Measurement Guide
6
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Greentherm T9800/9900 SE/SEO/SEi Application Manual
12.2019 | Bosch Thermotechnology Corp.Data subject to change
3 Water Heater Sizing
This section describes guidelines on how to select the right tankless water heater
size for a particular application. More detailed information is contained in the
installation manuals. Download manuals at www.boschheatingandcooling.com.
3.1 Sizing Tankless Water Heaters
Rule of thumb sizing
Table 1 provides a general rule of thumb when sizing for most residential
applications. For multi-unit applications or for a more detailed sizing method, use
the Maximum Power Demand Calculation below in conjunction with the charts on
the next page.
Maximum Power Demand Calculation
Step 1: Measure the maximum flow rate.
Measure the fl ow rates at each fi xture that will be used simultaneously and
add them together. If only one application will be used at a time measure
each fi xture and use the maximum fl ow rate observed.
Possible measuring technique: using a known volume container, record
the fi ll time. Perform the calculation below to determine the flow rate. For
example, a 3.5 gallon fill time of 30 seconds is equivalent to 7 gallons per
minute (GPM).
Flow rate (GPM) =
Volume (gallons)
Fill time (sec)
sec
min
60
x
Step 2: Calculate the desired temperature raise.
Identify the ground water temperature for the installation location, fi gure 1.
Subtract this temperature from the desired hot water temperature to get the
degree rise.
For example, if the desired hot water temperature is 110 °F and the ground
water temperature temperature is 55 °F, the desired degree rise is 55 °F.
Step 3: Calculate maximum power required
Power (kBTU/hr) = Flow rate (GPM)
x
Temp Raise (°F)
x
0.501
Step 4: Determine the models and number of appliances required to
fulfill required power, by selecting a combination of appliance that meets
or exceeds maximum required power.
Example 1: Maximum required power calculated in step 3 is 150kBTU/hr.
Select one T9800/9900 160 model, since this model is able to supply up to
160kBTU/hr, which is more than the maximum required for the application.
Example 2: Maximum required power calculated in step 3 is 370kBTU/hr.
Select two T9800/9900 199 models, since this is model is able to supply
up to 199kBTU/hr, so two units will be able to supply 398kBTU/hr, which is
more than the maximum required for the application.
Figure 1
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Average Ground Water Temperatures