Step 1:
Determine the maximum number of devices you want to run and their total
flow rate. Then, add up their flow rates (gallons per minute). This is
the desired flow rate you'll want for the demand water heater. For
example, let's say you expect to simultaneously run a hot water faucet
with a flow rate of 0.75 gallons per minute and a shower head with a
flow rate of 2.6 gallons per minute. The flow rate through the demand
water heater would need to be at least 3.26 gallons per minute. To
reduce flow rates, install low-flow water fixtures.
Step 2:
Determine required temperature rise. To determine temperature rise,
subtract the incoming water temperature from the desired output
temperature. Unless you know otherwise, assume that the incoming water
temperature is 50ºF (By using a low temperature assumption you
ensure
that you will not undersize your tankless unit. If you live in a warm
climate your water temperature will likely be much higher.) For most
uses, you'll want your water heated to around 105 - 115 degrees. In
this example, you'd need a demand water heater that produces a
temperature rise of 55 degrees.
Step 3:
Sizing Example: An average shower will be between 104 and 106 degrees
and uses 2.6 gallons of water. Assuming your water temperature is 40
degrees coming into your home, and you want to produce enough hot water
to run 2 showers at the same time, what temperature rise would you need
to produce to accomplish this?
Answer:
You'll need to raise the incoming water temperature from 40 degrees to
105. You'll need to be able to heat at least 5.2 gallons of water. So
you'll need a tankless water heater that can produce at least a 60
degree rise in temperature at 5.2 gallons per minute.
Flow Rates
How
much hot water do you need at one time? Do you need to run 2
showers
at the same time or maybe a shower and a couple sinks? The chart
below
shows the range of water usage range and average water temperatures for
various fixtures. We suggest using 2.5 gpm for a shower and 1.0
gpm
for a bathroom as a reference point in determining your total
simultaneous water needs.
| Fixture |
Average Flow GPM
|
Avg. Temp. |
| Tub |
4.0
|
102
F |
| Shower |
2.5
- 3.0 |
104
F |
| Washing Machine |
2.0
|
120
F |
| Dishwasher |
1.5
|
110
F |
| Kitchen Sink |
1.5
|
110
F |
For
example, if you are running 2 showers at the same time, you will need 5
gallons of hot water per minute from the tankless water heater.
If you
were running a shower and the washing machine at the same time, you
would need 4.5 gallons per minute from the water heater. In
either of
these situations, you will want to size the unit that meets or exceeds
how much hot water you need at the same time.
Note: tankless
water heaters are intended to heat potable (drinking) water only and
the inlet water to a tankless unit should not be pre-heated.
Other Sizing Notes
Gas
tankless water heaters are able to produce a larger temperature rise
per gpm than electric models. Most demand water heaters are rated for a
variety of inlet temperatures. Typically, a 70ºF water temperature
rise
is possible at a flow rate of 5 gallons per minute through gas-fired
demand water heaters and 2 gallons per minute through electric ones.
Faster flow rates or cooler inlet temperatures can sometimes reduce the
water temperature at the most distant faucet.
Next, you should look
at your ambient incoming water
temperature. If you live in a cold climate, like New York, your
incoming water temperature will likely be much lower than if you live
in a warm climate, like Florida. Your best bet is to find out how much
temperature rise you will need in order for your hot water to reach the
desired heat. If the ambient incoming water temperature for your shower
is 65°F, you are using a 2.0 gpm shower, and you want to raise that
temperature to 115°F, you will want to look for a Tankless Water
Heater that will provide at least a 50°F temperature rise at 2.0
gpm (115°F - 65°F = 50°F). However, if your anticipate
additional simultaneous demand, such as the hot water from a sink being
used while someone is showering, you will need to add the sink's gpm to
the shower's gpm in order to determine your overall gpm demand and then
find the temperature rise necessary to meet your overall needs.
!!!!!!!!!!!!!!!!!!! -
Warm
Climates
!!!!!!!!!!!!!!!!!! -
Cool Climates
|
|
|
220v Point of use tankless water
heater
Natural Gas or Propane Gas
Tankless Water Heaters
Powerful 220v Electric Tankless
Water Heater
Designed for mulitple installation
110v Tankless Shower Heater
Ideal for RV and Campers
|