Water Heating Systems:
Installation & Performance Issues
A domestic hot water system consists of a heat source, a heat exchanger, a piping system, and plumbing fixtures. Most systems also have storage tanks.
Water heating consumes about 18% of energy used in an average household. There are several simple, inexpensive actions you can take to reduce it.
Discussions in this article include:
- Current State of Water Heating Systems
- Contributing Issues Related to Water Heating Systems
- Diagnosing Water Heaters
- Combustion Appliance Zones
- Customer Performance Targets for Water Heaters
- Choosing & Installing Water Heaters
- Post Water Heater Installation Testing
Current State of Water Heating Systems
- Electric water heaters account for around 3500 kWh (15%) of electricity use
- Gas water heaters account for around 230 therms (25%) of natural gas use
- Americans use 15-40 gallons of hot water per day per person
Contributing Issues Related to Water Heating Systems
Homeowners are largely responsible for wasted energy and premature failures of water heaters. Leaks are left unattended, insulation is missing, water temperature is too hot, maintenance is neglected, and water is thoughtlessly wasted.
Over time, the energy efficiency of water heaters and hot water plumbing fixtures are required by law to be improved. Manufacturers are trying a variety of methods to provide cost-effective appliances.
Advances in water heating efficiency are significant enough to consider replacing some water heaters every 10-15 years because the energy savings alone make sense.
Another major contributor to high energy use with water heating systems is the failure to consider life-cycle cost at the time of purchase. They include the cost of purchasing & installing equipment, maintenance, and operating costs. The focus tends to be on the price of the water heater alone.
Home Energy Audit: Diagnosing Water Heating Systems
Tip: Diagnosing gas, oil, and propane water heaters depend on Combustion Appliance Zone (CAZ) performance.
The process for improving water heater performance follows this path:
- Interview Homeowners
- Conduct Visual Assessment
- Select Water Heating Options
- Prepare the house for the Water Heater
- Implement Water Heater Improvements
- Post Water Heater Installation Testing
Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.
As they relate to water heating systems, the following questions are asked during the homeowner interview:
How many people live in the house?
What do you use hot water for?
What annoys you with your hot water system?
How long does it take to get hot water at various plumbing fixtures?
How do you conserve hot water?
Where are people scalded?
Are there any plumbing leaks?
Is there anything else you feel is relevant?
Answers to these questions help us evaluate the water heating system in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.
Conduct Visual Assessment
During the home energy assessment, various parts of the water heating system are observed.
Proper Installation. The first priority it to ensure the water heater is installed properly and in good condition. It’s not hard to backdraft smoke out of a water heater flue. There are at least 14 things to look at to evaluate a typical water heating system.
An extension of proper water heater installation is combustion safety. These principles are discussed in the Combustion Safety article.
Water Temperature. Water temperature is usually noted at the kitchen sink. When gas water heaters are installed, the temperature is compared to the setting on the water heater controller. Frequently, water heaters are set too hot because people try to improve hot water performance at the plumbing fixtures by raising temperature. Sometimes, the thermostat is broken.
Where water is known to be too hot at the water heater, we feel water heat at tubs and showers to determine if the anti-scald features are set.
Checking water temperature is important because one-fourth of burn unit patients were scalded by their home’s hot water system. Tap water scald burns account for 7-17% of childhood scald burns.
System Layout. Sometimes a lot of wasted hot water because the water heater is too far away from some plumbing fixtures. It also not uncommon to find water pipes run underneath the slab, where temperatures are cool and constant. It may make sense to use recirculation or add small, local water heaters.
Data Labels. Much of the performance data needed to evaluate water heaters is contained in two required labels: manufacturer’s data label and energy use label.
Of particular interest are the manufacture’s name, trade name, model number, serial number, capacity, fuel type, and first hour rating (recovery efficiency). From this information, the age of equipment, energy efficiency, presence of heat traps, and home suitability can be determined.
Insulation. For whatever reason, we tend to leave the tank and pipes uninsulated. Anywhere from 0-70% of water heating energy is lost while waiting to be used.
Showerheads. Showerheads installed before 1994 are known to use more water than current showerheads are allowed to.
Two types of showerheads are in use. The kind that tends to make steam with fine streams of water are frowned upon because they easily increase moisture levels in bathrooms.
Arreators. Faucet screens should be present to reduce water use. There initial purpose was to slow flow rate to avoid sink splashing. There use can cut water consumption in half.
Plumbing Fixtures. Dishwashers and washing machines use hot water. ENERGY STAR models are preferred. Really old units need to be replaced right away. Other units can wait until they need to be replaced. Newer units may use less water.
Water Leaks. Water leaks account for 17% of indoor water use. Some portion of the amount is hot water. They need to be found and fixed.
Control Devices. There are a few types of control devices available to help lower energy consumption: timer clocks, computerized controllers, and recirculation pumps. Control devices tend to lower the temperature during periods of non-use.
The only diagnostic testing of a water heating system is for gas, oil, or propane water heaters, especially when the flame could be touched. This is done as part of evaluating combustion appliance zones, which contain water heaters and furnaces.
Select Water Heating System Options
Until replacement of the water heater is needed, most of our recommendations are about making better use of the system you have.
Replacement. Replacement may be recommended when there appears to be problems with the water heater, there are problems with the way it’s installed, it makes the house safer to live in, or the unit is really old.
