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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8429
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Contractor recently replaced 3ton gas pack, house is on crawl

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Contractor recently replaced 3ton gas pack, house is on crawl space. He used round duct adapters in the 13"x16" duct openings on the unit --- then connected my existing rectangular ductwork to the side of the unit. The round duct adapters are surely restricting the airflow - right? I also have severe condensation problems in the crawl space ductwork (never had any issues before). Could this be related to the reduced airflow?
Welcome to Just answer!

Judging by your mention of condensation on the duct work, he used too small duct work to attach to the 13 x 16" rectangular openings.

The square to round transition restricts flow slightly, it is generally not a problem. However if the round duct attached is not the same square inch opening, then there is significant restriction... that is most likely the case here.

Tell me what the round duct *inside diameter is. and how long those duct runs are.

Use just one thermometer, read the air temperature at the return air grill, and then at several of the cold air supply grills. I need those numbers.

We can go from there until you the full and complete answer you need.

Customer: replied 3 years ago.

Thanks for the reply.

I've attached a couple photos.

CabinetDuct4a.jpg is looking towards the rear of the unit towards the crawlspace. Instead of removing the standard 16"x13" cover --- he cut a hole and installed a round flange. CabinetDucta.jpg is a close up looking through the round flange and shows my existing rectangular ductwork. In other words, he installed a round flange - on the supply and return sides - but did not connect anything to the flanges??? Instead of utilizing the entire 16x13 areas he restricted them to smaller round openings???

Hello again

You need about 150 square inches of return air duct diameter, if the duct is over 20 feet long per ton of air thats about 2.6 square feet, or 375 square inches of duct cross section. Thats for the *return air duct. Thats running an air flow velocity in the return air ducts of 600 feet per minute..(that all varies according to the rest of the duct sizes and length of runs and the type of blower in the air handler... and the local climate.

If you have 2 ea 12 inch return air ducts you have about 65% of the return duct size you need to have a properly running system.

That can be restricted *AT the furnace some, but not for the longer run of duct... the contractors mistake was assuming the opening in the furnace could be used to size the return air ducts. The longer the duct run the more resistant it is to air flow.

There are legally mandated manuals required for those calculations required to meet Title 24 calculations.


The supply air ducts can be about 25% smaller because if fan performance characteristics.. higher velocity is permitted.

If you can tell me the return air temperature at the air handler (return air grill) and the temperature of the cold air at several of the cold air outlets.. I can be a bit more specific.

Good design normally allows for air no colder than 55F in the supply air ducts, water begins condensing well below that, in the 45F range.. The differential between supply and return air should be between 18 and 20 degrees with the house at 72F inside.

My guess is that you are running a differential close to 30F... that is proof that there is not enough air flow.

If you choose to rate my assistance so far *positively, I can hold the question open for any follow up you might need.


Phil and other HVAC Specialists are ready to help you
Customer: replied 3 years ago.


Thanks for the info --- i'm gathering the data you suggested and will continue tomorrow.

Thats a good plan, I will be looking forward to the numbers.

Customer: replied 3 years ago.


With thermostat set at 74:

Temp at hallway return filter: 74

Temp at floor register: 59


The blower also makes a pulsating sound at a rate of once per second. Sounds like it is struggling?


There are jumpers on the control board and the contractor has previously set the cool blower to the max speed. I have 2ea 16x20 filters for a return filter area of 640sq in -- but there is very little suction when I change them.


I actually measured run times last nite:

Cooling for 22 min, off for 5 min.

Cooling for 16 min, off for 7 min.

Cooling for 16 min, off for 8 min.

Cooling for 17 min, off for 7 min.



Any ideas?

Hello again, thanks for the data.

I need you to repeat the first test with the room temperature and cold air out let temperatures... this time with the thermostat set as low as it will go.

I also need the brand and model number of the outside unit, and the brand and model number of the inside unit (so I can check to see if you have a modulating type of blower motor or not.)

And... if you can tell me what the air temperature outside was when you did those run time readings I will be able to tell if the unit was cycling normally but with it relatively cool outside or if it was cycling normally with it 105 degrees F outside.

And... I need to know how long the 12 inch return air ducts are... and I need to confirm that are 2 of them.

If I only have partial data, the rest of the data cannot be evaluated properly
. All these things change radically throughout the day, depending on what the outside air temperature is, and many other factors such as the return air duct length, and if the thermostat is set to force the unit to run all the time or not.


so far though it looks like the filter sizing is plenty large, which would account for the lack of heavy suction against them.. along with possibly insufficient return air duct size.

Customer: replied 3 years ago.


The outside temp was approx 82 degrees.

Remember we're talking about a package unit --- there is no inside unit.

The basic model # of the unit is R6GF 3ton , 2 stage cooling, 2 stage heat.

The round duct in the photo have NOTHING attached to them.

I know this sounds crazy but it is like you take a package unit, install round flanges on the supply and return as if you were installing in a mobile home, but instead you connect the existing rectangular ducts to the side of the unit.

The pictures might help me explain it.

At any rate - I highly suspect these round flanges are restricting the airflow.

Hello again,

It sounds like one of the mans crew set the unit up to connect round ducts but did not punch out the side panels where he fit the round flanges.. and that when the contractor arrived he saw the error in that and used the alternate supply and return rectangular knock outs.

Make sure that there is no air flow, in or out, through the round flanges.

The temperature differentials you gave indicate the unit is operating normally.

Stay in touch as necessary.

Customer: replied 3 years ago.


Thanks for the input. I've filed an official complaint with the state hvac board. They will perform an inspection. If problems are found then I'll get estimates for repair, and the original contractor is supposed to pay for them. I hope this will happen in the next few days and will let you know what happens.

Also - there has always been a pulsating sound when cooling. Of a whim, I replaced the thermostat (RTH8320) with the previous honeywell single stage thermostat. The pulsating sound stopped. I then reinstalled the RTH8320 and the pulsating sound started again. Is it possible that the thermostat is pulseing the unit? I'm thinking of putting an osciliscope on the cool and fan terminals to verify. Your thoughts?

Hello again, thats a good plan.

Regarding the fan and thermostat pulsation issue. It is difficult to fathom what on earth is going on there. Put the pulsation causing thermostat back on, and include that in the inspection.

I will be very interested in how all this turns out.


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