HVAC opening for return air flow sanity check

fenstonMarch 10, 2012

Have an inspection report that claims the return air flow for the HVAC system may be too small and the contractor (ASI Hastings) should have opened it before installing. I checked out ASI and they appear to be a very reputable company from what I can tell.

This leads me to the question. Is there a spec on these units (this one is a split system) for minimum cutout and even though this looks bad in the inspectors report it may be within spec? I just can't believe this company would do something this wrong so just wanted a sanity check from the web! The unit is pretty new (around 4 years old) Thanks.

Here is a picture of the opening:


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You need to provide more information before you can get any feedback.

How large is this furnace? (Model number would help).
Is there an AC condenser or heat pump, if so how many tons? (Model number would help).
What are the demensions of opening in the picture?
What size filter is currently being used?

Is this inpection report for a purchase of a house? If you have concerns you may want to hire an HVAC contractor to do a thorough inspection. The typical home inspectors are not qualified to do this type of work, and often get it wrong.

    Bookmark   March 10, 2012 at 8:59AM
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Here is what I know right now. This is not a heat pump system and this is from an inspection report for a purchase of a house. I have already agreed to buy this place "as-is" so this is not a huge deal for me. I was just trying to plan my time after the close and whether I would have to try to enlarge that opening for better performance.

You write:

"The typical home inspectors are not qualified to do this type of work, and often get it wrong."

I really suspect this is the case. I just can't see a qualified company like this screwing something up this bad. I suspect the opening size needed is directly proportional to the size of the duct leaving the unit and pumping air into the house so as long as this hole has a larger square footage than the square footage of the duct I suspect I will be fine. Also suspect that these replacement units are built to accommodate these situations where the opening in the floor is smaller from the old unit and allows the installer not to have to muck with the installation site (gives more options). This is just a hunch (I'm an EE!, not an ME). Anyhow, thanks for that comment.

Have an appt today to get some carpet and paint estimates so will try to get full answers to the questions above. If things still look good after the groups comments I will just have ASI do all follow on work after the close of escrow.

    Bookmark   March 10, 2012 at 10:18AM
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Alright, just did my final self inspection before the close. So here are all the details I could find:

1) Inside York Affinity 8T Series model PT8C20L120UH11C ... above that was another model number ... PT9660-C210AP 410

2) Outside York Unit H1ZD060S06A ... says 9lb 1oz R410a

Filter size is 20x25x1

Measured cutout below inside unit is 19 3/8" x 12 1/4"

Do I need to enlarge that cutout for better efficiency? (would suck since I would have to remove a 2x4 brace in the way) ...

Any thoughts appreciated. I am hopeful I can just leave it alone. I can't see how this much stuff could be installed with something so basic wrong but never know.


    Bookmark   March 10, 2012 at 8:07PM
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From the model numbers I think you have a 120K BTU input furnace and 5 ton AC. It this is true then your return opening and a filter are half the area then need to be. Below is a link to a York furnace document. It states for your furance you need two 16 X 25 filters with an opening of at least 463 square inches.

The furnace blower is not getting a sufficient amount of air. Have you heard the furnace and AC in operation? The air rushing through the small opening must be very noisy. The blower may fail prematurely due to the add stress of the high static air pressure. You should fix this if you want to equipment to operate properly.

Here is a link that might be useful: York Furnace data

    Bookmark   March 10, 2012 at 9:03PM
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Very Weird. This can't be right. The 20x25 filter fits perfectly (and there is a label saying 20x25 on the unit). No way this thing could accept two 16 x 25 filters.

I have not heard this unit in operation.

    Bookmark   March 10, 2012 at 10:07PM
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Found the following link and trying to reason my way thru this. So for a hole that is 12.25 x 19.375 at 2000CFM you get air velocity of 20.2 ft/s ... I think if I were to remove that 2x4 (best I can do to increase opening) then I could get a hole that is around 15.25 x 19.375 ... with this same calc the air velocity drops to 16ft/s


Can't find a spec on this air velocity factor and whether the maximum air velocity at the maximum CFM is in an acceptable range. Anyone, bueller?

