Tankless Water Heater Terms and Definitions

This page was designed to help you become comfortable with the terms used in tankless water heaters. Most of these terms can also be used for typical tank type heaters as well.

If you don’t see a term please scroll to the bottom of this page and let us know. We will gladly try and find the explanation and most likely add it to this list.

Definitions And Terms Related To Tankless Water Heaters

Tankless Water Heater

Tankless also called on demand water heaters. This refers to any type of hot water maker that does not use a holding tank to store the water. Tankless heaters have been used in Europe for decades and are finally now becoming more widely used in the U.S.

Single Point Use

Single point use refers to a tankless unit that heats one fixture or maybe two. A small unit tucked into a bathroom would heat both the sink and tub and called a single point use heater.

Multi Point Usage

This generally refers to a single tankless water heater providing your entire home with hot water. Much like a large old tank style heater. These new units will provide enough o demand hot water for your entire household


This simple refers to the gallons per minute that water passes through the fixture. If you your shower requires 3.5gpm then you would want a tankless unit that can provide at least 3.5 gallons of water per minute to the shower head.

Temperature Rise

The temperature of water as it enters your home is usually much colder then we would like for home use. The difference between the incoming temperature (I.E 50f) and the desired temperature (I.E 120F) gives you the temperature rise.

Flow Restrictor

Many newer homes or those with copper pipes have very strong water pressure. That pressure determines how fast the cold water passes in to your tankless heater. If your pressure is to strong a flow restrictor will reduce the pressure giving your unit the correct amount of water passing through to heat to your happy temperature.

Direct Vent

Many tankless water heaters are gas operated (natural or propane) these units need to be vented to the outside. Modern technology has made it possible for these units to be direct vented which mean no need for a chimney the unit can be vented right out the side wall of your home. This method is safe and reduces installation cost as well as increase the possible locations for your unit.

Minimum Flow Rate

Minimum flow rate is the least about of water (gallons per minute) that must enter the heater before it will ignite. Every mfg has its own MFR.

Gas Meter

If you use Natural Gas you all ready know that the meter is the device that your gas supplier uses to keep track of your usage the same way your electric meter does. However most gas meters have a maximum cubic foot flow. Check with your gas provider to make sure your meter is large enough to handle your new tankless water heater as well as a furnace at the same time.


British Thermal Unit is the international method of measuring heat. It is the amount of energy to increase one pound of water one degree.

Ground Water Temperature

Ground Water Temperature also called GWT is the actual temperature of your water supply as it enters your home. This is the base temperature that you will need to know to help determine what size water heater you need.

Scald Protection

A concern for children and elders is scalding. There are two basic ways to protect your loved ones from the serious issue of scalding. First is a high level limit. This allows you to set the maximum temperature that your unit will produce. The second protection device available on some tankless water heaters is a device all a digital control panel. This remote control style controller is mounted in the bathroom and you punch in the temperature that you want your shower to be and that is the temperature that your water will come out of the tap. Both these types of protection will work to guard you from scalding.

Condensing Water Heaters

When using a fuel fired water heating system a large portion of the energy is lost up the flue. In a modern condensing water heater the gases that would normally be sent out the vent are recaptured and circulated until more of the heat is released back into the unit. The remaining discharge is barely warm to the touch. This type of water heater will give you an efficiency ratings up to 98%.


HVAC is a acronym for ” Heating Venting Air Conditioning. It is a term that has come into its own over the last 15 years or so. It refers to looking at your home heating and cooling system as a whole rather then individual pieces. Increasing the efficiency of your appliances through better venting will not only save you money it will improve the air quality in your home.  Read more on HVAC

Outside Mount

Some manufacturers make models of tankless water heaters that can be mounted outside. This is designed for folks that maybe want to heat an out door spa or a outdoor shower by the pool. It is also designed to withstand the elements of the outdoors for people that may not be able to use an indoor direct vent system.

Lock Out

When you are doing work on your tankless heater. The first thing you must do is turn off the power at the breaker. Do not rely on just turning off the switch. The use of a lock out is finding a way to prevent any other person from accidentally turning the power back on. This can be as simple as a piece of tape across the breaker with a tag saying out of service for maintenance.


No not the trucking company UPS stand for uninterruptable power supply. It is a secondary power source that comes online if you have a home power failure. For an in depth answer please read U.P.S


No we are not checking the tires. PSI however does mean the same thing every where. Pounds Per Square Inch. This is what we call water pressure. So when you say you have good water pressure you are talking about P.S.I.

Point Of Use

Similar to Single Point Use which refers to a tankless heater being used for say one bathroom ( a tub and sink). A Point Of Use is usually limited to one faucet like a sink. It also usually refers to a heater that is a mini tank style. Most point of use heaters are two to four gallon units. They are designed to give small fast short usage of hot water to a single location.

Flow Rate

Flow rate has a couple of different meanings. Usually flowr ate refers to the number of gallons of water flowing from the tap in a minute. In the case of tankless water heaters it also refers to the numbers of gallons a heater can increase the temperature to the desired degree in a minute.

Utility Pump

A utility pump is a generic term for a small usually inexpensive portable pump. For the use in cleaning a tankless water heater we suggest a submersible pump which as the name indicates means te pump can be placed in a bucket of liquid without causing damage to the pump.


An in-line-filter as called a pre-filter is located on the cold water intake side of your tankless heater. It is designed to trap minerals before they enter your tankless heater. This filter can be easily removed and cleaned. Is should be done yearly along with your whole tankless annual cleaning.

Energy Factor

Energy Factor or EF is a formula used by the tankless industry to rate the cost and efficiency of a tankless heater. Almost like MPG is to a car. EF will not only help determine your annual cost of running a tankless heater but it also helps determine the efficiency that the unit is converting fuel to energy. The federal guideline is a 0.82 rating is required to qualify for the tax credit. This information is usually available on the unit as you shop or ask your dealer.



Join In The Conversation

Hey, we love to hear from our site visitors, hint-hint... That's You!
Join in and share your comments below.