redsky49
Well-known member
For those interested, I have written a short description:
Infiltration?? That’s easy. Just use the basic residential formula:
Q = AL√(CSΔt+CWU2)
Well, maybe I can simplify it for you. First let’s define some terms.
Outside air that is intentionally brought into an interior space is referred to as ventilation.
Outside air that enters your interior space unintentionally is referred to as infiltration.
Ventilation occurs either by natural (open window, for example) or mechanical means (fans). In addition, there is the random air flow through building cracks and other openings. Occupied spaces are intentionally ventilated due to health and safety requirements.
But we are most concerned with the unwanted entry of outside air – infiltration.
The formula above describes the relationship of two primary factors – size of openings to the outside, and differential of pressure (and/or temperature or humidity) between indoors and outdoors.
Calculating accurately both the amount and effect of infiltration can be difficult and various methods have been used over time. This has been a challenge in the industry, but there is hope for the layman, and that brings us to the “crack” method.
Of all the possible ways to estimate infiltration, this is the easiest though it is not without perils. If your overhead door faces the prevailing winds of the Kansas plains, this will likely not work for you. Same as for those who live at the top of Mt. Washington, or north of the Arctic Circle. For the rest of us, by exercising some common sense, this will be a reasonable approach.
In the “Crack” method you have two approaches. If doing a whole house, you will need to determine the total amount of cracks subject to infiltration (doors, windows, top plates, etc.) on the most leak prone side and end. Note that not all four sides are used – air enters on two sides and exits on the others.
If calculating a single room, i.e. a garage, use the room total crack.
So, once you have added all the windows, doors, etc., and have a total length of envelope crack, another formula:
Qs = 0.018 x B x L x (ti - to)
where
Qs = heat gain (or loss) - sensible
0.018 = stack effect of a single story house
B = leakage per ft. of crack (from chart) for component and wind velocity
L = length of door or window crack
Ti = temperature inside
To = temperature outside
The component leakage chart was compiled from ASHRAE, and I do not have permission at this time to reproduce it. It can be viewed several places on line such as page 42 of this document:
http://www.wbdg.org/ccb/DOE/TECH/doe_v2.pdf
In the real world, all it takes is one careless tradesman to screw up all the best efforts of designer and manufacturer. If close enough is all you require, then the rough estimates provided in an earlier post will be adequate. Note however, that I described new, tight fitting components, not twenty year old articles. Also, under extreme conditions, the estimated air flow will exceed the calculated amount, sometimes by several factors.
Infiltration can be a sizeable portion of your cooling and/or heating load, and as such should be carefully considered in your calculations. It is only one component though of the entire load and should be part of a carefully considered approach to accurately determining your actual heating or cooling requirements.
Note that subscripts are not shown correctly on the formulas. Importing a text document lost all the subscripts. My apologies.
As always, offered only as opinion
Infiltration?? That’s easy. Just use the basic residential formula:
Q = AL√(CSΔt+CWU2)
Well, maybe I can simplify it for you. First let’s define some terms.
Outside air that is intentionally brought into an interior space is referred to as ventilation.
Outside air that enters your interior space unintentionally is referred to as infiltration.
Ventilation occurs either by natural (open window, for example) or mechanical means (fans). In addition, there is the random air flow through building cracks and other openings. Occupied spaces are intentionally ventilated due to health and safety requirements.
But we are most concerned with the unwanted entry of outside air – infiltration.
The formula above describes the relationship of two primary factors – size of openings to the outside, and differential of pressure (and/or temperature or humidity) between indoors and outdoors.
Calculating accurately both the amount and effect of infiltration can be difficult and various methods have been used over time. This has been a challenge in the industry, but there is hope for the layman, and that brings us to the “crack” method.
Of all the possible ways to estimate infiltration, this is the easiest though it is not without perils. If your overhead door faces the prevailing winds of the Kansas plains, this will likely not work for you. Same as for those who live at the top of Mt. Washington, or north of the Arctic Circle. For the rest of us, by exercising some common sense, this will be a reasonable approach.
In the “Crack” method you have two approaches. If doing a whole house, you will need to determine the total amount of cracks subject to infiltration (doors, windows, top plates, etc.) on the most leak prone side and end. Note that not all four sides are used – air enters on two sides and exits on the others.
If calculating a single room, i.e. a garage, use the room total crack.
So, once you have added all the windows, doors, etc., and have a total length of envelope crack, another formula:
Qs = 0.018 x B x L x (ti - to)
where
Qs = heat gain (or loss) - sensible
0.018 = stack effect of a single story house
B = leakage per ft. of crack (from chart) for component and wind velocity
L = length of door or window crack
Ti = temperature inside
To = temperature outside
The component leakage chart was compiled from ASHRAE, and I do not have permission at this time to reproduce it. It can be viewed several places on line such as page 42 of this document:
http://www.wbdg.org/ccb/DOE/TECH/doe_v2.pdf
In the real world, all it takes is one careless tradesman to screw up all the best efforts of designer and manufacturer. If close enough is all you require, then the rough estimates provided in an earlier post will be adequate. Note however, that I described new, tight fitting components, not twenty year old articles. Also, under extreme conditions, the estimated air flow will exceed the calculated amount, sometimes by several factors.
Infiltration can be a sizeable portion of your cooling and/or heating load, and as such should be carefully considered in your calculations. It is only one component though of the entire load and should be part of a carefully considered approach to accurately determining your actual heating or cooling requirements.
Note that subscripts are not shown correctly on the formulas. Importing a text document lost all the subscripts. My apologies.
As always, offered only as opinion