How Forced Air Works

ProblemSolver

I began working in this business 35 years ago. My first 6 years, I worked next to some really good veteran tin bangers. A couple of these guys had some influence on the first book written on Duct System Air Flow. To this I was blessed; these veterans didn’t necessarily teach me how to do the job, they taught me how to think; how to look for the question and not the answer because if you can find the right question, the answer is simple to find with a little research. It was this type of problem solving these veterans used and taught me. But as with any mentor, it is their hope that their students take the teachings and improve on them; and that is exactly what I have done.
Most people and HVAC techs believe that forced air conditioning is about defeating a temperature in a room. Think about it... a 2 story home; in the summer time, people close most of the registers on the first floor in hopes to send more air up to the second floor. Also, contractors attempt to better cool the 2nd floor by adding 110 volt booster fans to the 2nd floor runs. Why is everyone working with the supply air runs to solve a problem? And really, the problem does not get resolved! If anything it creates more problems by reducing the amount of air through the evaporator coil. So, there has to be a solution to this problem, but where do we look for it? What is the correct question to resolve this problem? So, I began looking for the question, and then it came to me; “where have we not looked?” THE RETURN AIR SYSTEM!
I have to go to work now. I will continue this another time. Have a good day!

Rich_49

Balanced airflow and Low ESP of the duct system are key . Less than .5 " would not give the feeling of forced air .
Can't wait to hear more . Just a conversation that took place earlier this year ;
https://www.linkedin.com/groups/1110797/1110797-6004086671514091520

JUGHNE

During a change out of oil furnace to heat pump, as I studied the return air system on a very old system I found the main RA floor grill had been covered over by the carpet layer. The HO had been told that "people don't use return air things anymore, see nothing comes out of it"....(it was easier to cover than cut out properly).
It was surprising that even with the cobble-jack existing ductwork that HP compressor lasted over 35 years. Most people will not be that lucky!

ratio

What an a**h***! I understand incompetence, but that's just plain malicious.

Bob Bona_4

Lol. Past summer I tracked down a 12x30 floor return, all panned out, covered by the hardwood floor installers, 15 years ago. 1 cracked HX 5 years in, compressor death at 12 years, evap pan melted at 13 years. Nice.

ratio

Unbelievable. Like I said, I understand incompetence, hell I'm as incompetent as the next guy. But to screw it up on purpose, that just doesn't make any sense.

JUGHNE

But you know that carpet is a really effective filter
Many people with rooms not heated well are amazed that you can blow air into a room and it cannot leave it will not bring heat. For some as this is explained you see the light bulb come on in their heads....for many others the bulb remains dim. If you remind them of why there were 3" cut off the bottom of the doors in the single wide trailer they just left then the bulb glows a little, sometimes.

A friend was considering and did eventually buy a double wide prefab, "built so accurately with laser cut studs, etc." I said ask them if there is a return duct in each bedroom and they assured him there was.
Well there was and it was a transfer grill/duct into the center hallway to feed the grilled closet door return. The first time the furnace started the HO thought there was a truck coming down the hall way. But anyway the doors weren't cut off on the bottom....it still sounds like a trailer house furnace when running.

Jamie Hall

Thinking about it... we steam guys like to think that steam is a little esoteric. But, in fact, really getting a forced air system to work right and be designed and installed right is, in my humble opinion, every bit as difficult if not more so. The biggest difference is that a steam system will make nasty noises and otherwise misbehave if it's wrong, and a forced air system simply won't be comfortable!

ProblemSolver

The low pressure Forced Air Systems cannot defeat the undesirable temperature in each room. How forced air works is it exchanges the air in each room: Remove the undesirable air from the room and replace it with the newly conditioned air. This is what makes all low pressure forced air systems work, but the challenge to this is: getting the air exchanged in every room in the house equally according to the size of the room and distance from the blower. “Distance from the blower”, very important!
Thanks to Dan Holohan’s books, we all know that high pressure goes to low pressure - every single time; it’s one of the laws of physics, here on Earth! So, if high pressure goes to low pressure; then trying to increase the supply air to a room is like putting the carriage before the horse and making the horse push the carriage with its head. We should be increasing the return air draw from the room, and then by the law of physics, the supply air has no choice but to follow the low pressure; or fill-in the empty space created by removing air from the room. Therefore, every problem with air flow in a low pressure duct system BEGINS with the return air.
Look on an air duct calculator, it will tell you that a stud space (3-1/2” x 14-1/2”) will give you just under 200 CFM. Every duct designer I have talked to says, that stud space is what balances the amount of air the return can draw. This is the furthest thing from the truth. Every setting on an air duct calculator is based off a certain velocity and noise criteria. We can easily get 500 CFM’s through that stud space by an increase in velocity.

