Guide to Characteristics of Types of Hot Water Heaters InspectAPedia® -
Comparison of alternative water heater type life expectancy, operating cost, maintenance cost, purchase cost, capacities, hot water quantity, hot water recovery rates, and water heater safety
How to compare various features of hot water heating methods: electric, gas, high efficiency gas, oil, high efficiency oil, solar, tankless coils, etc.
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Here we discuss how to compare the operating characteristics such as safety, capacity, life expectancy, and operating cost of alternative methods of providing domestic hot water for washing and bathing.
Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.
A Comparison of the Relative Life Expectancy of Different Types of Water Heaters
Here is a rough guess at the relative life expectancy of these types of water heaters, provided all other water heater life factors (discussed at AGE of WATER HEATERS) are the same (comments are invited Contact Us)
Electric water heaters (though individual heating elements may fail and need to be replaced before the tank fails)
Gas-fired water heaters, High Efficiency Models (LP gas or natural gas)
Gas-fired water heaters (LP gas or natural gas)
Oil-fired water heaters
Solar water heaters have a relatively high purchase cost but probably lower maintenance cost than oil and gas fired equipment
Conditions Affecting Water Heater Life
Keep in mind that conditions besides the type of water heater can dominate its life expectancy. Some of these water heater life expectancy factors include:
The duty cycle or level of usage of the water heater (more use = shorter life)
The chemistry of the water being heated (aggressive or corrosive water = shorter life)
The construction of the water tank, methods and materials used (look at the water heater warranty period; longer warranty probably means a more durable water heater tank)
Proper installation of the water heater; improper fuel or water piping connections or inadequate chimney venting on fossil-fuel fired water heaters can lead to safety problems or leaks and early heater failure. Improper location or improper position of the water heater (such as mounting some water heaters horizontally) can lead to early failure of the heater as well as voiding the manufacturer's warranty.
Proper maintenance; almost no one we've encountered drains and services their water heater on the schedule recommended by the manufacturer
For complete deathless of water heater life and things that affect how long a hot water heater will last, see our full article at AGE of WATER HEATERS.
How to Evaluate the Present and Future Relative Operating Cost of Different Types of Water Heaters: definition of therms
Heating fuel costs vary among different communities, states or provinces, and countries. In most of North America and in some of Europe it's more costly to heat water by electricity than oil and more costly to heat water by oil than gas.
However exceptions occur, especially where one or another energy source is regulated or subsidized by government, or where cheaper sources of electrical energy may be available such as in areas where hydro-electric plants are common.
Certainly, a modern high efficiency oil or gas water heater will cost less to operate than older models of water heater using the same fuel.
Compare the Energy Guide sticker data to compare the relative operating costs of alternative water heaters that you are considering. Our sample water heater energy guide sticker above provides the following data that you can compare among other water heater alternatives:
Annual energy use measured in Therms per year For example, our high efficiency AO Smith natural gas water heater shown at High Efficiency Water Heaters uses an estimated 242 Therms/Year
Estimated annual operating cost in dollars. The water heater we are using as an example has a (standardized) annual operating cost of $146.
Naturally this number is "wrong" in that the amount of hot water individual families will use varies among households, and the price of various energy sources (electric, gas, oil) is continuously increasing.
But this is the measurement we want: a stable number that permits comparison among alternative types of water heaters.
Where the particular water heater model falls in the whole range of water heater annual energy usage rates. (Least energy usage = 220, most energy usage = 288), so you can see not only how your water heater compares with other models, but where your water heater falls in the energy efficiency range.
What is the Definition of a Water Heater Therm?
In the U.S. natural gas is sold to consumers sometimes measured in therms (th). Therms is also used in Energy Guide ratings useful for comparing alternative water heater operating costs.
A therm is the amount of natural gas used to generate 100,000 BTUs of energy.
A therm (th) is thus also a measure of heat content, or a measure of energy usage.
Since this is a simple fixed number, it's easy to compare gas-fired water heater efficiencies.
Example: If water heater A uses 1 therm to produce 100 gallons of hot water heated up from 40 degF to 120 degF, and water heater B uses 2 therms to produce the same 100 gallons of hot water heated up from 40 degF to 120 degF, then water heater A will cost half as much to operate as water heater B. In other words, the fewer therms/gallon or therms/estimated-annual-usage-in-gallons, the higher is the efficiency and lower the operating cost of a given water heater.
