Should we have new ducts installed with our new a/c?

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jwblue
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Should we have new ducts installed with our new a/c?

Post by jwblue »

Our home was built in 1982. We are purchasing a new air conditioning unit.

Should we install new ducts? They will be R6 flexible ducts.

Will there need to be any damage done to our home to replace the old ducts?

Would there be any money saved by installing new ducts?
crake
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Re: Should we have new ducts installed with our new a/c?

Post by crake »

As a new homeowner with a house built in the 70's I'd be curious to hear the answer to this question. I've never heard of ducts needing to get replaced. It is also my understanding that flex duct is less efficient than rigid duct.

Is there something prompting you to consider this?
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jwblue
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Re: Should we have new ducts installed with our new a/c?

Post by jwblue »

crake wrote:As a new homeowner with a house built in the 70's I'd be curious to hear the answer to this question. I've never heard of ducts needing to get replaced. It is also my understanding that flex duct is less efficient than rigid duct.

Is there something prompting you to consider this?
The HVAC installer suggested it.

We did recently have our ducts cleaned but was wondering if the ducts still might be dirty.
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jwblue
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Re: Should we have new ducts installed with our new a/c?

Post by jwblue »

This is the reply I received on another forum.
If you have existing metal round ducts then I would keep them and reseal and re-insulate them if needed. They are far superior to flex ducts for air flow, longevity, and leakage.

As far as damage goes unless you have to tear out walls there shouldn't be much of a concern.
crake
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Re: Should we have new ducts installed with our new a/c?

Post by crake »

I don't know how far along you are but I would personally avoid this contractor. It seems like they are either incompetent or trying to fleece you. I would also look to an unbiased source for info about duct cleaning. It seems that the jury is still out if it provides any benefits and some people think it may be harmful. Read here for an unbiased source http://www.epa.gov/iaq/pubs/airduct.html

It is disappointing that you can't just trust a contractor but you always have to do your own research and get multiple quotes. I had one guy try to sell me a 25k basement waterproofing system for a basement that doesn't leak. He told me a lot of scary stories and showed me pictures of what would happen if I didn't pay him. I'm glad I didn't.
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tractorguy
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Re: Should we have new ducts installed with our new a/c?

Post by tractorguy »

I've no experience replacing a furnace of this age but speaking as an engineer, I'd ask the contractor why he wants to replace them. Unless you have noticed problems with balance (ie some rooms don't get warm while others are too hot), are buying a new furnace that has a much higher capacity blower, or have reason to think there might be mold in them, I wouldn't bother. Unlike furnaces which have had significant technical changes, the technology of heat ducts hasn't changed. You basically have a certain amount of cross sectional area that is needed for a given air flow. As others have said, flexible ducts have more air resistance than rigid ducts (of any shape) so they need to have more area to properly balance the same airflow. If you replace rigid ducts with flexible ones that have the same area, you are actually going to make the furnace work harder.

Dirty ducts should not be a reason to change them. I good cleaning will get rid of anything that is likely to be blown into the house by the new furnace. The only exception to this is if you have had water in the ducts and have mold in them that can't be cleaned properly. I'd expect this to be very rare.
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crake
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Re: Should we have new ducts installed with our new a/c?

Post by crake »

Just want to add that I do not believe all contractors are bad. You can find some that will work with you rather than against you if you do your due diligence.
pshonore
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Re: Should we have new ducts installed with our new a/c?

Post by pshonore »

If the ducts were installed during the original construction of the house, it could be difficult to get them out. Plus I think it would significantly add to the labor cost.
kcb203
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Re: Should we have new ducts installed with our new a/c?

Post by kcb203 »

Are the existing ducts insulated? If not, that would be a good reason to do it.

hvac-talk.com has a great moderated forum in which pros give out sound advice. They won't discuss pricing or DIY, but otherwise I've gotten great advice there.
scotthal
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Re: Should we have new ducts installed with our new a/c?

