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How much hot water does bath or shower use?
water consumption bathtub
Take showers instead of bath.

Does not include maintenance, installation, purchase, repair, replacement of water heater... which can substantially add cost to final energy numbers.
All costs and decisions are energy.


Regular bath uses 15+ gallons hot water 
Princeton 60" x 30" x 14" insulated bathtub
Filled to overflow, the tub holds 42 gallons according to American Standard.
Note: 60" x 30" x 14" is rough-in dimension of bathtub... and not interior space filled by water. 
Note that many bathtubs are larger, such as 60" x 32' x 16" and these tubs hold proportionally more water. 

Body displaces water, so bathtub is not filled to overflow when taking bath.
>Moderately large person sitting in tub displaces about 20 gallons, so if tub holds 42 gallons, then amount of water used for bath is roughly 20+ gallons of water.
Summary: 20 to fill tub + 20 for displacement = 40 gallons which rises almost to overflow when person sits in 60" x 30" x 14" bathtub. Water level where person sits is 8-1/2" deep; measured next to hip of person sitting in tub when water level reaches overflow. Water level measured at tub drain is 1" deeper, or 9-1/2".
Typical water temperature of bath or shower is 103-105°F. Typical water heater thermostat is 120-125°F. Typical incoming cold water temperature might be 50-60°F.
If 5 gallons of 50°F water are added to 15 gallons 120°F water, it yields 20 gallons 102°F water.
So the hot cold ratio can be approximated: 75-80% hot to 20-25% cold

Resources:
Compare gas and electric water heater timers
Off peak water heater options
Set timer chart for electric water heater
9 ways to save with water heater
How to increase amount of hot water

Buy:
Bathtubs and shower stalls at Amazon 
Electric water heater rated 240 Volt 4500 watt will heat 21 gallons of water per hour based on 60 degree rise in temperature/ 240 Volt 3500 watt will heat 16 gallons per hour.

Gas water heater will typically heat 41 gallons per hour based on 60 degree rise in temperature. For example if water inside tank is 60 degrees F at start time, and you want 120 degree water, this would be a rise in temperature of 60 degrees.
The 20 gallon shown above was measured after a normal bath. Measuring technique: using small bucket to fill 5 gallon buckets (to within 3/4" from top rim of bucket same amount when you open 5 gallon paint). 

Measurement shows 20 gallons of water is used for each bath.... 
.... estimate 15 gallons of hot and 5 gallons of cold. 
The hot-cold estimate 75% based on 105° bath and based on position of handle when filling tub. 
Temperature of incoming cold water is colder in winter and warmer in summer. Amount of cold water added to hot also varies winter-summer. Plus water heater energy-consumption varies winter-summer as more energy is needed to heat colder water. To average the variables, I estimate an 'average' year-round cold water temperature of 60 degrees. See groundwater temperature map

Hot water consumption of 15 gallons for each bath is estimated based on 120 degree thermostat setting. If water is hotter than 120 degree, then less hot and more cold would be used... but the energy and cost to heat water would be comparable when baths are same temperature, with slight energy savings per bath if thermostat is set lower. Also using mixing valve located at water heater can reduce energy consumption.
Formula for heating water inside tank:
Gallon Per Hour (Electric) = (KW x 3413) divided by (temp. rise x 8.25) or (KW x 414) divided by (temp rise.)
Let's say you have 5500 watt elements.
let's say incoming water temperature is 65°F and you want to heat tank to 105°, or 40° rise in temperature.

5500 watt or 5.5Kw elements x 3413 = 18771.5
40 degree temperature rise x 8.25 = 330
So in this specific situation, the element would heat 56.88 gallons per hour

This number is for EACH element, but does NOT consider heat loss from tank, room temperature, or thickness of insulation etc.
Remember, with residential simultaneous tank, only ONE element is ON at one time. 
Upper element is ON, or lower element is ON, or BOTH elements are off. At no time are both elements on.
Cost of 1 bath:
60x30x14 insulated bathtub
Assumes Temperature rise 60°
Incoming water temp 60°, raise to 120° hot water inside tank
Assumes 15 gallon of hot water needed for regular bathtub
Total 42 gallon bathtub to overflow
Displacement 20 gallon for person
Total water consumption 20 gallon
Hot water 75% to 80% with thermostat set at 120°
Total hot water consumption for this bath is 15 gallons

These estimates are conservative, based on fairly large person in tub, and tub not filled to overflow
Also this is the small bathtub with outside dimensions 15x30x60
Cost of hot water for 1 bath/ Tank Electric water heater
It takes .146666 Kwh to raise 1 gallon 60°
.146666 Kwh x 15 gallon = 2.19 Kwh per bath

Divide 2.19 Kwh by efficiency rating of water heater, then multiply by cost per Kwh hour you pay.

