Tankless Water Heater Specifics

Deciding what kind of tankless heater to go with depends on
a couple of things:

* The flow rate, or amount of water you'll need heated at
one time
* Temperature rise, or the difference between your
groundwater temperature and the desired output temperature

The Federal Energy Policy Act of 1992 set flow limits at 2.2
gallons per minute (GPM) at 60 pounds per square inch (PSI)
for household water fixtures. Some people also use aerators
to further limit the flow of water. Tankless manufacturers
size their units based on the temperature rise needed for
a given flow rate.

To calculate your flow rate, add up the GPM for the household
water fixtures you'll need at one time:

* Bathroom faucet - low-flow faucets use 0.5-1.5 GPM.
Standard post-1992 fixtures are set at 2.2 GPM. Faucets
before 1992 fall between 3.0 and 5.0 GPM.

* Kitchen faucet - pre-1992 fixtures use between 3.0-7.0
GPM. The post-1992 standard remains 2.2 GPM, and kitchen
faucets don't use aerators, so there are no low-flow numbers.

* Shower - low flow rate is between 1.0-2.0 GPM. The 1992
standard remains 2.2 GPM. Pre-1992 heads fall between
4.0-8.0 GPM.

Now figure out your temperature rise by calculating the
difference between the temperature of your groundwater and
what you'd like the end result to be. For instance, if you
have a groundwater temperature of 70 degrees and you like
your showers to be a pleasant 110 degrees, that's a rise of
40 degrees. Your ground water temperature is roughly the same
as your average yearly air temperature.

Once you have your temperature rise and know your flow rates,
then you ­know what size and what kind of heater will work
best for your needs. It's important to remember in this
calculation that you'll be measuring the amount of hot water
you'll need at one time. Tankless systems never run out of
hot water, but if you want to turn on every fixture in your
house at the same time, the hot water will be split among
them. So estimate the number of fixtures you think you'd need
at one time -- chances are it won't be every fixture.

Let's say you live in an older home that has been partially
remodeled. You estimate that you'll need to heat water for
your kitchen faucet, one bathroom faucet and two shower heads
at one time. One of the shower heads is newer and meets the
1992 standard, while the other is older and has a flow rate
of roughly 5.0. The rest of your fixtures also meet the 2.2
standard. Add 2.2 + 2.2 + 2.2 and 5.0 for a total flow rate
of 11.6. You live in Miami, so your groundwater temperature
is roughly 72 degrees and you like your showers at 100
degrees. This means you should look for a tankless system
that can heat 11.6 GPM at a rise of 28 degrees.

Gas- and propane-powered heaters typically provide more juice
than electric models and are generally used for whole-house
systems. Electric models are more common in point-of-use
scenarios, although sometimes people prefer to use two
electric heaters in parallel instead of one larger
gas-powered unit. If you want a shower in your pool house or
hot water for an outdoor kitchen, you might be a good
candidate for a small electric tankless heater.