Author Topic: AC air temp - inside the vent - hotter in one room vs. all others  (Read 3143 times)

Offline wizedup

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The AC air temp inside the MBR vent / register is between 8 - 10 deg F warmer than all other vents in the house (measuring inside the vents / ceiling boxes). So MBR doesn't cool as well as other rooms (even larger ones).

I wonder if the only / most likely explanation is, there IS a split in the flex duct, sucking in attic air?
Never found any evidence leaks, splits anywhere on the MBR duct, or its connections.
Inspected every inch of it (several times), but can't see past the insulation covering the duct.

MBR is one of the shortest duct runs. The temp at Kit. vent - the longest duct run is still 8 - 9 deg F colder (it's probably 3x the MBR run) .
All duct work, plenum, evaporator coil & air handler are in the attic (as most homes in this region).

- MBR flex duct is 7 in. Same as the 2 ducts supplying den / breakfast area.
- Plenty of air volume at the MBR vent (far more than at den / breakfast area).

- The MBR vent / grill is 8" x 14" (max), covering the ceiling box (box comes thru a high wall, not ceiling). Same grill size as in den / breakfast area.
- 11.5 ft duct run (near the shortest to any room).
- Duct is sealed at both ends w/ mastic & zip ties (@ collar off the plenum & at ceiling box).
- Same type duct as most rooms.

Couple of differences in MBR duct vs. others (I doubt would cause this much difference) :
- MBR duct collar comes off the top side of the rectangular plenum (duct board).
- All others come off the plenum sides, including an approx. 12 - 14 in. sq., fiberglass duct board chase - that feeds some more distant rooms.

- a few ft of MBR duct is strapped up closer to the rafters, to allow crawling under it, then across the air handler.
- MBR duct is closer to the gas central heater / air handler than all other ducts (parallel to it, but not directly over).

The MBR duct leaves top of plenum, rises toward rafters, then back down to attic floor.
That puts the MBR duct maybe 4 ft. higher above the ceiling joists, than any other duct (and that's only at its highest point, for a few ft out of its total run).

But the attic is well ventilated w/ continuous ridge vents & plenty of soffit vents. I can't detect any "hot spots" in the attic.


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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #1 on: June 24, 2015, 01:05:49 PM »
Is the MBR flex covered by attic insulation or is it strapped above the attic insulation?

Normally when using flex we can't install balancing dampers and all flex runs should be the same length (around 10') to ensure our airflow is balanced.

If the MBR flex had a breach it would not suck in air.  The vent has a positive pressure so it would simply leak air into the attic.

A side or top take off can both be used and won't really affect airflow.

I assume the MBR flex is already insulated but if it's not covered with attic insulation like the other flex ducts then it could be losing hot or cold air before it reaches the wall vent.  It could also be that the airflow is too high.  You may want to try reducing the airflow thru the MBR flex which should increase the cooling airflow temperature.  You could install a small section of 7" metal duct with a volume damper in the MBR flex and adjust the airflow down until the supply temperature increases.  Just make sure that metal duct gets insulated after the damper is adjusted.  Or replace the MBR flex with one as long as the others rooms.  Just leave the flex coiled in the attic and the airflow should be similar to the other rooms if the duct is the same length and size.

Offline wizedup

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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #2 on: June 24, 2015, 08:56:36 PM »
You may want to try reducing the airflow thru the MBR flex which should increase the cooling airflow temperature.
You're saying, reducing airflow thru the duct should lower temperature at the vent?

Why would more air volume and / or velocity - moving thru (MBR) duct raise temps at the vent, by a whopping 10 deg F?
In just 11.5 ft run of 7" duct?  I'm just asking.
** How to determine optimal balance between colder air at the vent, but much less air volume into the room? 
The former will improve overall room cooling; the latter will decrease room cooling?  Or is that incorrect?

All the (flex) ducts have dampers, incl. MBR.  I used them to balance the rooms - yrs ago.  And, cooling in all other rooms is fine.
Maybe ? was a mistake to allow more air into the MBR / vanity? 

