Thread: No Return Air Plenum?

How many see this type of install where there is no plenum box for the return duct to be secured to.

After sitting in a 16hr. HVAC education class this past weekend, I found out this is a real common problem. It forces the air to fast through the evaporator coils per the instructor.

Any comments?

Rick

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The speed of the air is determined by the fan speed.
A return plenum just offers a method of duct attachment.

I am possibly misunderstanding what they were trying to say.

Yes velocity of air over coils to high or to low can cause problems based on the coil type and unit sizing in relation to the rest of the duct design.

Was the course offered by a manufacture rep who is an engineer, a factory sales rep, a Lic HVAC installer or other?

Rick there are several things that may be good or no so good about it. The bottom line is, you won't know for sure without proper testing. This would include measuring Total External Static Presssure, delta T, blower amp draw, combustion analysis, super heat & subcool in cooling mode, fan RPM, and air balancing measurements at the registers both supply and return.

Sometimes this is done in an attempt to make up for otherwise poor duct design/ execution. For instance, you take a TESP and find your RA is too restricted. By ducting right into the blower plenum without a filter may give you a little tweak. I would see it as a red flag to other problems but it is not a true problem unless proven by measurements.

Originally Posted by Bob Harper

Total External Static Presssure.

Bob how do you measure that? With a barometer?

The feet per minute (fpm) is also determined by the duct size. A plenum is used to increase the area and lower the velocity. You would need to look at the installation manual to see what the specified air flow rate is into the unit. You can also calculate the area of the duct and see if there is a CFM rating on the unit. The airflow should be around 500 fpm.

I wouldn't see it as a problem necessarily, assuming that the connection is airtight and permanent. The coil velocity will be the same regardless of the size of the incoming duct. If the unit is pulling 2,000 CFM and it happens to speed up thru the return duct the velocity will lower once it reaches the coil face.

IMO, not a problem.

TESP is typically measured using a manometer and a Dwyer 303 static pressure tip inserted into the return plenum just before the filter and in the supply hopefully before the coil then add the two numbers disregarding + - signs. The rating plate will state the max. static pressure but it is usually 0.5 wci. If you have too much, your amp draw increases costing more to run and your airflow drops off. Too little static pressure, the blower can over-rev, which throws off your delta T, move too much air for the system, etc.. If you lower fan speed, you increase heat transfer. Howver, combination heat & cool split systems typically are set at around 400-500cfm per ton of cooling. On simple units, the heating mode makes do with this higher fan speed, while some modern units use dip switches to ramp back on heat. You typically need more airflow for comfort and dehumidification in a cooling mode whereas in heating, you want better heat transfer with less air blowing around to cool you by convection.

The location of the blower motor would have some effect on efficiency and air flow with regards to where the RA enters. Regardless, you don't want turbulence but a smooth transition with no eddies.

Again, you won't know unless you test. This is something for an HVAC tech and beyond the scope of a home inspection.

It looks to me like with the duct conected directly to FAU at the filter locacation that air over filter is uneven. IMC and IRC address this issue. "Ducts shall be constructed to allow an even distribution of air over the entire filter."

Originally Posted by Garry Sorrells

The speed of the air is determined by the fan speed.
A return plenum just offers a method of duct attachment.

I am possibly misunderstanding what they were trying to say.

Yes velocity of air over coils to high or to low can cause problems based on the coil type and unit sizing in relation to the rest of the duct design.

Was the course offered by a manufacture rep who is an engineer, a factory sales rep, a Lic HVAC installer or other?

The instructor is a very reputable licensed HVAC contractor in our area.

rick

Originally Posted by Rick Hill

"Ducts shall be constructed to allow an even distribution of air over the entire filter."

But what does that mean? In reality the velocities that we are dealing with will provide even enough airflow. As an air filter loads it tends to distribute air to the areas of least resistance anyway.

There are a multitude of problems shown in that photo, but connecting the return duct directly to the air handler is not one of them.

If there is an air speed problem it is because the proper testing was not done and the blower speed was set, or left set, at the wrong speed.

On more modern air handler units with variable speed motors controlled by computer chips, the fan speed is a function of the programming for the computer circuit.

Jerry,

This instructor stated that without a return air plenum that the fan motor does not "load up" as intended which in turns causes less air flow.

He stated with a plenum it made the fan pull the air through the system as intended.

He said basically it is done by HVAC contractors only trying to save a few bucks by building out a return plenum box.

