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Question DetailsAsked on 6/26/2015

How much should it cost to add a new return, replace ductwork and rebuild plenum in a 1600 sq ft home in FL?

Size of outside unit is larger than original and doesn't "play well" with the rest of system. Already had to replace coils (rusted out) and return air is insufficient. Need to rebuild plenum (larger) for return, build plenum for intake, extend ductwork to back of house for additional return and run new ductwork throughout. There are 12 vents.

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2 Answers


New duct work throughout, $3000.00 ,but is that needed? Rust is not the cause of a coil leak. Add a return, $250 to $600.00


Answered 5 years ago by BayAreaAC


Sounds like the BayAreaAC numbers are for the HVAC contractor only, and probably assumed a crawlspace run because that sounds low to me for a slab-on-grade house reducting. And undoubtedly is NOT including drywall, carpenter, painter to do the repairs after the work.

As he said, the rusted out evaporator has nothing to do with the duct sizing.

You did not say if the contractor did the Manual D calculations to determine what duct sizing/capacity changes were required. Also, whether he looked at whether a duct booster fan and/or parallel return plenum would work.

For cases like this, especially for a house that likely does not have a basement ceiling to run the ducting freely in, commonly it is more economical and less destructive to add an in-duct booster fan or larger primary air mover (if furnace can handle it). Or put in parallel ducts - either to split the existing system into two branches so the existing duct adequately handles part of the system and the new ducts the rest (plus the extension), or sometimes in parallel splitting the airflow to increase total air carrying capacity without tearing out the existing and putting in larger ducts which might not fit worth beans.

Another solution done at times like this is to not use the furnace fan for the A/C at all - rather, put in a separate air handler unit for the A/C with a more powerful fan that can move the air as needed in the existing system (after adding the new extension), and feeding that airflow into the existing HVAC duct with few changes other than a dampered wye that shuts off one source or the other depending on which is being used. Increases your electricity cost a bit for the larger fan but can be much cheaper installation cost, and typically almost or completely eliminate the disruption and ripping out of drywall that upsizing the whole house ducting requires. This assumes of course that your existing ducting is not rusted out, which would require replacement of it anyway.

This solution can also address the rusted out evaporatorand inadequate return air issues - which likely occurred because the airflow (caused by the furnace fan) past it was not adequate for the A/C capacity, so it resulted in too cold an evaporator, so excessive condensation and maybe even icing, which a larger dedicated air handler could solve. Sometimes a larger furnace fan can solve this too - especially if two-speed - one for furnace and A/C post-shutdown coil dehumidification running, higher power for A/C running mode. To figure out what is needed takes a competent A/C system configuration designer, not just a repair tech - so either an HVAC detailer at an architect or engineer's office (usually overkill for a normal size house system) or an HVAC company with an actual system design person on staff.

Answered 5 years ago by LCD

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