I probably figured, since only one vent (7" duct) supplies the MBR & adjoining vanity area, needed more air volume from the one vent. 
Total floor area of MBR & vanity = 232 SF.
Both have sloped ceilings.  In MBR, ceiling slopes from 8' at outside wall, up to 13.6' on opposite wall (which is where vent is).

ALL runs of flex duct are strapped above the attic insulation.  The other flex ducts are all strapped - about 6" - 12" above the ceiling joists.
None are covered by the loose, attic insulation (except at the ceiling boxes).

Yrs ago, I wrapped & aluminum taped an extra layer of duct insulation, over all flex ducts' existing factory-applied insulation.  Because the factory insulation was so thin/ low R-value.
As said, the MBR duct is only one that arches up toward rafters (to allow getting under / past it).
But it's only "nearer the rafters" for a few ft of its total run.  At it's highest point, it's only ~ 3 - 4 ft higher (above joists) than all other ducts.
Today, I measured same attic 94 F, both at apex of MBR duct, and ~ 1 ft above ceiling joists.

Much more air comes out of MBR vent, than the 2 vents in the den / breakfast.  But, den & breakfast Rm are much farther from the plenum. 
The den, breakfast rm, DR, Kit - are all supplied thru a 12" x 12" duct board chase, coming off side of plenum.
Then flex ducts (varying lengths) come off the chase, to those room vents.  (I believe they all have dampers).

All 3 BRs, both baths - have flex duct straight off the plenum (using metal collars in the duct board plenum).

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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #3 on: June 24, 2015, 09:32:30 PM »
Swap the MBR flex with the flex from a side takeoff.  It's possible some return air is bypassing the evaporator coil which could be lowering the supply temp out of the top take off. 

Offline wizedup

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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #4 on: June 24, 2015, 10:40:11 PM »
OK.  Is there an easier way to check the air temp, coming off the top of plenum (very near the evap coil), vs. a duct (collar) on side of the plenum?  Given they're all sealed w/ mastic? 
I want to find an answer, but it'd be a chore to remove the mastic, on 2 ducts / 2 collars, just to check temperature difference (w/o knowing there is any).

Just an idea - maybe use a probe digital thermometer & drill / poke tiny holes in the plenum - near the "test" collars?
Seal the holes later.  Or maybe there's a better way?

I didn't have time to close the damper on MBR duct (could tmrw), but tried restricting flow using the adjustable vent (all the vents are adjustable).
In brief test, didn't seem to lower temps at the vent.  But restricting flow at the vent may not have same effect as using a damper?

I've wondered a long time if air into the duct, coming off the plenum top, very near the evap coil, could actually be warmer.
Guess it depends on how much air mixing happens inside the plenum - if any, before entering the MBR duct.

Offline wizedup

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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #5 on: June 28, 2015, 06:08:30 PM »
Measured temps inside the plenum 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 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 MBR flex duct connection to wall box (again) - from attic side & removed 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 & 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.

Offline wizedup

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Re: AC air temp - inside the vent - hotter in one room vs. all others
« Reply #6 on: July 14, 2015, 08:02:15 PM »
I may see why temp to the MBR is warmer (even inside the plenum). Don't know if there's any decent short term solution, except replace plenum (& probably air handler, coil, ducts).

See pic - how top of plenum is even with or tad higher than the coil's outer case.
On the sides & bottom, the outer surface of plenum is smaller than the coil's outer case.

Someone (maybe several) mentioned return air could be bypassing the coil, between TOP of plenum & top of coil. That could be right. Unless the top of coil is made differently than it's sides (where the plenum fits against it).

On the bottom, the plenum is considerably higher than coil's outer case. Much more so than the side toward camera.

I don't know if these coils have "flanges" or something for the duct board to rest on / against.
Or, the installer takes his best shot, making sure the coil's centered, and the plenum fits it correctly.

Based on how much higher the bottom of plenum is than bottom of coil, compared to the top, only guessing the installer didn't block up the coil high enough.

Air bypassing top of coil would explain everything I've observed & measured temps being warmer on the top of plenum, right next to the coil, vs. much colder on plenum sides.

Short of replacing system; else removing all transition, then adjusting height of coil to properly fit against plenum, not sure anything (easy) can be done.