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Moving A/C supply vents - cost savings?

Posted by SoonerDave (My Page) on
Fri, Jul 20, 12 at 14:51

I have a 13-year-old home that I built new, but did not realize during construction that my builder's HVAC contractor took the cheap route and installed my A/C supply vents in (generally) the middle of each room rather than near a window on an exterior wall. And I was too stupid at that time to realize what had been done.

A couple of local AC guys were stunned to see how my vents were laid out - no vents near the greatest heat transfer. One guy said it was clearly a matter of a contractor going cheap and quick rather than right. A look in the attic clearly shows one main, large conduit with clearly a minimum number of branches for the room vents. Two of our bedrooms have their supply vents about 36"-48" from the return air, and I've noted that cold air is getting sucked right back up into the system the way it is.

What it boils down to is that while my A/C works and the house stays cool, its costing a small fortune presumably because my vents are really trying to cool essentially the majority of the space in the house rather than cooling the heat entering through the windows.

Bottom line - I need to move the vents to the walls with outside windows in nearly every room in the house. A local A/C contractor won't touch the job this summer, but said he'd probably be willing to tackle it for $1K later in the fall.

My question is this: Is there any way to predict or model just how much I stand to save by moving these vents so I can gauge the payback time? I (now, years after the fact) know this installation is terribly inefficient.

I was planning to add extra insulation, but I can't really do that until the vents are moved because guys crawling in the attic would squash the new insulation.

Any thoughts or feedback would be appreciated.


Follow-Up Postings:

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RE: Moving A/C supply vents - cost savings?

Are the rooms comfortable now?

If they are not comfortable moving the vents may make them have a more uniform temperature at an increase in heating and cooling load and higher operating costs.


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RE: Moving A/C supply vents - cost savings?

I'd look at new supply grills that have adjustable
fins. there are 4 way adjustable grills that have
two large areas of adjustable fins and two smaller
areas. there are three way adjustable grills.
you may have to go to hvac supply & ask to look
at some of the different types of grills, or shop
online.

for example:
http://www.valueac.com/a6sehacocere.html

better supply grills can throw the air where you
want it to go.

this would be a diy without moving ducts.
then invest in duct sealing of your existing ductwork.
mastic sealing ducts is a quick payback and can
improve comfort easily.

best of luck.


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RE: Moving A/C supply vents - cost savings?

"while my A/C works and the house stays cool, its costing a small fortune presumably because my vents are really trying to cool essentially the majority of the space in the house rather than cooling the heat entering through the windows."

This is not logical. If you are comfortable, leave it alone. If not, try what energy suggested. I'd lose the contractors who said you'd save energy.


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RE: Moving A/C supply vents - cost savings?

This is not a problem, all my registers (both in and out) are located in the middle of the ceiling. 2 feet apart. I have 3-way grill, the big grill side blows most of the cold air toward one side of the room and form a nice circulation and come back from the other side. I have never had any cooling issues.


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RE: Moving A/C supply vents - cost savings?

Simply replacing the registers as suggested
is good advice.

If the duct work is in the attic there's a
good chance you could increase your A/C
electric bill because lengthening the ducts
will increase the heat gain from the hot attic
to the ducts because of the extra surface area
of the ducts.

37 year veteran HVAC professional

Here is a link that might be useful: Heating, Ventilation and Air Conditioning Help and Advice


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RE: Moving A/C supply vents - cost savings?

If it is working why fix it? I wouldn't move the ducts - I would switch out the registers as stated above. Sounds pretty expensive to do a total rehaul of the vent system.


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RE: Moving A/C supply vents - cost savings?

Wow, lots of feedback. Let me try to digest it a bit.

I must admit to some serious confusion on the feedback here, and I guess part of that is my own ignorance.

Let me offer some additional information. My kids' rooms on the east end of the house are *always* substantially warmer than the rest of the house. One room has a supply vent located not 36" from the return duct, forcing him to keep the door closed to at least try to keep cool air in the room - to say nothing of the fact that there's no vent throwing against either window in the room.

Same goes for my daughter's room - nothing throwing against either window, although she's at the opposite end of the hall and not affected quite so much by the return vent.

The supply vent in the master bedroom is 12 feet away from the windows, and perhaps four feet from the return vent in the opposite end of the house. We've taken to stuffing a towel at the bottom of the door to keep the cold air from going up the return.

I understand that different grills might help direct the airflow a bit, but when the fundamental arrangement of the ductwork is wrong, eg wrong side of the room, 10-12' wrong, how can a directional grille overcome *that much* of a problem?

