AC temp at ONE vent 10 deg warmer than all others

Problems related to residential installations.

AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Thu Jun 25, 2015 4:50 pm

The AC air temp *inside* the MBR vent / register is always between 8 - 10 deg F warmer than *inside* all other vents; day or night. So MBR doesn't cool as well as other rooms (even larger ones).
Air temp has been lower at (inside) that one vent for several yrs (possibly forever). Other rooms cool fine.

No leaks, splits anywhere on the MBR 7" flex duct, or connections to plenum collar or ceiling box. Inspected every inch many times.

MBR 7" flex duct is one of the shortest runs -11.5 ft. Temp inside the Kit. vent - 3x longer run than MBR - is still 8 - 9 deg F colder.
- Plenty of air volume at the MBR vent (MORE than all others - even rooms w/ same size ducts / vents).
- All ducts, plenum, evaporator coil & air handler are in attic (as most homes here).
- All ducts are suspended *above* the attic blown insulation.

- MBR vent / grill is 8" x 14". Same duct & vent size as den / breakfast area (one vent each). Those rooms cool fine, with less vent air flow than MBR vent.
- All flex ducts are sealed w/ mastic & zip ties - @ plenum collars & ceiling boxes .

Couple of differences in MBR duct vs. others -
- MBR duct collar is ONLY one coming off the * top * of rectangular plenum (duct board). About 4 " downstream from where evap coil & plenum join. (? can't post image links on this forum?)
- All others come off plenum sides; including a 12 x 12 in fiberglass duct board chase - feeds some more distant rooms.

- a few ft of MBR duct is strapped up closer to the rafters, to allow crawling under it. But * NO * measured attic temp difference near MBR duct vs. others.
- Attic is well ventilated w/ continuous ridge vents & plenty of soffit vents.
Thanks.

MBR duct at plenum (edit url / remove asterisks, to view pic) h*t*t*p*i60.tinypic (dot) com/2dv4x1w.*j*p*g
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- AC temp at ONE vent 10 deg warmer than all others

Postby Freon » Fri Jun 26, 2015 12:48 pm

Possibly there's a poltergeist in the house.

If you were to measure the air velocity at the supply register, I suspect there would be a difference from the other ducts. My theory is the travel time may be partly responsible for the temperature difference. But 8 to 10 degrees is significant.

My suggestion would be at the plenum takeoff for the MBR 7" duct use a 45 degree metal elbow. By clipping the plastic duct back inside the insulation you can stretch the insulation over the elbow. It's possible the 7" duct at the plenum is being crimped. Flex duct has to be carefully installed to avoid crimps and the resulting reduced air flow.
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- AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Sun Jun 28, 2015 12:19 am

Freon -

> If you were to measure the air velocity at the supply register, I suspect there would be a difference from the other ducts.

Not sure if you meant more or less velocity at the MBR vent than others? I suspect you meant the MBR may have LESS volume / velocity than others (that's not the case).
As mentioned, the MBR vent has MORE air volume (& feels much stronger) than all others. Without actually measuring air flow, the MBR vent feels twice as strong as other vents w/ the same duct size - or any in the house.

Now, the other vents' dampers may have been closed a bit (or not), but they still cool the rooms & they are much colder than the MBR, even though they're flowing less.

It's definitely not pinched or constricted (or wouldn't blow so hard), nor has any of the plastic inner duct exposed w/o insulation covering it.

>My theory is the travel time may be partly responsible for the temperature difference. But 8 to 10 degrees is significant.

Doesn't make sense, given my clarification. There'd be less travel time in the MBR duct, given its short length, and the high volume coming out.
And, the vent w/ longest run (~ 3x that of MBR) is still far colder.

> My suggestion would be at the plenum takeoff for the MBR 7" duct use a
> 45 degree metal elbow. By clipping the plastic duct back inside the
> insulation you can stretch the insulation over the elbow. It's possible the
> 7" duct at the plenum is being crimped. Flex duct has to be carefully
> installed to avoid crimps and the resulting reduced air flow.

