Worth it to replace ducts when central air is replaced?
June 10, 2016 8:26 AM   Subscribe

My central air system for the upstairs has croaked and I am replacing the chiller and air-handler. I have two quotes in hand from my chosen vendor: one for a 2-ton chiller (using the existing flex ducts, circa 1994) and the other for a 1.5-ton unit plus new, R-8 ductwork. The cost difference is pretty big; will the smaller-capacity unit save me enough in electricity costs to pay for the duct work?

The sales rep isn't pushing very hard for the deal with a smaller unit and new ductwork; he only says that new ducts would be more efficient. He said the existing ducts in the attic look like they were installed well.

My wife wonders if the old ductwork is dusty/moldy/yucky inside; a couple of our kids have allergies to dust. I am sympathetic to this, and suspicious of the maintenance by the P.O.

We are in New England, in a Dutch Colonial from the mid-80s; the air-handler sits up in the unheated attic.

But replacing the flex ducts will cost an extra $1500, and I don't have much confidence that the smaller unit will save me enough money in its lifetime to cover that. (It's a SEER-13 "Silver" air conditioner from American Standard.)

Can anyone offer an opinion, or a way to help make the choice?

(Yes, I saw this recent post, but it seems to discuss why to do it; I am interested in the economics of the question.)
posted by wenestvedt to Home & Garden (8 answers total) 3 users marked this as a favorite
Today's best air conditioners use 30% to 50% less energy to produce the same amount of cooling as air conditioners made in the mid 1970s. Even if your air conditioner is only 10 years old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model.

Proper sizing and installation are key elements in determining air conditioner efficiency. Too large a unit will not adequately remove humidity. Too small a unit will not be able to attain a comfortable temperature on the hottest days. Improper unit location, lack of insulation, and improper duct installation can greatly diminish efficiency.

This ROI calculator might help you crunch the numbers and come up with meaningful figures.

You should also look for rebates or incentives programs. When my AC died in a 20 year old house, I got a rebate on a heat pump of about $1000 and the electric company financed the installation. So, my payments were billed with my electric bill. My energy savings were so substantial that it covered most of the cost of the loan payments. I think my total bill went up by about $10/month and the entire house was dramatically more comfortable.

My wife wonders if the old ductwork is dusty/moldy/yucky inside; a couple of our kids have allergies to dust. I am sympathetic to this, and suspicious of the maintenance by the P.O.

If you are routinely spending money on allergy medication and doctors visits, new duct work may reduce that expense, possibly dramatically. Moving from an old duplex to a new apartment dramatically reduced the amount we were spending on medical care. Suddenly, my budget was not spinning crazily out of control. My budget stabilized and I stopped living in fear of where we were heading financially.

This is something notoriously hard to calculate, even after you actually experience it. But maybe sit down with your wife and come up with some guesstimates on how much is currently being spent on OTC meds et la. If that figure were reduced by 50%, would it make a meaningful difference here towards defraying the cost of the new duct work?
posted by Michele in California at 9:03 AM on June 10, 2016 [1 favorite]

sheet metal ducts are more amenable to cleaning than the flex stuff is. If you are worried about allergies, this might be worth considering.
posted by Dr. Twist at 9:29 AM on June 10, 2016 [1 favorite]

If you have uninsulated duct in an unconditioned attic, you are most likely losing a lot of energy into the attic: I would definitely replace the duct with insulated duct. Is replacing with solid duct an option? Well sealed solid duct is superior to flex duct in pretty much every way.

Also, SEER 13 is pretty minimal. I would ask for a quote for a more efficient unit.

If you have dust allergies, you will want to look at filtration and ventilation options. Do you have a fresh air supply to the house? An HRV? How well sealed is the house? There are plenty of things to consider for indoor air quality, but the ducts are probably pretty low on that list unless you have any specific reason to be concerned (why not go up in the attic and take a look inside some ducts?).
posted by ssg at 9:30 AM on June 10, 2016

Response by poster: I think the existing ducts are flex duct that is wrapped in insulation. (I've been up there myself and seen them.) I have called the salesman to ask whether he is proposing flex ducts or metal ones.
posted by wenestvedt at 9:42 AM on June 10, 2016

There's not nearly enough information here to know if the increased efficiency will be worth it. We don't know what your existing ducts are like, what your usage patterns are, how big your house is, what the price difference between the two packages is, or how fast it would need to pay off to be worthwhile to you. So this isn't really an answerable question.

However, better duct insulation has benefits beyond just efficiency. Poorly-insulated ducts can generate condensation in the summer, and inside your walls and ceilings is the last place you want that. Also, rigid ducts are better for your system than flex ones as they generally allow for smoother airflow. They also last longer. New ducting is also good for air quality, as you mention above.

Does any of that apply in your specific case? I have no idea. I'd push your HVAC person a bit to give you more detail. Let the new ducts guy know that you're interested in their proposal but it's $X more than what someone else is proposing to keep the existing ducts and get a bigger compressor, and you want to know what you're getting for that extra cost. They're supposed to be the expert here, and they have more information.
posted by Anticipation Of A New Lover's Arrival, The at 9:50 AM on June 10, 2016 [1 favorite]

Oh sorry, I missed that the difference was $1500. It's still impossible for us to say how quickly that would pay off given only the above info, though.
posted by Anticipation Of A New Lover's Arrival, The at 9:52 AM on June 10, 2016 [1 favorite]

A 1.5 ton unit may run more often than a 2 ton unit to cool your living space so there may be no energy/money saved. You need to do a load calculation and determine how many tons of cooling you need. Size the unit based on those calculations.

If the existing ducts are insulated and installed well, why replace them. You can do a visual inspection of the existing ducts with a scope to see if they're dirty.
posted by LoveHam at 12:10 PM on June 10, 2016 [2 favorites]

Response by poster: Once we committed to the job, the sales guy pretty much admitted that the ductwork was fine, and so we're glad we didn't spend the extra $$$.

With the heat wave we've been having in New England lately (with more to come!!), we're glad we came to a decision and had this ready in time. :7) Thanks for all the advice, everybody!
posted by wenestvedt at 7:54 PM on July 23, 2016

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