Relative purchase and maintenance cost of water heaters
In order of cost from least-costly to most-costly to purchase and maintain we estimate this order of types of water heater:
Electric water heaters
Gas-fired water heaters, High Efficiency Models (LP gas or natural gas)
Gas-fired water heaters (LP gas or natural gas)
Oil-fired water heaters
Solar water heaters have a relatively high purchase cost but probably lower maintenance cost than oil and gas fired equipment
Remember to Include Water Heating Method Conversion Costs When Changing Water Heaters
Don't forget that if your plan to change the type of water heater, say from electric to oil or gas-fired, involves changing to use of a new fuel that you do not already have present at the property, extra costs will be involved to install the fuel tank, piping, and controls.
The new LP gas tank installed at this home where a gas fired water heater was installed to replace an electric unit added several hundred dollars to the installation cost.
Be sure to add those infrastructure costs as well as an estimate of the varying rates of change in energy prices among different fuels (electric, gas, oil, solar) into your analysis of the lifetime operating cost estimates when comparing alternative means of making domestic hot water.
Our opinion is that the only energy sources whose price is not going to increase in the coming decades are sources such as solar or wind energy. It is plausible to expect the infrastructure of those systems may actually decline when more people begin to purchase them and their production volume increases.
Hot Water Quantity: A Comparison of the Relative Total Hot Water Capacity of Different Types of Water Heaters
Any water heating method that stores hot water in a tank will provide about the same total hot water capacity if the tank is the same size. This includes electric, gas, oil fired water heaters and side-arm coil water heaters or range boilers, all of which use a water storage tank.
Instantaneous water heaters, also called tankless water heaters, can provide effectively endless hot water but usually in limited quantity to the one or two fixtures supplied by the heater.
Tankless coils provide a more limited quantity of hot water, depending on the amount of heat stored in the heating boiler (a function of its size and materials of construction - steel has less heat storage than cast iron), the temperature of the incoming water, the use of mixing valves, and the flow rate through the coil.
Most tankless coils are rated as capable of providing a specific temperature degree rise across the coil, from input to output, at a specified boiler temperature and at a specified water flow rate in gallons per minute (often 5 gpm). If the water flow rate is faster than specified the coil will not heat water to as high a temperature.
Relative Hot Water Recovery Rate/Speed of Different Types of Water Heaters - What is the recovery rate of a water heater?
The hot water heater recovery speed of a water heating system refers to how quickly the water heater can re-heat incoming cold water.
The standard measure of domestic hot water heater recovery rate is the number of gallons of water than can be heated to 140 degF in one hour. (There must be an underlying assumed-temperature of the incoming water).
A water heater that has a recovery rate of 35 gph will make hot water faster than a water heater that has a recovery rate of only 20 gph.
On newer water heaters you'll see the water heater's recovery rate printed on one of the information labels affixed to the equipment. (Also see
Particularly if your water heater has limited total hot water quantity, and if you expect high hot water usage (such as a large family all wanting to take long hot showers one after another), the recovery speed of the water heater can be an important factor in evaluating its adequacy.
There is some subjectivity in this list. We've encountered a large family of six who said they never ran out of hot water although they were all living in a home whose hot water was provided by a single tankless coil on an old heating boiler.
We've encountered a small family of two adults who complained that they never had enough hot water even though their home was served by a 40 gallon oil-fired water heater.
A lot depends on just how people use hot water: how much at once, how fast the flow, how long the flow per person, etc.
As we illustrate with Carson Dunlop's sketches shown here, in order of speed of re-heating or hot water recovery time, listing slowest-recovery time to fastest recovery time we'd list water heater types as follows:
Range boilers
Electric water heaters
Gas-fired water heaters
Gas-fired water heaters, High Efficiency Models
Solar water heating systems (depends on weather, season, and sun exposure)
Tankless coils (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
Indirect-fired or tankless water heaters (depends on heating boiler size, construction [steel/cast iron], thermal mass, and fuel type [oil/gas])
Oil-fired water heaters
Instantaneous water heaters
Don't confuse water heater recovery rates (how fast we can heat water) with water heater operating costs, which we discuss at Water Heater Operating Cost Comparisons. Recovery rate is measured in gallons per hour or gph. Water heater operating costs are compared using a standard measure of energy cost in therms.
