National Oilheat Research Alliance 39
The heating capacity of an indirect heater
depends on the size of the tank, the size of the
heat exchanger, boiler water temperature supplied
to the heat exchanger, and the flow rate.
The efficiency of indirect water heating is
directly related to the efficiency of the boiler.
Sizing Water Heaters
Unlike space heating, the need for hot water
will vary widely between houses and families,
and oversizing may not have a significant efficiency
penalty. Most manufacturers provide
recommendations regarding water heater sizing
and installation practices. For example, see:
http://accusize.bockwaterheaters.com/IntelliSizing/
NewProject.aspx
This web page is shown as Figure 8. If manufacturer
supplied information is not available,
NORA reccomends using the AHRI first-hour
sizing method.
In addition to the size of the heater in gallons,
there are a number of considerations
which must be taken into account when determining
which water heaters best fit the needs
of each individual application. For example, on
a spec sheet, there are a number of capacities
listed that must be considered before a model is
selected.
First hour rating is the amount of hot water
that a fully heated water heater can deliver
in the first hour. This is normally shown as
gallons per hour at a 100°F temprerature rise,
unless stated otherwise.
Figure 6
Figure 7
Figure 8