Post by scotthal »

Was the house originally set up to have A/C? Ducts installed solely for forced air : furnace (vs heatpump) heating may not have sufficient cross section for the larger air flows typically required for A/C.
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crake
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Re: Should we have new ducts installed with our new a/c?

Post by crake »

kcb203 wrote:Are the existing ducts insulated? If not, that would be a good reason to do it.

hvac-talk.com has a great moderated forum in which pros give out sound advice. They won't discuss pricing or DIY, but otherwise I've gotten great advice there.
From my limited knowledge I believe that heat loss through ducts is actually minimal. Insulated ducts are generally used to prevent condensation more than to improve efficiency. It seems that any theoretical gain in efficiency would mostly be countered by the increased resistance to flow of the flexible ducts when compared to rigid.

Even if we do assume that there is an efficiency increase in insulated ducts the next question is what would the pay back period be. I would imagine installing new duct work would be a multi-thousand dollar project. You would need a very large increase in efficiency to make that project worth it. It seems that many contractors use energy efficiency to up-sell unneeded upgrades. Unless you can do an ROI calculation and see if the upgrade is worth it I wouldn't do it.

I was looking at getting new kitchen appliances and was trying to justify it by saying they would be more efficient. Out of all the common appliances the only one with a <5 year pay back time was the fridge so that is the only item I'm planning on replacing. In the future I might upgrade the rest but I can't justify it by saying its going to save me money. It would be a purely luxury expense.
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6miths
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Re: Should we have new ducts installed with our new a/c?

Post by 6miths »

Replace ducts?! Seriously?
'It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so!' Mark Twain
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theduke
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Re: Should we have new ducts installed with our new a/c?

Post by theduke »

I had my heat pump and ducts replaced about five years ago. My previous ducts were flexible, not in very good condition, and the insulation was an R-2. The ducts are run under my house, and no damage was done. Whether it was worth it to replace the ducts, I can't say. I would probably do it again under the same situation. If my ducts were rigid, I would not replace them.
Dag
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Re: Should we have new ducts installed with our new a/c?

Post by Dag »

What type of ducts are currently installed? If they a rigid, they probably should not need replacing. However if they are flexible ducts, some do deteriorate over time especially the nonmetallic type. The plastic these are made out of can tear, causing excessive air leakage. This will impact your electric bill. I would guess that the metallic type would be more robust.

I'd replace flexible duct with rigid metal duct rather than flex duct. Flexible duct needs to be properly installed, it can easily be installed improperly.

This site, askweldin.com , has great information on duct of all sorts. Check out the section on flexible duct, but also check out the other information. You'll get a decent education on duct.
placeholder
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Re: Should we have new ducts installed with our new a/c?

Post by placeholder »

I had the ducts replaced a couple of years ago but the original ones were made out of ductboard (essentially dense styrofoam) that were leaking so badly that much of the cool air was going into the attic and even retaping didn't really take care of it but it was a relatively simple job since all the ductwork is in the attic and not in the walls or floors.
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walkabout
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Re: Should we have new ducts installed with our new a/c?

Post by walkabout »

If rigid metal, you are probably ok not replacing. One poster upthread did make a good point about duct sizing. If your ducts were originally sized only for forced air heating and you are adding (or it was added since the system was installed) central AC, then the ducts might not be sized adequately. If the metal ducts are accessible, you should consider having them sealed (with mastic at all seams) and insulated. An earlier poster is correct about "saving" your heated/cooled air with insulation. The insulation (and sealing) does do that, but it will also prevent sweating of your ducts (during cooling season) where the ducts pass through unconditioned space (eg under a house, in attic, in garage).

If flexible, then you might consider replacing. You should only replace if there is a reason to replace (ducts have deteriorated, they are leaky, insulation is in poor shape, critters have chewed on them, etc). Your contractor should be able to give you a good reason why he thinks they need to be replaced.

If duct board, you might consider replacing for the same reasons you might consider replacing flex duct.

