This page covers how estimate the DIY blower door flow rates, and then how to use these flow rates along with the house depressurization measurements to estimate flow rates and air changes per hour at a 50 Pa house depressurization (the blower door standard). Estimating natural infiltration from the 50Pa flows and the resulting heat loss are also covered. The section ends with a an example that covers both infiltration and other heat losses along with fuel costs and carbon emissions.

These topics are covered on this page:
Before getting into all this, you might ask: Why do I care?
While the page is kind of long and may appear a bit daunting, its all very simple stuff, and I've provided small spreadsheet to plug into for some of the calculations.
In most of the methods listed below I use Pa (pascal) as the unit for pressure measurements. If you pressure meter reads in inches of water, just multiple by the inches of water reading by 250. For example if your meter reads 0.2 inches of water, then the pressure is Pa is 0.2*250 = 50 Pa.
For the half hp furnace blower that I have, I measured flow rates at each speed using detailed velocity survey that is a fair bit of work and requires an accurate velocity measuring device. I found that the flow rates I came up with were close to the published flow rates for a half hp Fasco blower on Grainger. This table shows the two:
Flow rates in cfm for my blower compared to A Fasco blower, and the average of the two. 
For your blower, I'd suggest one of three approaches to estimating the flow rate at each blower speed:
You don't have to worry about flow rate changing as the indoor to outdoor pressure changes, because the the furnace blower flow rates (at least the ones I've seen) change very little over the range of pressures used in blower door tests.
Part of a standard blower door test is to estimate the flow rate out of the house when the blower door has depressurized the house to 50 Pa (0.2 inches of water). Knowing the flow at 50 Pa depressurization will allow you to compare your home to other homes that have been measured at 50 Pa, and is also the basis for estimating your actual infiltration rate and heat loss due to infiltration.
Case 1: Blower door can reach 50 Pa depressurization:
If your furnace blower can depressurize the house to 50 Pa or more, then I would
just interpolate the flow at 50 Pa from the flow at the blower speeds that come
in above and below 50 Pa. For example, if high speed (speed 4) results in
62 Pa depressurization on the manometer with 2470 cfm flow, and the next lower
speed on the blower results in 44 Pa on the manometer, and 1873 cfm flow, then
the flow at 50 Pa by interpolation is:
cfm@50Pa = (50 Pa  44 Pa) / (62 Pa  44Pa) *(2470 cfm  1873 cfm) + 1873 cfm = 2072 cfm
Case 2: Blower door cannot reach 50 Pa depressurization on high
speed:
If the highest speed of the blower does not get you up to 50 Pa of
depressurization, then you can extrapolate the result for the highest blower
speed.
Use this equation to estimate the flow at 50 Pa based on the pressure you measure at the highest fan speed:
Flow Factor = ( (50 Pa) / (Pressure at high speed) ) ^ 0.65
That is take the ratio of the 50 Pa to the pressure in Pa at the highest fan speed, and raise it to the 0.65 power.
Multiply the Flow Factor by the flow at the highest blower speed.
Why the pressure raised to the 0.65 power? You might tend to think that the flow out of the house increases in direct proportion to the pressure change (linearly), but in fact, the flow rate changes as the pressure difference raised to a power. That is, Flow = C* dP^n where C is a constant that depends on hole area, and the n exponent depends on the details of the flow through the holes  for laminar flow, n is 1, and for fully turbulent flow, n is 0.5. It has been found that an average for real homes of n = 0.65 gives good results. 
For example, if your blower on high speed is only able to depressurize the house to 42 Pa, and your blower on high speed has a flow of 2470 cfm, then the estimated flow at 50 Pa is:
Flow Factor  (50/42)^0.65 = 1.12 That is the flow at 50 Pa will be about 1.12 time the flow at 42 Pa
Flow at 50 Pa = 1.12 * 2470 = 2770 cfm
This is often referred to as CFM50.
This small spreadsheet allows you to just plug your pressures in to get the estimates....
Note that for case 2, the closer your furnace blower can get to a depressurization of 50 Pa, the less error there will be in extrapolating to 50 Pa.
