Demand Water Heaters

Demand Water Heaters

Demand Water Heaters, also called “instantaneous” or “tankless” heaters, use an electric element or a gas burner to heat water.
When a hot water tap is turned on, cold water travels through the unit, triggering a flow sensor and causing the burner or heating element to turn on. A supply of hot water is produced as long as the water continues to flow. When the flow stops, the burner/element turns off. Unlike a standard tank-type water heater, no hot water is maintained in the unit when there is no water flow, and no energy is used to keep a tank of water hot when no hot water use is occurring. These systems generate no standby heat loss, and thereby you save energy and money

Quick Links:
How to buy?
Energy-Saving Tips
What are my Options?
How much can I save?

How to buy?

When purchasing a new demand water heater, you should consider factors including size, efficiency, cost and fuel type.

Sizing a Demand Water Heater. Demand heaters range in size from a small under-sink unit to a large unit able to supply an entire house with hot water. Unlike traditional storage tanks, these models are compact enough to fit in small places.

The ratings for tankless demand heaters are based on the maximum flow rate and the expected water temperature rise. The amount of hot water that can be supplied at a given temperature is dependent on the heating capacity of the burner or heating element. As the flow rate increases, the temperature of the water is reduced.

To calculate the necessary Flow Rate, identify the number of hot water devices in use at a certain time and then add up their flow rates (gallons per minute).
 

This sum is the suitable flow rate for your demand water heater.

To calculate the temperature rise, subtract the assumed incoming water temperature (typically about 50ºF) from the set outgoing temperature (120ºF). In most cases such as the example above, you will need a demand water heater that can generate a temperature rise of 70 ºF.

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Energy-Saving Tips

The overall efficiency of a water heater is measured by the Energy Factor (EF). The EF is an indicator of how much fuel was used in relation to the amount of hot water actually produced. This figure is based on the following:

Recovery Efficiency: How efficiently the heat from the energy source is transferred to the water.
Standby Losses: The percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks).
Cycling Losses: The loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes.
Demand hot water heater systems essentially burn fuel only when needed or “in demand”. By having little to no standby heat loss, these units waste less energy and can greatly improve the efficiency of hot water production.

Demand hot water heaters are reliable, and when operated correctly, provide sufficient hot water for even large homes. Although the limits of a single demand water heater can be easily reached, you can install multiple units connected for simultaneous uses of hot water.

Installing separate demand water heaters for certain appliances is another option to secure your family’s hot water needs and as well save money on energy costs.

Typically, demand water heaters provide hot water at a rate of 2–5 gallons per minute and gas-fired demand water heaters produce higher flow rates than their electrical counterparts. But as long as the rate of hot water use is within its capacity, a tankless water heater can provide unlimited hot water.

Demand hot water heaters work best when the need for hot water is uninterrupted for a period of time. There is a short start-up time after water flow is sensed by the unit at the beginning of each draw of water, and the unit shuts down when water flow stops. If the flow of water is interrupted, there will be another start-up period when flow begins again. This can result in alternating hot and cold water being delivered to the water outlet.

Demand hot water heating systems have a life expectancy of over 20 years, compared with 10-15 years for traditional tank systems. They are made up of replaceable parts that are also less subject to corrosion.

Tankless water heaters work better when paired alongside other efficient hot water uses. Low-flow faucets and showerheads are simple adjustments that you can make that will reduce the flow-rate and in turn lower your energy costs.

Caution: Demand hot water heaters require a minimum flow before they will start up. Make sure your fixtures and water-using appliances have flow rates greater than the unit’s minimum flow requirements.

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What are my Options?

You will need to choose the fuel type that will be used to power the tankless heater. Units are powered by Natural Gas, Propane, or Electricity.

Gas-Fired Demand Hot Water Heaters:

Have specific venting, combustion air, and gas line requirements.
Various gas-fired demand models with an EF of .80 are eligible for federal tax credits depending on the manufacturer.
Are generally more powerful than the electric hot water heaters, and are used to provide hot water to the entire house.

Electric Demand Hot Water Heaters:

Require a relatively high electric power draw to heat water quickly. Most residential wiring will not support a demand electric model sized to provide hot water for an entire house.
Voltage: Most units available will accommodate 110V, 120V, 208V, 220V, 240V, and 277V.
Amperage: Different electric demand units will have various requirements in amp draw. Ensure that your homes electric system can support the electrical demand of the specific heater you are considering.
Circuit Breaker: Make sure that you have a circuit or circuits that will supply your heater with sufficient energy. Putting your new demand system on its own circuit or circuits may be needed.
Generally, Electric-Demand Heaters are most appropriate for small hot water applications, such as a single bathroom or a powder room with no bathtub or shower.

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How much can I save?

A Demand Hot Water Heater System can provide the following efficiencies:

24%–34% more energy efficient than conventional storage tank water heaters for homes that use 41 gallons or less of hot water each day.
8%-14% more energy efficient for homes that use up to 86 gallons of hot water each day.
27%–50% more energy efficient when installed at each hot water outlet.
You should also consider the distribution of hot water and the location of your home’s water outlets. Heat is lost from hot water piping, especially if uninsulated. To reduce these distribution losses, hot water systems should be as compact as possible, with water outlets grouped together, and piping runs kept as short as possible.

In terms of installation and cost of the system itself, demand water heaters are more expensive than tank systems but have lower operating and maintenance costs.

Demand Hot Water Heaters can be fueled by natural gas, propane, or electricity. The type of fuel you select to power your system will affect the operating costs. For example, electricity costs as much as three times more per Btu than other fuels such as gas. Regardless of fuel type, tankless systems conserve energy by only using it when in demand, which results in a lower monthly energy bill.

The chart below provided by the American Council for an Energy-Efficient Economy’s (ACEE) Cost Analysis compares different types of heaters powered by natural gas.
 

1. Purchase costs include our best estimates of installation labor and do not include financial incentives.
2. Operating cost based on hot water needs for typical family of four and energy costs of 9.5¢/kWh for electricity, $1.40/therm for gas, $2.40/gallon for oil.
3. Future operating costs are neither discounted nor adjusted for inflation.
4. Estimates for tankless gas water heaters are based on the federal EF rating method, which may over-estimate the efficiency of tankless water heaters in houses.

Demand water heaters range from $200 for a small under-sink unit up to $1,000 for a large gas-fired unit that can generate five gallons per minute. As a consumer you can assume that the more hot water the unit produces at a higher flow rate, the higher the cost of the unit.

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