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# Can I simulate the cycling process of a boiler?

For now I want to simulate boilers with different turn down ratio, but from the source code, seems like when the demand is lower than the minimum part load, EnergyPlus will operate the boiler at 20% capacity (let's say turndown ratio is 5:1) because it can't modulate to lower loads.

$$FuelUse = \frac{TheoreticalFuelUse}{NormalisedBoilerEff}$$

$$TheoreticalFuelUse = \frac{BoilerLoad}{BoilerEff}$$

is that mean the way EnergyPlus simulating boiler consumption can not achieve the cycling process when load below the turndown ratio? If it can, any advice?

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The boiler will operate at the minimum part-load ratio until the water outlet upper temperature limit is reached. At that point the boiler will cycle off.

Boiler:HotWater,
0.2,             !- Minimum Part Load Ratio
1.0,             !- Maximum Part Load Ratio
100.,            !- Water Outlet Upper Temperature Limit {C}


The code shows:

// Limit BoilerOutletTemp.  If > max temp, trip boiler off
if ( BoilerOutletTemp > TempUpLimitBout ) {
BoilerOutletTemp = Node( BoilerInletNode ).Temp;
}

OperPLR = min( OperPLR, BoilerMaxPLR );
OperPLR = max( OperPLR, BoilerMinPLR );

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@rraustad gave you the gist of what E+ is doing when cycling the boiler. But as far as I know, E+ doesn't specifically take any purge losses into account when cycling the boiler.

It is common for forced-draft (non atmospheric) boilers to go through a post-purge cycling at the end of a firing event, and a pre-purge cycle before ignition occurs. These are done for safety reasons, air is blown in order to remove any left-over gases. The length of the pre and post purge cycles varies from one burner to another and from experience ranges from 30 secs up to 2 minutes. This aspect is paramount in what is described as "short cycling". Your fuel use might go up by several percentage points if you end up cycling too often.

Now, some of that will be captured if you set a bad efficiency at the min PLR.

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