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# Indirectly heated water heater with a backup electric heater - IndirectHeatAlternateSetpoint

I've been trying to figure out how to the IndirectHeatAlternateSetpoint option works for "Source Side Flow Control Mode" on the waterheater:mixed model in EnergyPlus. The scenario I would like to simulate is a water heater that is indirectly heated by a boiler that also has a backup electric heater. The tank setpoint is 60°C with a 2°C dead band. I only want the electric resistance heater to run if the tank temperature drops below some threshold (let's say 55°C).

Based on the documentation for the waterheater:mixed model, it seems like using the IndirectHeatAlternateSetpoint should allow me to do this, however I haven't been able to figure it out. Using the 5ZoneWaterSystems.idf example file to test this with the following setup:

1. Set the the "Setpoint Temperature Schedule Name" for the water heater to the lower threshold temperature schedule (55°C) with a 2°C dead band and 10000 W heater capacity

2. Set the "Source Side Flow Control Mode" to IndirectHeatAlternateSetpoint

3. Set the "Indirect Alternate Setpoint Temperature Schedule Name" to the the 60°C schedule name

My expectation was that the the electric heater would turn on only once during the example simulation, however what I get with the above configuration is that the tank temperature is controlled to 55°C and both the electric heater and the source side heater run at the same time.

Reversing the schedule names results in the water heater being controlled to 60°C and both the electric heater and the source side heater run at the same time.

What am I missing here? Any help would be greatly appreciated.

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I don't think you're wrong anywhere in this case, but here are a couple potential things to check:

First, I'd watch closely what the loop setpoint temperature is. It appears to be 56C. You probably want to do something like Indirect Heat setpoint is 56 C, and set the tank heater's setpoint temp (cut-off) to something like 55 or 54 C, so that you have a deadband in between the two.

But this is likely not going to suffice: most importantly you want to check carefully the sizing parameters and resulting component sizes. Indeed, I see that this example file is largely autosized, and I would expect you'll get problems from that. You do want to make sure that technically the source loop can provide most of the required heat and that you have an adequate amount of storage capacity (m3).

That being said, I do find that in most cases it's a lot easier to have two WaterHeater:Mixed or one HeatExchanger:FluidToFluid + one WaterHeater:Mixed. You put the tank (with no heating element) or HX served by the boiler loop first in the Plant equipment list, and the other one - the backup with electric heater - second, and you make sure the PlantLoop has a Load Distribution Scheme set to SequentialLoad so that the load falls on the source loop first, Then only if not sufficient the backup heater will turn on.

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Thanks for the response Julien. I'll have to try your recommended approach.

( 2018-03-20 09:43:14 -0600 )edit

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