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Sten
Re: Heating/Cooling Return Air Question

You can balance your air flow with the dampers but restricting to much supply or return will cause your compressor to work harder than it's should. A/C units are sized by tonnage and that is converted to the CFMs that your house requires by square footage. If you close off to many supplies it's not moving the air that it was designed to move. Same with returns.

Brookworld
Re: Heating/Cooling Return Air Question

Sten: thank you. . . I want to know more.

I do have a hard time understanding how all of this may be stressing the compressor.

Room A/C's have 2-3 fan speeds, different CFM's.
In my central A/C, there is one fan speed, and if I close off air registers, my expectation is that the CFM provided by the fan doesn't change, but that the velocity will increase at the registers that remain open (i.e, more air output at the open registers which is what I want).
Practicably, the registers can't be fully closed (I have 3 registers that have dampers, but some air leaks through). However, my hope is that more air is then pushed to the 2nd floor registers because the 1st floor registers are now 90% closed. I will say that the velocity (and flow) is only marginally higher at the registers on the 2nd floor. There may be other factors, like leaks in the duct work or that the ducts are not insulated in the attic (I haven't checked as I just moved in 6 weeks ago).
The returns are not changed as they don't have dampers.
I'm using a relatively high MERV filter (with ridges & higher density material to capture inside particles) which is probably reducing air flow.
What I'm describing seems like a typical situation in 2 story houses with a finished basement and a 1-zone system -- to get the 1st floor comfortable, the basement is very cold, and the 2nd floor warmer, depending on how many registers, and unless registers are closed, the temperature differences are even more severe. I lived in Townhouse where a southern BR had only 1 register, and it was hot in the summer -- no excuse for this bad HVAC design in ALL tract housing.

fvultee
Re: Heating/Cooling Return Air Question

I agree with your thinking and my thinking is, so what if it shortens the lifespan of the unit. I rarely live anywhere for more than 2 or 3 years, so if it dies at year 10 instead of year 15 i'm not that concerned.

Today is the big day, i'm going to bust a hole in the wall of my guestroom and install another return duct right into the return air plenum (the main plenum, not a branch of the existing smaller plenum return that goes to my second floor). My hope is that by giving the unit more air to use, and by having some airflow return from the 1st floor, that the temperature differences between the first and second floor will be reduced.

Still cold as hell downstairs, hot upstairs. Best case would be a multizone system, but i'm doing this on the cheap.

Anyone know an easy way for a novice to cut a rectangular hole in sheet metal? I've seen those circle cutters used with drills/dremels, however I'm guessing the return air plenum I buy is going to be rectangular...

Sten
Re: Heating/Cooling Return Air Question

Your A/C system works by pressure, you have the high side and the low side, or liquid and vapor. The liquid travels to the Evap Coil but just before it gets there it goes through a metering device, be it a TXV , piston or cap tube system. As soon as it goes through the metering device it changes state and becomes a vapor, goes through the Evap coil and returns to the Compressor and repeats the process (short version). The refrigerant becomes very cold after it changes to a vapor and the blower motor has to move air through the coil in order to maintain the proper pressure. If your not moving enough air the coil will freeze be it a lot or a little, remember that the condenser doesn't know if the coil is freezing and is going to keep on moving or try to move the refrigerant. The harder it is to move the higher your head pressure or liquid line pressure will be and that takes a toll on your compressor. Believe it or not it's not rocket science but there is a science involved with A/C. This is a very short version and I really don't want to take up the space and get into saturated vapor and subcooled liquid. If your really interested just google those two and something should come up. I hope this helps you at least a little.

There are Low and High Pressure switches that will shut you A/C off if the pressures go to low or to high, sort of a safety but not all Condensers have them.

fvultee
Re: Heating/Cooling Return Air Question
fvultee wrote:

I agree with your thinking and my thinking is, so what if it shortens the lifespan of the unit. I rarely live anywhere for more than 2 or 3 years, so if it dies at year 10 instead of year 15 i'm not that concerned.

Today is the big day, i'm going to bust a hole in the wall of my guestroom and install another return duct right into the return air plenum (the main plenum, not a branch of the existing smaller plenum return that goes to my second floor). My hope is that by giving the unit more air to use, and by having some airflow return from the 1st floor, that the temperature differences between the first and second floor will be reduced.

Still cold as hell downstairs, hot upstairs. Best case would be a multizone system, but i'm doing this on the cheap.

Anyone know an easy way for a novice to cut a rectangular hole in sheet metal? I've seen those circle cutters used with drills/dremels, however I'm guessing the return air plenum I buy is going to be rectangular...

