AC Temp at one vent 10 deg warmer than others

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  #1  
Old 06-28-15, 05:17 PM
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AC Temp at one vent 10 deg warmer than others

The AC air temp *inside* the MBR vent is always 8 - 10 deg F warmer than inside all other vents; day or night. So MBR doesn't cool (as well).
This vent's been warmer for several yrs (maybe longer). Other rooms cool fine.

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

MBR 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.
- More air volume at MBR vent than most others - even rooms w/ same size ducts & vents.
- All ducts, plenum, evaporator coil & air handler are in attic.
- 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 flow than MBR vent (but they're 9 - 10 deg colder).
- All flex ducts are sealed w/ mastic & zip ties - @ plenum collars & ceiling boxes .

Differences in MBR duct vs. others:
1. MBR duct collar is ONLY one off the * top * of rectangular plenum. Collar is about 4 " downstream from where evap coil & plenum join. *(see note later).

2. All others come off plenum sides; including a 12 x 12 in. fiberglass duct board chase - feeds more distant rooms.

3. Of the rooms that have entirely flex duct runs (not partially using a chase), the 11.5' MBR 7" is the largest dia.
The den & Brkf Rm also have 7" flex duct / boxes, but most of their run is thru the 12x12 chase, then couple ft. of flex.

4. A few ft of MBR duct is strapped closer to the rafters, to allow crawling under. But * NO * measured attic temp difference near the highest part of MBR duct vs. others.

* (note on #1): Measured temp inside plenum w/ analog probe thermometer.
- Measured next to MBR collar & by another collar - on plenum side.
- Inserted probe same distance on each.
- NO plenum temp difference measured - either location.
+++
- Attic is well ventilated w/ continuous ridge vents & plenty of soffit vents.
Thanks.

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  #2  
Old 06-29-15, 03:43 AM
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I would relocate the MBR flex. Get it down lower, away from the roof.
 
  #3  
Old 06-29-15, 11:52 AM
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Thanks.
Relocating it lower down may be good - in theory (or not).
Again, no measured attic temp differences - at at highest point of MBR duct, vs. 6 - 12" above ceiling joists.
Whether on a hot day, or after sundown.
Sharp bend may be another matter.
One could argue, the duct near the roof gets more sun radiation heating.

Except, MBR vent is ALWAYS ~ 10 deg warmer, day or night. Sun radiation obviously isn't main issue.

I could lower it temporarily as a test (to eliminate sharp bend).
See if magically lowers temp at the vent.

Permanently lowering it (much) presents real problems.
- At 1 - 2 ft off attic floor, it'd be right in front of heater burner & blower inspection / access panels. Right where service tech (or me) has to kneel to work on the unit. Not good.
- 3 or 4 ft above floor, it'd really block getting across the air handler, to evap coil lines, drip pan drain, other ducts & rest of attic. Also not good.

- Unless rout the duct WAY around, adding several ft. Guessing? another 6 - 8 ft, likely more - plus another couple turns / bends.
- Moving it - much - toward camera (in pics) puts it over the attic stairs.
Stairs could be relocated - 3 - 4 ft back from air handler (allow room to step over the lowered duct). Before anything that drastic, need hard evidence the sharp bend is main issue.
 
  #4  
Old 06-29-15, 02:40 PM
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It could be because the duct is connected too close to the E-Coil & there is some air bypassing the coil that that duct is picking up. I'd move it downstream further & on the side if possible.

The type of takeoff collar can be important, I'd want one that covered a larger area of the main duct shaped to angle the air efficiently into the duct; also one with a short (TEL) Total Equivalent Length Rating.

Plus the fact that the duct has turns in it & goes up & down which creates resistance & slows air velocity...since it has a short physical run it should be made to deliver air as cold as the others.

Get an air balancing expert out there to evaluate the situation; my ideas may not solve the issue.
 
  #5  
Old 06-29-15, 06:12 PM
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I'll do easiest steps 1st.
Lowering the existing duct (straightening bends) is quick.
And checking the damper's full open (doubt that's the issue).
Maybe those will do it.
If being straight & away from rafters has no effect, on to plans B, C, D...

But I've slightly closed dampers on 1 or 2 other rooms (AC too cold & heat too hot). And that didn't raise their AC vent temps by 10 deg (or any). That's what dampers are for.


could be because the duct is connected too close to the E-Coil & there is some air bypassing the coil that that duct is picking up.
I've seen that hypothesis.
Problem there is, I checked temps inside the plenum - inserted an analog probe thermometer.
Right by the MBR collar & one on the plenum side (that fed a room w/ "cold" vent).
- No difference in temp by either collar.

