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WATER HEATERS
Tankless Hot Water Heaters

Tankless water heaters offer a number of advantages over storage type units, but they also have some disadvantages that shouldn’t be ignored. They are common in Japan and in Europe. In the U.S. they are not in widespread use, especially in the northern regions.

Tankless heaters heat water only as it is used, and do not store water for potential use. They are activated by the flow of water when a hot water valve is opened. Once activated, they provide a constant supply of hot water at their rated capacity.

They are sold in varying sizes and capacities, from single point-of-use units to whole households. They are also installed as booster units for dishwashers washing machines, or other high-demand appliances. They are sometimes connected in parallel for increased hot water demand.

Efficiency
Unlike storage type heaters, the heat exchanger in a tankless heater is a copper-finned device connected to the water line, made to provide maximum surface area and very high efficiency.

As is the case with storage-type units, those heated with natural gas or LPG are much more efficient and economical than those heated by electricity. A flue or vent pipe is required to release combustion gases.

The burner is designed to distribute flame evenly over the entire heat exchanger for maximum effect. The amount of energy used is always proportional to the volume of hot water being used.

Some types of heaters are thermostatically controlled and can vary their output temperature based on the water flow rate and inlet temperature. Some units have electronic remote controls for precise temperature control from various points inside the house.

Thermostatically controlled units work well in conjunction with a solar heater because less energy is required when the water is pre-heated.

The typical residential unit is activated after 3 gallons of cold water has passed through it. Flow rates vary between 1.2 gpm to 6.9 gpm with input BTU’s ranging from approx. 44,000 to 175,000 respectively.
Tankless units alter the flow rate to maintain constant preset temperature. In winter months when incoming ground water is cooler, the flow is reduced. If the temperature is set to a higher level, flow is also reduced.

Even the largest whole house tankless gas models cannot supply enough hot water for simultaneous, multiple uses of hot water (i.e., showers and laundry). Large users of hot water, such as the clothes washer and dishwasher, need to be operated separately, or multiple units installed.

Larger homes, or those with high demands, require the installation of commercial sized units, or multiple units which increase cost considerably, and drastically reduce payback. (many commercial units require a storage tank to maintain water temperature)

Sizing
The size of your tankless unit will be determined by the peak demand for hot water, and the incoming temperature of the water supply.

Choose a manufacturer that can supply the demand level you need, and the model that can properly heat the water.

Choosing the Manufacturer
To determine the proper on-demand water heater for your needs, list the number of hot water devices you expect to have open at any one time, and add up their flow rates (see chart below). Then select a manufacturer that makes a unit that meets your peak demand.

Appliance Water Flow Chart *
Source	Flow per Minute
Faucets:	0.75 gallons (2.84 liters) to 2.5 gallons (9.46 liters)
Faucets: Whirlpools, Soaking Tubs:	8 gallon (30.28 liters) to 35 gallons (132.48 liters)
Low-flow showerheads:	1.2 gallons (4.54 liters) to 2 gallons (7.57 liters)
Older standard shower heads:	2.5 gallons (9.46 liters) to 3.5 gallons (13.25 liters)
Hi-Performance Shower Valve (each):	5 gallon (18.92 liters) to 15 gallons (56.78 liters)
Clothes washers and dishwashers:	1 gallon (3.79 liters) to 2 gallons (7.57 liters)
* Flow Rate Will Vary By Type of Faucet, Size of Supply Line and Water Pressure

Choosing the Model
Most on-demand water heaters are rated for a variety of inlet water temperatures. Choose the model of water heater that is closest to your needs

For most hot water uses, the water temperature should be 120° F (49° C), or 140° F (60° C) for dishwashers without internal heaters.

The typical residential on-demand heater can raise the water temperature 50°F (10° C). Larger and commercial units can can produce a higher temperature rise.

Setting a residential on-demand heater for more than a 50° temperature rise will reduce the flow of water.

According to the U.S. Department of Energy the largest gas-fired demand water heater can supply only 3 gallons per minute at a temperature rise of 90°F (from 50° to 140°, for example) -- not enough for two simultaneous showers.

Electric models typically have even lower output. As a result, demand water heaters are better suited for low-use applications like vacation homes, rather than full-size homes with two or three baths.

To determine how much of a temperature rise you need, subtract the incoming water temperature from the desired output temperature. If you are not sure of the incoming water temperature, assume that it is 50°F (10° C).

Some manufacturers of residential tankless heaters claim that their product can match the performance of any 40 gallon (151 liter) tank heater. However, most Amercian households (3 or more people) will find the 40 gallon comparison much too small. (for a chart of storage tank heater size requirements see: Rheem Hot Water Heaters)

Custom showers with multiple shower valves will require a dedicated water heater (often a dedicated 100 gallon capacity heater is needed), or a commercial size heater for the home.

Advantages

1. No storage tank is required.
   This conserves space but also assures that the water won’t run cold simply because the tank has been drained of hot water.
   
   
2. Flexible Installation
   Due to their size, some units can be mounted on a wall or in a cupboard.
   
   
3. Reduced Enegry Consumption
   Stand-by heat loss is virtually eliminated. This reduces energy usage by about 20%.
   
   
4. Longer Life Expectancy
   Most tankless units have a life expectancy of more than 20 years, compared to the 15 to 20 year life of storage heaters. Many parts are even replaceable without replacing the entire unit.
   
   
5. Consistent Temperature
   Water is generally delivered at a consistent temperature from the tap and won’t cool down over time.
   
   

Disadvantages

1. Limited Heating Capacity
   Tankless units alter the flow rate to maintain constant preset temperature. In winter months when incoming ground water is cooler, the flow is reduced. If the temperature is set to a higher level, flow is also reduced.
   
   
2. Flow rates cannot be meet a surging demand, such as when a washing machine is turned on while someone is in the shower. Flow will be reduced to both outlets.
   
   Even the largest whole house tankless gas models cannot supply enough hot water for simultaneous, multiple uses of hot water (i.e., showers and laundry). Large users of hot water, such as the clothes washer and dishwasher, need to be operated separately, or multiple units installed.
   
   
3. A minimum flow rate is required to activate the unit. Depending on the unit, it may not be possible to generate a trickle of hot water.
   
   
4. Gas units require larger flue pipes and gas supply lines than do storage type heaters. Electric units require heavy gauge wiring and large circuit breakers. They may use less energy overall, but when they are heating, they use much more energy than conventional units.
   
   
5. Tankless units are not in widespread use, so parts may not be readily available nor may be the expertise to repair them.
   
   
6. These units tend to be very expensive relative to storage type heaters, and the higher the flow rates, the more expensive they are. According to figures released by the U.S. Departent of Energy, Small point-of-use heaters that deliver 1 gallon (3.8 liters) to 2 gallons (7.6 liters) per minute sell for about $200. Larger residential, gas-fired tankless units that deliver 3 gallons (11.4 liters) to 5 gallons (19 liters) per minute cost $550-$1,000.