This article describes how we are changing the EnergyPlus infiltration metrics under the hood that will affect Sefaira simulations.

If you're interested in just knowing how infiltration works in Sefaira, read this article.

Summary

The EnergyPlus infiltration parameters change on February 18. 2019. There are 2 changes.

1) We're changing the choice of infiltration algorithm from BLAST to DoE-2

The changes bring Sefaira in line with more up to date research on infiltration. They reduce the impact of temperature differences and heighten the impact of wind on infiltration flow rates.

Because these 2 parameters are so dynamic, it's hard to exactly communicate how the changes will affect your project. Most tests show changes will be fairly limited but don't be surprised if results change slightly on a rerun. Changes to all options should be comparable.

2) We're clarifying the pressure drop for facade infiltration, with 2 options - 50Pa and 75Pa.

We get loads of support tickets about infiltration and in particular people want to understand whether the value is at 50Pa or 75Pa. This is because different jurisdictions test infiltration with different blower-door tests (some at 50Pa, some at 75Pa).

So to make it easy, we've provide both options in the dropdown to clarify what the external pressure is for each.

Detailed Explanation - Changing from BLAST to DOE

We don't take changing inputs like this lightly, but we get a lot of feedback from users that they are confused about the extent of infiltration loads in their model, or dubious of the results.

In response, we decided to research whether or original inputs were still reasonable. We found that current research had found that the variables we were using in E+ for infiltration were outdated. Because lots of customers have contacted us about this issue, we decided it was better to move to a more current set of inputs.

Infiltration in EnergyPlus is worked out in each time step based on a range of coefficients that are affected by both wind speed normal to the facade and also (in some cases) the temperature difference between inside and outside as well as a baseline infiltration constant. 

Note none of the infiltration calculations in E+ are modified by building pressurisation.

Sefaira's pre-Feb 11 algorithm was the BLAST coefficient set which allowed for fairly generous infiltration flows to be driven by temperature differences. 

We have switched from the BLAST to the DOE-2 methodology. The DOE 2 methodology:

• does not have a constant value (so there is no longer a baseline infiltration)
• does not have a temperature coefficient (so temperatures differences don't drive infiltration)
• has a higher wind speed coefficient (so wind has a higher effect).

To read more about the differences between the variables and how E+ calculates infiltration, see these documents:

• Infiltration Modeling Guidelines for Commercial Building Energy Analysis, K. Gowri et al, 2009, US Department of Energy. 
• E+ reference documentation on Infiltration, republished online with permission by BigLadder software.