Often air flow is expressed in Air Changers per Hour. This is the how many times the blower door flow at 50 Pa could fill the total volume of the house in one hour. It is equal to the blower door flow for one hour divided by the house volume in cubic feet.
ACH50 = (CFM50)(60) / (House Volume)
For example if a house is 40 ft by 50 ft by 8 ft high,
then its volume is 40*50*8 = 16,000 cubic ft.
If its blower door flow at 50 Pa is 2200 cfm, then the Air Changes per Hour
(ACH50) are:
ACH50 = (2200 cfm)(60 min/hr) / (16000 cf) = 8.25 ACH at 50 Pa depressurization
The main value of the ACH50 number is that it allows you to compare the ACH50 of your home to other homes out there, and get some idea how it stacks up and what your potential for improvement might be.
ACH50 values for real home vary over a wide range:
The numbers above are from a combining a few sources on a Google search of ACH for homes  you can probably find more.
Remember that these Air Change per Hour numbers are measured at 50 Pa depressurization of the house and are much larger than natural infiltration ACHes (see below).
The air flow at 50 Pa that you estimated above is much higher than actual natural infiltration rates. The 50 Pa rate is about the inflation you might get if the wind was blowing at your house at 25 mph from all sides.
The Factor of 20 Method:
Estimating the natural infiltration rate from the 50 Pa rate is inexact and subject to quite a bit of variation, but its probably the best estimate you will be able to get without running a tracer gas test on you house.
The quick method to get natural infiltration is to divide the the
50 Pa infiltration rate by 20.
For example if your estimated 50 Pa infiltration rate is 2770 cfm, then your estimate natural infiltration rate would be 2770/20 = 139 cfm.
The Air Changes per Hour for this example would be (139 cfm) (60 min/hr) / (16000cf) = 0.52 ACHnatural.
This is nice and simple, but does not take into account things like your climates effect on infiltration, the height of the building and the degree to which the building is sheltered from the wind.
The LBL Method:
A more accurate way of estimating natural infiltration from the 50 Pa infiltration rate that was developed at LBL (Lawrence Berkley Laboratory). This method takes into account: some of the features such as height of the house, wind and wind shelter, etc.
Natural infiltration is calculated as:
ACHnat = ACH50 / N
where:
ACHnat is the natural infiltration rate
ACH50 is the blower door flow rate at 50 Pa depressurization as measured in the
blower door test
N is a factor that depends on the climate and characteristics of the house 
see below.
The factor N is:
N = C * H * S * L
where:
C is the climate factor and depends on both the wind and
temperatures for the climate (see map below)
H is a factor to adjust for the height of the building
S is a factor to adjust for how well the building is sheltered
L is a factor that adjusts for average crack size
Climate Factor:
Read the Climate Factor (C) from the map below.
Climate Factor
Height Factor:
The height factor adjusts for the height of the building. Tall buildings have a stronger stack effect, and this increases infiltration compared to short buildings.
Number of Stories  1  1.5  2  3 
Height Correction Factor (H)  1.0  0.9  0.8  0.7 
Sheltering Factor:
Houses that are well shielded by trees or other buildings have lower infiltration rates. The Shelter Factor (S) adjusts the infiltration rate for sheltering.
Degree of Shielding from Wind  Well Shielded  Normal Shielding  Exposed 
Shielding Correction Factor (S)  1.2  1.0  0.9 
Crack Size Correction Factor:
Infiltration depends on the size of the holes or cracks in the house. This seems like a difficult thing to estimate, but one way is to assume that homes built more recently have smaller cracks. That is, tighter construction goes with the Small Cracks end of the table.
Size of Cracks and Holes  Small Cracks  Normal Cracks  Large Cracks/Holes 
Crack Size Correction Factor (L)  1.4  1.0  0.7 
As an example, suppose we have a 40 ft by 50 ft ranch home with 8 ft ceilings in Bozeman, MT. The homes was built in the 90's. A blower door test determined that the infiltration at 50 Pa is 2900 cfm. The home is located in a very open area with no nearby homes or trees for wind protection.