A Comparison of the Relative Safety of Different Types of Water heaters
In the LP gas fired water heater shown in our photo, the installer had trouble fitting the draft hood atop the heater since there was then not enough overhead clearance to route the flue out of the building. The installer has thoughtfully left the draft hood he removed, an important safety device, sitting atop the water heater as a reminder that this is an improper installation.
There is just too much subjectivity to make a sensible answer to this question. Some people are frightened by the prospect of a natural or LP gas explosion in a building.
But while it's difficult to set heating oil on fire and so explosions based on the fuel character may be less likely than with an LP or natural gas system, an oil-fired water heater runs at much higher burner and exhaust flue temperatures and so can prevent other types of fire risk.
Most critical with any water heater are that the heater is properly installed and includes the required safety controls such as those responsible for limiting temperature and pressure. Any water heater should include a temperature and pressure relief valve installed of the proper type and at the correct location. Gas-fired devices also should include flue-gas spillage detectors. Oil-fired devices should also include a fire-safety valve that shuts off fuel to the appliance in case of fire.
We guess that proper installation, inspection, and maintenance make more difference in the safety of a water heater than the innate differences among heater types, fuels, etc.
Here is a list of hot water heater safety devices and features:
Pressure/Temperature Relief Valves: Water heaters (and any pressurized tank) require a pressure and temperature safety relief valve or TP valve as well as an extension or discharge tube. In Carson Dunlop's sketch at left the pressure relief valve is improperly installed on the far side of the pressure reducing valve at the water heater. This is a very dangerous installation. See our Guide to Heating System Boiler Temperature and Pressure Relief Valves.
Spill switch on gas flues: Gas fired water heaters should have a flue gas spillage switch that shuts off the gas valve if needed
Gas shutoff valves: Gas fired water heaters in some jurisdictions require an automatic gas shutoff valve that responds to high water heater temperature.
Oil shutoff valves: oil fired water heaters should always have their own oil safety valve: a special valve installed on the oil line near the water heater and which will automatically close and stop the flow of oil to the water heater should a fire occur. Watch out: often in a building where there is an oil fired heating system as well as an oil fired water heater, the installer may have installed the safety valve only at the oil tank or only at the heating boiler or furnace.
Flame shields: Gas or oil fired water heaters include a flame shield around the port used to inspect the burner - it must remain in place.
Combustion air: any fossil fuel fired water heater (oil or gas fired) requires combustion air. The volume of air intake to the utility room where the heater is installed depends on the BTUs of the device and the type of fuel. Watch out: installing a gas or oil fired heater in a small confined space whose door is shut and which lacks adequate combustion air is unsafe.
Combustion gas venting by a chimney: any fossil fuel fired water heater (oil or gas fired) requires safe venting of the combustion products, through a chimney or in the case of high efficiency heaters, through a plastic vent line. Watch out: venting a small gas fired water heater through a large old masonry chimney may be unsafe. See CHIMNEYS for details.
Temperature settings: on the water heater, setting the temperature very high in an attempt to obtain more hot water can be dangerous and lead to serious scalding burns for the building's occupants unless water tempering or mixing valves or other temperature safety controls are provided and properly set at the plumbing fixtures.
At a temperature setting of 100 degF or below most water heaters are unlikely to scald an occupant; more than 5 minutes exposure at 120 degF are required to produce 2nd & 3rd degree burns on adult skin.
Hot Water Scalding Burn Warning:Mixing Valves contains a table of hot water temperatures which are safe and which explains the risk of second and third degree scalding burns that occur at different water temperatures and exposure times.
Working space: Water heaters require safe working space around the device regardless of the heater type
Damage protection: Water heaters in garages should be protected from mechanical damage such as being struck by an auto
Regular safety inspections: Water heaters should be periodically inspected for safety and evidence of damage or leaks. An example of an unsafe water heater is one whose relief valve has been leaking, modified, or blocked, or a heater whose exhaust flue lacks proper fire clearance distances.
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Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
A.O. Smith water heater safety sticker: 182734-000 Rev.00 observed on the gas fired water heater shown at High Efficiency Gas Water Heaters
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
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