The other thing that's really got me confused now is that of the three different A/C types that have been through my house, *each one* has pointed out that our ductwork/ventwork is wrong. Two of the three companies didn't even want to bid on fixing it, so it wasn't like they were trying to sell me some bill of goods. But what I'm reading here is that the ductwork is fine - even when the last guy through told me that the ducts cooling the largest rooms in the house are attached to the wrong output side of the blower.

Not at all meaning to argue or dispute the kind advice offered here, but I'm just incredibly frustrated. I believe I'm churning out entirely too much kWh in AC operation unnecessarily, esp. compared to other homes of comparable size (and some much older age), and I'm trying to figure out where the disconnect lives.

As a comparison, my mom's 2500-sq-ft house built 38 years ago, with an A/C compressor that's about 10 years old, and kept to about 77 degrees costs her about *half* what it does me to cool. The local power company charts indicate I'm burning up about 50% more electricity that comparable houses in my area, and the only practical explanation is cooling costs. And when I couple that with three different vendors telling me my ductwork was incompetently laid out, and that my ventwork is fundamentally flawed, its hard for me not to see that as a significant source of the problem.

Again, I'm not at all trying to argue with the more learned folks here. I'm just very, very frustrated on what's going on. Is the rule of thumb about putting supply vents on the outside walls/near windows just a bunch of sales nonsense?

FWIW: I'm in central Oklahoma, where summer temps routinely reach 100 or better.


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RE: Moving A/C supply vents - cost savings?

Just to be clear, you have a return in every room that has a supply?

It seems like having the supplies washing windows with air is a real waste, though that is not saying that they should not be located near the highest load in a room. I think that a directional vent could do a lot. Since it is a pretty inexpensive item, buying one to see if it does is a pretty economic experiment. I'll be interested in the response from AC pros on those points.

Poor comfort from poor distribution is related to energy cost because you have to over cool your home to get comfortable. It is not always the same thing however. You might have two very separate problems. Your high energy costs could be something as simple as a very leaky return air duct sucking in outside air or a leaky plenum or ducts blowing out conditioned air. (Note that both of these also result in blowing or sucking in other areas to make up for the leaks.) Leaky ducts systems are very common.

Don't concentrate on adding insulation without considering air sealing first. Here are some questions for starters. Is your attic access well-sealed? Do you have a lot of recessed lighting? Do you have an attic ventilation fan? Those attic fans can suck the cool air right out of your living space if things are wrong. I'd consider hiring an energy ratter to look at your house. They can identify the low-hanging fruit for energy savings and know local programs for energy-saving rebates from utilities and local government. Look at Resnet for starters.


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RE: Moving A/C supply vents - cost savings?

we can only answer based on the info you give us.

back 25+ years ago when ductwork was installed
for heating only..they located supplies near windows.
note..heating only.

now that we design for heating AND cooling..it is
a different day.

13 years ago..double windows were around. what type of
windows did you chose to install? single metal, double
metal..double vinyl..low e?
granted windows are the weak point of the wall..but
newer windows with air space insulators and coatings
to reflect heat make a substantial difference.

some of the things we don't know:
we don't know how big your house is..
what it's configuration is..
how big the hvac system is..
please define substantially warmer
childerens rooms in degrees.
what type of ductwork do you have..flex duct?
hard pipe ducting?
house on slab?
hvac equipment type info..
is this a new issue, or has this comfort problem
and high utility cost been an issue all along?

enlighten us as to window choices you made,
insulation levels & types and hvac info.
this will help us to help you.

ionized raises several valid points in his last
post, although I'd steer away from an energy ratter..
LOL!
there are many things that contribute to comfort
and high utility costs.
we have bare bones information so far. so
we will bear with you, and you bear with us..
and maybe we can get some solutions for you.

several things stand out in your latest post
besides the addtional information..
one is this " Is the rule of thumb about putting supply vents on the outside walls/near windows just a bunch of sales nonsense?"
there is a manual for design of ductwork. the information
in this manual is applied to your specific layout
of your specific house.
I've not heard of a rule of thumb about placement.
rules of thumb get us all into trouble when you try
to apply this nonsense to different houses.

"The supply vent in the master bedroom is 12 feet away from the windows, and perhaps four feet from the return vent in the opposite end of the house. We've taken to stuffing a towel at the bottom of the door to keep the cold air from going up the return."

first..the opposite end of the house is 4' away from this
supply?? what??
second..cold air doesn't rise. hot air rises.
the return is in the ceiling?