*For example, typical pre-2015 electric water heater efficiency is 96%:
So 2.16 Kwh ÷ .96 = 2.25 Kwh x 15¢ per Kwh = cost 34¢ per bath

*For example, typical 2015 electric water heater efficiency is 99%:
So 2.16 Kwh ÷ .99 = 2.18 Kwh x 15¢ per Kwh = cost 33¢ per bath
Cost of hot water for 1 bath/ Tank gas water heater
1 therm = 100,000 BTU
Cost per therm .42-1.10
1 BTU to raise 1 lb of water 1°F
15 gallons of water = 125 lbs
Raise 125 lbs water 1° = 125 BTU
Raise 125 lbs 60°F = 125 BTU x 60 = 7500 BTU
1 BTU = .00001 therm

*For example, typical pre-2015 gas water heater efficiency .61
So 7500 BTU ÷ .61 efficiency = 12295 BTU for 1 bath
12295 BTU x.00001 therm = .12295 therms per bath
Cost 05¢ to 14¢ per bath

*For example: typical 2015 water heater
So 7500 BTU ÷ .675 efficiency = 11111 BTU for 1 bath
11111 BTU x .00001 therm = .11111 therms
Cost 05¢ to 10¢ per bath depending on cost of therms
A water heater holds 20/ 30 / 40 / 50 / 60 / 80 gallons of hot water depending on size of tank. 
Fully heated 40 gallon tank gives you 2 baths and a shower at one time. 


Safety grab bar
Use safety grab bar on bathtub
Buy:
Safety grab bar at Amazon
Bathtub safety grab bars
Bolt on safety grab bar
Shower rods Amazon

Energy cost of 1 bath using 45 gallon hot water: (10 gallons cold)
Cost to raise 45 gallons of cold water from 60 degrees to 120 degrees inside a water heater tank:
Typical electric water heater 96% efficiency pre April 2015= 6.88 Kwh per bath 
Typical gas water heater pre April 2015

Typical electric water heater 99% efficiency post April 2015= 6.66 Kwh per bath
Typical gas water heater post April 2015

None of these costs factor maintenance, installation, purchase, repair, replacement of water heater... which can substantially add cost to final energy numbers.
All costs and decisions are energy.
Typical walk in tub uses up to 55 gallon hot water
There are different size walk-in bathtubs
Example American Standard fill height of water is 4" lower than deck of tub
Seat height from floor 17"
From seat to fill height is 12.5" (you sit in 12.5" water vrs 8.5" in regular tub). 
From textured floor to fill height is 29"
My estimate is 80+% Hot to Cold ratio/ higher ratio for large tubs that take longer to fill.
Filled to overflow:
60 L x 37 W = 80 gallons...25 displacement for person = 55 gal final fill/ 45 gal Hot
60 L x 32 W = 70 gallons...
25 displacement for person = 45 gal final fill/ 35 gal Hot 
50 L x 30 W = 60 gallons...25 displacement for person = 35 galfinal fill/ 28 gal Hot
48 L x 28 W = 45 gallons...25 displacement for person = 20 gal final fill/ 16 gal Hot
Displacement for regular bathtub is 20 gallon for moderately large person.
Displacement for walk-in tub is 25 gallons, or 5 gallons more than regular tub, because about 5" more of person's torso sits inside the water.


