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Water Heaters
Choosing a Water Heater for Optimal Performance | Available Options | Electric | Heat Pump | Basic Gas | High Efficiency Gas Condensing | Tankless Gas | Solar Thermal Hot Water | Durability and Quality | Installation and Maintenance | SustainabilityQuick Facts
- Electric water heaters are generally much more expensive to run than gas water heaters
- Condensing gas water heaters extract heat from exhaust gases and are much more efficient than basic models
- Modulating water heaters can have a varied heating output and are even more efficient
- Can be used in conjunction with hydronic heat (radiant or forced air) for added home energy efficiency
- Sealed combustion appliances are safer because they pull air from the outdoors, not the house, and backdrafting of combustion gases (like carbon monoxide) is not possible when installed correctly
- Can function as a backup for solar thermal hot water
Components
A basic water heater has two main components which correspond to the two main functions (water heating and storage). Higher quality and more efficient models can have additional components which increase system efficiency.
BurnerProvides heat in a water heater | TankStores the water |
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Choosing a Water Heater for Optimal Performance
There are several types of water heaters and each is appropriate for different applications. To choose a water heater that will deliver optimal performance, energy efficiency and cost, consult a home performance expert. An expert will be able to quantify the needs of the home, taking into account
- Size
- Occupancy
- Water usage
- Water quality (hardness)
- Maintenance
Instead of simply installing a model identical to the existing heater, the expert should determine optimal:
- Seasonal water usage
- Energy output
- Tank capacity
- Flow rate (in first hour rating or GPM)
- Energy efficiency rated by energy factor (EF)
- Thermal efficiency (what percentage of energy used is actually transfered to the water as heat)
- Heat loss of the tank (quality and thickness of insulation)
Along with specifications about the layout of the home and the desires of the home owner, the following options can be considered:
- Integrate hydronic forced air or hydronic radiant heating with a properly sized and highly efficient condensing (sealed combustion) gas heater
- Conversion from more expensive electric or oil fuel sources to gas
- Conversion to tankless gas heater (appropriate gas supply, venting and water demand), if only small amounts of hot water are needed
- Benefits of solar thermal hot water heater which can be appropriate for most homes, especially those with high domestic hot water use
Available Options
Electric | Heat Pump | Basic Gas | High Efficiency Condensing Gas | Tankless Gas | Solar Thermal Hot Water
Electric
- Heats water through electric resistance heating elements located inside the tank
- Less expensive to buy than the cheapest gas water heaters (a low initial cost)
- Much more expensive to operate than gas (high long term operating costs)
- Less efficient overall than efficient gas water heaters (despite better efficiency rating)
- Electric resistance is a very expensive way to generate heat
- Less efficient than gas if electric generation and transmission losses are counted
- Generally emits more CO 2 than gas heaters due to this inefficiency, depending on local electric utility power generation and transmission
- Replacement with high efficiency gas or heat pump heaters can save money overall due to lower operating costs
Examples include:
Heat Pump Water Heater
- Uses an electric compressor and evaporator coils to transfer heat from ambient air to the water
- New to the American water heater market
- Uses one half to one third the electricity as electric resistance heaters, due to higher efficiency
- Additional cost compared to electric resistance heater can be recouped through energy savings
Examples include:
Basic Gas Water Heater
- Less expensive to buy than high efficiency condensing models
- Can be 40% more expensive to operate than gas condensing models *Note
- Standards have improved the insulation of the most basic water heaters
- They still lack some technologies which vastly increase energy efficiency, such as closed combustion and a condensing heat exchanger
- Consist of
- An insulated tank
- An electric ignition burner under the tank
- A flue running up the middle of the tank
- Exhaust flue to outside air (must be metal since exhaust gases are quite hot)
- Danger from flammable gases since open combustion burner is open to outside air
- Exhaust gases can be “direct” or “power” vented for very modest efficiency gains (7%) and decreased risk of fire from gases
Examples include:
- Bradford White® Defender Safety System® High EF Gas Water Heater
- GE® Gas Water Heater Model GG50T06AQJ00
Tankless Gas
- Provides hot water on demand and does not store hot water
- Burner comes on when hot water is requested at the faucet
- Heat exchanger quickly and effectively transfers burner heat to water
- Avoids the loss of standby energy used to keep the water hot (usually between 120° and 160°)
- Hot water does not run out since there is no tank to be emptied, but hot water flow rate will decrease with increased hot water demand
- Water may run cold if the flow of water demand surpasses the maximum flow rate of the water heater
- Only turns in when hot water is being used
- When it does turn on, it uses much more energy than storage heaters (and burns more gas), since it must heat the water right away
- Most appropriate for households
- With water efficient fixtures, but which allow enough flow to trigger the burner
- Which use hot water infrequently and in moderate volumes (and thus which may have the highest stand-by losses with a storage tank water heater)
Common concerns:
- Hot water pressure drop when flow rate cannot keep up with demand
- “Cold water sandwiches”: alternation of hot and cold water as the heater works to keep up with hot water demand, or as demand drops enough to turn off the burner
- For the highest levels of efficiency in cooler climates install a high efficiency condensing water heater in combination with the home’s space heating system (hydronic forced air or hydronic radiant heat).