Rick

Rick,

The only way for the air handler to move the air is for the blower to "pull the air through the system", there is nothing else there to move the air, so I may not be understanding what he is trying to say. The return air duct attached directly to the air handler is going to cause the blower to pull the air through the system.

Also, as I recall, I do not recall having seen manufacturer's installation instructions which require a return air plenum as you are describing, plus, for the return air plenum to affect anything the return air plenum would need to be precisely sized, and that would be addressed in the installation instructions.

Not saying that he is wrong, only that I think he is not right ... ... or that he left something out from what he was trying to say.

Jerry,

This whole thing has me questioning this then.

Why would some contractors install a return plenum when only (1) return duct is present? and then we see this type of installation?

Rick

Rick,
The return plenum is really nothing more than an adapter to connect the duct to the air handler. There may be some alteration of the air flow as it expands into the plenum as it passes into the air handler box and it may be possible that there may be a reduction in the friction at that point but I would not think that it would be significant.

The return plenum is also created to contain the filter.

Less duct fabrication is always a cost savings.

Only caviet would be that the return duct size and location does not cause a restriction on the fan motor cabinet.

Jerry, could you tell us some of the things your seeing in that pic? Thank you Brian

From the left ...
- The flexible duct is not supported and strapped within 18" of its connection to the attachment collar at the evaporator unit. Flexible duct needs to be supported at bends also.
- The evaporator unit is behind that metal plate, where is the access panel on this side, the service side? The service platform is required to be on the service side, and the access walkway needs to lead to that area.
- There is no trap in the condensate line coming out of the evaporator unit.
- The primary condensate line is not insulated, it will likely cause condensation to drip onto the ceiling.
- There is no secondary condensate line from the evaporator unit (there are options to eliminate this, but I don't see those options in the photo).
- the Type B gas vent is not properly supported.
- The NM cable really should be protected from physical damage when hung out like that, the NM cable could have been routed from back behind the furnace unit and then under the unit.
- There is no sediment trap in the gas line.
- The flexible duct coming out the right side, the return air duct, is not supported properly (just the the other flexible duct is not supported properly).

Those will work for starters, anyone see something else to add to the list?

Figured I should add this one: Where does the condensate line come from on the left? Are there two units using the same 3/4" condensate line?

Thanks Jerry, much appreciated! Brian

Jerry you have great eyes "but" the gas line is connected to a "T" fitting and the leg is vertical. Wouldn't that configuration also be basically the same as a sediment trap? Gas is being run along the horizontal run on the floor then the unit draws off the supply vertically. Any particles would remain along the run on the floor.
How would you configure that piping, standpipe, 90 elbow, horizontal pipe, drip leg with "T" and appliance connector?
Always I appreciate what you contribute to the forum.

A plenum is just another piece of ductwork as far as the airflow knows. I don't see any difference in the setup in the OP picture and the way a gas pack is connected. A gas pack nominally does not have a plenum...just some sort square to round adapter...normally a flat adapter. Also the ductwork leaving the unit has as much to do with the amount of airflow as the return does.

Mike, there is no *trap* here. Sure, the gas takes off vertically but there is no trap as required by the code.

One correct way to pipe this would be to have your main branch run past the furnace higher up with a tee branching off to the furnace. This leg drops down to a shutoff, sediment trap with a horizontal branch off to a ground union then the appliance connector and thus to the applinance valve. The union can be vertical or horizontal.

The flexible appliance connector is looped forming a trap. Should condensate pool, it can block the flow of gas to the appliance causing underfiring, sooting, CO, delayed ignition or failure to run.

HTH

Rick, I agree with your instructor... sort of.
Although in your example picture I don't see an issue due to the straight shot and simplicity.
A return air plenum is a good design feature since it allows for pressure equalization between multiple ducts, smoother air flow, etc.
I figure the instructor is used to talking to A/C guys, not inspectors and what he is talking about is good design and installation practice not a real black and white prohibition.
To really understand duct design in general, a semester course is really needed along with a little wind tunnel demonstration, IMHO.
The instructor is not wrong, just maybe talking to the wrong crowd.
While it is good to know, I don't see much application for HI's.

The need for an air return plenum at the intake end of the air handling unit pictured can be based upon the "theory" that some kind of volumetric buffer for the incoming air is needed in order to achieve the required/desired airflow volume and/or operational noise level.

