What Size Tankless Heater?
Sizing a tankless water heater
The size tankless hot water you need depends on two things: ground water temperature and how many gallons per minute you will be using. To be accurate the temperature reading should be winter.
Step 1
Calculating the temperature rise
This is the difference between winter groundwater temperature and what is required at the faucet or showerhead.
For example:
115 F degrees (shower) - 50 F degrees (entering the house) = 65 F degrees temperature rise.
Step 2
Flow rate
This should be based on the number of outlets that could be running at the same time. This is calculated in gallons per minute. A watch, and a measuring can will give this number.
Here are some common uses:
An average shower uses about 30 gallons of water
A clothes washer uses about 30 gallons of hot water
Average person uses 2 gallons shaving
Average person uses 3 gallons teeth brushing
Average person uses 4 gallons washing hands and face
Average person uses 5 gallons preparing a meal
A leaky faucet looses about 20 gallons per month
A clothes washer uses 7-10 gal with warm wash, cold rinse
A clothes washer uses 30-35 gal for hot wash, warm rinse
It takes about 8-10 gallons for a dishwasher
It takes about 12-15 gallons to hand-wash dishes
Keep in mind that the more gallons per minute, the more expensive the installation will be. This is a situation where talking it over with the family about changing water use patterns can save money.
Step 3
Putting temperature rise and flow rate together
Even knowing how electric and gas compare is not be enough to make a decision, because there are other factors such as the amount of hot water needed and your local climate. This is the temperature rise/flow rate factor.
<insert flow rise +map illust>
The chart on the left shows how as RISE increases, FLOW (gpm) drops off . At maximum power, an electric tankless heater will probably produce no more than 5 gpm FLOW. With a 60 degree RISE, no more than 2 gpm FLOW rate.
This US map may help with the ground water question.
Where you live is just as important as what your hot water needs are. Together they will determine the size heating system you need.
Then you can factor cost and utilities to make the right choice.
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