"last guy through told me that the ducts cooling the largest rooms in the house are attached to the wrong output side of the blower."
maybe its just me...but wrong side of the blower?
what?
granted the ducts closest to the coil/ahu (depending
on if you have ac & gas or a/c & elec strip heat
or heat pump..none of which we know) the ducts that
tap off closest to the equipment will get the colder
air, but you size ducts according to the size of the room
..larger rooms get larger ducts, smaller rooms get
smaller ducts. this is determined by another manual
calculation that sizes the duct based on size of room,
heating/cooling requirements of that room, and
amount of air to be supplied to that room.

just like load calcs for the house..which determine what
size unit your house requires, there are calcs to
determine design & size of ducts.

when you built the house..did you get any type of load
calcs, design of ductwork or anything more than
a rule of thumb sizing?

if you can give us more info..maybe a rough drawing
of the layout of the house..and provide the info
that we don't know it would be very helpful.

its been a long day for me..problem house
with a rat poop covered plenum and cuts around
supply ducts that I could put my hand into.
so don't take offense if I've not been as
tactful as I could be. this isn't my intent.

best of luck.


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RE: Moving A/C supply vents - cost savings?

The companies who visited your home should have done a heat load calculation and measured the air flow in the problem room.

If they didn't at least do that, they are guessing just like the rest of us.

Doctors don't diagnose until they know the vital signs.

Call a company and tell them you would like them to do the following:

1) Manual J heat loss calculations
2) Static pressure test of your duct system
3) Measure air flow from registers
4) Check pressures and temperatures of a/c refrigerant
5)Take both dry bulb and wet bulb supply and return temperatures.
6) Have them present the report

"Testing before diagnosis - Diagnosis before prescription"

37 year veteran HVAC professional

Here is a link that might be useful: Heating, Ventilation and Air Conditioning Help and Advice


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RE: Moving A/C supply vents - cost savings?

Here's some follow-up information on our home:

SQ FT: 2272, built in 1999, built slab-on-grade, wood framing, masonry exterior. Originally ceiling insulated to R-38, but confident that real value has dropped due to settling in intervening 13 years. Home faces north. Located in central Oklahoma, where summer temps routinely hit 100+.
Double-pane sealed windows. Have had to replace 5050 bay window in kitchen eating area due to failed seal. Replacement is double-pane, argon-filled.
Central A/C: 10 SEER RUUD unit installed new with home. Natural gas furnace. No heat pump. Ductwork in attic, all insulated flex - R value on flex shows R8 on larger ducts, R6.5 on smaller.

Children's rooms on east side run 3-4 degrees warmer than bedroom/bathroom on west side in summer. If master bedroom is 76, children's rooms reliably run about 80.

Already plan to add more insulation; have quote in hand to have 7" of additional fiberglass blown in. However, holding off on any such installation if there exists a need to rework ducting, as it seems pointless to blow in new insulation only to have it mashed down and moved by workmen moving ductwork.

I don't have the ability to upload a sketch of the home right now, but I'll do so tonight. In the meantime, I'll try as I can to offer this clarification on home configuration:

Two bedrooms on east side of house, bathroom between; bedrooms connected by hall with large ceiling return vent; master bedroom/bath utility on west side. Return vent just outside bedroom door in hall (total of two for entire house). Living room, kitchen, office, and gameroom in the middle. Supply vents are in each room - two in living room - and are mounted either in middle or near wall *opposite* room windows.

The supply for the master bedroom is located immediately adjacent to the door. Return duct is immediately *outside* that door. When AC runs, cold air is drawn from vent and up through return, so we've taken to closing the door and stuffing a towel in the gap at the bottom to try preserving the cooled air.

Local contractor came yesterday to evaluate ductwork, and concurred with prior two vendors with notion that ductwork is improperly laid out, and additional offered that supply vents to living room are not attached to system properly, connected to low velocity side when they should be connected to high velocity side.

I know of no load calculations or evaluations done by the HVAC contrator at the time of construction. I've already heard feedback from one contractor who said our builder should never have let his HVAC contractor get away with laying out the ductwork as he did, which he characterized as "the cheap, easy way out."

I had suspected our energy use was too high the first summer we lived there, but I was too stupid to know the right questions to ask. I contacted the electrical contractor as an ignorant first step, and he said "watch your TV less, dude." Since he convinced me I was more or less an idiot for asking, I just decided I had to accept what I saw on the electric bill, and blame it on a bigger house. I've had enough anecdotal evidence from other folks with comparably aged/sized/built homes now to conclude I wasn't such an idiot after all; I'm burning too much juice, on the order of $100-$150 *minimum* too much per month, but I also admit I'm (obviously) painfully stupid/uneducated about the details of HVAC to know precisely where the loss is occurring. When I came across multiple folks tell me my ductwork was screwed up, a light bulb went on, and here I am.

