3 Steps to Sizing A Tankless
Hot Water Heater
Finding the size tankless water heater that’s right for your needs is a simple 3step process that will give you what you need to estimate waterheating capacity.
Working With Water
If you’re just starting your research, you’ll see these terms frequently used:
· “flow rate” – this is the amount of hot water coming out of your heater unit
· “gallons per minute” (GPM) – this is the measurement of your flow rate. So, when you see instructions using the term GPM, they’re talking about the amount of hot water the heater is producing per minute.
· “PSI” – just to confuse the issue, spec sheets will list PSI (water pressure measured in pounds per square inch) which translates into how much pressure you have to force the water through the pipes. The greater the PSI, the higher the GPM. But there is more to it, read on.
You can determine the GPM (if you have a strong stream, your PSI is fine) and your Temperature Rise with measuring pail, a watch and a thermometer.
Step 1. Find the Temperature Rise
The temperature of the water as it enters your house is called “ground temperature. “This can be anywhere from 35 F degrees to 90 F degrees. The difference between summer and winter can be 30 degrees or more. This difference between ground water temperature and showerhead is the Temperature Rise.
Measure:
1. the temperature of the water as it enters your house (that’s “ground water temperature”) during your coldest winter months
2. how much water you typically use simultaneously throughout the house (this is your usage amount or flow rate) so you’ll know the maximum demand you’ll be placing on the system at any one time.
Step 2. Calculate the GPM
What you’ve got is something that can be plotted on a chart. It will show that as Temperature Rise goes up, gallons per minute goes down. Let’s use as an example a tankless heater with max flow rate of 5 GPM:
1. Summer – temp rise from 70 F to 115 F = 4 GPM
2. Winter – temp rise from 50 F to 115 F = 2.8 GPM
(mouse over above images)
A typical shower usually uses about 1.5 to 2.5 GPM, the tub, washing machine or dishwasher about 2.0 to 2.5 GPM. Always factor in a faucet, which is about .75 to 1.25 GPM, because it never fails that someone always turns one on while someone else is in the shower. Using these numbers, you should get an idea of what your flow rate is.
When you determine your usage, it’s very important to be objective. Try to schedule hot water use as a series rather than simultaneous – single use works better for tankless, but more GPM or higher Temperature Rise is what more powerful models are there for.
If you can find ways to minimize multiple usage and take advantage of water saving measures you can save money. Big tankless hot water heaters cost more and will surely be more expensive to install.
You now have numbers for
1. GPM )Flow rate)
2. Temperature Rise
The third step is to determine how powerful a water heateris needed to meet this obligation.
Step 3. Calculating Kilowatt Hours
A tankless electric model may give a clue by its model number, which could be something like SuperDuper BX240. This model number usually refers to kilowatts, as in 24 kilowatts of electricity it will use when operating at full force. This is the maximum amount of energy it can draw.
At about 40 F degrees temperature rise it will be producing its maximum flow rate. The important thing to know is the flow rate for your particular situation.
As an example
If the ground water temperature is 75 F degrees (summer) and the temperature leaving the heater is 115 F degrees then the temperature rise is 40 degrees and a 24kw heater will be producing its max.  close to 4 gallons per minute.
If the temperature rise is 50 then gallons per minute for this same heater would fdrop off to about 3.3 gallons per minute.
You can get a pretty good idea of what size hot water heater you will need by using this formula:
Flow rate (gpm) x Temperature rise (F)
—————————— = Heater size (in KW)
6.83
For dealing with this gallons per minute issue there are two options:
1. Look for a bigger heater (burn more fuel) or
2. String your water use back to back( lower gpm.)
Actually there is a third option  look at economical energy saver page.
