How Forced Air Works

Harvey Ramer

4Johnpipe said:

@Harvey Ramer sounds like we need zone valves on returns for HVAC...hmmmm

Return dampers operated proportional to the respective area's static pressure and current air conditions, would be ideal.

SWEI

The controls would not be inexpensive, but definitely a fun project for the right client.

Tim McElwain

Problem solver please come to the Northeast. Every warm air system I have ever serviced has had insufficient return air. I for years based my answers to folks who asked about how much return air they needed to an old manual from the National Warm Air institute (1928), in that manual it stated that ideally with a blower operated system the minimum return air should be 1 1/2 times larger than the supply. Very simple and I do not claim to be an expert on all this in fact I really think on both the hydronic side and the warm air side we are becoming to complex, a good way to go "is keep it simple stupid", and just so as to not offend anyone the stupid is referring to myself.

Tim McElwain

Problem solver do you know Carter Stanfield?

ProblemSolver

SWEI said:

Got a photo or a sketch of this? Near-zero sheet metal experience here but I'm interested in learning how we might improve things for the furnace customers.

ProblemSolver

Tim McElwain said:

Problem solver do you know Carter Stanfield?

No, why?

SWEI

Thanks, that does make sense. Could be a bit of a challenge on some box configurations, but worth the effort.

Tim McElwain

I wondered if you had read Carter's manual? Seeing you do not know him then I am sure you have not read his manual.

ChrisJ

Tim McElwain said:

I wondered if you had read Carter's manual? Seeing you do not know him then I am sure you have not read his manual.

Fundamentals of HVACR?

ProblemSolver

SWEI said:

Thanks, that does make sense. Could be a bit of a challenge on some box configurations, but worth the effort.

I am replacing 4 furnaces & 3 A/C's in one house, and it's a ranch. I will take pic's of installing the return pan to give everyone an idea. Here is my drawing to have the pan made-up.

Oh, by-the-way; I remove the blower to install the pan. It makes it a whole lot easier - all around.

ProblemSolver

Are there limitations on the type of pictures I can down load?
I tried downloading a "jpeg" picture last night, and the down load sign showed it downloading for quite-a-few minutes and nothing. So I gave up...

JUGHNE

Your pan is, I believe, an excellent idea. When servicing blowers you always notice the one side with the cutout has more dirt/dust than the other. Curious, why no cross break, I realize it sits on the floor but is there another reason?

ProblemSolver

Stephen Minnich said:

@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?

Hi Stephen, in response - the return creates a negative pressure (or, low pressure). Control what the return draws, and you control the amount of air that comes into the room. HIGH PRESSURE GOES TO LOW PRESSURE!

Tinman

You felt it necessary to use all upper case letters to state an obvious fact? Feel better? There are very few homes with forced air that can afford to have return air restricted in any way. Almost all of them are lacking in return by a large percentage to start with so I won't be that guy making it worse.
install the correct amount of returns in the first place and you're golden. And in terms of pressurizing a room? When was the last time you designed/ installed forced air in a home that didn't have a wide open floor plan??? It's not important that we agree.

ProblemSolver

JUGHNE said:

Whenever I change one of these out I use the old supplies for returns, usually add more and cut in floor regs on the perimeter.

Right- on; @JUGHNE . But think about how to control the amount of air each return can draw, and still using the same over-sized floor return air grill. -- Put a blocker in each return air joist, and cut a hole in the blocker the size needed for the amount of air needed.

ProblemSolver

Stephen Minnich said:

There are very few homes with forced air that can afford to have return air restricted in any way. Almost all of them are lacking in return by a large percentage to start with so I won't be that guy making it worse.

I apologize; when I used the word, "restricted", I was talking about, restricted from what installers normally do. The sizing is not a restriction from what we should be doing; but a restriction from how we have been taught. The sizes I share with you are a guide; not an exact science.

JUGHNE

The 50-60's houses have wall stack 3 1/4 x 12 or 14 for supplies.
The older original gravity flow systems have ornate supplies on the inside walls, sometimes back to back with a bedroom. They might get the equivalent of a 7 to 10" round return. In this type of house the people don't want more cuts than necessary and the modern baseboard grill doesn't fit in with the old décor. So the liv rm/din rm may be the main RA for the system. So the dampers should be locked in place according to the room size.

ProblemSolver

Tim McElwain said:

Problem solver please come to the Northeast. Every warm air system I have ever serviced has had insufficient return air. I for years based my answers to folks who asked about how much return air they needed to an old manual from the National Warm Air institute (1928), in that manual it stated that ideally with a blower operated system the minimum return air should be 1 1/2 times larger than the supply. .

In 1928, the blowers were belt driven. Direct drive blowers don't have the same requirements as belt drive blowers did.