The Evolution of Hot Water Generation Systems -Ron George 18 / SEPTEMBER 2018 Geysir at Haukadalur, Iceland ( Ron George, 2016)
INFOCUS Water Heater Terminology Appliances that heat water for domestic use have been called hot water generators, geysers, boilers, and water heaters. The term geyser is a slang term that has its origins in Iceland, and it is a common slang term used in Great Britain. I prefer the term water heater, as that is the term used in the United States of America. Hundreds of years ago, the term geysir (Icelandic spelling) was used in Iceland to describe hot water that would periodically gush skyward from pools of hot water near volcanic regions. The word geysir is derived from the Icelandic Old Norse language, geysa, which means to gush. very hot rocks near the volcanic magma deep in the ground. When the water hits those rocks, it boils and flashes into steam in a violent steam expansion. The expanding steam forces the water above it to spray out of the crack in the rocks, causing the geyser effect of hot water and steam at the geyser s surface vent. In Great Britain, around the time solid-fuel burning stoves were invented, a geyser-effect was created when water was poured into the hot water holding tank of the stove. The water would have flashed into steam and bubbled out of the hot water holding tank like the hot water and steam gushing up from the geysirs in Iceland. This is likely from where the slang term geyser came from when referring to an early water heater. Since water heaters do Geysir at Haukadalur, Iceland. ( Ron George, 2016) The formation of a geyser is due to rock formations that allow water to seep deep into the Earth s crust. Generally, all geyser field sites are located near active volcanic areas, and the geyser effect is due to water flowing deep into the ground through cracks in the rock formations. The water then flashes into steam when it is exposed to not act like geysers anymore, I will simply refer to them as water heaters. After the invention of indoor piping and plumbing, the heating of water moved away from a compartment on a stove and to a storage tank sitting on top of a fire-box, or in the flue which used solid-fuel (wood or coal) to heat the water. SEPTEMBER 2018 / 19
Seven of the Most Popular Types of Water Heaters 1. Storage Tank Water Heater 2. Tankless Water Heater 3. Heat Pump Water Heater 4. Solar Powered Water Heater 5. Condensing Water Heater 6. Hot Water Generator/ Heat Exchanger Water Heater 7. Copper Fin-Tube Water Heater Wood Stove with Hot Water Tank These early water heaters had very little in the way of controls, and the water often boiled and flashed into steam, probably very much like geysers. The early storage tanks were riveted steel, and when the outlets were plugged, or lids were secured, the vessel would rupture and even explode. Early controls included limiting the amount of fuel added to the fire, using a damper to limit the combustion air to the fire, and adding temperature and pressure relief valves. Riveted tanks fell out of favor and welded tanks became the norm, and glass and other corrosion resistant lining materials were added to both eliminate corrosion of the hot water tanks and the rusty water that it produced. Insulation was added to limit radiant heat loss. Thermostats were added to turn on and off the burner, and the staging of burners was introduced in order to control the heat input. Today, there are many different methods and controls for heating water for domestic use, including: Water heater type; fuel type; piping accessories; temperature gauges; combustion controls; temperature and pressure safety controls; flue gas sensors; high temperature limit switches; insulation types; refractory (heat resistant) linings; flame arrestors; thermostatic tempering devices on the outlet piping; Thermostatic and pressure balancing mixing valves at fixtures, etc. The following is a general discussion of each type of water heater type. Electric Storage Tank-Type Water Heater Source:https://www.aplusplumbingprofessionals.com/information/ water-heaters/components-water-heater/ 20 / SEPTEMBER 2018
INFOCUS calculation to determine the amount of hot water required for the system during the peak hot water usage period. Often, the peak hot water usage period can last an hour in a single-family residence or small commercial buildings. The peak hot water usage period can last several hours for a hospital or hotel. To maintain both an adequate quantity of hot water and to avoid the dual risks of microorganism growth, such as Legionella, and scalds, it is a good idea to keep guest room hot water systems and other utility hot water systems separate. Also, include two or more storage tank water heaters in the event that one goes down, so there will still be hot water during a repair period. An interview with the owner of the building about how hot water will be used is important in understanding the hot water usage patterns. Fuel Gas, Storage Tank-Type Water Heater Source:http://infowiring.today/whirlpool-hot-water-heater-diagram. html. 1. Storage Tank Water Heater: Storage tank water heaters are the most popular type of water heater in the United States. They are the least expensive type of water heater, and they can deliver a constant temperature of hot water over a large flow range. When coupled with temperature actuated mixing valves, inline thermostatic mixing valves, point-of-use thermostatic or pressure-balancing controls, temperatures can be maintained to control both the risk of scald for the end-user and microorganism growth in the tank and throughout the hot water distribution system. The