A key question is whether or not your ductwork is accessible. If accessible, and if replacement is warranted, then there should be no additional damage.

It is worth noting that you can probably have an energy audit done on your house (preferably not by someone that is trying to sell you "fixes" for problems indicated by the audit). Maybe your local or regional utility does this (or partners with people that do). One test seals all of the registers in your home and blows air into the HVAC system. The pressure is measured. This test gives a very good idea of how leaky your ducts are.

I'll give a brief version of the spiel that I've given on this forum before:
1. Make sure that your new unit is sized according to a Manual J calculation, not simply by the size of the old unit. It is not necessarily the case that the old unit was correctly sized in the first place.
2. Bigger (more tons) is not necessarily better. People often think they need more tons then they really do. They think a good test is how fast the house can be cooled from temp X to temp Y and the faster the better. If a system is too large, it can cool your house so quickly that not enough humidity is removed. NO "rules of thumb" like 1 ton per 500 sq ft. Get the calculation.
3. You should prefer a system that runs a little longer as your temps will be more even and your humidity will be lower. Lower humidity should actually allow you to run your AC at a higher temp and still be comfortable (eg 75 at lower humidity might be as comfortable as 72 at higher humidity).
4. Consider a media cabinet or a 4-5" pleated filter that can fit in the return (if that is where your filters go now). The deeper pleated filter lasts longer (up to a year) and provides less resistance to air flow, thus making your unit work not as hard as the 1" pleated filters do. IMO, the 1" fiberglass filters (or should I say "filters") (the ones for $1.99, or some low price, at Lowes) are worthless. They won't filter much. Note that filter serves two purposes: make it easier for you to breathe (remove dust and particulates) and to protect your unit (keep dust and grit out of your air handler).

Good luck!
songman52
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Re: Should we have new ducts installed with our new a/c?

Post by songman52 »

We had new ductwork installed when we replaced our units a few years ago. The old ductwork was an amalgam of tubes running off several connections. The new ductwork runs with just two main runs off the plenum, then individual rooms connected to the mains. It's much more efficient, and worth the extra cost (I believe about $1200.) The temperatures in each room are more balanced now. It's installed in the attic area or our older (60 yrs.) home, so access was not difficult. The ducts are rigid, well insulated, except for a couple of short runs to two rooms with flexible ducts. We did get four or five estimates beforehand.
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pennstater2005
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Re: Should we have new ducts installed with our new a/c?

Post by pennstater2005 »

6miths wrote:Replace ducts?! Seriously?
Our house was built in 1952 and the ductwork installed had way too many 90 degree turns and elbows used and was poorly sealed. I've tried to seal what I can see but the forced air that comes out of the vents is very weak and it's not the blower as it has been tested and is fairly new anyway. I will need new, more efficient ductwork especially if I ever have central AC installed. It's going to be a PITA.
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runner26
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Re: Should we have new ducts installed with our new a/c?

Post by runner26 »

6miths wrote:Replace ducts?! Seriously?
"This article describes Owens Corning Valueflex™ brand gray flex duct failures that appear as loss of the gray plastic duct covering due to brittleness that appears to be caused by exposure to heat (such as in a hot attic), with references to product failures by several manufacturers of flexible duct work, , including ATCO™ Ruber Products, Alloy Systems™, Goodman™ flex duct, Owens Corning™ flex duct, Owl™ flex duct.

We believe that none of these defective flex duct products is currently sold (2010) but both may be found in older homes. Note: not all Owens Corning flex duct products share this defect and disintegration problem."

http://inspectapedia.com/aircond/Owens_ ... x_Duct.htm
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6miths
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Re: Should we have new ducts installed with our new a/c?

Post by 6miths »

I guess building codes around here are (or have been) a bit different. I can't imagine the ducts in any house that I have ever lived in or worked on needing replacement due to failure of the ductwork or improper installation. Live and learn.
'It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so!' Mark Twain
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