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Fresh Air Requirements and Furnace Operation

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.