storage tank water heater is generally selected with a tank that is sized to store enough hot water to satisfy the peak hot water usage period for a particular building type. Larger systems generally use circulating pumps to circulate hot water closer to the fixtures (point-of-use). This minimizes hot water waste, which occurs in uncirculated systems. Generally, for uncirculated storage tank water heaters, about 70 percent of the water in the storage will be at a useable temperature. After 70 percent of the water in the tank is consumed, the incoming cold-water mixes with the remaining water such that the last 30 percent of water in the tank is not at a temperature that would be considered useable. Thus, the selection of a tank size requires a Most storage tank water heaters utilize a thermostat element located near the bottom of the tank to turn on and off a gas burner or an electric heating element. The cold water can either enter the top of the storage tank through a dip tube that terminates near the bottom of the storage tank or the cold water can enter the storage tank on a side inlet near the bottom of the tank. This allows cold water, which is more dense and heavier than hot water, to remain near the bottom of the tank until it is heated. The thermostat senses the cold water entering the bottom of the storage tank and turns on the energy. The thermostat turns off the energy when the thermostat temperature setting is satisfied. Most thermostats have a wide range between the on and off temperature (15 degrees Fahrenheit, (8.33 C) plus or minus from the set-point). This can lead to fluctuating temperatures on storage tank water heaters, which can allow the hot water temperatures at the top of the tank to rise as much as 30 degrees Fahrenheit (16.6 C) or higher above the thermostat set-point. In circulated systems, the stratification of hot water within the storage tank is minimized. For this reason, many commercial projects utilize a master thermostatic mixing valve that conforms to industry standards to control distribution system temperatures. Industry standards, like the American Society of Sanitary Engineering (ASSE) standard, entitled: ASSE 1017: Temperature Actuated Mixing Valves for Hot Water Distribution Systems, assure accurate control of the hot water temperature delivered to the hot water distribution system. Master temperature actuated mixing valves are not mandated in the U.S. Codes, but when they are installed, they must comply with ASSE 1017: Temperature Actuated Mixing Valves for Hot Water Distribution Systems. The smaller valves control temperatures to within plus or minus SEPTEMBER 2018 / 21
3 degrees Fahrenheit (1.66 C) of the set point with a flow rate of 3 Gallons Per Minute (GPM) (11.3 LPM) to 5 GPM (18.9 LPM); medium sized valves control temperatures to within plus or minus 5 degrees Fahrenheit (2.77 C) of the set point with a flow rate of 5 GPM (18.9LPM) to 40 GPM (151.4 LPM); and, larger valves control temperatures to within 7 degrees Fahrenheit of the set point with a flow rate in excess of 40 gpm (151.4 LPM). Because a storage tank water heater can have the temperature rise as much as 30 (16.6) degrees above the set point of the water heater thermostat, a system will be much safer with a temperature actuated mixing valve located near the water heater serving the hot water distribution system. Make sure you read, understand, and follow the mixing valve manufacturer s installation instructions. There are specific piping arrangements to address heat traps and to control temperatures within the recirculation systems. The return line should be split after the circulation pump and routed to the cold-water inlet to the mixing valve and to the cold-water inlet of the water heater. Temperature gauges should be located on the water heater outlet, the mixing valve outlet, and the tempered water return before the cold-water connection. Tankless water heaters generally do not have master mixing valves installed because they are sized to only heat the water up to a usage temperature. Emerging technologies include digital mixing valves that can control temperatures accurately within one-degree Fahrenheit. Digital mixing valves can also record the system temperatures and provide for a user-interface to be able to see or adjust the system temperatures from a remote terminal or from a hand-held cell-phone. To reduce the risk of scald at the point-of-use, shower valves that comply with the ASSE standard, entitled: ASSE 1016-2017/ASME A112.1016-2017/CSA B125.16-17: Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/ Shower Combinations are mandated in the U.S. Codes. The standard has a requirement for a maximum temperature limit-stop adjustment to allow the installer or building owner to adjust the valve to reduce the maximum hot water temperature that can flow from the valve to a safe temperature that will not scald the bather. Two handled shower valves with on/off valves for the hot and cold water are prohibited in the United States in new construction because of the temperature fluctuations associated with pressure disturbances, which can cause sudden temperature changes or what is known as thermal shock, which in turn can lead to scald injuries. Two handled shower valves with a thermostatic element and handles for temperature and volume are acceptable if it complies with the Industry standard. Tankless Gas, Water Heater Source:http://www.workingre.com/wp-content/uploads/2013/10/ tankless-water-heaters-ne.html Tankless Electric, Water Heater Source:https://www.amazon.com/ECO180-Self-Modulating-Multiple- Tankless-territory/dp/B0089WW89M 26 / SEPTEMBER 2018