Factors to consider for purchasing water heaters are considered in the article, Improving Water Heating Systems.
Key Improvement Factors. The key to improving the existing water heating system is to reduce.
Reduce -- water use
Reduce -- waste
Reduce -- standby losses
Reduce -- distribution losses
Reduce -- water temperature
The other key factor is maintenance.
Reduce Water Use. Beyond limiting water use, there are ways to reduce the amount of water used at plumbing fixtures.
Showerheads. Where showerheads are suspected of being installed before 1994, replacements are always recommended. Water-saving showerheads can save 200-400 kWh or 8-14 therms
Faucet Arreators. Adding or replacing faucet strainers can cut water consumption at the sink in half.
Dishwashers. ENERGY STAR dishwashers consume less energy. All modern dishwashers allow the water temperature to be set between 110-120 degrees and still get the dishes clean. The heating element used to dry the dishes also heats water to 140 degrees.
Washing Machines. Front-loading washing machines consume less water than top-loading models. Front loading machines require 1/3 the water of a top load one.
Front loaders have the advantage of washing more clothes, leading to fewer loads. In addition, front loaders wring water out of clothes better, leading to shorter drying times.
Almost all residential front-loading washing machines are ENERGY STAR rated.
Reduce Waste. Fix drips and water leaks. Replace plumbing fixtures as needed.
Drips. 1 drip per minute is about 4.335 gallons every 30 days. Count the number of drips per minute and multiply by 4 or 5 gallons to find the waste per month.
Leaks. Immediately stop all water escaping from the plumbing system to protect the structure from water damage and protect indoor air quality.
Reduce Standby Losses. A lot of heat loss from water heating systems is from normal heat transfer mechanisms: conduction, convection, and radiation.
Where it makes sense, less energy will be completely lost if the water heater is inside the living space of the home.
Tank Insulation. Adding a water heater blanket can save 4-9% on water heating energy costs or about 1% of total energy consumption.
Before doing so, ask your water heater’s manufacture about the impact on its warranty. Some companies object because the labels are covered. Many will provide stickers to put on the jackets.
Insulated Pads. Electric water heaters directly transfer heat to what it is installed on. Rigid foam covered with plywood makes a great stand while limiting heat loss to the floor. Under certain circumstances it may make sense to relocate the water to accommodate this.
Pipe Insulation. Pipe sleeves, the same stuff pool noodles are made of, can deliver water 2-4 degrees hotter to the plumbing fixtures than it can without it.
Heat Traps. Heat traps keep hot water from circulating by convection in the first two feet of water pipe. Most modern water heaters have them built in. Heat traps can be purchased in plumbing departments of building supply stores. Otherwise, plumbers can make external heat traps.
Automatic Controls. Timers and computers can reduce standby losses. However, insulating the tank is cheaper and more cost effective.
Reduce Distribution Losses. Everyone complains about the water wasted while waiting for hot water to arrive at plumbing fixtures.
Recirculation. Sometimes it makes sense to recirculate hot water back to the storage tank for reheating. This avoids wasting water, which can be 3-5 gallons every time hot water is needed. This practice ensures hot water is instantly available. Just be sure to insulate the hot water pipes first.
Local Water Heating. Sometimes it makes sense to heat water where it is used. Kitchens a long way from water heater is a good example.
Instantaneous water heaters (e.g., tankless) or small storage tanks will work. Recirculation is eliminated and the tendency to increase water temperature is reduced.
Relocate Water Heater. The water heater should have short pipes to the plumbing fixtures. Sometimes, out of convenience, a water heater is placed well away from the fixtures. Where possible, relocate water heaters where performance improvements are possible.
Reduce Water Temperature. Hot water, suitable for hygiene and household activities, is defined as 110-120 degrees. The upper limit for electric water heaters is 125.
Water heater life is prolonged by reducing the temperature to 120 degrees. 160-degree water has 8 times the scale in it than lower temperature water. There is less scale, sediment, corrosion, and standby losses.
The problem with lowering the temperature to 120 degrees is that bacteria can grow in it, particularly Legionella. The incidence is higher with electric water heaters than it is with gas-fired ones because water does not heat from the bottom of the tank. The elderly, young, and immune compromised people are more prone to this risk.
Current recommendations by the World Health Organization (WHO) is for water to be heated to 140 degrees and tempered with anti-scald devices at the plumbing fixtures. Temperatures this high will eliminate this bacterium.
Maintenance. Water heaters last longer when maintained. There are at least two maintenance intervals to follow: one for sediment and the other for sacrificial anode inspection.
Sediment. Sediment collects in every water heater to varying degrees. Modern improvements lead to less build up. Sediment can cause gas water heaters to overheat and burn out the tank. Otherwise, sediment can destroy the tank.
A recommendation is to replace the drain at the bottom of the water heater with a 3/ 4 inch ball valve to allow for better flushing. When sediment is a problem, the water heater should be flushed once a year.
Removing sediment prolongs the life of your water heater and plumbing appliances.
Sacrificial Anode. There is rod that takes the brunt of ionic activity in the water. Once it’s gone, expect the water heater to need to be replaced in 2 years or so. This 30-40 inch rod should be inspected every 2-5 years, depending on water hardness. Replacing them as necessary can extend the life of the water heater 8-10 years.