Comments on: How Many Tons of Air Does Your HVAC System Move?

By: Robin Boyd

Robin Boyd — Tue, 24 Oct 2017 23:45:38 +0000

This is a great conversation
This is a great conversation to have. As you have mentioned several times, there are many other variables to moving air. One of the things mentioned is that by running a 1,000 cfm blower continuously, you are “probably” wasting energy and “possibly” creating a mold factory. But what about the positive side to running a blower continuously?
First and foremost, duct sizing must be proper for the system. Secondly, I only recommend constant air flow when an ECM motor capable of having the volume of air reduced dramatically during times when the system is not calling for cooling or heating. Now we have totally different conditions for moving the same air. These differences alone, and you know there are many others, “can” reduce energy usage due to reducing system cycles and “may” reduce humidity conditions that cause most issues with mold if that ECM blower is a variable speed controlled model that is set up to utilize less volume of air during calls for cooling. Less air, colder coil, more condensing of moisture in the air; greatly offsetting condensate blow off conditions that may keep humidity levels higher with the use of a PSC motor. As for the mold? Mold is in the house no matter what. If we have any chance of reducing mold spores in the home it is by filtering it out of the air. Air filters, as all IAQ products, only work when the air is moving through the system. This is especially important for home that have attic cooling systems that are not used for heating during heating season. That is when moisture collects in the ductwork in the attic, condensing in low lying ducts and making for a wonderful place for mold spores to grow.
Then there is the comfort factor. When there is a constant movement of air, preferably low enough velocity that it is not even noticeable, there is less stratification of heat in multi-level homes. My home in the very humid but not really that hot Mid-Atlantic area of the country has a heat pump system that is designed to operate with a nominal 1,000 CFM of air in the cooling mode. There are two levels to my home with a bonus room with a vaulted ceiling, so I have a variety of conditions in my home. By operating my system in the cooling mode at 700 CFM with a Variable speed blower motor, along with the use of a properly metered but otherwise oversized indoor coil (rated at 4 ton nominal for 13 SEER ratings), my system reduces moisture in my home very well when in the cooling mode. By leaving my blower set to constant on at 30% of cooling capacity when the system is not calling for cooling, my filtration and UV light system are in constant operation and the temperatures do not vary from one part of the house to another. By setting up the air movement in this way, even though I have technically reduced the SEER “rating” of the system, my energy costs are slightly lower, my humidity levels are much lower and my comfort level is about as good as anyone could expect.
I merely wanted to point out that we need to take each specific application into consideration before being negative about constant air flow in HVAC systems. My set up is obviously not for every application, but in my area of the country it more applicable than not.

By: RoyC

RoyC — Tue, 24 Oct 2017 22:10:05 +0000

In reply to abailes. When I said "lift" the air 1
When I said "lift" the air 1 in of water, I used a bad metaphor. The ideal power required for a blower or fan to move air is the volumetric flow rate multiplied by the pressure rise of the blower with some appropriate units conversions. If you multiply 1000 cfm by 1 in.w.c. for a typical residential blower to overcome the coil, filter, duct, etc. losses, the ideal power is about 120 W. Note that the density is irrelevant! A real blower requires 3 to 4 times that amount due to blower and motor inefficiencies (overall efficiency of around 25 to 33%). As for temperature rise through that same blower, it would be about 0.4 F for an ideal blower and 3 to 4 times that for an actual blower and motor. By the way, I learned this in my engineering classes, not my physics classes ;-)

By: Lee H

Lee H — Tue, 24 Oct 2017 21:10:27 +0000

Steve Waclo...about 1.6F
Steve Waclo...about 1.6F

By: Steve Waclo

Steve Waclo — Tue, 24 Oct 2017 16:21:19 +0000

Allison, a question.
Allison, a question.

To what extent does energy expended by a typical PSC fan change the temperature of air moving through an HVAC system?

“It depends” is not an acceptable answer. 🙂

By: Mark Johnson

Mark Johnson — Tue, 24 Oct 2017 16:20:57 +0000

So unless I slipped a decimal
So unless I slipped a decimal, one cubic foot of air weighs more than one ounce! I never suspected. Thanks.

By: abailes

abailes — Tue, 24 Oct 2017 13:32:13 +0000

In reply to RoyC. Glad to hear the tornado

Glad to hear the tornado missed you, Roy. That kind of air movement gives a whole new meaning to weight of air.

At the beginning of the article, I mentioned work and energy but didn't explore that topic further after the calculation. Of course, anyone who's been through an introductory physics class may recall that it's possible to move things with weight with no expenditure of work or energy. As long as you move it horizontally and with no friction, you don't have to do any work. But if we have to push air through ducts that have friction or if we have to push it up from the basement to the first or second floors, then we have to use some energy to make it happen. Now, if someone will invent frictionless ducts...and then not do stupid things with them.

By: RoyC

RoyC — Tue, 24 Oct 2017 13:15:38 +0000

I live in Oklahoma. We get a
I live in Oklahoma. We get a lot of tornadoes here. Last weekend, one missed our house by a few miles and hit the local casino. (Perhaps I live right!) The damage was minor, but it was only an F1 tornado. So yea, I am aware of the “weight” of air.

It’s kind of amazing that we can move that house air with a half-horsepower motor, but we are only “lifting” it by about an “inch” (of water).