Modeling indirect or sidearm water heaters with a boiler

[Ryan Moore]

How do I model and indirect or sidearm water heater with a boiler?

[Ryan Moore]

There is a simple method to use for this answer if the tank fits into the assumptions listed. Then, there is a more complicated answer if the tank is significantly different. See the 2 different methods below:

Here is a simple workaround for sidearm heaters, based on support documentation for EERE standards:

Assumptions:

40 gal metal tank
1 in. (2.56 cm) of HFC-245fa insulation
intermittent pilot ignition device

Procedure:

Set Recovery Eff (RE) to system efficiency of boiler
Set EF = .65*RE + 12.6

Here is the more complicated workaround:

Select a standard tank water heater.
Set the tank size and btu/hr capacity
Set the Recovery Efficiency to the AFUE of the heating system boiler
Use the attached worksheet to estimate the EF
Enter tank size
Enter boiler AFUE for Thermal Efficiency
Estimate thermal losses for the tank as Btu/hr or %/hr (percent of difference between ambient and tank temperatures)
The %/hr could be measured directly as follows:
Measure hot water temp=Ti
Turn off heat source for 1 hour
Measure hot water temp=Tf
%/hr = 100*(Ti-Tf)/(Ti-Tambient)

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