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How much does it cost to run gas water heater?
09 to 11 per bath... but no easy answer
EF (energy factor) is NOT efficiency, it is a way to find efficiency Read
Generally: Buying 40 gallon can save $9+ per year over 50 gallon ... ?
Buying gas water heater that is too small can overwork heater and shorten lifespan
The new energy efficient water heaters do not pay back Read

gas meter on gas water heater Install meter on Gas water heater
Record exact usage of BTU of gas appliances.
3/4" pipe thread connections. Use yellow teflon tape or gas-approved sealant.
No excess tape or sealant inside pipe. Use black pipe or yellow flex line. Keep pipes clean during assembly.
Install in vertical position with inlets pointing up as illustrated.
12.2" Tall x 8.5" Wide x 11.2" Deep
No power source required
Digital readout in cubic feet
Remote Readable option (pulse output) using EKM-25IDS

EKM gas meter
Approved gas pipe sealant
Yellow gas line

EKM gas meter
Gas meter spec sheet with optional remote monitor specs
Gas water heater timer
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Gas water heater timer

Integrate with your home-automation module, zwave, wifi, insteon.
Or use daily or weekly plug-in timer to set schedule.
Gas water heater timer adjusts dial on gas control valve, rotating between ordinary thermostat setting and 'vacation' setting.
The water heater is never completely off, so there is no short-cycling issue when operation resumes, plus vacation mode at 50 prevents water heater from freezing.
No-tool installation. Works with older style mechanical thermostats, and newer Honeywell thermostat, fitting both propane and natural gas heaters.

Gas water heater timer at American Pacific
$5 coupon code GENO

Gas water heater timer
Parameters of this page
Parameters of this page: Information below is about using efficiency rating of gas water heater to calculate cost for 1 standard bathtub, size 60" x 30-32", that consumes 17 gallons hot water, 20 gallons total water.
Typical shower or bath is 104-110F or less, but not more. Typical water heater thermostat is set at 120F, giving a ratio of 85-90% hot and 10-15% cold if thermostat is not out of calibration, and single handle faucet cartridges are not leaking Read about crossover. For more accurate percentage, use cooking thermometer to measure temperature with full Hot water coming out of faucet, and then test again at bath or shower temperature. Repeat test after 4th person takes a shower. Repeat test in different seasons since cold water temperature varies. Buy: Cooking thermometer
From this information, you can estimate daily cost of heating water, applying the same calculations to showers (9 gallon+ hot water).
To find the total for your shower habits, plug drain, take ordinary shower, remove water using measuring cup, then multiply by 85-90% hot-cold ratio.
Similar calculations can be made for laundry by filling empty laundry tub then use measuring cup to manually empty tub. Estimations can be made for other hot water usage like washing hands. Of course toilets and outdoor faucets are not connected to Hot water and should have no effect.
This page does not attempt to estimate cost for average family of 4 for entire year, as seen on yellow sticker usually located on side of heater.
Nor does this page consider cost of purchase, maintenance, repair, and warranty differences. All costs are energy, so individual decisions affect total cost. Resources: Which is best gas water heater   How to maintain water heater

5 things you need to know to do the calculation for your water heater:
1) if your heater is pre-April 2015 or post April 2015. 
2) gallon size of heater as shown on product label, for example 30, 40 or 50 gallon.
3) efficiency rating of heater (see charts below)
4) price of BTU from your gas bill
5) incoming cold water temperature measured by thermometer at different times of year (water is colder in winter). Resource: Average ground water temperature
EF (Energy Factor) for water heaters is a formula

If flame is not burning crisp clear blue, with specs of yellow and red, then water heater is not running at full efficiency. If flame is lazy yellow, then clean burner parts, check for draft and incoming air supply etc
Troubleshoot Gas water heater
How to clean gas water heater burner
How to maintain water heater
Efficiency ratings are approximate at best and unreliable at worst. Energy Factor is a comparative measure of two simulated conditions: How efficiently the unit uses fuel, and heat retention during 'stand-by'. Testing is done on a new water heater by the water heater manufacturer over a 24 hour period using methods and standards set by DOE (Department of Energy).
In April 2015, the energy standard was changed from the previous standard set in December 2012. Read .pdf The amazing science of marketing and self-regulation assures the public that all products meet the DOE standard.