Volume of home = (40ft)(50ft)(8ft) = 16000 cf
ACH50 = (2900 cfm)(60 min/hr) / (16000 cf) = 10.9 air changes per hour at 50 Pa
Picking the factors:
Climate Factor from the map above for Bozeman is 17 to 20  use C = 18.5
Since this is a 1 story home, the height factor from the table above is 1.0  H = 1.0
The home has no wind shelter, so the wind shelter factor from the table is 0.9  S= 0.9
Since the home is fairly recently built, use Crack Size Factor of 1.0  L = 1.0
So, natural infiltration is:
ACHnat = ACH50 / (C * H * S * L)
= (11.9) / (18.5 * 1.0 * 0.9 * 1.0)
= (11.9) / (16.7) = 0.71 air changes per hour natural.
While going through the LBL method will give you a better estimate of natural infiltration, based on field tests, its still subject to a lot of variation.
Air infiltration results in heat loss because the cold air that infiltrates from outside must be heated up to inside temperatures.
An easy way to estimate the heat loss and added fuel costs due to infiltration is to use this Home Heat Loss Calculator... This will not only give you the heat loss due to infiltration, but will also give you conduction heat losses through the walls, floors, ceilings, and windows so you can get an idea of how the infiltration loss compares to other losses.
To use the calculator, you will need the volume of your house, which is the floor area of
the house multiplied by the average ceiling height. So, for a 2000 sf house
with 8 ft high ceilings the volume would be (2000 sf)(8ft) = 16000 cubic feet.
You will also need the natural infiltration rate for your house (from
section above, and the Heating Degree Days for your area  the calculator has a
help file on how to look this up.
See the Full Example just below for an example of calculating infiltration and other heat loss and fuel costs.
This is a full example of calculating heat loss and fuel cost for an example home, including infiltration and other heat losses.
The house:
 Rectangular, single floor with crawlspace under, and measuring 40 ft by 50 ft with 8ft ceilings.
 Total window area is 100 sf (5% of floor area) and the windows are double glazed with an R value of 2 (U=0.5)
 Two doors at 20 sf each with an R value of 4
 Insulation levels are: Walls R20, Ceiling R40, Crawl R20
 Infiltration is taken from the example above: 12 ACH50, and 0.7 ACHnat
 The house is located in Bozeman, MT and has 8100 Heating Degree Days (HDD)
 The heating fuel is propane at $2 per gallon which is burned in an 80% efficient furnace.
The Home Heat Loss Calculator provides help files for estimating R values, HDD, etc.
Going down the inputs to the calculator:
 Design outdoor temp is looked up in the help file  and is 20F in nearby Butte.
 Heating Degree Days for Bozeman are looked up using the help file  8100HDD for Bozeman.
 Fill in the heating fuel, cost per fuel unit, and furnace efficiency  Propane at $2 per gallon, and an 80% efficient furnace in this case.
 Ceilings: Area = 40*50 = 2000 sf, Rvalue = 40 (for example 12 inches of cellulose) the help file provides help on figuring out your R values.
 Walls: Gross Wall Area = (40+50+40+50)*8 = 1440 sf, less window and door area of 140 sf gives net wall area of 1300 sf, all at R20.
 Windows & Doors: window area = 100 sf for area and R2 for the R value. Doors are 2@ 20sf each for 40sf and are R4 doors.
 Floors: Area is same as ceiling at 2000sf. The conditioned crawl space is insulated to R20, but since the crawl space is buffered from the outside conditions, the R value is often doubled to account for this  so, R40.
 Slabs: no slab  enter 0 for perimeter.
 Infiltration: House volume is (2000sf)(8ft) = 16000 cf. Based on the example above, use 0.7 for natural Air Changes Per hour from the example above.
 Internal Heat Gains: Enter 2 for two occupants ( I suppose we could include the dog and use 2.5)
Now click the Calculate button, and you should get:
So, in this case, Infiltration is the largest single het loss, accounting for about 47% of the total heat loss, and for $1065 in propane costs per year, with nearly 7000 lbs of CO2 emissions.
This may or may not be true on your house, but you can go through the same procedure and find out!
Note that this simple heat loss calculator makes many assumptions, and I'd feel like it was having a good day if it comes within 20%  but, its better than a guess and the answers are likely accurate enough guide decisions on which insulation or sealing projects are worth pursuing on your home.
Back to the main page on the DIY Blower Door...
Gary February 26, 2013