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I currently have an over-sized furnace for my home square footage. I'm happy with the duct's performance, they are quiet.

The furnace is 27 yo and I'd like to replace it with a heat pump and air handler. To keep the project cost down, I would prefer to keep my existing ducts.

I would step down to a 2.5 ton system from the 4 ton furnace - what would I expect to happen if I keep the ducts?

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Definitely maybe... Sorry but it depends on whether or not the existing ducting will work with the new unit. So you should take that into consideration when selecting a new air handler. Many units are similar but not identical and so you may have to have an "adapter" fabricated to mate up the new unit with the old ducting.

Why do you think the old unit is oversized? It would be unusual to oversize like that because a larger unit is more expensive and a waste of money if it's larger than the needed size. You should have an HVAC professional evaluate your home and make a size recommendation.

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  • It's actually fairly common to have oversized HVAC gear -- most resi HVAC is "rule of thumb" sized, a practice that doesn't take into account today's envelope performance improvements – ThreePhaseEel May 4 at 11:47
  • I realize that but 2.5 -> 4 is 60% oversize. That seems extreme to me. – jwh20 May 4 at 11:53
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    I agree. But keep in mind that the tools used to size HVAC systems today are a lot better than back-in-the-day, when many systems were sized by anecdotal advice, rules-of-thumb, then "add 25% to the calculations for a CYA". – SteveSh May 4 at 11:55
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    @SteveSh -- yeah, the old rule of thumb was 500sf/cooling ton, but modern envelopes need 1/2 to 1/3rd of the capacity that rule predicts, and sometimes less. It's why a proper Manual J calculation is an absolute necessity for anything that's built better than minimum energy code, envelope-wise.... – ThreePhaseEel May 4 at 23:56
  • Sorry I forgot to mention that this is a single story 1,350 sqft home. Thanks a lot for the info; our HVAC contractor will be installing new ducts considerably smaller than the existing ones. – Leo Romanovsky May 12 at 21:21

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