In the past, manufacturer's product specification charts showed EF efficiency as a decimal, such as .58. My files contain three older specification charts, Chart1 Chart2 Chart 3. The old charts clearly show that 30 gallon water heater has higher efficiency than 50 gallon heater. Smaller is better.

Today, manufacturer spec sheets generally show EF as a decimal. But often they show a whopping EF .675 ... a huge improvement over the old .58. Except their water heaters don't have efficiency of .675.

EF (Energy Factor) for water heaters is a formula where a percentage of the tank gallonage is subtracted from the EF .675 rating to calculate the efficiency decimal.
The new result is 30 gallon water heater has higher efficiency than 50 gallon heater. Smaller is better, same as the older water heaters. The only difference between old and new heaters is .02 efficiency improvement, a solid achievement, but misleading when manufacturer's post EF .675 without explanation.

So there it is: actual efficiency of tank is always below the EF number shown on manufacturer's charts because you have to subtract size of tank to find actual efficiency decimal.
No worries, see chart below for the old 2012 and new 2015 decimals.

Efficiency ratings after April 2015
Is gas clean burning
Which is the best gas water heater
Manufacturers also sell 'commercial grade residential' heaters with a larger BTU burner.
The purpose is to put more heating capacity into less-than-55-gallon size water heater.
The 'commercial grade residential' heater has a different DOE testing method that results in different efficiency that does not fall within calculations shown below.
Find efficiency rating
Efficiency of gas water heater
The older 2012 Standard test was based on thermostat set at 135F
The new 2015 Standard test is based on 125F to reduce standby cost of keeping tank hot throughout day.
Other changes in past several years include thicker insulation around tank, adding short piece of insulation over TP valve, and heat trap nipples that stop heat from traveling up the pipe and out of heater. Flue damper reduces amount of heat escaping up flue pipe, but flue dampers consume electricity. How gas water heaters work
The future of water heating is solar. Build your house so panels can be installed facing south. Locate water heater and hot water usage points near solar panels. Convert AC water heater to DC
Energy factor is NOT efficiency rating. There is a subtraction factor based on size of water heater.

To find efficiency for 50 gallon heater, you take EF .675 and subtract (.0015 x tank volume measured in gallons). The result equals .592 .... so the efficiency for 50 gallon tank, manufactured after April 2015 is .592 or rounded up to .60 as seen on bottom row of chart.

The 50 gallon gas water heater has EF .675, but it has .60 efficiency. The 30 gallon has EF .675, but efficiency of .63.
So 30 gallon has higher efficiency than 50 gallon, even though spec sheets show same EF for both units. Source for chart AO Smith

True efficiency of heater has numerous variables.
Efficiency declines over time with all appliances. Thermostat calibration is approximate . Other variables include elevation above sea level, quality and temperature of incoming air supply, air supply to burner, ambient room temperature, venting distance, vent obstruction, condition of heater, water hardness and quality, scale build up, etc. The more adverse factors that apply to your heater, the lower the actual efficiency.
Troubleshoot gas water heater
Higher temperatures from global warming force air and oxygen molecules farther apart, reducing complete combustion of fuel, causing lower efficiency. Except higher ambient temperatures mean water has to be heated less anyway.
Gas water heater parts 
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Gas water heater
Basis for calculations:
1 therm = 100,000 BTU
Cost per therm estimated for use on this page April 2017/ $.69 at residential retail price with taxes.
1 BTU will raise 1 lb of water 1F at sea level, but requires more BTUs for higher elevation.

Cost of hot water for 1 bath using Tank gas water heater:
Assume 17 gallons of water for typical bath x 8.34 lbs per gallon = 142 lbs
Raise 142 lbs water 1 = 142 BTU
Assume incoming water supply is 60 F and tank thermostat is set to 120F, or 60 rise in temperature. Incoming water temperature varies by season, latitude, elevation and other factors.
Raise 142 lbs water 60F = 142 BTU x 60 = 8520 BTU
1 BTU = .00001 therm

Example 1: typical 2012 50 gallon gas water heater efficiency .55, and this is generous efficiency for older water heater with dirty combustion parts and build up of sediment on heat transfer surface.

Calculation for 2102 50 gallon: 8520 BTU .55 efficiency = 15490 BTU for 1 bath
15490 BTU x.00001 therm = .15490 therms per bath x .69 per therm = 10.7 per bath
So 10.7 per bath, at sea level, depending on cost of therms.
Same calculation for 2012 30 gallon with .58 efficiency = 10.1 per bath

Example 2: typical water heater manufactured after April 2015, for 50 gallon heater, and let's say new heater still has full efficiency of .60.