Back to work, I’ll continue this tomorrow. What I have to say next is what I developed, and it’s what makes the whole system work.

ProblemSolver

OK, I didn't develop it, it was always out there, I just recognized it and began using it.

Rich_49

Do you actually use the building assemblies as part of the duct system ?

ProblemSolver

Jamie Hall said:

Thinking about it... we steam guys like to think that steam is a little esoteric. But, in fact, really getting a forced air system to work right and be designed and installed right is, in my humble opinion, every bit as difficult if not more so.

Right on, Jamie. But with this new info I am sharing, it literally solves the mystery of the problems with forced air. And the only thing that needs to change is how we think about it. In-other-words, somebody got it wrong a long time ago, and we all just followed it.

ProblemSolver

Rich said:

Do you actually use the building assemblies as part of the duct system ?

Here in Detroit, we use the joist spaces and stud spaces as a cavity for the return air; most homes have basements.

Tinman

Thats been against code in the Chicago area for a long time. Let me cut to the chase. In an ideal forced air system, the amount of return (cfm) should equal the amount of supply (also cfm).

As far as panning or using stud spaces, that's one of the very biggest problems with system inefficiency. A recent study showed that homes in the Midwest with forced air have a system efficiency of 40-50%. I'm MEAA HVAC SAVE Certified. One of the best courses I ever taken on forced air, a real eye opener.

ChrisJ

If people viewed forced air the same as forced hot water they would have an easier time. It's no different, you're circulating air instead of water but it's still a big loop, or loops.

You need a return, be it a duct, leaks under doors or floors, walls etc or outside the house where you'll bring in a ton of dry cold air, the air will circulate one way or another. The last of course being the reason many relate forced hot air with dry heat.

As is likely well known, I want absolutely no part of it and feel it's a poor way to heat or cool a structure, but it's still an art just is hot water and steam.

Rich_49

No doubt you do . Does not matter where you are located , it is a terrible practice .
Like I said above and Stephen verified , balanced & Low ESP is the only way to make garbage not stink . Spend the money and put tin in those bays or find a more appropriate route where you can .

Tinman

And seal those tin joints with mastic. And if the temperature drop from point A to point B within the duct is too great, insulate the duct. This is assuming it all started with a load calculation using Manual J and a duct design using Manual D.

Rich_49

Don't be bound by the constraints of the program (MJ) either , be not afraid to enter what you know . The program does have restraints and may not recognize what you intend .

Steamhead

Forced-air is now, has been, and probably always will be a race to the bottom- who can throw it in fastest and cheapest. These guys don't care if it actually works- once the project is finished and they get their money, they're outta there.

I'm with ChrisJ- steam or hot-water will always rule.

Rich_49

It can be done well and is here to stay . Might as well throw the real information out here for those that may desire to excell at it . It is here to stay so gotta make an effort

Steamhead

Rich said:

It can be done well

That may be, but when you have developers, flippers and others who don't care if it works or not, it is as I said just a matter of fastest and cheapest. You will never get these people to do it any better.

ChrisJ

Rich said:

It can be done well and is here to stay . Might as well throw the real information out here for those that may desire to excell at it . It is here to stay so gotta make an effort

Not necessarily.
Hydronics can win especially as people expect better and better efficiency.


There are three things about hydronics that can win people over.

Silence, comfort and cost to operate.

Forced hot air has won the battle, but not the war.

Bob Bona_4

We have been on board with duct pressure testing with or without the equipment in place (new install) 4 years now. Combination mastic and UL Hardcast 1403-181BFX gets it done. Even Pittsburgh seams are sealed. Southwark has "airtite" collars, we seal them further. Independent party test.

The days of using joist bays for ducting are long over.

Rich_49

Chris ,

You know I am a wethead but the fact remains we are not gonna get rid of this trash . As long as someone is only gonna look at the bottom line it is here for the long haul .

ChrisJ

Steamhead said:

Rich said:

It can be done well

That may be, but when you have developers, flippers and others who don't care if it works or not, it is as I said just a matter of fastest and cheapest. You will never get these people to do it any better.

And if a proper forced air system is compared to a proper radiant system is there really that huge of a price difference?

Enough to offset the discomfort, noise and lower efficiency?

Rich_49

LMFAO ! Try finding people to do either correctly . I absolutely hear what you are saying but the fact still remains that finding the proper people is the difficulty . They only find the right people @ HeatingHelp after they've been had .

njtommy

I a believer that slow and steady wins the race with forced air heat and hydronics.

Forced air will never go away. We deal with large commercial buildings on a daily basis and schools. You need proper air exchange every hour. Some of the new schools we do have co2 sensors in the class rooms and based on the number the dampers for fresh air and exhaust air modulate open or closed.

