[Bldg-sim] Modelling Infiltration for Low Energy Houses

[Shaun Martin]

Hi Hayes,

 

The ZoneInfiltration:EffectiveLeakageArea approach in E+ might give you
better results as it is tailored for residential buildings.  It accounts for
stack, wind and sheltering better than the ZoneInfiltration:DesignFlowRate
method, which is used more for larger building models.  

 

Regardless of the method, the bigger problem is the infiltration schedule.
You could start with the schedules they have in eQuest.   The operation of
the HRV will affect the infiltration schedule.  And, on larger buildings, of
course, infiltration in core zones can be set to zero.

 

On a 600m3 house, ACH50 = 1.0 is roughly 166 L/s.  This is about 55 L/s at
10Pa,  and translates to an effective leakage area of about 0.45 cm/m2.
These are what I would use as design numbers.   From experience, ACH50 = 1.0
(even 1.5) is a challenging target to reach and can't be reached unless you
use a blower door during construction.  

 

Hope this helps,

Shaun

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Hayes Zirnhelt
Sent: Monday, May 27, 2013 9:40 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modelling Infiltration for Low Energy Houses

 

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 Coefficient

B =     0.03636,                 !- Temperature Term Coefficient

C =    0.1177,                  !- Velocity Term Coefficient

D =    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.Sc

EcoOptimal Building Energy Analysis

hayes at eco-optimal.ca

www.eco-optimal.ca <http://www.eco-optimal.ca/> 

T: 778.961.0086

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