Though good tips, I'm guessing the last comments (45 deg elbow; crimped flex duct; reduced flow) wouldn't still be your suggested solutions (or suspected cause of the higher vent temp)?
There's just no getting around how much warmer that vent is, despite putting out far more air than any other.

I intend to measure the temps inside the plenum, at the MBR collar (on top of plenum) & near 1 or 2 collars on side of plenum.
See if air temp inside the plenum (at those points) actually differs.
If it does, there's the answer. Move the MBR duct to the plenum side.

If no temp difference inside the plenum, then the MBR vent being hotter could be related to air flow (too much??), or a problem w/ the duct itself (that can't be seen).
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- AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Sun Jun 28, 2015 6:52 pm

Measured temps inside the plenum today w/ analog dial thermometer - 5 in. metal probe.
Wanted the plenum's inside temp, at the level where it enters the collars.

So didn't insert the whole 5 in. of the probe, but inserted it the * SAME distance * for each test.
Not inserting it fully may have skewed the actual readings, but not relative temp differences between 2 areas of the plenum.

** No internal plenum temp difference measured - from right next to the MBR duct collar vs. another collar off the plenum side. And the room VENT for that collar off the plenum side - was 9 - 10 deg colder than MBR vent (today).

Inspected the MBR flex duct connection to wall box (again) - from attic side & taking off vent.
Almost no cold air leak detectable in attic, between flex duct & wall box.
Inside the wall box - looks normal (to me). It's old, but the fabric covered insulation is in place on all inside surfaces.

Any gaps between sheet rock & edge of metal box were already filled w/ silicone.

Later, I'll see if the MBR duct damper is even partly closed. Can't be closed much, or flow wouldn't be as strong as it is.
Unless the damper's partly shut, AND opening it drops its vent temp - close to other vents, may be time to look at replacing the duct.
Unless someone thinks of something else.
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- AC temp at ONE vent 10 deg warmer than all others

Postby Freon » Mon Jun 29, 2015 2:03 pm

You seem to be doing everything possible to find the cause. If you think the velocity at the MBR is the same as the other registers, then I am stumped.

The velocity is important because with increased velocity you get less time in the duct to gain heat. Since you have a 7" duct to the MBR and I assume 6" ducts to the other rooms, with the same cfm (volume flow) the velocity to the MBR would be less. Due to air pressure within the duct you'd lose cold air to the attic space, not gain hot attic air. And the pressure variation at various places within the plenum can be significant affecting air flow volume.

Keep digging. It seems that 5 to 10 degrees of lost air temperature should be from a very obvious flaw. You could switch the tap on the plenum for the MBR trunk line. You may need a reducer at the plenum (7" to 6") but there may be a noticeable difference.
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- AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Mon Jun 29, 2015 5:24 pm

Thanks. Don't know about real MBR vent velocity vs. others (by velocity, you mean ft/sec, not CFM).
Haven't used a meter.
But the MBR force of air & the perceived volume (w/ or w/o vent grill in place) is much greater.
Especially compared to say, Den & Brkf Rm vents, that have ~ 2 ft of * 7" flex *, coming off the 12x12 chase, going to that end of house.

Air from Den / Brkf Rm whispers out their vents (though I may ? have partly closed their dampers @ some point, to balance heating /cooling of rooms).
Comparatively, it blasts out the MBR vent.
But, for a given room w/ given heat load, sufficient CFM of air, at a *sufficiently cool* temp, is what cools a room. Not velocity. As you said, good velocity in the duct lessens heat gain.

Theoretically, there could undetected leaks in the MBR duct (I did add a 2nd layer of foil back duct insul, and taped all seams WELL).
But, unless leaks weakened the vent flow drastically (compared to other rooms), doesn't seem like it'd drop vent temp by 10 deg.
And flow from MBR vent seems far greater than others, not less.
Leaks might reduce CFM at that vent, where the room didn't cool. But we're talking air temp inside the vent being much too low.

1st, I'll temporarily drop the elevated section of MBR duct - straighten it & remove most curves (esp. the sharpest one).**
Make sure damper is full open (though that vent blows lots of air).
See if that lowers vent's temp - at all.

If no effect, the bend in the duct (or closeness to rafters) isn't the issue.

