Dear Marcel and Liam,
If the home is of older construction or constructed by someone
not familiar with low infiltration construction practice … it would not
be unusual to have 1.0 -2.0 air changes per hour infiltration. I recall
some ASHRAE data from years back which confirmed this.
On another note, infiltration does not occur at a constant
rate. It is affected by the pressure difference between outdoors and
indoors. Although I’ve used a constant average rate in the past
(similar to your approach, Marcel), it has only been for a rough evaluation.
I have not studied the impact of different approaches with E+,
but I think the air change per hour approach with coefficients that allow
variation with wind speed is a good one. It allows selection of a “base”
rate, which is then modified according to wind speed. The coefficients
below were taken from the E+ I/O Reference manual and originally came from BLAST
(I think). Another advantage of this approach is that, for many climates,
the wind speed (and therefore infiltration) is lower in the summer months, higher
in winter months … which causes the infiltration to vary something like
reality.
“Flow/Exterior Area” is theoretically perhaps
the best method, but past experience suggests that it’s almost impossible
to predict. Most of my past experience is with industrial buildings,
where average heating season infiltration rate varied between 0.1 and 0.5 air
changes per hour, so I tend to stay in that range.
Finally, the fact that the room remains warmer than setpoint
probably has little to do with the infiltration. Rather, your cooling
system air flow rate or coil capacity is probably too low. I always report
outdoor air flow rate, mixed air temperature, coil leaving temperature, fan
leaving temperature and leaving temperature setpoint for each air handler to
see what is happening at each timestep. This lets me know whether each
component is performing properly.
The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653
From: EnergyPlus_Support@xxxxxxxxxxxxxxx
[mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Liam O'Brien
Sent: Monday, July 19, 2010 11:06 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Air infiltration due to natural
leakages
Hi Marcel,
It looks like you set this up right. But note that 0.5 ach is actually quite
high depending on your building type. I suspect that the 0.5 ach is what was
measured at some higher pressure (like 50 Pa) differential during a leakage
test. In reality, under normal circumstances, you can expect much lower
infiltration. It might be closer to 1/20th of this (0.025 ach). See
"Estimation of infiltration from leakage and climate indicators" by
Sherman (1987) for details.
You can test to make sure infiltration is working by outputing the following
variable: "Zone Infiltration Air Change Rate"
Liam O'Brien
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx,
"marcel.jekel" <jekel.marcel@...> wrote:
>
> Hi,
>
> I am a beginner on EnergyPlus and am trying to model a small house with
> an infiltration rate due to natural leakages of 0.5 air changes per hour
> (ACH).
> I have set up the following in the idf file:
> ZoneInfiltration:DesignFlowRate,
> Air_Leakage, !- Name
> ZONE ONE, !- Zone Name
> Constant, !- Schedule Name
> AirChanges/Hour, !- Design Flow Rate Calculation Method
> , !- Design Flow Rate {m3/s}
> , !- Flow per Zone Floor Area {m3/s-m2}
> , !- Flow per Exterior Surface Area {m3/s-m2}
> 0.5, !- Air Changes per Hour
> 1, !- Constant Term Coefficient
> , !- Temperature Term Coefficient
> , !- Velocity Term Coefficient
> ; !- Velocity Squared Term Coefficient
> Where the "Constant" schedule was obtained from the
"schedules" data
> set.
>
> I have looked at the results of the simulation with the above in place
> (there are no warnings), but it seems that the room remains too warm. I
> am aware that 0.5ACH is not much but I am suspicious that this is not
> being recognised. Have I correctly set this up?
>
> Thank you in advance
>
> Marcel
>
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