    Bookmark   March 10, 2012 at 10:53PM
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Found this in the users manual:

1. Air velocity through throwaway type filters may not exceed 300 feet per minute. All velocities over this require the use of high velocity filters.

20.2ft/s is 1213ft/min which is greater than 300 feet per minute so a high velocity filter must be used.

Now see the maximum velocity for user areas at the link below. I think this whole issue resolves to noise levels with the air moving faster. That cut out hole in the bottom of the unit is pulling air from two large vent holes in the living room. The maximum velocity at 1213ft/min is much less than the maximum velocity for user areas shown below at 2000ft/min for the air flow rates in CFM this unit would be pulling.

I am now thinking the ONLY advantage to getting that opening a bit larger is the air velocity goes down a little bit (so less noise) but unless I am missing something I highly suspect this is the only issue. Any other thoughts here? I think ASI didn't do anything wrong based on this analysis but I am a EE by trade, not an ME so possibly still missing something here.


User Areas

Offices, receptions, lounges and similar

Air Flow Rate Maximum Velocity
(m3/h) (CFM) (m/s) (ft/min)

    Bookmark   March 10, 2012 at 11:17PM
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mike_home writes:

"It states for your furance you need two 16 X 25 filters with an opening of at least 463 square inches."

It doesn't really say that but the 463 square inches factor is correct.

Finally figured out what I was missing. This unit gives multiple options for return air flow. You have the bottom and both sides. Both sides can accommodate the 16 x 25 filters. The manual states:

"2. Air flows above 1800 CFM require either return from two sides or one side plus bottom."

OK, this is coming together. The unit is designed for retrofit. It gives you multiple options to come up with the 463 square inches needed. Unfortunately ASI didn't utilize them and I bet the homeowners said they would deal with the extra return later (based on what I see going on in this house so I can't slam ASI too much here). ASI could have walked away from the install though and said we either do this right or walk away but understand how this happened.

So basically they have a solar heater, a big 50 gallon water heater and this HVAC unit all stuffed on a platform. The solar heater needs to go, the water heater moved far left which then leaves a big place for a hole in the middle. Then I can vent the left side of the unit using a right angle connection thru the platform and right underneath the platform is the vent to the living room.

Now this is the great part:

Bottom opening is 12.25x19.375 and just one side would get me 14x23.5 ... total is ~566 square inches which is a comfortable margin above 463. And don't need to mess with that 2x4 in the way or the structure of the platform! Sweet!

My previous comments about the air velocity I think are true but added stress on the blower sounds like the result of this since it is clearly out of spec based on the 463 factor. I wonder if they can hard wire that unit for lower CFM ... run it slower and then this is not an issue perhaps. But the house is pretty big 2468 so running the unit at half rate seems pretty lame.

Think I will just make the space and then call ASI back to fix this properly. But have to get space to the left of the unit on the floor before even bothering to call!

At least I have a game plan now! Thanks mike_home!

    Bookmark   March 11, 2012 at 8:28AM
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Furnaces with 5 tons of cooling will typically have returns attached on both sides of the unit. This is why two filters are recommended. Most houses which have 5 tons of cooling have duct capacity issues.

As a general rule, you need 350-400CFM of air flow per ton of cooling. If you slow down the air the coil will freeze up. You need about the same amount of air flow for a 120K furnace. If you slow down the air flow the furnace will overheat and trip the limit switch.

    Bookmark   March 11, 2012 at 9:46AM
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Thanks again Mike,

Lucky for me this unit can have the returns on both sides or one side and bottom. One side of the unit in my case is directly against a wall. Super lucky I can make room on the left side to get this done properly. I will beat up ASI when the time comes. Seems fair.