INFOCUS 2. Tankless Water Heater: Tankless Water Heaters are also referred to as ondemand water heaters. Tankless water heaters come in a wide range of price points. The best tankless heaters can cost many times more than the cost of a comparable storage tank water heater. Compared to other types of water heaters, tankless water heaters offer: a. A continuous flow of tempered water b. Space savings c. Less stand-by heat loss Stand-by heat loss or heat/energy loss of a water heater to the surrounding ambient air in 24 hours is a factor used by BEE (Bureau of Energy Efficiency) to decide on the star rating of a water heater. Newer fuel-fired storage tank water heaters have increased insulation thickness. Also, there are new low voltage technologies that control flue dampers on storage tank water heaters. These low voltage technologies have combustion efficiencies close to 99 percent. This eliminates the cooling effect that occurs in older atmospheric combustion storage tank water heaters, which allows room air to draft up through the water heater flue, creating a cooling effect during the time the burner is off. With flue dampers to minimize this draft effect in fuel fired storage tank water heaters, the stand-by heat loss of newer storage tank water heaters is much less now than in the past. However, monetary savings associated with less standby heat loss in tankless water heaters will usually not compensate for the higher initial costs associated with a tankless water heater over the life of the water heater. These higher costs include: 1. A higher purchase price 2. A larger cost for electrical installation for electric tankless water heaters 3. A larger cost for the installation of fuel gas piping for gas-fired tankless water heaters However, stand-by heat loss is often promoted as a benefit for tankless heaters, but the energy saved from stand-by heat loss from a tankless water heater rarely adds up to the amount of money spent to purchase and install a tankless water heater over a storage tank water heater. Top-of-the line tankless water heater models cost 6 to 7 times more than the cost of a top-of-the line storage tank water heater. The simplest tankless water heaters utilize electricity and work by a flow switch that activates a heating element with a fixed energy input. The tankless water heaters often come with a throttling valve that can be adjusted to reduce the flow rate to a flow that allows enough temperature rise to reach the desired temperature. With a fixed energy input, the temperature will change as the flow changes. When the flow switch activates, the heat input/output will be at its highest temperature rise. As the flow increases, the temperature will drop off. The flow rate through the simple tankless water heater must be at least enough to activate the flow switch, so these types of tankless water heaters often have problems at low flows or when they are manifolded together in a parallel piping arrangement where flow can be split between multiple water heaters. For this reason, tankless heaters without sophisticated controls should only be used on one fixture. When a tankless water heater serves a shower and other fixtures, sudden changes in temperature can occur when more than one faucet or water usage occurs at the same time, as multiple usages cause a change in the flow rate through the water heater. There will always be some level of temperature fluctuation with changes in flow rates with tankless water heaters. This is more common in the less expensive models, and it is controlled to some degree, but not eliminated, in the more expensive models. This is accomplished with staged burners, modulating burners, multiple staged heating elements, flow control valves, and digital controls. The more sophisticated models can vary the energy input, usually in two or three stages, to help control the outlet temperature, but the outlet temperature will still vary somewhat with changes in flow. These issues are also manageable in the less expensive models, but only if the tankless water heaters are used for a single fixture, where only one fixture will be flowing at a time. All tankless water heaters must be sized for the peak flow rate in gallons or liters per minute. Although tankless water heaters can be set to control for scald risks, the risk of Legionella is still present as a small amount of warm water still remains in the water heater piping. The temperature in a tankless water heater rarely gets to a disinfecting temperature or a temperature that will control the growth of microorganisms, including Legionella. SEPTEMBER 2018 / 27
Heat Pump, Water Heater Source:https://www.energystar.gov/products/water_heaters/high_efficiency_electric_storage_water_heaters/how_it_works 3. Heat Pump Water Heater: These hybrid water heater/heat exchangers recover heat from a variety of sources and generally have a supplemental electric heating element for peak periods. Heat pumps can recover or transfer heat from the air, ground source wells (vertical), ground source loops (horizontal), water source, or waste heat source, such as steam condensate or hot flue gas with the use of a heat exchanger. This means that electricity is only used to move heat from the source to the water, instead of using electricity to generate heat. Heat pump water heaters can use up to 60 percent less electricity than storage tank water heaters. Because the heat pump is on the top of the tank, vertical clearance is required. These hybrid water heater/heat exchangers are very expensive. They typically have a larger storage tank than a conventional storage tank water heater because the energy input is smaller. Solar, Water Heater Source:https://www.solarpowerworldonline.com/2011/10/smart-solarthermal-drainback-system-installs-for-half-the-price-of-other-models/ 28 / SEPTEMBER 2018
