Water heater types and selection considerations
There are many types of water heaters, and there are many sizes and fuel options available. The number of types, sizes and fuel combinations create many options when it comes to choosing a water heater for an application. This is not an endorsement for any one type of water heater over another, as each type of water heater design has an application where it may be best suited.
Storage type water heaters offer a storage tank with a burner assembly under the tank or electric immersion heating element or heat exchanger in the tank. Compared to instantaneous water heaters, storage type heaters have lower energy input, and they allow the stored volume of water to heat up over a period of time. The amount of stored hot water is generally sized to store an amount of hot water sufficient to meet or exceed the peak demand of hot water for the facility type.
Generally, the peak demand for a water heater will last anywhere from 15 minutes to two hours depending on the type of facility. A typical residential water heater will generally handle a peak demand of hot water use. The different types of storage type water heaters include basically all water heater types which can be coupled with a storage tank.
The advantage of a storage tank is it allows a lower BTU/hr or lower electrical input. The disadvantage of a storage type heater is if it is not circulated it can experience thermal layering or stratification. This creates a layer of cooler water near the bottom and hotter water will rise to the top. The stratification condition is eliminated in storage type water heaters when there is a circulation pump installed on the water heater.
Tankless water heaters
Tankless water heaters, also known or promoted as demand-type, instantaneous or semi-instantaneous water heaters, theses heat water directly with and without the use of a storage tank. The term “tankless” is not accurate because all heaters have some volume of storage. The term “tankless” was coined by manufacturers of the small point-of-use type water heaters with very small input levels in response to code requirements for temperature and pressure (T & P) relief valves in the top six inches of a water heater tank. For the point-of-use or tankless types of heaters, they have such a small volume of water that a steam explosion is not likely. They coined the term “tankless” to indicate there is no need for a temperature and pressure relief valve, although they should have a pressure relief valve.
Typically, the heat input for tankless water heaters is equivalent to the maximum design flow rate and design temperature rise for the application. There are tankless water heaters with sophisticated temperature controls which vary input energy, and there are tankless water heaters with no temperature limiting controls using a simple flow switch for operation.
The smaller, simpler units are designed for small flows and small temperature rises (usually about 35 F). They heat up water in the branch piping, which should be close to ambient temperature, and they only raise the temperature 35 F from 70 F up to 105 F for hand washing. If the incoming cold water is lower, they may not provide hot water for the application. Flow is very important for these small instantaneous/tankless water heaters.
The temperature rise and type of controls for electric and gas units vary from 35 to 70 F from manufacturer to manufacturer. Tankless heaters generally do not heat water up beyond 105 F; although some models have very high energy input rates. These types of heaters can work well in smaller applications. It is more challenging with larger peak flow rate applications with variable flows rates.
Tankless water heaters have advantages of space savings and minimal stand-by heat loss. The disadvantages of a tankless/point-of-use water heater are the initial cost ($2,200 to $3,500 whole house tankless heater cost) for the more sophisticated whole house residential units is about five to seven times the cost of a high efficiency tank type water heater ($450 to $800 residential tank cost).
Heat pump water heaters
Heat pump water heaters generally remove heat from any heat source and transfer the heat from the source in a reverse refrigeration process to transfer the heat to a hot water storage tank using the refrigerant hot gas instead of generating heat directly for providing hot water.
Heat pumps can remove heat from ground water and in some cases remove heat from the air, but they may need electric heating elements as a back-up during winter months and during peak demand periods.
Solar water heaters
Solar water heaters use the sun to heat up a glycol solution through a heat exchanger and provide heat for a hot water storage tank. Solar water heaters should never have the domestic water in the solar panel piping. This is because the solar panels can be turned off for a significant portion of the year during winter months, creating a large dead leg of stagnant water.
Legionella cases have occurred in solar system piping that did not utilize a heat exchanger to separate the solar piping circuit from the domestic hot water piping system. With separate piping systems, glycol can be added in the solar loop to help prevent freezing of the solar piping and solar panels.
