 50 degrees.

Tankless Water Heaters vs. Traditional Water Heaters

Making the decision to invest in a new water heater isn't easy. After all, a water heater is an investment in one's home that is designed to last for numerous years. Tankless systems offer many benefits, but should be carefully weighed against individual needs.

 Traditional Water HeaterTank less Water Heater
Maximum capacity

40 - 80 gallon tank

Limitless - tank less systems never run out of water

Heating time

20-60 minutes


Installation cost

$100 - $200

$200 - $500

Unit cost

$300 - $1,000

$700 - $1,500

Year-round use?



Energy use

High use: gas or propane

Low use: gas, electric or propane

Environmentally friendly

No: excessive energy use & release of greenhouse gases

Yes: less energy & no release of greenhouse gas in electric models


Whole-house heating

Whole house and point of use


10+ years

15+ years

Tax credits available



Space required

Substantial floor space

Minimal space; can be wall mounted

Home risk

Possibility of flooding



For families worried about the environment, want reduce energy bills, and are tired of being stuck in a cold shower, tankless water heaters are highly effective solutions. These efficient systems allow a personalized approach to hot water, ranging from point of use units designed to keep hot water flowing in one location to whole house fixtures that can always keep your water warm.

Offering benefits like energy tax credits, a long life, and energy efficiency, millions of families across the country can benefit from the numerous advantages of tankless water heaters.

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