ProblemSolver

Air travels to, or from, the place of least resistance; just like water does. And I’ll bet your mind is on the supply side of the system as you apply this idea. So how does this idea apply to the side of the blower that draws air? At the blower, the velocity of air flow is very high compared to the furthest end of the return air trunk line. The further you get away from the blower the weaker the velocity gets; but equally, the closest you are to the blower the stronger the velocity gets. Based off of this, the blower will draw “all the air it can get” from the closest opening first (remember, the velocity is the strongest here), then it draws "all it can get" from the next closest opening, and so-on down the line. Whether you’re cutting a hole and using the joist space, or piping off the trunk line to your return gill, a hole is getting cut! Most system’s I’ve seen, the holes are pretty much the same size (here in Detroit, they cut the largest hole they can get). Because the blower will draw all it can get from the closest opening, and because the closest opening sees the greatest velocity, that first hole should be smaller than what an air duct calculators says to use. On my jobs, if the hole is just above the return air drop, and it’s a short run, I’ll cut a 4” hole (I typically see about 150 CFM’s, give-or-take). As we move further from the blower the holes (or pipe) get a little larger. If I’m doing a two story home, the largest hole I’ll cut for any 1st floor R/A run is 6”; and all the 2nd floor runs get between 8” to 10” holes. What I am doing here is restricting what the blower can get from the first floor runs (by taking into account the strength of the velocity); thus it has no choice but to also draw air from the second floor to get the total amount of air it needs. A ranch home is no different; the return air at the end of the trunk line should be your largest hole, just like the closet return air to the blower should be your smallest hole; but keep in mind, Velocity! Now you are controlling the amount of air being drawn out of each room, and because high pressure goes to low pressure - every time, you are also controlling how much air goes into the room. Balancing air flow BEGINS at the RETURN AIR TRUNK, and knowing what the velocity is based off how close to the blower the total distance of the run is.
There are deeper details than just this that I use, but this is the heart of it. And if you think I’m done, not yet! The next thing I want to talk about is how to give the furnace the air it needs. Yeh, I said “give” the furnace. It still has to do with Return Air. You can’t push any more air than what can you pull.

ChrisJ

@ProblemSolver No, I'd never look at just the supply side, as I said, it's a loop. It has to be a loop otherwise the room would explode or implode. The entire thing must be viewed as a loop or it'll always be a failure.

I had an argument several times with a friend that was selling some stupid fan that was designed to dry out basements better and cheaper than a dehumidifier. All it did was suck conditioned air from the living space down through the basement and blow it outside.

I told him I didn't want to be sucking damp nasty hot air through my house and he assured me multiple times it didn't do that. I asked him where the air was coming from, he said no where. So I said "So the house just implodes?" He still after many years just doesn't get it.


This is the same reason I refuse to buy a portable air conditioner. I can't stand the idea of using my cooled dry air to cool the condenser and then blow it outside and then sucking hot damp air in. It seems completely bass ackwards even worse than putting a compressor and condenser under a refrigerator.

ProblemSolver

ChrisJ said:

@ProblemSolver No, I'd never look at just the supply side, as I said, it's a loop.

So I said "So the house just implodes?" He still after many years just doesn't get it.

Don't miss the point here, the suction side of the blower performs differently than the supply side of the blower; therefore, it gets balanced differently. In the return air trunk and its runs, your dealing with a different velocity at each run; but in the supply air trunk, your dealing with trying to maintain an even back pressure and volume of air - not velocity. We typically shoot for about 200 CFM's per single R/A grill. If I cut a 7" hole in the return trunk close to the return air drop, thus close to the blower, to service a 1st floor single return air grill (6 x 14); based on velocity, the blower will get about 400 CFM's from that 7" opening. But if I cut a 4" hole, based on the same velocity, the blower can only get about 200 CFM's.

The only reason there are rooms in the home that don't heat or cool well, is because all the holes cut into the return air trunk are very close to being the same size, and usually bigger than they should be. The blower will draw as much air as it can from the closest openings first; and with all the holes being approximately the same size, and even over sized, the blower will typically get all the air it needs from the closest R/A's and completely ignore the one's that are the furthest away. I have found over the years of using this that the only place you can balance the return air system is at the trunk; not the stud space, like most contractors in my area believe.

ChrisJ

ProblemSolver said:

ChrisJ said:

@ProblemSolver No, I'd never look at just the supply side, as I said, it's a loop.

So I said "So the house just implodes?" He still after many years just doesn't get it.

