[Bldg-sim] Modelling Infiltration for Low Energy Houses

[Hayes Zirnhelt]

Hi,
I've created a detailed multi-zone model of a low energy home in Canada in EnergyPlus. I am looking at the peak heating load results and finding that the infiltration predicted seems quite high. I'd be interested to hear how other building modellers are approaching infiltration, and any general thoughts anyone has on this situation.
The model is based on the design - the house has not been constructed yet, but it is assumed that an ACH at 50pa rating of 1 can be achieved. EnergyPlus has a simple infiltration modelling option, where the air leakage can be input as ACH@ regular pressure (I've assumed this is equal to 1/20 = 0.05).
EnergyPlus's basic infiltration model can adjust the infiltration rate for delta T and windspeed, using the following formula:
Infiltration = I_design * {A + B (T_zone - T_outdoor) + C*windspeed + D*windspeed^2 } The big question I have is what coefficients should be used? EPlus provides three suggestions: 
1. coefficients from BLAST:
A =     0.606,                   !- Constant Term CoefficientB =     0.03636,                 !- Temperature Term CoefficientC =    0.1177,                  !- Velocity Term CoefficientD =    0;                       !- Velocity Squared Term Coefficient
2. coeffs from DOE: 0, 0, 0.224 (windpeed), 0
3.Default: 1,0,0,0 which means a constant ACH with no modification.
The BLAST coefficients seemed to be the best to me because I think wind and temperature difference should be accounted for and there should always be some basic infiltration. However, using these coefficients at the peak heating load condition (delta T ~ 40C, windpeed = 27 km/hr) the infiltration rate is tripled from the base 0.05 ACH, and the total load from infiltration equals 2.2 kW, which is the largest component of the heating load.
For a relatively tight house this seems like an unrealistic value to me, but I do not have much to compare it against.

Thanks,
Hayes
Hayes Zirnhelt, M.A.ScEcoOptimal Building Energy Analysishayes at eco-optimal.cawww.eco-optimal.caT: 778.961.0086 		 	   		  
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