Sorry for the ignorant questions, folks. Wish I knew more than I did. I'll put together a sketch of the floor plans I drew and post later tonight.


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RE: Moving A/C supply vents - cost savings?

energy_rater_la,

"ionized raises several valid points in his last
post, although I'd steer away from an energy ratter..
LOL! "

I am just a Joe homeowner so yielding to when reasoned and knowledgable advice is given is easy. In this case, however, SoonerDave stated that his home is using a substantially higher amount of energy compared to "similar" homes. Doesn't that demand the attention of someone from your tribe?


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RE: Moving A/C supply vents - cost savings?

"watch your TV less, dude."

LMAO!

Dude, you will get more help with the advanced description and the diagram will probably help too. You don't seem so ignorant to me and pretty smart. You are starting in good place and asking the right questions. Some of the answers you have so far are not making sense, like high velo and low velo side. I am not getting that.


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RE: Moving A/C supply vents - cost savings?

Ionized..I was poking you about spelling rater
ratter. yes, raters are my tribe.

looking forward to seeing the sketch.
can you post a few pics of ductwork in attic.
plenum, close up of duct takeoffs and
general duct pics. maybe close up of return.

I really don't think it is duct location that
is your issue. old school contractors who did
heating only or back in the day of single wood
windows would locate next to exterior walls.
nowdays, hvac is heating & air conditioning.
a/c supplies more air than heating.

the rule of thumbs that you keep asking for
are what gets people into trouble. once it is
understood that there are calculations to determine
these things rather than just a box fit for
every house..then you get solutions.

glad to see you went hvac-talk route. good
info from pros there.

best of luck.


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RE: Moving A/C supply vents - cost savings?

I was overcome by events last night and didn't get the sketch or the attic pics up. Will do tonight, although it may be late :) And based on what I'm reading, I'm less pursuaded that the ductwork is the problem.


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RE: Moving A/C supply vents - cost savings?

I was overcome by events last night and didn't get the sketch or the attic pics up. Will do tonight, although it may be late :) And based on what I'm reading, I'm less pursuaded that the ductwork is the problem.


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RE: Moving A/C supply vents - cost savings?

For those that might still be following the thread, a local AC contractor I found through a referral came out to my home last night, reviewed the setup of the house/attic ducting, and concluded that my overarching problem is return-air ducting, leading to the air-starvation of the system. That's inducing higher compressor head pressure and a (much) greater than expected current draw from the main A/C unit. Although he suggested a couple of supply vent changes, he clearly offered that the return ducting was an overarching problem that really made the others (better grills, moving supply vents) secondary. So it looks like that's where we're heading.

Thanks.


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RE: Moving A/C supply vents - cost savings?

good!


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RE: Moving A/C supply vents - cost savings?

Does that makes sense to you, energy_rater_la? I'm trying to get as many sanity checks as I can at this point :)


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RE: Moving A/C supply vents - cost savings?

seems to me..and this is based on replies from
hvac-talk thread that I've been following..
that the contractor didn't take sufficient measurements
to determine that this is the whole problem.

it may well be a part of it, but more information
should have been gathered to make it a total
solution.

you are on the right path..asking good questions
and getting good input.
don't settle for a partial solution,
you've lived with the problem long enough.

best of luck.


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RE: Moving A/C supply vents - cost savings?

Did anybody measure the static pressure? I suggest doing a measurement before any changes are made. If it it high then the theory of insufficient return air is plausible, but it may not be the only problem.


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RE: Moving A/C supply vents - cost savings?

"that the contractor didn't take sufficient measurements
to determine that this is the whole problem. "

It can often be a complete waste of time to make any further measurements on a system that is already starved for return air.

The return air restriction is going to reduce the flow and alter the pressure in the whole system.

Some design defects can only be solved serially.


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RE: Moving A/C supply vents - cost savings?

energy_rater_la: Well, if you're still following the thread on hvac, the contractor who visited my house found the thread as well, an dis explaining why he reached the conclusions he did. He's standing behind what he asserted, so I honestly don't know what to do next. Some of the gurus there think the system is overcharged, while my contractor says its air starvation. Maybe the real answer is that the system, poorly designed, requires less refrigerant charge to do the job it is presently asked to be doing, and expanding the ducting inherently solves that problem. I just don't know. All I do know is that I'm the dumbest guy in the room, caught between multiple experts who seem to be saying conflicting things (or I'm just not smart enough to understand how they really all agree) - either way, my ignorance is costing me $. And now I'm afraid I've unintentionally insulted the contractor merely because I had posted a summary of his conclusions on that thread, as a courtesy to the responses others had posted.

