Disappointed in new A/C performance


Old 09-12-17, 05:54 PM
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Disappointed in new A/C performance

I own a 700 sq ft park model trailer in Mesa Arizona. My 30 year old package air conditioning unit finally gave out. I replaced it with a 1.5 ton new unit. The new unit runs constantly after the outside temperature reaches about 95. At 110 the inside temp climbs to about 78. Still comfortable but I worry about the unit.

I do not know the capacity of the old unit, but it was physically smaller and had smaller input output flanges. Old one has 10” round flanges the new one has 12” flanges. The return and feed ducts for my trailer are 10” flex insulated round ones. It is noticeable that the amount of air flow with the new unit is less. The old unit could hold inside temperature at 75 even at outside temps of 115.

I do not have a lot of windows 70 sq ft and I have changed them all to double pain. The walls are all r11, the ceiling is r19 but the main trailer 400 sq ft has blown in insulation that has compressed over time. The floor is r19 but the addition has 200 ft with no insulation. The ceilings are 7 ft. The original 3 floor vents are feed by a 10 in flex going into a sealed 6x14 floor joist bay. I added 3 more vents buy splitting off the 10 in tube with a 10in tube. All 6 registers are 4x10. The return air is on the floor 3 ft from a register with a 14x20 filter.

I thought the 1.5 ton unit would be plenty large but it is marginal. Can anyone suggest things that will help my new units performance.

Thanks Will

Last edited by PJmax; 09-12-17 at 06:46 PM. Reason: removed duplicate text
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Old 09-12-17, 06:36 PM
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10" flex is only good for around 300 CFM. You need 600 CFM for 1.5 tons.
You'll need to increase duct size to 14".
Is the new unit rated for mobile home use?
Old 09-13-17, 08:25 AM
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Yes it is built to replace the compact size of older mobil homes units. It is a "package unit", everything in one package.
The flanges on the unit are 12".

Revolv - 1.5 Ton Cooling - Packaged Air Conditioner - Manufactured Home - 14.0 SEER - Horizontal

Old 09-13-17, 09:50 AM
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The duct connections on the unit do not dictate the final duct size. The actual duct size needs to be calculated for length, static loss, CFM and material.
You should be moving 350-400 CFM per ton of capacity.
Use this chart-

Old 09-14-17, 10:53 PM
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I got a free piece of 12” flex duct that I installed for the return air.

It helped some but it is not 14” like you advised, but there is not enough clearance for 14” flex.

On the supply side I feed 3 4x10 registers with 10” flex, and split off at the ac unit to a 8” flex that feeds 2 more registers. With wheel, axles, sewer pipes, supports, and clearances I do not see anyway to easily fix the undersized duct problem on the supply side.

Is there any gain to improving the return side more without fixing the supply side? Does this just create an imbalance? On the supply side I can replace all but 10 ft with rigid 12” round ducting, and use a rigid 90 elbow so all the flex will be almost in a straight run. I did stretch the flex which should help.

The blower in the ac unit has 5 power settings. For ac it is set on 2 and for heat it is set at 4. I can definitely feel the difference in airflow between the settings. In Arizona there is very little humidity, so would it hurt to crank the blower up?

The owner's manual has a chart that tell the cfm with the different setting for external static pressure drop in inches water column. 0.1 through 0.8. I’m not sure what this means, but I’m hoping I can turn the blower up to off set the undersized ducts.

At 110* the trailer was 78* which is tolerable so perhaps i should just call it good enough.

Thank for the responses and help.

Old 09-19-17, 03:51 PM
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For a properly sized unit to cool well, the ducts have to be sized correctly. Low flow causes capacity loss and oversizing masks that.

Going from 10 seer or less to 14, the unit will be physically larger.

Check the temperature difference between supply and return at the package unit.

Depending on humidity level, if it's much above 18 to 22 you have an airflow problem.
Old 09-21-17, 07:59 AM
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WwI have ordered the parts to get both the supply and return up to 600+ cfm using the chart that Roughneck supplied.

On the return side i’m coming off the unit with a 12x12x8. The 8 will go to a new return in the bedroom. The 12 will go to the existing return. 480+ 160 cfm should do it.

On the supply side i’ll come off with a 12x12x12 one 12 reduces to a 10 then feeds a 14x5 duct in a floor joist bay. This duct feeds 3 registers 2 to one side and 1 to the other side of the entrance point. The other 12 reduces to a 10 after 12’ then splits into two 8” ducts that feed one register each. I hope this will give me more than the 600 cfm requirement.

All the ductwork is flex.

I hope that 5 4x10 registers will give enough flow.

The original filter for the 12” flex return is 14x20 with 1.5 sqft of grill. I do have room to increase both of these if necessary.

I'm in arizona so humidity is not a problem.

If i have overlooked anything please let me know.

I will not do this work untill it cools down here some. It is still around 100 and would not want the ac to be down in the afternoon.

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