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Have two new 5 ton Infinity air conditioning units with gas furnace in residential installation. Repeatedly getting "change filter" warning. Indicated CFM is 1400 on high cool. This seems low to me. Shouldnt the CFM be closer to 400 to 450 CFM per ton? Could the problem be caused by undersizing the ducts? What CFM is recommended for the 5 ton unit on high? Hank Haeseker, PE

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Yes, you are correct... the air flow should be at least 400 CFM/Ton for good efficiency. 350/ton if you have a dehumidification mode....
But under 300 is totally unacceptable...
The problem probably is the ductwork.....or check that a number of your vents are not restricted or shut off by dampers...sometimes when balancing a system many of the dampers will get shut down to force air flow to the fartherest areas and this will cause low air flows...
Also take a good look at the return duct...this can cause many problems.....especially if it is undersized.
The problem I am having with all of this is that with the units you have the blower motor is a ECM motor that will make up for many mistakes by move air at higher static pressures and also overcoming many of the problems with undersized ductwork....
So I am also wondering if the motor is not programmed correctly...check that out also...

At any rate the installing company has a responsibility to make sure the installation is working correctly...A call to Carrier directly will get you action....if the original company will not correct the situation then get the name of another company from Carrier and have them send them out the access the system and make a recommendation on fixing it...
Carrier does not want a bad rap on theri equipment so they will often hire another company to fix the job if it is botched up...

Rattle chains till you get you problem corrected!!!


Posted on Oct 15, 2010


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