Examples include:
Condensing High Efficiency Gas Water Heater
- Cost more than basic models
- Uses sealed combustion which is safest and most efficient
- 30% lower operating costs can recoup the extra investment over time
- Does not use a straight flue or open combustion so there is no risk from flammable gases
- Heat exchanger coils transfer combustion heat to the water much more effectively and thoroughly
- Combustion gases are cool when exhausted
- Electronic controls allow for precise adjustment of the water temperature
- Optimal for hydronic heating applications due to powerful and precise burner
- Some units have modulating burners to match varying hot water needs
Three significant benefits:
- Efficiency can be increased by 30% because heat in exhaust gases is recovered instead of being thrown away.
- Installation becomes simple and functioning becomes much safer because no heat resistant metal flue is needed to exhaust burning hot gases; instead, the cooled exhaust gases can be vented through PVC pipe, reducing necessary clearances from combustible materials.
- Sealed combustion eliminates possibility of backdrafting and carbon monoxide poisoning when the flues are installed correctly (per manufacturer instructions).
The very top tier high efficiency water heaters will also use modulating technology. This means that the boiler component of the water heater has a variable heating output, instead of a simple on or off operation. Top tier water heaters will have a sensor which calculates optimal heating output based on the temperature of the water in the tank. For example, if the water only needs to be heated up a small amount, the boiler can fire on low, which uses much less energy than firing on full power when it is unnecessary.
Examples include:
Solar Thermal Hot Water
Some high efficiency water heaters are made to work in conjunction with a solar thermal hot water system in warm regions. The water is directly heated in the collectors and delivered to the tank for storage, or more commonly, closed loop systems use a heat transfer fluid to move the heat from the collectors and into the storage tank (through a heat exchanger). For either system the gas burner component is considered supplementary or back up heating power.
Solar thermal hot water systems are complex and can work in a variety of ways, primarily
- Internal Heat Exchange Systems
- External Heat Exchange Systems
- Drain Back Systems
- Thermo-siphon Systems
Manufacturers include:
Energy Factor and Thermal Efficiency
The values in the following table are representative of general ranges in efficiency levels. Actual efficiency levels and operating costs will vary with make and model of the water heater as well as the household plumbing layout and hot water usage. It is mostly important to note that efficiency factors cannot directly be compared across heaters with different fuel sources, particularly for electric models which do not account for losses in generating the electricity; this is partially reflected in the higher yearly energy costs for electric models compared to gas models.
Average Energy Costs and Carbon Emissions for Water Heaters
| Energy Source | EF (Energy Factor) | Annual Energy Cost | . Annual Tons CO 2 . |
| New Standard Electric | 0.9 | $520 | 3.4 |
| Heat Pump (electric) | 2.2 | $213 | 1.4 |
| Solar w/ Electric Storage | 2.0 | $234 | 1.5 |
| New Standard Gas | 0.6 | $304 | 1.4 |
| Condensing Gas | 0.86 | $212 | 1.0 |
| Energy Star Tankless | 0.82 | $223 | 1.0 |
| Solar w/ Gas Storage | 2.0 | $91 | 0.4 |
| Conventional Oil | 0.55 | $655 | 3.2 |
Sources for carbon emissions: EPA , EPA .
Note: Based on average usage. Actual emissions will vary with actual usage and the carbon intensity of the local power utility.
Return to Choosing a hot water heater
Durability & Quality
Quality water heaters cost substantially more than basic models, but they can be easier to install, much less costly and much safer to run and they should last longer and need less maintenance than basic models. As with any product, manufacturer warranties are a sign that the manufacturer stands by the quality of the product.
Installation and Maintenance
Tankless
Since it does not have a tank, a tankless water heater takes up much less room than a heater with a tank. However, some things to keep in mind are:
- installation requires an electrical connection
- upsizing of the natural gas line is usually required to 3/4” or greater, depending on burner output and distance to the gas meter
- many units require stainless steel vents and condensate management
- certain models can be mounted outdoors
- always include a flush kit during installation to enable yearly maintenance
- a water de-scaler or water softener is needed in areas with hard water
Basic Gas Water Heater
The basic models consist simply of an insulated tank and an electric ignition burner which needs venting for the hot exhaust gases through a metal flue. These models are less expensive to buy, but they are more dangerous to operate (due to open combustion producing hot exhaust gases and the risk of explosion due to leaking of flammable gases) and much less efficient than more expensive high efficiency water heaters.
High Efficiency Gas Water Heater
Installation can be more straightforward than for tankless units and functioning is much safer than basic gas water heaters because no heat resistant metal flue is needed to exhaust burning hot gases; instead, the cooled exhaust gases can be vented through PVC pipe. Some other things to keep in kind include:
- an electrical connection is required
- condensate management (such as drain pipes) is necessary
- exhaust termination location is critical on some units as they can be noisy
Solar Thermal Hot Water
Installation of these systems is more complex since the solar collector must be installed on the roof. A home performance expert or solar installation provider will need to install these systems. Annual maintenance is typically suggested for these systems.
Sustainability
Selecting a long lasting high-quality water heater will assure highly efficient production of hot water, meaning that energy is not wasted and carbon emissions due to water heating will be greatly reduced. A solar thermal hot water system will produce even less CO 2 since it also relies on renewable, clean solar energy. In addition, high quality products greatly reduce the need for replacement after a few years due to malfunction and deterioration. The longer home building materials last, the less they will be discarded in landfills. Also, it is important to take the entire product lifecycle into account, which means that it is better to buy products from companies which use responsible and sustainable manufacturing and employee treatment practices.