Now, I didn't insert the thermometer 5" probe fully, but inserted it the same depth by each collar, on repeated tests. Wanted to test the temp right where it entered collars (not 4" below them).
Part of the probe being exposed to attic air may've raised actual readings, but would be the same change for either collar, since I inserted it the same depth each time.

Could there be a flaw in that testing method? Maybe, but inserting the probe several in. past / below where air actually enters a collar is also not ideal. I could stick the probe thru side of duct, just above collar, but I'm not crazy about punching holes in the plastic duct. Partly from fear of starting a hole that turns into a rip.

Anything's possible, but I'd say by inserting the probe the same depth by each collar (several times each), always with identical readings, seems fairly conclusive. But maybe not.

I'd want [a collar] that covered a larger area of the main duct shaped to angle the air efficiently into the duct;
Don't follow "cover larger area of the main duct." You mean, a longer neck on the takeoff collar, extending farther into the duct? I get the angled part (45 deg, etc.).

Or, just mean that if the duct needs to leave plenum at ~ 45 deg, then the duct will fit better over an angled collar w/ less bending?

"Air Balancing Expert" - they ALL claim to be experts (over the phone). No one that's looked at it had a clue. How to find a true balancing expert, or what they might cost?
 
  #6  
Old 06-29-15, 07:08 PM
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If identical temps are entering the duct, it would have to be picking up heat somewhere on the way to the diffuser. It is a real puzzle...
 
  #7  
Old 06-29-15, 08:03 PM
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Well... I said the plenum temps I measured by each collar were identical.
If there's a flaw in the technique, well...

Just now, had an idea - to check tomorrow. The location of the MBR wall vent in relation to return air chase. They are very close - will investigate if any air flowing between them. Builder made plenty of mistakes on lots of stuff - anything's possible.

Tonite, dropped the MBR duct straps - eliminating the bulk of that sharp bend, but it's still not perfectly flat, straight. Now, it's a much more gradual curve.

Verified the damper was always wide open (but damper's old & F'd up - like me).
Took s lot of cussin & decades experience of messing w/ old, broken stuff to get it back to fully open.

So (appears) not to be a shut damper, or the sharp bend (by itself).
Could be - somehow - air off the top of plenum AND close to the evap coil is actually warmer. But 10 deg??? That's a lot, inside a tiny box where air is really moving & bouncing. Still, all other things considered (& examined), this seems more plausible than many ideas.

Could also be they didn't stretch the MBR duct nearly as tight (due to arching it way up, then down) - creating more friction from NOT fully extended it. I have trouble believing extra friction from not being fully stretched (over 11.5 ft), raises temps by 10 deg, when air blasts out the vent.
 
  #8  
Old 07-08-15, 08:48 PM
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Mystery solved? Probably.
Internal plenum temps directly between the MBR collar (on top of plenum) & the e-coil are considerably higher than by collars on the side of the plenum.

5" thermometer probe inserted fully,
* between MBR collar & e-coil = 71 F (partly affected by attic temps).
* Between a collar on plenum side & e-coil = 63 F (* also partly affected by attic temps). * Because, temps at all room vents are colder than 63, except for MBR.

That 8 deg temp difference is almost the same as between MBR vent & others.

Can theorize why, or just move the MBR collar.
Best guess - some air bypassing e-coil, at top, near the MBR collar.
Or, turbulence from collar being inches from the e-coil.

I'll move the MBR collar to the plenum side, farther from e-coil.
And use higher R-value duct, making sure there aren't sharp turns.
 
  #9  
Old 07-09-15, 05:28 AM
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Mystery solved? Probably.
Internal plenum temps directly between the MBR collar (on top of plenum) & the e-coil are considerably higher than by collars on the side of the plenum.

5" thermometer probe inserted fully,
* between MBR collar & e-coil = 71 F (partly affected by attic temps).
* Between a collar on plenum side & e-coil = 63 F (* also partly affected by attic temps). * Because, temps at all room vents are colder than 63, except for MBR.

That 8 deg temp difference is almost the same as between MBR vent & others.

