Replace ducts or not?

[Kira Smith]

One of our two AC units died. Given the age of the furnaces and remaining AC unit (all over 20 years old) we decided to look at replacing everything at once. We've never had issues with not being cool or warm enough and haven't had hot or cool spots at any time. The current AC units are 3.5 tons. We'll replace with 4 ton 16 SEER Daikin units (I don't think Daikin makes units in half tons).

Two individual who gave us estimates recommended we keep the existing metal ducts. One guy even said not to let anyone talk us into replacing the ducts because they were engineered when the house was built in 1974. The third guy comes in and says the ducts are actually undersized and need to be replaced. Of course he said it was more important to get the correct size as opposed to having metal ducts.

Are there other questions I should ask to determine which view is probably correct? TIA!

 

[pseudorealityx]

The higher SEER units are typically variable speed units, and rarely come in half ton increments. That's across most manufacturers.

It's hard to tell online if the ducts are under/oversized. You would need to ask them for a manual J calculation, and then compare the airflow required per room to the duct work serving that particular room.

For the guy who wants to replace everything, I would certainly ask for manual J to see what he's trying to do.

Another thing... "typically" residential ductwork is sized pretty small, typically because there simply isn't a ton of room to locate it, and because it saves a few bucks. Combine that with the idea that you're adding more airflow (4 ton vs. 3.5 ton) and it "could" be an issue.

 

[Kira Smith]

Thank you!

 

[BD1]

As for replacing, a sketch of what is current and what they intend to do.
My brother in law had a forced air ducted system. Nice rectangular ductwork through out basement supplying upstairs.
Contractor convinces him to replace. Well he did, contractor ran one main rectangular duct. The rest was ALL 6'' round ! There was runs as long as 20 feet ! Real piss poor job.

 

[zmaxmotorsports]

Just don't let him run any of that nasty flex duct.

 

[racintj]

Good chance that it is undersized, since it was installed in the 70's. Larger duct is typically more quiet as well. High efficiency units don't like to be choked down with small ducts.

Metal is definitely a luxury and best, but there is nothing wrong with a correctly engineered, sized, and installed ductboard and flex system.

 

[Uncle_Charlie]

Be glad you're not in my shoes. The ductwork is trapped between the original 10 ft ceiling and the 8 ft sheetrock ceiling that was added in the seventies. My AC compressor (also from the seventies) died in the summer of 2013. The ductwork has started to pull apart where the 40 yr old duct tape has dried out. When the furnace is running, heat is wasted in the two ft dead space. When it all gets replaced, I'll have to swap out the 40 yr old furnace, AC compressor, and run new ducts through the crawl space. I'm afraid to ask what that'll cost.

 

[rsanter]

Often the old ducts are actually very good, however you need to evaluate them to be sure.

Use a dust blaster to verify that there are no leaks, if there are then fix them.

One bad thing is the old ducts are not insulated. So insulate them on the outside of them.

Be sure they are sized to what you will need currently. If they are then keep them

Bob

 

[404]

What did the AC die of? Bad relay?
Suggest keeping the old duct. Insulate with duct board if feeling ambitious.

 

[volleyball]

So you are replacing a system that worked fine. So it had to be pretty well suited to your space. Was it noisy? Did the blower blow too hard? If it were fine, then your ductwork is good. I would use duct sealer and seal all the joints now. Also insulate any supply that goes outside the space. Either attic or basement.
You may have undersized ducts that leak enough that it doesn't blow so hard at the outlets. Not to say you cannot change things after the new system is installed.
You may want to go with the 3 instead of 4 ton especially if you have added insulation.
A smaller system that runs most of the time may be cheaper to run than a bigger system with setback timing. Depends on your lifestyle and how much you can deal with temp swing.
Round ducts are actually functionally the best to use. Rectangular are a compromise.

 

[Kira Smith]

Thanks everyone!

The compressor motor on one AC unit died. The other unit is 23 years old and was put in at the same time. One furnace is at least 23 years old, and the other is 10-12 so we figured it was time to look at replacing everything to be more efficient and compatible.

We never had any issues with the old systems. Right now, only the one system is working. We lowered the temp a bit and shut the plantation shutters on the windows in the area that the broken unit covered. Between that and fans it's really okay at night, although the guy who wants to redo the duct work kept telling us our house is really hot. We both work at home and are here all day and we were quite comfortable.

I'm going to pass on the duct work being replaced since the current duct work is insulated, we didn't identify leaks and he does want to use flex ducts, not metal.

 

[volleyball]

Flex ducts are terrible. Their ease of installation is there only good point. And since they are so easy, why bother with a pro? Especially a pro wanting your money.
What temps are you getting? Maybe what you consider comfortable may not be typical.

 

[magicrat]

Im a pro and if someone buys the flex ill install it cheap......Like 50 bucks a run and throw away the old.....But gooduck finding someone like that.....Ive seen some guys charge over 1500 for like 8 runs....And check youtube you can do it yourself.....I agree with 404 above...keep old duct......add insulation if u want

 

[Cypherian]

Fiber duct is ok but as it ages it breaks down some and you end up with more dust in the house. Before anyone jumps me on that umm yep I know this for a fact as in my own home . For the record I did install commercial HVAC systems long ago ( Think Pittsburgh lock system of joints and s s-clips) when fiber duct came out it was not allowed for food areas etc. I am sure it has changed but there is not a style/type/weave etc of fiberglass that does not break down over time unless it is totally encapsulated in resin. Ask as stated for a duct size study and perhaps size the unit to the duct since it has worked all this time, with newer / efficient systems you may be able to match it up.

Cypher