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Boiler behavior in E+ when boiler demand below minimum PLR

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

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:

    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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answered 2015-10-26 13:17:55 -0500

updated 2015-10-26 13:20:15 -0500

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

Julien Marrec's avatar Julien Marrec  ( 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.

Neal Kruis's avatar Neal Kruis  ( 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.

Julien Marrec's avatar Julien Marrec  ( 2015-10-27 11:49:41 -0500 )edit

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

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Last updated: Oct 26 '15