Monthly Archives: December 2012

Fresh air is a necessary component of human health. That’s a biological no-brainer. And, decades ago when a person was inside their home, they still got a lot of fresh air  through the process of infiltration. Air leaked into the house through cracks near doors and windows and other places in the building, and infiltrated into the living space.  So, infiltration is a nice way of saying that the contruction of buildings wasn’t exactly tremendous, and they leaked a lot. I recall one place I lived in. On cold winter days, if you held a match up next to an electrical outlet on a exterior wall, it would go out. Yes, we were very healthy. Cold and shivering maybe, but healthy.

But, of course, time has moved on and buildings are built much tighter now, which means that we still need fresh air to be brought brought into a building; it’s just that now, we have to do it on purpose.

And, of course, the question that is often raised is, “How much fresh air do we need?”

In commercial buildings, there are specific numbers relative to the amount of fresh air an HVAC system needs to bring from the outside and circulate along with the return air throughout the structure. In the case of residential applications, though, things are somewhat different. Since there is really no specific requirement that states the exact amount of fresh air necessary, a ‘rule of thumb’, which says that it’s generally accepted that a person needs 15 CFM (cubic feet of air per minute) of fresh air, applies. And, when it comes to applying another rule of thumb to the building itself, another generally accepted factor is that the number of occupants in a building will be calculated as 1 per bedroom, plus 1.

So, what this boils down to is that in a three-bedroom home, we would consider that there will be four people that need fresh air.

And, 15 x 4 = 60 so the total fresh air requirement for the building would be 60 CFM.

So, when you remove the filter from a return plenum in a gas furnace installation in a new home and look closely, you’ll likely note some kind of system that allows fresh air to be drawn directly from the outside and delivered into the return of the furnace cabinet. It’s quite common to see a 6-inch round flex duct of some sort connected to the return, and the other end is just open (screened in order to keep critters out) to the outside.

Of course, when this outside air is drawn into the structure while the furnace is operating in the  heating mode, the air that’s coming is a going to be cold. And, that means that we need to account for that additional load on the furnace when it’s sized so that it will be able to handle raising the temprature correctly even though we may be bringing in outside air that’s 20-degrees (which is what we consider pretty cold here in Arizona), so that’s the number we would use in a total heat formula in order to determine just how much of an additional load we’re creating with the building’s fresh air system.

I’ll give you an example of that next time.

Learn from yesterday….Live for today…..Look forward to tomorrow.


Depending on which study you read, it’s been estimated that somewhere between 58 and 72 Percent of air conditioning systems are not functioning as designed due to problems with the air handling system. Regardless of whether or not we accept any given number presented by any given study at the time, we have to admit that it’s important that the air flow through any HVAC system be proper in order to accomplish the necessary heat exchange and achieve the desired comfort level. And when it comes to forced-air gas heating systems, technicians can accomplish the task of ‘wrapping their head around’ the concept of proper air flow by understanding the fundamentals of the properties of air….and two formulas.

As far as the properties concept goes, since we always have to start somewhere with a standard, when we consider the properties of air we understand that two of these standard points are:

1. Sea level (14.7 PSI….meaning 14.7 pounds per square inch of atmospheric pressure pressing down on the earth, referred to as 1 Atmosphere of Barometric Pressure). 

2. A temperature of 68-degrees Fahrenheit.

Of course, a change in temperature or pressure will bring about a change in the air itself, but at the standard conditions mentioned above, we can derive two factors, which are:

1. Air will have a Density of 0.075 lbs. per cubic foot.

2. Air will have a Specific Heat of 0.24 BTU’s.

Which brings us to our first formula that HVAC technicians need to understand relative to air flow. When we apply these two factors in a formula along with a factor of time (one hour….expressed as 60 minutes in the formula), we can calculate what is known as The Sensible Heat Factor of Air.

0.075 x 0.24 x 60 = 1.08

And, that number that we arrived at can now be applied within the second formula, which is:

CFM = Qs / 1.08 x TD

And, once we are reminded that the factors in the formula are:

CFM = Cubic Feet Per Minute

Qs = Sensible Heat In BTU’s Per Hour

1.08 = Sensible Heat Factor of Air

TD = Temperature Difference Between Return and Supply Air….

….We can undersand that a manufacturer designs a gas furnace, this formula, along with other factors, are used to determine the size of the squirrel-cage blower and the horsepower rating and speed of the motor that ultimately determines the amount of air flow through the furnace cabinet.

To illustrate the formula, we’ll consider a specific gas furnace and plug in the necessary numbers to make it work.

Our equipment will be an 80,000 BTU furnace that has an efficiency rating of 80%, which means that the actual heat output (Sensible Heat Factor in BTU’s Per Hour) can be calculated as follows:

80,000 x 0.80 = 64,000

And we’ll also determine that our return air temperature will be 70-degrees, and that our supply air temperature will be 130-degrees, which means we can calculate our TD as follows:

130 – 70 = 60

As our next step in explaining the process, we’ll again present the original formula as….

CFM = Qs / 1.08 x TD

 …Which means that with our numbers plugged in, it reads as:

CFM = 64,000 / 1.08 x 60

…Which works out to:

CFM = 64,000 / 64.8

…Which means that what we have to do is divide 64,000 by 64.8, in order to arrive at:

987.65432 Cubic Feet of Air Per Minute

All of which boils down to the fact that this particular furnace needs 988 CFM traveling through its cabinet, along with properly designed and installed duct system, in order to accomplish the proper temperature rise, which accomplishes the intended goal of proper operation and achievement of the desired comfort level in the building.

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