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 = BoilerLoad / BoilerNomCap;
OperPLR = min( OperPLR, BoilerMaxPLR );
OperPLR = max( OperPLR, BoilerMinPLR );

@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.