If you were to examine the open intake end of various units you would note how little clear and free space actually exists right at the intake opening on some units. Many (but not all) units will have the blower assembly literally fitted right at the plane of the intake "opening" leaving no small percentage of the intake "opening" obstructed. This is done by design because the size of the air handler must be kept compact by the manufacturer in order to be able to get the one piece units into and out of attics and tight mechanical closets.

A possible result of this design constraint however is that incoming air from an attached lateral duct hits the blower housing head on right at the face of the intake opening and creates a zone of turbulence and back pressure. The resulting turbulence then backs up into the connected lateral duct before redirecting itself around the obstruction. This zone of turbulence and back pressure, right at the mouth of the connected duct, potentially represents some degree of impediment to airflow and can also increases blower operation noise.

Now imagine the presence of a suitably sized return plenum installed at the intake end of the pictured air handler that acts as a buffer to help ameliorate this impediment to flow. The change in cross section and volume from the round duct to the larger plenum serves (to some degree) to reduce the incoming air’s velocity before impact with the blower housing but more importantly provides extra volume needed for the created turbulence to be suitably expended and muffled. The end result is reduced impediment to airflow when compared to the design pictured above and a likely reduction in the level of operational blower noise.

So the question remains…Is a return plenum required or even needed at the intake end of this or any other similar unit?

As usual we would consult the unit manufacturer’s installation instructions for the answer. If the manufacturer is concerned about possible airflow problems related to a direct duct connection to the their unit, then they should specify the requirement for a plenum in the installation documents. In my experience however most remain silent on the matter and tend to rely on the field installers to design an appropriate supply and return duct system. The field designers and installers however do not always follow the "best practices" in the industry (as we all know).

Nonetheless there is hope and we should keep in mind that air handling units are designed with the intent that they will operate within an expected range of "normal" parameters. The understanding that air return filters for example will become increasing loaded over time is built into the equipment design. This also means that most air handling equipment can tolerate a "less than perfect" duct design and will still perform satisfactorily (albeit not perfectly). We should not however look to use this as an excuse to push the limits of any inherent "fudge factor" that may or may not exist as part of the equipment’s original design.

Bottom line...As a stand-alone issue I personally do not see this as being a significant problem in the majority of cases. In a system that is "marginal" in several other respects however, this could be one of the many straws that ultimately breaks the camel’s back.

Return air plenum is not required. Bottom return air is alot better than pulling from one side of the furnace, the blower motor and CFM is more efficient. Duct work is sized by the tonage of the A/C system and number of heat runs in the house. You need 400 CFM per ton of cooling, if it is a 2 1/2 ton condenser than you need 1000 CFM of air. The supply duct would need to start with a 20x8 duct off the plenum and the return air needs to be 10x 20 standard return air drop or 14" round minimum to handle 1000 CFM of return and supply air. In the picture I also didn't see an electrical disconnect. Jerry is correct in the list he provided. If you have an attic horizontal like this one and the insulated round is comming off a plenum, you need to make sure there is enough round insulated supply duct to handle 1000 cfm of air. 1=6" round is 110 cfm, 1=7" round is 160 cfm, the register boot needs to be taken into account also. The blower needs to be sized for the correct speed tap for 1000 cfm. Most furnaces are sized with either a 1200 cfm blower of a 2000 cfm blower on high speed. Look at the manufacturers info for the blower / cfm speed tap requirements. The heat is on med-low and the fan switch is on low. As long as the heat speed is within the temperature rise on the name plate, you are good. This is too much information for such a simple issue. The return air is OK the way it is.

Originally Posted by Rick Hurst

How many see this type of install where there is no plenum box for the return duct to be secured to.

After sitting in a 16hr. HVAC education class this past weekend, I found out this is a real common problem. It forces the air to fast through the evaporator coils per the instructor.

Any comments?

Rick

Rick I think something might have been misunderstood ( No offense )
Manufactures do allow the duct to be connected to the FAU.
I would assume the issue was the velocity of air through the "filter"
You want the speed of the air through the filter to be about 1/2 of the speed through the duct for it to do a good job.

i.e You need a plenum box for the return filter/grill. This way you can have say a 20x25 return grill = 500 sq" and the duct connected the plenum is say 18" which = 255 sq inches. Which would reduce the air speed through the filter to about half of the duct.

If I found an attic installed FAU ( as pictured) that did not have a ceiling return air grill with a filter / plenum box that would be an issue. imagine having to go into the attic to replace the filter?