Thermal Management In Green Design
There are two physical types of water heaters that are currently in use in the United States. The most common is known as a tank type water heater. In this traditional heating tank design there is a large tank that holds water and uses a heating mechanism to warm it. To provide hot water on demand the tank continually heats the water in order to maintain a constant temperature. The energy lost in this process is known as standby heat. Inside of the most common residential tanks there is anywhere from 40 to 60 gallons of hot water pressurized at around 50 to 100 psi. Standard water heater tanks can hold water at approximately 180oF, however it is recommended that the maximum temperature normally be limited to 140oF to avoid the risk of scalding. As the water is heated from underneath the tank, the warmer water will rise to the top of the tank and then exit through the “heat-out pipe” to go wherever it is needed. The water that leaves the tank will always be the hottest in the tank at any time because of the heat rising principle of thermodynamics. The other type of water heater is the tankless water heater which offers a more cost-efficient and longer lasting alternative to the standard water heater, as well as energy conserving.
The Thermal Management Difference
The main problem with a standard tank style water heater is that it loses a fair amount of energy to standby heat loss as well as travel distances from the heater to the fixture.To counteract this loss, the heater will cycle on and off to keep the water at the desired temperature. In this process, it uses an unnecessary amount of natural gas. Tankless water heaters eliminate this problem because they only heat the water on demand as it is needed. Flow sensors located at the heater are activated when the water passes through, letting it know when to come on and heat. Instead of using a natural gas heating system, a tankless water heater utilizes a powerful heat exchanger in order to quickly transfer the thermal energy into the water. The heat exchanger in a tankless water heater is similar to the heat exchanger utilized in a refrigerator, as explained in last week’s blog, except the input and output are switched. This allows for instant hot water only when demanded by the fixture in use. There are two types of tankless water heaters, each with their own purpose. The first is known as a point-of-use heater and is a much smaller device with a smaller heat exchanger that can be installed in a specific place, such as under the sink. This is beneficial because it nullifies energy loss due to lag time, or the time it takes for the water to reach its destination after being heated. The second type of tankless heater is known as a whole-house heater and would be used for wide spread on demand heat. However, the lag time for this type of water heater can be significant, especially in large homes, so even though the heating bill is shrinking, the water usage bill may be increasing. Most point-of-use heaters will utilize an electric heat exchanger while whole-house systems will use either natural gas or propane.
Benefits and Drawbacks to a Tankless Water Heater
While not every home is a prime candidate for a tankless water heater, many would benefit from it. For instance, since a tankless heater never runs out of hot water, there would not be issues if there is an unexpected high demand for hot water. Tankless water heaters also take up less space, can last 5 to 10 years longer and are much more efficient, especially since there is no standby heat loss. The large drawback to a tankless water heater, however, is that it will cost about three times as much as a traditional water heater, and depending on the model you may need to add a natural gas line or an additional electrical circuit to your house.
[i] While the initial cost is greater with a tankless water heater, the long term cost savings more than make up for the difference.
[i] How Stuff Works “How Tankless Water Heaters Work” Charles Bryant