Well not ideal, but my do it yourself is done. I couldn't fit a rectangular return line back to the main plenum, so i opted for an 8" round line with elbow. Cut a hole in the main return duct (not easy since I don't have the right tools), installed, insulated, taped, and voila. Where the temp differential between the first and second floor used to be 8 or 10 degrees, it's now down to about 2 degrees difference when the system is running. I know an 8" line isn't ideal, but I'm quite happy with the results nonetheless. Thanks for all your help everyone, much appreciated!

Sten
Re: Heating/Cooling Return Air Question
fvultee wrote:

Well not ideal, but my do it yourself is done. I couldn't fit a rectangular return line back to the main plenum, so i opted for an 8" round line with elbow. Cut a hole in the main return duct (not easy since I don't have the right tools), installed, insulated, taped, and voila. Where the temp differential between the first and second floor used to be 8 or 10 degrees, it's now down to about 2 degrees difference when the system is running. I know an 8" line isn't ideal, but I'm quite happy with the results nonetheless. Thanks for all your help everyone, much appreciated!

They have oval which is made for tight spaces but still I'm glad to see that it's working better and it's amazing what the proper air flow will do.

fvultee
Re: Heating/Cooling Return Air Question
Sten wrote:

They have oval which is made for tight spaces but still I'm glad to see that it's working better and it's amazing what the proper air flow will do.

They actually used oval ducting in a space under the house, but my choices were limited to what Home Depot had on the shelf, and so I went with an 8" pipe directly into the main return box. Anyone know how many CFMs move thru an 8" pipe? :)

Sten
Re: Heating/Cooling Return Air Question

That 8" pipe should move between 150-200 CFMs

This should help

ROUND DUCT SIZE ESTIMATE
Flexible Duct Round Metal Pipe

Duct Size Design Airflow
5" 50
6" 75
7" 110
8" 160
9" 225
10" 300
12" 480
14" 700
16" 1000
18" 1300
20" 1700

Duct Size Design Airflow
5" 50
6" 85
7" 125
8" 180
9" 240
10" 325
12" 525
14" 750
16" 1200
18" 1500
20" 2000

Flex duct = .05" on most metal duct calculator Round metal pipe = .06" on most metal duct calculators

Design
CFM 4" CFM 6" CFM 8" CFM 10" CFM 12"
60 6x4 60 4x6 90 4x8 120 4x10 150 4x12
90 8x4 110 6x6 160 6x8 215 6x10 270 6x12
120 10x4 160 8x6 230 8x8 310 8x10 400 8x12
150 12x4 215 10x6 310 10x8 430 10x10 550 10x12
180 14x4 270 12x6 400 12x8 550 12x10 680 12x12
210 16x4 320 14x6 490 14x8 670 14x10 800 14x12
240 18x4 375 16x6 580 16x8 800 16x10 950 16x12
270 20x4 430 18x6 670 18x8 930 18x10 1100 18x12
300 22x4 490 20x6 750 20x8 1060 20x10 1250 20x12
330 24x4 540 22x6 840 22x8 1200 22x10 1400 22x12
600 24x6 930 24x8 1320 24x10 1600 24x12
650 26x6 1020 26x8 1430 26x10 1750 26x12
710 28x6 1100 28x8 1550 28x10 1950 28x12
775 30x6 1200 30x8 1670 30x10 2150 30x12
40 21/2 x10 1300 32x8 1800 32x10 2300 32x12
70 21/2 x14 1400 34x8 1930 34x10 2450 34x12
150 21/2 x30 1500 36x8 2060 36x10 2600 36x12
100 31/2 x14 2200 38x10 2750 38x12
220 31/2 x30 2350 40x10 2900 40x12
3050 42x12

NW HVAC Tech
Re: Heating/Cooling Return Air Question
Sten wrote:

If your not moving enough air the coil will freeze be it a lot or a little, remember that the condenser doesn't know if the coil is freezing and is going to keep on moving or try to move the refrigerant. The harder it is to move the higher your head pressure or liquid line pressure will be and that takes a toll on your compressor.

This information is not accurate:
Inadequate airflow across the indoor coil will cause LOW PRESSURE conditions in the system resulting in indoor coil frosting/icing which perpetuates the problem by obstructing the airflow even more. Without the proper airflow across the indoor coil and the insulating effect of the frost the refrigerant will not absorb the heat it needs to boil int a complete vapor. when liquid returns to a compressor that was designed to compress vapor if it trys to pump liquid it can be mechanically damaged. The primary problem with what is known as floodback is that the liquid refrigerant will mix with the oil in the compressor and cause bearing washout and failure. The scenario is similar to pouring gas int the crankcase of you car engine.

Sten
Re: Heating/Cooling Return Air Question

My bad, if air is not moving through the condenser coil you get high head pressure.

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