Don't miss the point here, the suction side of the blower performs differently than the supply side of the blower; therefore, it gets balanced differently. In the return air trunk and its runs, your dealing with a different velocity at each run; but in the supply air trunk, your dealing with trying to maintain an even back pressure and volume of air - not velocity. We typically shoot for about 200 CFM's per single R/A grill. If I cut a 7" hole in the return trunk close to the return air drop, thus close to the blower, to service a 1st floor single return air grill (6 x 14); based on velocity, the blower will get about 400 CFM's from that 7" opening. But if I cut a 4" hole, based on the same velocity, the blower can only get about 200 CFM's.

The only reason there are rooms in the home that don't heat or cool well, is because all the holes cut into the return air trunk are very close to being the same size, and usually bigger than they should be. The blower will draw as much air as it can from the closest openings first; and with all the holes being approximately the same size, and even over sized, the blower will typically get all the air it needs from the closest R/A's and completely ignore the one's that are the furthest away. I have found over the years of using this that the only place you can balance the return air system is at the trunk; not the stud space, like most contractors in my area believe.

Instead of cutting different size holes, why not install dampers that can be fine tuned?

ProblemSolver

@ChrisJ , were talking residential, nothing much larger than 3,000 sf. why spend the extra cost of dampers, when all you have to do is cut the right size hole?

ChrisJ

ProblemSolver said:

@ChrisJ , were talking residential, nothing much larger than 3,000 sf. why spend the extra cost of dampers, when all you have to do is cut the right size hole?

Things change.
For example lets say the entire house is carpeted, sans kitchen and bathroom etc. They rip the carpeting out of 2 rooms and put in hardwood so now there are gaps under doors.

Also different people like different things. I like my bedrooms cooler and bathrooms warmer.

I like to have control over things and a fixed hole doesn't provide that. Dampers are plenty cheap enough that I'd want them if I absolutely had to have forced hot dust. Hell, give me a way to have thermostats control the airflow to the rooms automatically!

Why does commercial always get the proper systems and residential always gets the short end of the stick?

njtommy

Forced hot dust. Lol.

Commercial systems have their far share of problems.
But I'm a big fan of VAV systems with hot water reheat coils or water source heat pumps ( just not the placement of them in tight spots

Tinman

I'm having trouble accepting whats being shared in this thread as fact.

@ProblemSolver - Your suggestion for sizing return branch ducts is completely contrary to Manual D. Please correct me if I'm wrong.

@ChrisJ - I know you're a steam guy but I've never heard of a damper being installed in a return duct and don't know why anyone would.

Bob Bona_4

Not sure the purpose of this thread.

hvacfreak2

Stephen Minnich said:

I'm having trouble accepting whats being shared in this thread as fact.

@ProblemSolver - Your suggestion for sizing return branch ducts is completely contrary to Manual D. Please correct me if I'm wrong.

@ChrisJ - I know you're a steam guy but I've never heard of a damper being installed in a return duct and don't know why anyone would.

I am 30 years into the hvac trades , I have come to believe that return air flow and comfort seem to parallel each other ( residential and commercial ). One would install dampers into portions of that system for the same balancing purposes as any other. Speaking residential cooling , I would say that sun rooms are an example that one might wish to have an isolation damper on a return duct. Single duct systems that serve more than one level can benefit from seasonal change over balancing dampers on the return side ( h-c systems ). And if I ever design for individual returns in each room ( which is ideal ) I would provide balancing dampers in those branches.

ProblemSolver

Stephen Minnich said:

I'm having trouble accepting whats being shared in this thread as fact.

@ProblemSolver - Your suggestion for sizing return branch ducts is completely contrary to Manual D. Please correct me if I'm wrong.

Couldn't tell you Stephen Minnich, I have never used the manual D to design the trunk line or the branch runs. If the manual D does not take into account the velocity on the return side of the blower, then I would have to say Yes, the manual D falls short.

Please keep in mind, I didn't read this somewhere; I have been using this idea for over 15 years and I learned it by way of trial & error. All the veterans that taught me, also learned by way of trial & error and they passed it down to me.

ProblemSolver

ChrisJ said:

, give me a way to have thermostats control the airflow to the rooms automatically!

this whole thread has been about controlling the air to the room by controlling the air being removed from the room. If you want a cool room in the winter season, then put a magnetic sheet over 70% of the return air grill. Never shut off a supply or return 100%, you will stress out the equipment.

Tinman

@hvacfreak2 - I absolutely agree that the proper amount and placement of returns is critical. In terms of balancing returns, what you're suggesting is balancing airflow back to the furnace and/or air handler. Why would anyone want to do that? High/Low returns is a different discussion.

@ProblemSolver - I choose to believe that Manual D does not fall short. I've been doing it that way since I came into this awesome trade a very long time ago. Its ok that we'll just have to agree to disagree.

hvacfreak2

Return duct sizing should be 600 fpm for 400 cfm per ton for trunk ducting ( per my ACCA class years ago ).