Duct Work Repair or Replacement

Does fiberboard and flexible duct work have a limited lifespan?
How do you decide whether to replace it or repair it?

When my parents had a new AC system installed last year, they kept the old duct work which is 30-35 years old. This may not be related, but the system seems to be cooling very slowly (three hours to drop from 80 degrees to 77 degrees). The air temperature at a ceiling vent was about 70 degrees in a room that was about 77 degrees.

The duct work in the attic has a few places where the rectangular fiberboard duct with aluminum foil on the exterior had lost its foil. I think the foil may have deteriorated in those places. I can feel air passing through the fiberboard where the foil is missing. In other places, the foil appears to have separated from the fiberboard so that the foil puffs out, but it doesn’t appear to leak.

There are a few joints that have come partially untaped and wouldn’t be hard to re-tape. At least one slip in joint (where the rectangular duct meets the air handler and on the duct side of an anti-vibration flex joint) is leaking notably (possibly because the fiberboard duct expands outward when the system is pressurized).

The smaller round ducts which have a foil covering and feed the ceiling vents seem to be mostly intact, but I haven’t inspected them closely or looked to see what’s going on inside the foil. I think these are flexible with fiberglass insulation under the foil, not steel-walled, but I’ll have to take another look to be sure.

What do you think I should do repair or replace?

To answer your repair or replace question?

Metal is always better and if have you flex ducts that are 35 years old they need to be replaced!

To answer your question about the temperature difference?

Are you equating the length of time it takes to drop the temperature to bad ductwork?
It could be that there are some ductwork problems but the length of time it takes to drop temperature is not necessarily a good gauge of problems. It would be expected that if your a/c was properly sized the pull down time could be normal. For maximum efficiency, especially in an area that has high humidity longer run time is a good thing.

When we install residential a/c systems we are careful to explain that when the a/c is used that it is not meant to have the unit turned on when the owners get home from work to a scorching hot house. Could this be the case with your parent’s a/c? If you expect hot weather turn on the cooling in the morning and set it for whatever their comfort level is. It will come on when the cooling load is lower and not be struggling with a large amount of heat.

The temperature split between your return air register and your intake register are only at 7 degrees which is not enough for sufficient cooling. Generally, you want between 15-20 degrees temperature differential between the return air and the supply air.

For more information contact McDonald Plumbing, Heating and Air at (916) 233-4194.

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