You might need larger water heater, or 2 water heaters to supply enough hot water
Resources:
New DOE standards April 2015
Maintain your water heater
9 ways to save
Two water heaters

Typical faucet delivers 2 gallons per minute.
Test your water flow using 1 gallon or 5 gallon bucket, and stop watch on cell phone.
In regular tub, person can lean back at 26 degree angle -or- displace more water by sliding torso deeper into water by lifting knees, and the water level can rise above overflow. 
Walk-in tub the person can lean back at 26 degree angle -or- displace more water by leaning forward to immerse torso into water which can raise water level above overflow.
It is important to calculate heat loss when filling large tub. Cup of coffee cools fast.
If faucet flow is 3-4 gallons per minute, then fill time takes 4-18 minutes depending on tub.
Anticipate using more hot water vrs cold water when filling walk-in tub because it takes longer to fill tub, so more heat is lost while waiting for tub to fill.
Offsetting heat loss during fill, is insulation value of walk-in tub vrs inexpensive steel tub, and deeper water in walk-in will hold temperature longer, and less overall surface area of walk-in will lose heat more slowly. 
Bubbler will cool water more quickly. Energy cost of running 1 Hp blower motor, and heat loss when using blower are not calculated, nor energy cost of any blower heater.
Floor loading 725 lbs for smallest size to 877 lbs for largest size. Subfloor reinforcement recommended and not included in energy calculations. All costs are energy.
Cost of 1 bath:
Large walk-in bathtub
Assumes Temperature rise 60°
Incoming water temp 60°, raise to 120° hot water inside tank
Assumes 45 gallon of hot water needed for walk-in bathtub

Total water consumption 55 gallon
Total Hot consumption 45 gallons

Cost of hot water for 1 bath/
Tank Electric water heater
It takes .146666 Kwh to raise 1 gallon 60°
.146666 Kwh x 45 gallon = 6.6 Kwh per bath

Divide 6.6 Kwh by efficiency rating of water heater, then multiply by cost per Kwh hour you pay.

*For example, typical pre-2015 electric water heater efficiency is 96%:
So 6.6 Kwh ÷ .96 = 6.875 Kwh x 15¢ per Kwh = cost $1.03 per bath

*For example, typical 2015 electric water heater efficiency is 99%:
So 6.6 Kwh ÷ .99 = 6.66 Kwh x 15¢ per Kwh = cost $1.00 per bath
Cost of hot water for 1 bath/
Tank gas water heater
1 therm = 100,000 BTU
Cost per therm .42-1.10
1 BTU to raise 1 lb of water 1°F
45 gallons of water = 375 lbs
Raise 375 lbs water 1° = 375 BTU
Raise 375 lbs 60°F = 375 BTU x 60 = 22500 BTU
1 BTU = .00001 therm

*For example, typical pre-2015 gas water heater efficiency .61
So 22500 BTU ÷ .61 efficiency = 36885 BTU for 1 bath
36885 BTU x.00001 therm = .36885 therms per bath
Cost 16¢ to 40¢ per bath

*For example: typical 2015 water heater
So 22500 BTU ÷ .675 efficiency = 33333 BTU for 1 bath
33333 BTU x .00001 therm = ..33333 therms
Cost 14¢ to 37¢ per bath depending on cost of therms
water consumption shower Shower uses 6 - 8 gallons of hot water (10 gal water total)

To check amount of hot water used during a shower taken in bathtub, stop the drain and then take shower.
After shower, measure the amount of water in tub, or roughly compare with the 20 gallon depth of water needed to take bath.

To check hot water usage when shower is taken in a shower stall, put stopper over drain, seal with duct tape. Tape will hold stopper in place when pouring water into shower. Add water to shower. Use measured amount of water to fill bottom of shower. Do not flood water over the top. This will give a baseline amount of water.
Then remove taped stopper.
Use stopper again during regular shower. No need to tape the stopper when taking a shower since water will be dropping from above and will not move the stopper. When water reaches same point as the measured test, remove stopper and release water. Estimate if you are half-way through shower, and then estimate how much water the total shower would use.

If temperature of water heater is set at 120 degrees, you can probably estimate 75% how and 25% cold. But more accurate way is to look at position of shower valve and see what percentage of cold water is being added to hot to temper the shower.

