Tankless water heaters, sometimes referred to as on-demand or instantaneous heaters, are a modern day alternative to the traditional 30- to 120-gallon tanks found in the average home.

How They Work

Unlike standing tanks, which maintain the temperature of a finite number of gallons, on-demand devices heat liquid only as it is used. Once a valve is opened, a flow sensor ignites the heating element of the device. Moving through lines that coil around the heat exchanger, water is delivered at the desired temperature. When the valve is closed, the flow sensor turns off the heating element.


Due to their design, from an efficiency standpoint, tankless water heaters have the potential to conserve more energy and, as a result, save money. Whether you are at school, at work, or away for the weekend, your heating tanks are perpetually bringing their contents back to the desired temperature, wastefully burning energy. On-demand devices, however, remain idle in the same instances.

Tankless water heaters have other advantages over their older counterparts. An average tank is floor mounted and about two feet in diameter and four feet in height. Instantaneous ones can be mounted on a wall inside or outside the home and range from as small as 10 by 7 inches up to 30 by 20 inches, making for substantial space savings. These devices also lower the risk of damage that may result from a failed or ruptured tank and generate less exposure to dissolved toxic metals from hot water that may accrue from standing in a tank for extended periods. Another advantage is temperature control, lessening the likelihood of spikes that a stored unit might generate.

Finding The Right On-Demand Device

Before rushing out to purchase one of these replacements for your voluminous storage heating tanks, a few things should be taken into consideration to find a unit that suits the needs of your home. The first thing is to establish the temperature of the liquid coming into a home. The average desired temperature of hot water is 110 degrees. If it enters the home at 50 degrees, a device is needed to sustain a 60-degree rise in temperature as it relates to the flow rate. Thus it is important to know the flow rate as well, which is how many gallons per minute liquid comes out of the tap. Finally, along with the average number of gallons used per day, these factors can determine the size and scope of the model needed.

Tankless water heaters use either gas or electricity as heating elements. Gas is more efficient, as it heats at a faster rate, but it is more complicated to install since it needs a gas line and ventilation. A gas-heated device, however, is the obvious choice for a family that uses a fair amount of heat on a daily basis. Electric on-demand types, though much easier to install, are less capable of handling high volume. However, they are more than adequate for a home or apartment of one or two people.

Though tankless water heaters are more expensive than tanks, if approached and used properly, their efficiency can easily outweigh these costs. The only other drawback is the temptation to remain in the shower till your skin is of a prune like consistency, which many may be likely to do. Limitless hot showers may be a dream, but temperance is a virtue.