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# Boiler behavior in E+ when boiler demand below minimum PLR

I'm unclear about what E+ does when the load on the boiler for a given timestep in below the Minimum Part Load Ratio

I'm running a simulation using the following:

Timestep,6;
ConvergenceLimits,
1;                       !- Minimum System Timestep {minutes}


I'm requesting a bunch of output variables at the detailed frequency for my boiler, which is set to have a Minimum Part Load Ratio of 0.2 and to be ConstantFlow.

I'm getting output that is at 6 timestep per hour. (I would have thought I'd get system-level output, down to the minute then...)

When plotting just 2 call for heat during low load conditions, I can see that the two have a different Heating Demand Rate, but the same energy is used... This is confusing to me, I was assuming E+ would do something similar to eQuest, that is to calculate a "Fraction of timestep boiler runs" (timestep=hour in eQuest) or something similar.

Can someone explain to me what E+ does when the load on the boiler is below the minimum boiler load?

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## 1 Answer

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When the demand is lower than the minimum part load, I believe EnergyPlus will operate the boiler at 20% capacity because it can't modulate to lower loads. The boiler does not cycle to meet the lower loads, it just outputs its minimum. The result is hotter than normal outlet temperatures.

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## Comments

Thanks Neal. Hum, I'm pretty disappointed on this one. You said 'believe', are you confident?

( 2015-10-27 04:33:55 -0500 )edit

I'm pretty confident. Here is the line in the source code that sets the outlet temperature based on the minimum part load ratio. If you want to simulate the boiler cycling below 0.2, then I'd recommend dropping the minimum PLR to zero. The boiler will technically modulate down to that level, but this is nearly identical to just using the timestep average flow rate. If you'd like to reflect increased gas use related to cycling, you can modify the PLR curve.

( 2015-10-27 09:33:58 -0500 )edit

Hum. Noted. Another approach is to use 60 timesteps per hour, but it has a big effect on other calculations and on simulation time. My ultimate goal was to try and write an EMS program to include burner purge losses, I guess this is not going to happen.

( 2015-10-27 11:49:41 -0500 )edit

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Asked: 2015-10-26 12:26:44 -0500

Seen: 185 times

Last updated: Oct 26 '15