Can theorize why, or just move the MBR collar.
Best guess - some air bypassing e-coil, at top, near the MBR collar.
Or, turbulence from collar being inches from the e-coil.

I'll move the MBR collar to the plenum side, farther from e-coil.
And use higher R-value duct, making sure there aren't sharp turns.
I like your analysis. Those factors can have a big effect on the air going to the MBR. I believe U said the airflow velocity was also higher to the Master bedroom?
 
  #10  
Old 07-09-15, 10:17 AM
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U said the airflow velocity was also higher to the Master bedroom?
Yes, by "relative comparison" to other vents, esp. same 7" size ducts & same size vents. On a ladder, with or w/o grills on wall boxes. Not a scientific measurement, but a noticeable difference.

** A BIG question is, whether the adjoining MBR + Vanity / dressing room, needs > 1 vent, and / or larger than 7" flex?
* Whether (say, 8") that Y's to 2 vents, or 2 runs of smaller flex, to 2 vents, etc.

Because of higher load factors in those 2 rooms vs. the other large rooms.

* Biggest load factor - MBR / Vanity have sloped ceilings - follow the 2x6 rafters, the full width of both rooms (max 13'6" Ht).
Only these 2 rooms have NO attic to vent air; AND - just 6" batts of rock wool (now 39 yrs old).

From attic side - batts don't look seriously settled, but still < R-19. Not much for hot, humid South. On hot days, those rooms heat more & faster, than others. Even w/ MBR facing N side.
In evening / morning, they're very close to temps in other rooms.

* Only 1 vent supplies both rooms (250 SF, total, incl. closets). MBR & Vanity are separated by 7' high wall. Above 7', it's open - up to the sloped ceiling.
There's 3' wide passage (no door) between MBR / Vanity. So, not perfect circulation between them.

* Vanity "room" has (1970's) 6'x6', single pane, alum. sliding glass door; faces N. Somewhat leaky by today's stds, plus 39 yrs old.

HVAC business (owner) coming tomorrow - give est. on moving the MBR collar / duct. Maybe ? he can actually measure flow / CFMs from current 7" & tell if need considerably more than now, for given load factors.

From forums feedback (say they're "pros") - NO one agrees on CFMs or # vents needed in these rooms.
 
  #11  
Old 07-09-15, 04:48 PM
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I agree that it's way too high at the peak of the roof. The ductwork is probably hot to the touch. Get it lower or use Flex with a higher R Rating.
 
  #12  
Old 07-10-15, 03:12 PM
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Contractor (owner) came. Decision is now whether to spend $ - in Summer, on a 39 yr old air handler, plenum & duct system, that probably should be replaced this Fall. If only because of heater's age.

Didn't try to sell a new system or bunch of extra work (refreshing).
Price quotes:
* Move the MBR collar; new collar w/ damper; new flex (~16 ft) to existing MBR boot / vent = $65 total.
Yes, it's easy access location, quick job for pros. My retail materials cost would probably be that much.

* Install a new (additional) 6" duct run (? ~ 20 ft), start collar w/ damper, wall boot, vent grill = $225 total.

But, this is PEAK AC repair & installation season.
Maybe some HVAC pros (in areas w/ hot summers, & v. mild winters) can comment, to replace air handler, plenum, ducts - how much lower quotes might be in "off season" vs. peak cooling season?

Guessing, quotes could be hundreds less in Oct / Nov?
Here, mild heating season starts ~ mid Dec.

Yes, be nice to get the MBR a little cooler, more dehumidified now (not unbearable). Paying ~ $300 for that now may be OK, if lower bids on partial system replacement in Oct / Nov, offsets $300 now - on an old system. (Still cools rest of house fine - MBR is OK at night.)

Contractor looked at MBR / Vanity Rm layout & plenum, ducts.
He was stumped why MBR vent temps are warmer than others. Did not think that (part) of the duct near rafters was why. Besides, MBR vent's still warmer at 10 PM. Also, short run - heavily insulated.

He didn't "disbelieve" plenum temp measurements were ~ 8 F higher at the MBR collar than on plenum's side. Just that "it shouldn't be so." Saw no obvious problems to explain it.

Said, even if get the MBR vent temps back down near all others, did NOT think one 7" duct / one vent was ever enough, given load factors & size of these 2 rooms.
His suggestion - add 2nd duct / vent to MBR - 6", for extra CFMs the rooms really need.
He'd move the MBR collar to plenum's side, if I wanted.
 
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