Shell and tube, semi-instantaneous or indirect water heaters
Shell and tube, semi-instantaneous or indirect water heaters can use gas, oil or electric, heating hot water or steam as a heating media in a heat exchanger or heating coil inserted into a storage tank or piped together with a storage tank and a circulating pump. Shell and tube heaters come in many designs with multiple pass designs for efficient heat transfer.
Copper fin tube heaters are an indirect type of water heater, and they come with high efficiencies, and typically there is a 35 F temperature rise through the water heater. This requires the use of a circulating pump and a storage tank where the water is circulated two to three times in order to satisfy the temperature rise requirements for a 100 F temperature rise.
Some designs of shell and tube, semi-instantaneous water heaters or heat exchangers are sized to overheat the water, and they utilize a temperature-actuated mixing valves on the outlet of the water heater to provide a consistent outlet temperature to the hot water distribution system. This allows for a constant hot water temperature as flow through the heat exchanger fluctuates with demand.
Advantages are space savings without a storage tank, although they can be coupled with a storage tank and circulating pump for significant peak load applications or to reduce the energy input requirements. A disadvantage is that without a mixing valve, or other sophisticated controls the outlet temperature will fluctuate with changes in flow or demand. Shell & Tube, Semi-Instantaneous or indirect units may require high BTU per hour or high energy input requirements for higher flow rates because the peak demand must be satisfied instantaneously.
Direct fired water heaters
Direct fired water heaters are relatively new on the market, although they have been around for a while, they are mostly used in process applications. The direct fired units have a spray of water over a combustion chamber full of metal spacers or filler material. The burner injects the flame directly into the same chamber as the water.
Fuel selection considerations
When selecting the best type and model of water heater for your application, consider the following.
The fuel type or energy source you use for water heating will not only affect the water heater's annual operation costs but also its size and energy efficiency. Investigate the cost of all of the available fuel sources. Then after a hot water load calculation is done, a comparison of the annual energy operating costs can be made.
To provide the building with a sufficient hot water supply and to meet peak demands and maximize efficiency, you need a properly sized water heater. A domestic hot water load calculation should be done to determine the peak hourly demand for storage type water heaters and the peak flow in gallons per minute needs to be determined for instantaneous water heaters.
To maximize the energy and cost savings, you should investigate the energy efficiency of a water heater before specifying it.
Water heater fuel type
Fuel type and its availability in your area may narrow your water heater choices. The following is a list of water heater options by fuel or energy source:
Electricity is widely available as a fuel source for storage, tankless or demand-type, and heat pump water heaters. Electricity can also be used with combination water and space heating systems, which include tankless coil and indirect water heaters.
Fuel oil is available in some areas of the U.S. for conventional storage water heaters, and indirect combination water and space heating systems.
Geothermal energy is available to those who will have or already have a geothermal heat pump system installed in their homes for space heating and cooling.
Heating hot water is generally available in larger buildings with heating hot water systems and heating hot water boilers. Heating hot water is generally between 180 F and 210 F and can be used for heating the domestic hot water. It is important to determine the BTU/hr load for the peak period requirements so that the boiler can be sized to handle the heating loads, domestic hot water loads and any process loads as required.
Natural gas is for conventional storage and demand (tankless or instantaneous) water heaters, as well as combination water and space heating systems, which include tankless coil and indirect water heaters.
Propane is for conventional storage and demand (tankless or instantaneous) water heaters, as well as indirect combination water and space heating systems.
Solar energy is available throughout the U.S. — most abundantly in the Southwest — for solar water heaters.
Steam, where available for heating domestic hot water, is generally supplied up to about 15 psi pressure which is equivalent to about 250 F. If steam pressures above 15 psi are to be used, consider a steam pressure-reducing station if higher pressure steam should be used with a double wall heat exchanger. Other safety controls should also be considered to prevent higher pressure steam from entering the domestic hot water system. If a heat exchanger fails at 15 psi, the steam system would be flooded with water. If the heat exchanger fails with high pressure steam the water piping would be filled with steam. Dual fuels combining any two of the above fuels is an option in special applications where it may be important to have a continuous supply of hot water.
Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. He can be reached at: office 734-322-0225; 755-1908; and website www.Plumb-TechLLC.com.