Set water heater timer for 20-30 minutes for hot shower


Resource:
Family chart for setting timer

Buy:
Shower stalls at Amazon
Home energy savings books at Amazon
Shower head flow rate Shower head flow rate
Typical bath or shower is 105° F
Shower shut off
Shower shut off
Product that can save money: use shower shut off
Save 20¢ per month
Stop drips.
If shower drips when valve is off, and valve is off in center position, or if double handle faucet leaks on hot side, then hot water is leaving tank and new cold water is entering tank.
Reduce flow.
Reduce consumption.
If shower head is turned off between lather and rinse, this action will save water and save hot water
Remove shower head. Apply teflon tape to threads. Install shut off. Apply teflon tape to shut off threads. Re-install shower head.

Buy:
Shower shut off at Amazon
Resources:
Water-loss-with-one-drip/ pdf
hot water consumption Clean shower and doors with White vinegar
or CLR calcium and rust remover
Buy:
CLR shower cleaner Amazon
Shower stalls at Amazon
Pour bath water into rain barrel Pour bath water into rain barrel

Use 5 gallon bucket to empty tub and pour into rain barrel
Cut off top of 55 gallon drum and leave lip around edge for screen
Cover top with hardware cloth and then window screen to keep out mosquitoes
Add 2 pints Hydrogen peroxide occasionally to disinfect/ deodorize

Buy:
Order 1" bulkhead fitting, and drill 2" hole, uses 1" IP
Bulkhead fitting at Amazon
55 gallon barrel Amazon
Rain barrels Amazon
Price of water What is water worth?
If you had to carry each gallon from the river up to your 2nd floor bathtub, when would you start to conserve?
Average home uses 1/2 acre-foot water per year.
135,000 gallons = 27,000 5-gallon buckets
Read about growth and electric consumption
Rain barrel
Larger image
Set rain barrel higher than 550 gallon tank
Use bulkhead fitting to tap into rain barrel
Use 1-1/4" PVC water line to connect rain barrel to tank
Tank must be on flat concrete pad.
Cover top of rain barrel with hardware cloth to protect animals from falling into tank.

Use gutter to channel water to rain barrel.
Pour bath water into 55 gallon barrel using 5-gallon buckets
Low pressure water timer

See example chart
Old sizing chart
Post 2015 heaters require high tech for sizes over 50 gallon
Take showers instead of bath. Save 10 gallons hot water each time.
To achieve same hot water consumption as previous heater withut buying more expensive heater install 2 water heaters, cut consumption, maintain heater, turn up thermostat temperature

Two water heaters
9 ways to save with water heater
Maintain water heater
How to adjust themostat
Ways to increase hot water
Calculate first hour delivery


For large bathtub, use 80 gallon first hour
What size water heater?

Size chart shows First Hour delivery rating
First hour is capacity of tank at peak-usage times.
For example at night when everybody is taking a shower or bath

Increase amount of hot water with changes and maintenance
How to adjust temperature
How to increase amount of hot water
How to maintain water heater

Homes with garden tub, whirlpool bath, or oversized tub = first hour 80 gallon

Avoid oversized bathtubs.
It's not cheap to heat all that hot water.
Buy a spa.
Drive to ocean and support tourism.
What size water heater for oversize bathtub?
Large bathtub / whirlpool tub

Recommend 80 gallon first hour delivery
Electric: minimum 60-to-80 gallon size just for that one tub
Gas: minimum 50 gallon size just for that one tub

Electric heat pump water heaters generally cannot produce enough hot water.

Tankless promise unlimited hot water, but temperature of water is 120 F and water in tub may be cool by time tub fills. Cost of maintenance and repairs never pay back. All costs are energy.
Troubleshoot Rheem tankless

safety chart
Safety resources
How to adjust temperature on water heaters

180° scald is potential fatal injury
scald chart
How to install gas water heater

How to install electric water heater

Troubleshoot gas water heater

Troubleshoot electric water heater

Support the economy:
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Hybrid / heat pump/ water heaters at Amazon

Resources:
How to install gas water heater
How to install electric water heater
Troubleshoot gas water heater
Troubleshoot electric water heater
Point of use water heaters
Resources:
Troubleshoot Gas valves with manuals
Icon gas valve

How to replace water heater element Waste King disposal
How to wire tankless electric


Install point of use water heater
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