INFOCUS 4. Solar Powered Water Heater: A solar powered water heater draws energy from the sun. It relies on roof-mounted or ground-mounted solar panels as its energy source. The energy is typically transferred to a closed loop system that contains a freeze-resistant heattransfer liquid, like glycol, which then heats the water in the tank via a heat exchanger. It is not a good idea to run the domestic water through the solar panels unless the entire system is made of materials suitable for potable water. There have been many illnesses, such as Legionnaires Disease, that have been attributed to water that has sat stagnant in solar panels for long periods before being opened to the hot water system. Therefore, the heat transfer liquid in the solar panels should be in a closed loop. It is also not a good idea to have potable water in the solar panels in areas subject to freezing because, in freezing weather, the water can freeze, expand, and damage the solar panels or split the piping. These solar powered water heaters are very expensive, and it can take 25 to 40 years to have the system pay for itself. Hot combustion gases pass through heating coils in the cold-water feed to the hot water storage tank. As the combustion gas passes through the heating coils, moisture condenses out of the combustion gas. Most of the heat from combustion goes into the water, causing condensation to form in the heat exchanger. This type of water heater will typically have a continuous trickle of condensate draining from the water heater. Therefore, a drain should be located adjacent to a condensing water heater, and consideration should be given to ensure that the flue gas materials and drain materials will not corrode. Minimal energy is used to heat the water for a building. In the USA, this style of water heater is not readily available in smaller models and works best for applications that need a tank capacity of over 50 gallons (189.2 L). Plate & Frame Heat Exchanger/HW Generator Source:http://www.virginiaheattransfer.com/content/heat-transfer/hotwater-generation Condensing Water Heater Source:http://fuzzle.me/31-fantastic-condensing-gas-water-heaters/ 5. Condensing Water Heater: Condensing water heaters are high efficiency water heaters that use hot combustion gas to pre-heat the water, which is held in a tank much like the storage tank water heater. Shell & Tube Heat Exchanger/HW Generator Source:https://www.armstronginternational.com/products-systems SEPTEMBER 2018 / 29
Shell & Tube Heat Exchanger in Storage Tank/HW Generator Source:https://www.indiamart.com/decent-engineering/hot-water-generator.html#hot-water-generator 6. Hot Water Generator/ Heat Exchanger Water Heater: Hot water generators can be plate & frame heat exchangers, shell & tube heat exchangers, or any other heat exchanger design where two different fluids flow past each other transferring heat from heating hot water, a heat transfer fluid, or steam to heat domestic water. These types of heaters can be sized for instantaneous flows, or smaller heat exchangers can be coupled with a storage tank and recirculation piping to allow for a volume of stored hot water. Temperature controls typically operate a control valve that opens to let the heating fluid into the heat exchanger and turns on a circulating pump. An aquastat is generally located in the hot water storage tank to sense the hot water temperature and call for heat when needed. Copper Fin-Tube Source: http://www.spiralfinnedtube.com 32 / SEPTEMBER 2018
INFOCUS Copper Fin-Tube Water Heater Source: http: www.lochinvar.com 7. Copper Fin-Tube Water Heater: In many larger commercial buildings in the USA, watertube boilers are used with copper fin-tubes that are routed over a gas or oil-fired burner to absorb heat from the burner and direct it into the flow of domestic water flowing through the tubes. Copper fin-tube heaters are typically piped with a hot water storage tank. They are rarely used by themselves (i.e., without a storage tank) in a domestic water system because of a very specific flow velocity and temperature rise limitation associated with this style of heater. Copper fin-tube water heaters generally only raise the hot water temperature about 35-degrees Fahrenheit (19.4 C) with each pass through the heat exchanger. Therefore, the water will have to pass through the water heater two to three times to reach the desired storage temperature. An aquastat in the hot water storage tank senses the hot water temperature. When the tank temperature drops, the aquastat senses the lower temperature and turns on the circulating pump and starts the ignition sequence in the copper fin-tube water heater. After a brief purge cycle, the burner fires and continues firing until the desired hot water temperature in the tank is reached. Because of the pre-determined flow velocity that sets the 35-degree Fahrenheit (19.4 C) temperature rise, these heaters rarely, if ever, scale-up. Ron George, CPD, President of Plumb- Tech Design & Consulting Services, LLC is involved in Plumbing, Piping, Fire Protection and HVAC System Design & Consulting Services; 2D AutoCAD & 3D Revit CAD Services; 3D Building Information Modeling (BIM); Forensic Investigations of Plumbing, Piping, Fire Protection & Mechanical System Failures; Litigation Support, Technical Report Writing; Code & Standard Consulting; Training Seminars: Plumbing, Mech., Fire Protection Design, Codes & Standards. SEPTEMBER 2018 / 33