Calculation for 2015 50 gallon: 8520 BTU .60 efficiency = 14200 BTU for 1 bath.
14200 BTU x .00001 therm = .14200 therms x .69 per therm = .098 or 10 per bath.
Cost 10 per bath, at sea level, depending on cost of therms.
Same calculation for 2015 30 gallon with .63 efficiency = 9.3 per bath.

Troubleshoot Gas water heater
\Cost to operate gas water heater .pdf
-This webpage can only apply simple 1-or-2-dimensional math to a complex multidimensional problem encompassing a huge swath of usage patterns, conditions, etc, making the gas meter shown at top of page the most reliable answer for any gas appliance.
-The analysis on this page shows .7 (almost a penny) per bath improvement with new water heater vs old water heater.
-If your water heater has 2% increase in efficiency, and your house takes 3 baths per day x 365 days x 1 = $10.95 per year savings for baths.
-Other hot water usage for home is not estimated, but the numbers show 2015 efficiency standards provide a savings (if manufacturer's product actually meets the standard).
-It also shows household can save money by reducing hot water consumption and installing the smaller, more efficient water heater Resources: calculate peak demand   9 ways to save with water heater   Which is best gas water heater
-250 million people in US x 1 bath per day x 1 = 250 million = $2.5 million saving per day?

Why the new energy standard is suspect:
-Aside from reliability of self-tested products, the older 2012 energy standard is based on 135 thermostat setting. The new 2015 energy standard uses lower setting of 125 to achieve the higher efficiency. A lower thermostat setting means you have less hot water in the tank. It reduces first hour delivery, so the water heater might run longer to keep up with demand. This factor might eliminate the expected savings shown on this page. Calculate first hour  Ways to increase amount of hot
-All costs are energy, and new heaters have one-time up-front cost: they use more materials, arrive in larger boxes, take up more space and cost more to buy. On the back end, they cost more to replace. All costs are energy. What is the real energy savings if a heater saves $12 per year, lasts 12 years for a total saving of $144, but consumes more raw materials to manufacture and costs $200 more to buy just to claim higher efficiency?

-The new water heater standards illustrate an attempt to preserve the heyday of consumption by using 'more efficient' products. Instead of high cost efforts, why not sell a cheap do-it-yourself insulation kit, since most of the design difference between pre-and-post 2015 30-40-50 gallon water heaters is thicker insulation?

-My observation: Relying on 'the marketplace' to solve problems has often resulted in promoting boutique-priced rooftop solar panel installations, and awarding 'energy stars' to 4-person, 5000 square foot home because the massive windows are triple-pane, while overlooking inherent efficiency of DIY solar panels installed on a 6 person mobile home. Perhaps more do-it-yourself decisions, and less reliance on 'somebody inventing a new product' might be correct solution. 
Protect your investment:   How to maintain water heater
The future of water heating is solar panels: Convert AC water heater to DC
General information:
Install gas water heater
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Gas water heaters with power vent or flue damper consume 120 volt to operate flue damper or power vent. Power vent models have 1/4 HP motor. The cost of electricity is a consideration, as is the need to install dedicated outlet and surge protection. Troubleshot power vent water heater

Figuring cost ACCURATELY is a considerable task as seen above: Age, type of gas, cost of local gas supply, seasonal fluctuations in prices and temperatures, elevation above sea level, incoming air supply, efficiency, efficiency decline over time, cold water temperature, thermostat setting etc play a role in calculation.
All numbers for efficiency, BTU rating etc, are based on new heater under ideal conditions rarely attainable in real world.

Showing approximate cost of fuel consumption per bath does not take into consideration the cost of the heater. For example, 75 gallon condensing gas heater shows 74% efficiency but the total cost is misleading because of the increased cost of purchase, maintenance and repair for high tech heaters. Same with residential heaters, the purchase cost, maintenance and replacement cost have not been calculated, but they affect the real cost of heating water. How to maintain water heater

Each water heater has a BTU rating seen on the label that says how many BTUs are burned. For example commercial gas water heaters have large burners and are rated for higher BTU than residential heater. Installing commercial gas heater in home might consume so much gas that the household gas meter cannot keep up, starving other appliances of fuel and causing malfunction.
How to install gas water heater
Maintain water heater
Which is best gas water heater
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