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Kman1138 (Mechanical) (OP)
7 Feb 10 14:09

     I have a questions on estimating air changes per hour when estimating infiltration/exfiltration. I use loadsoft 6.0 for doing load calcs. I have a technologist who tells me not to enter in air values for infiltration and use a formula that another engineer they were working for showed them. I belive it is BTU=(Vol/60)*1.08*Delta T. I've looked all over trying to find out where this came from to no avail. Looking in the Load Calculation Applications Manual from ASHRAE, in Ch5 it states that "Experienced engineers often estimate infiltration by the air cahnge moethod."
     So my question is, what is a good way to estimate the number of air changes per hour that a space will experience.


Zesti (Mechanical)
8 Feb 10 6:00

There are so many variables...

The formula you have only uses volume, but this does not say anything about the geometry of the building/space (large or small windward facing walls).

It does not say anything about the number of doors and windows, it does not say anything about the way the building is constructed.

The list is endless and I guess this is way the only reference you could find is "Experienced engineers often estimate infiltration by the air cahnge moethod." in which the emphasis must be on the word "experienced"...

Guidelines over here have tables for different parts of the country (windspeeds), height of the building and quality of the construction. Numbers vary a lot and so it is still a bit of a guess.

In my "experience", infiltration of outside air is over estimated when it comes to most modern, well constructed buildings.

(Did you have a specific (type of) building in mind?)
AbbyNormal (Mechanical)
8 Feb 10 22:03
I tend to neglect it in favour of the ventilation

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

Zesti (Mechanical)
9 Feb 10 3:52

Windpressures can be far higher than roompressures from the ventilationsystem (hundreds of Pascals vs tens of Pascals).

So, neglecting infiltration might not be the right thing to do in all cases.

In general: heating systems tend to be to large in most cases anyway. Heat produced inside the building is never taken into account and extra capacity is put in to heat up the building after nighttime lowering of the setpoint and so there is always "spare" capacity.

Kman1138 (Mechanical) (OP)
9 Feb 10 11:26
Well, I do commercial buildings in Northern Alberta mostly. I could estimate by air changes per hour. I have no idea what would be a guess, 5 maybe, 10? Any suggestions would help.
Helpful Member!  VerneE (Mechanical)
9 Feb 10 12:05
My rule of thumb is to do the heat loss calculation of the building based on wall & roof "R" values. Then check to see what two air changes per hour of the building volume require for heating. If V is the building volume and Ti is inside temperature and To is outside temperature (in degrees F) the equation becomes


The 1.08 is the conversion of units & the specific heat of air combined together. V*2/60 converts two air changes per hour of building volume into cubic feet per minute.

If it is a tight building, like an office with good doors and vestibules, only one air change per hour can be used. If it is a warehouse or large high roof manufacturing building, three or four air changes per hour may be used.

I have found two air changes per hour usually is more heat loss than the building construction heat loss. You only need the extra heat on weekends or holiday shutdowns when the internal heat gains from equipment or activity & lights is not available.
Zesti (Mechanical)
10 Feb 10 2:51

VerneE writes:

"If it is a tight building, like an office with good doors and vestibules, only one air change per hour can be used. If it is a warehouse or large high roof manufacturing building, three or four air changes per hour may be used."

I guess construction methods can vary a lot but in my experience modern warehouses are very airtight. Therefore having 3 to 4 air changes per hour for a warehouse is way too much in my opinion. I use around 0.5 air changes per hour due to infiltration on large warehouses.
Heating up 3..4 ACH on a large warehouse is a huge heating capacity.
walkes (Mechanical)
10 Feb 10 9:27
Air change method is dependent on the configuration of the space.  If using ACH I use only the 10 foot perimeter of the space for the calculation, and then ensure that perimeter radiation is used to cover the heat loss.

My normal infiltration calculation is based on a airflow per square foot of exterior wall area.  The rate varies from 0.05 cfm/ft2 up to 0.17 cfm/ft2 depending on the tightness of the envelope construction.  It is an estimate as it will vary due to actual construction, wind pressure, stack effect etc.

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