** Dropping the MBR duct - temporarily - will cause more slack in flex duct & create (some) more friction, from rougher walls.
But probably no more (maybe less) than re-routing it - different path, which would add * many * more ft of duct, plus adding a couple of turns.
The MBR duct can't just be laid near attic floor, near the front of the air handler. Would be seriously in the way.
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- AC temp at ONE vent 10 deg warmer than all others

Postby Freon » Wed Jul 01, 2015 2:24 pm

I understand your intuition regarding the "feel" of the air flow at various supply registers. The velocity (ft/sec) will tell you how long the air is in the duct and that may tell you if it's long enough for significant heat gain. Personally I don't think so but I don't know for sure.

Leaks will only affect the cfm coming to the MBR. Pressure in the duct will cause you to lose cold air, not import warm air.

How does the supply air to the MBR return to the hair handler? If there's any restriction (closed door or long hallway) that might affect air flow into the MBR. A restricted return would increase static pressure in the room and thereby reduce the cfm from the supple register.

You have me stumped.
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- AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Wed Jul 01, 2015 7:16 pm

- MBR vent - path back to return air.
MBR door stays open. The door opens into central hallway, on same wall as return air grill. Barely 1 ft between MBR door frame & return air. So, very short return path from MBR to return air. Why would that affect the air temp AT the MBR vent?

I ruled out any air leaking between the MBR wall box & return air chase. They're close, but separated.

RE- air bypassing evap coil (around frame) & some bypass air sucking directly into MBR takeoff collar.
The one temp I didn't measure, was a spot * directly between * the downstream side of e-coil & the MBR collar. I measured at a point to the side of MBR collar, 90 deg away (on a circle) from the LINE OF SHORTEST DISTANCE, between the collar & e-coil. That could be one error in measuring method.

Also, the velocity inside the plenum, on the top, right next to e-coil (near e-coil frame), may not be as great. If true, while the MBR vent isn't starving, its velocity/ CFM may be somewhat decreased. For that large a room & vanity area, w/ high ceilings, vent flow probably needs to be maximum possible.

Temporarily, I put a box fan (on low) in MBR, near opposite wall from the MBR vent; blowing across MBR, back toward the MBR door into the hall. Also, MBR ceiling fan runs 24/7 (sloped ceiling, from 8' up to 13'6").
Box fan maybe helped getting MBR room temp down a degree or 2. Even w/o the fan, it's NOT like the MBR is 82 while rest of rooms are 75.
But no denying the MBR vent is warmer than others.

RE Velocity- True, don't know the MBR velocity (ft/sec). I understand your comments & puzzlement (I'm puzzled - just clarifying).
But I can compare the MBR to the Den & Brkf Rm (7" ducts) & identical vent style / vent size as the MBR (or vents removed) .

The Den feeds thru ~ 16 ft of 12x12 chase - off the plenum. Then a short ~ 2 ft run of 7" flex off the chase.
Den vent's perceived air volume / force / flow, out of same 7" duct size (to any half-mechanically inclined person) is much less than the MBR. With or w/o grills, on either.
Standing on a ladder in front of each, or many ft away.

With obviously more air volume from MBR vent as other 7" ducts (Den & Brkf Rm), its velocity has to be * or > than the Den / Brkf Rm, or else it couldn't blow that much more air. That's the key, comparing same size ducts.
The 12 x 12 chase (144 sq.in.) supplies * 4 *vents in Den, Brkf Rm, DR, Kit. Each of those 4 has flex duct off the chase (Kit being longest, maybe 10 ft of 5 or 6" flex).

It'll cost a good bit in supplies * just * to test a theory, to run the MBR duct either to an existing collar on the plenum side, or install a new collar.
I doubt any other collars (right off plenum) are 7" - they're all to smaller BRs & baths. For testing only, maybe some fitting to go from 6" collar (outlet) up to 7"?