I just want things to work right, that's all. Don't mean to offend anyone or challenge anyone else's expertise. Just know what I have right now is costing my family $$$ I think I need to try saving, and I don't know whom else to ask.


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RE: Moving A/C supply vents - cost savings?

well Dave, I see that the contractor posted his
findings so I wouldn't worry too much about
upsetting or offending anyone.
it isn't unusual for there to be conflicting
opinions, but the fact is that you and he are
the only ones who have seen the ductwork.
his discription of your return air setup is suspect.

I see he is advising you to wait out the summer
to do any duct/return modifications. if you can,
do.
it is awfully hot to be modifying ductwork this
time of year. a rushed job isn't a good job.
better someone take the time to properly install
and seal ducts.

it kinda bothered me that no mention of duct
or return sealing was mentioned. of course the
discussion is pretty technical. but I find that
leaky ducts/returns are often overlooked.

when you talk about the kids rooms and the 3-4 temp
difference, you mention west facing windows.
how many & what approx size are in each room?
what type of windows do you have?
single pane, double pane?
metal frames, vinyl frames?
clear glass or low-e glass?

if the temp difference is due to the west
windows..and it would be here in my area,
then there are a couple of things you could try.
block the afternoon sun from the exterior for a day or
two and see if this evens out
the 3-4 degree temp difference. wouldn't have
to look great, or be permanent, just stop the
solar gain.

if you can keep the heat from entering the house,
then the a/c doesn't have the added load to temper.
exterior shading is the best route.
rather than interior shades & curtains.
beat the heat before it gets inside.

if this works then invest in solar screens. yes
they are darker than normal screens, but they will
minimize heat gain.
if windows are double paned..you can't install window
tints as it will increase heat between panes and
blow the seal.

awnings and bahama shutters are other options but
to me..at this point solar screens would be more
cost effective.

right now you have more time than solutions,
so give it a try.

don't worry about riling those guys up..they live for
that stuff. Shophound & the guys have a good handle
on things, so let them talk themselves around.

I have to admire your restraint & patience.

you are aware that 1" pleated filters are very
restrictive? for now I'd change to a less
restrictive filter. at least then more air
will return to the unit.

hang in there.

best of luck.


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RE: Moving A/C supply vents - cost savings?

Thanks, energy_rater_la :)

I've all-but resigned myself to the fact that nothing is going to get done this summer, unfortunately. And I understand that working in an attic this time of year would be absolutely brutal if not borderline fatal, especially with temps in the OKC area this week slated to approach 110 *every day*.

The kids rooms are actually on the east side of the house, one on the northeast corner, the other on the southeast corner. Each has two windows (which I abjectly regret now); one facing east, the other facing the side the room is on. The windows are double-paned, so I was reluctant to put on any window tint/film.

I invested $50 in a temperature gun, and observed 90+ degree readings from the south window in the midmorning, and that's with a closed, lined curtain! Looks like another $50 may be invested in a Levolor cell-type shade that blocks at least some heat; we put two of those in our master bedroom (also on a south wall) and those two windows measure 77-78 degrees at the same time of day, so I figure *any* heat savings at this point is better than none.

I have no trouble changing to a different filter type if that will help, even a little.

I've got an uncle who is an extremely talented electrical engineer who was *really* spooked when I told him that the terminal block/wires on the A/C compressor had melted twice, and that the HACR pull-out breaker next to the A/C was scorching hot to the touch the other day. He thinks there's still a specific electrical problem going on that hasn't been found yet. When I told him about the block melting, he said "you're telling me that the block on the compressor with the terminals hooking up power literally melted?" He thought I had misspoken, and told him "Yep, when we opened it up, the wires were burned and separated, and the terminal was "semi-congealed" from the heat/current." This guy spent the better part of his career designing electrical power subsystems that are still on the grid in OKC to this day, so I have zero doubt about his
knowledge on the electrical side :)

'Tis a frustrating jigsaw puzzle, to be sure.


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RE: Moving A/C supply vents - cost savings?

The scorched and melted wires/terminal block correlate well to high head pressure and current draw.


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RE: Moving A/C supply vents - cost savings?

Sure makes sense to me, ionized. From what I understand, there can be a bunch of possible causes, just wish I were smart enough in HVAC to identify the most probable ones...


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