    Bookmark   March 11, 2012 at 10:16AM
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Don't forget the return area of the vent cover itself can suffocate the flow by 50%. I had nice size return vents on the walls but tiny holes cut through the bottom plate and subfloor so everything looked good but the actual flow was horrible but luckily the system leaked everywhere so nothing overheated. If I had sealed up the leaks without catching and correcting this I would have burned up the blower motor.

    Bookmark   March 12, 2012 at 12:24AM
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That is an excellent point. I have two large vent covers in the living room that air would be pulled from. I think if I estimate the number of square inches of "hole" space in the vent covers to be > 463 square inches then I should be ok. But I think those vent covers are original stock from the home and probably sized for the original unit which I believe was a furnace only.

I have a whole house fan that I bought from Airscape fans in my current home. The vent cover on it is pretty nice. Big open squares for lots of air to pass thru. Suspect I will need to replace the vents with something similar.

Again, you would think the HVAC vendor would correct all these issues during the installation. I will add this to my ever growing todo list for the new place. Thanks so much!

    Bookmark   March 12, 2012 at 1:09AM
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Might just try to "retrofit" the existing vent covers with a rotary tool to open them up a bit if a problem although lots of grinding and the end result would probably not look so good. Will see ...

    Bookmark   March 12, 2012 at 1:25AM
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Nothing like someone who can answer all of their own questions. Anyways I would think your return air should be 1200 sq. inches.

    Bookmark   March 12, 2012 at 9:05AM
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heatseeker - hahaha

Yeah - I'm just iteratively processing all the comments and getting there. OK - so return air at 1200 sq inches ... does this assume a typical vent configuration like this one at HD:


So 473 square inches per spec on the unit. Assume 2x that assuming 50% vent blocking. How are you getting 1200?

There are two large vents pulling air from the living room. Still need to measure those after the close but hopefully don't need to do major drywall repairs to enlarge and can do something with existing configuration.



    Bookmark   March 12, 2012 at 10:22AM
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What do you mean two large vents pulling from living room? Are these in addition to the other return. Why don't you add all of your return grills and add them together I bet they come up to 1200 square inches.

    Bookmark   March 12, 2012 at 12:05PM
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There are two vents in the living room that currently feed the bottom of the unit in the garage. If these are constricted then can't get the air moving thru back to the unit. But I think I understand what you are saying. You must be referring to the output vents where the air is coming out. I am referring to the input vent to the unit (which is currently way too small based on spec).

And there are two path ways into the unit. One thru the grill/vent and the other into the actual unit thru the floor. Both of these need to be opened up more for the size of my unit.

Airscape fans sent me this link for the cube grids, custom order. Sweet! A couple of these in place and I think the airflow should be 80% plus even with dust in the grill over time.


    Bookmark   March 12, 2012 at 12:47PM
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If you look at most grilles they are 6x14 or 6x30 or something close to that which is about twice the size of a 2x4 wall cavity or two combined to compensate for the grill itself.

    Bookmark   March 13, 2012 at 1:20AM
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I did a rough measurement of the two large ducts leaving the unit. One is 16" or about 201 sq in. One is 8" or about 50 sq in.

So how the heck can this possibly work. Even if I were to open up the bottom+side of the unit more to the 463 sq inches, the ducts leaving the unit only measure 251 sq inches. In vs out do not match. Is my unit simply over sized for the existing infrastructure? I'm in the process now of beating up ASI Hastings and York. Any thoughts appreciated.

    Bookmark   April 10, 2012 at 12:42PM
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According to ASI & York the unit is set to 1750CFM so only needs to be opened from the bottom. Said they only set them to 350CFM/ton to control humidity. They are going to fully enlarge the bottom cutout and fix my condensate line run which was previously run in front of the water heater.

I saw that these units are configurable CFM but not quite sure how that works. Based on the spec for the unit at 1750CFM only having the bottom cut out should allow the blower to operate w/o increased static pressure if I understand this correctly?

    Bookmark   April 14, 2012 at 9:03AM
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