Maybe I could "borrow" some duct from a box store, if don't mess it up. But not cutting flex duct to right length & stretching fairly tight, would add friction (somewhat offsetting potential improvement, from moving the collar from top of plenum to the side - farther from e-coil). Maybe a couple HVAC places I know, would give some scrap 7" duct. Just to test the theory.
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- AC temp at ONE vent 10 deg warmer than all others

Postby Freon » Thu Jul 02, 2015 2:04 pm

"So, very short return path from MBR to return air. Why would that affect the air temp AT the MBR vent?"

If the MBR door were closed stopping the return air from going to the return duct in the hall, the pressure in the MBR would increase thereby reducing the air flow into the room.

At this point I would double check your temperature measurements at the supply register. The room temperature is irrelevant, only the air temperature at the supply register in the ceiling. Be sure the probe is into the register.

I'd call an HVAC company for a repair estimate to your duct system. See what they say is the problem. Make sure it's a free estimate. See what their temperature readings are. Based on their assessment, you may want them to do the work, or not.
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- AC temp at ONE vent 10 deg warmer than all others

Postby RunningLow » Fri Jul 03, 2015 6:48 pm

Thanks again. Hmmm... "free * repair * estimates" on very small amount of ductwork, in July... in deep South. Where they have to analyze the situation (to their satisfaction) before even deciding on a plan.
Not sure that animal exists. Could be wrong. I could ask. Free estimates are more likely for replacing condenser unit, or major work.

** Either way, probably 85 - 95% solved the mystery of why MBR vent is warmer than others (maybe 100%).
Took more plenum interior temps today.
Outside temps in low 90's, exter. humidity ~ 60%, so attic was pretty warm.

1. This time I compared plenum temps next to MBR collar, along the line of shortest distance between collar & e-coil.
2. measured (again) to the side of the MBR collar, 90 deg away (around collar's radius) from the direct line between collar & e-coil.
3. And (again) by a different collar off plenum's side (that supplies a "colder" vent, in a smaller BR, on same end of house as MBR).

Plenum temp -
A) With thermometer 5" probe inserted FULLY into -
* on the SIDE of MBR collar (90 deg away from shortest distance to e-coil) * 69 F
* DIRECTLY BETWEEN MBR collar & e-coil (here, there's only ~ 3 - 3.5" between edge of collar & e-coil) * 71 F
* right next to a DIFFERENT collar, off plenum's side (on the side of collar closest to e-coil) * *** 63 F *** Quite a big difference.
I repeated each measurement, verified similar results. Allowed time for each reading to stabilize.
- even w/ probe fully inserted, readings were obviously still affected (some) by attic air. Several vent temps are ~ 58 - 59 F, even w/ similar outside temps as today.

B) With tip of thermometer probe inserted only ~ 3/4" past inside wall of -
* on the SIDE of MBR collar (90 deg away from shortest distance to e-coil) * 83 F
* DIRECTLY BETWEEN MBR collar & e-coil (here, there's only ~ 3 - 3.5" between edge of collar & e-coil) * 86 F (also fairly significant)
* right next to a DIFFERENT collar, off plenum's side (on the side of collar closest to e-coil) * 82 F
Again, repeated each of those, to verify consistent results.

Seems we have the smoking gun.
Biggest question now is how to route the MBR duct, from side of plenum to wall box (only one side of plenum still has room for it).
If run it as straight as possible across attic (not arching up near rafters, then back down to wall box), it'll be insanely in the way. Possibly get damaged from people trying to get past / under it, or working on air handler.

Any other route, that doesn't take it as close to the rafters, but still isn't a major obstacle (accessing air handler), will add MANY more ft of run, plus at LEAST (2) 90 deg turns (even if large radius).
A little like some 3 Stooges creation.

** I also wonder if the MBR / vanity area doesn't call for LARGER duct than 7" (and / or 2 room vents, Y'd off a larger duct), with total 232 sf (NOT incl. MBR closet), sloped ceiling (from 8' to 13.5'), AND both ceilings follow bottoms of 2x6 rafters (less insulation * more load)?

MBR & vanity area are separated by a 7' wall (above that is open, up to the sloped ceiling), and has 3' W x 8' H passage between them (no door). Now, there's only 1 vent (off 7" duct) supplying both MBR & vanity. The very small MBa (shower / toilet) - next to vanity room - has a vent.
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