# Theories on Temp Drop Please

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• 07-14-2013, 04:06 PM
Southern Mech
Quote:

Originally Posted by SonicExplorer
Update:

Today I pulled one register and the boot/box is flush to the drywall. I also put the thermometer up into the duct, well past the boot. Same result, still gaining about 5 degrees. So the boot is not the main source of infiltration. I also went over to the other neighbors' home who's house is exactly like mine, and who had her ducts re-fastened as well, and she has the same anomaly going on, gaining about 5 degrees between the AHU supply and the register of the shortest duct run.

So....mystery remains.

Its not a mystery. You are picking up heat from the attic. Cool the attic down. Add a attic fan.
• 07-14-2013, 06:07 PM
Shophound
Measure your temperature rise with the system running late at night. Then measure it again in the middle of the day. Post your findings here.
• 07-17-2013, 09:05 PM
SonicExplorer
Okay, here's the numbers. It has been a bit overcast during the days lately so the attic temps are about 10 degrees lower than normal, but at least you can get an idea....

Night: Attic Temp = 80f, AHU = 50f, Duct (< 10ft away from AHU) = 53f (Diff of 3f)
Day: Attic Temp = 115f, AHU = 52f, Duct (< 10ft away from AHU) = 56f (Diff of 4f)

From past measurements, the diff increases another 1f (total of 5f) when attic temps are at true peak of about 125f+ This is where I came up with the original comment that 5f is being gained between the AHU and the nearest duct run (less than 10ft). And to recap, the part that is thoroughly perplexing is that during the day when temps are measured at the FARTHEST duct (about 50ft) the total gain is only about 2 degrees more than the shortest run, meanwhile the SHORTEST run is gaining 5 degrees being less than 10 ft from the AHU. ???? From a layman's perspective, the logical conclusion seems that something near the AHU is introducing about 5f gain into the entire system.
• 07-18-2013, 04:22 AM
dan sw fl
...Very Low air flow?!

... Junction boxes and flex duct will ALWAYS be an issue.!

... A series of junction boxes is a True KILLER.!
• 07-18-2013, 08:17 AM
hvacrmedic
Quote:

Originally Posted by SonicExplorer
Okay, here's the numbers. It has been a bit overcast during the days lately so the attic temps are about 10 degrees lower than normal, but at least you can get an idea....

Night: Attic Temp = 80f, AHU = 50f, Duct (< 10ft away from AHU) = 53f (Diff of 3f)
Day: Attic Temp = 115f, AHU = 52f, Duct (< 10ft away from AHU) = 56f (Diff of 4f)

From past measurements, the diff increases another 1f (total of 5f) when attic temps are at true peak of about 125f+ This is where I came up with the original comment that 5f is being gained between the AHU and the nearest duct run (less than 10ft). And to recap, the part that is thoroughly perplexing is that during the day when temps are measured at the FARTHEST duct (about 50ft) the total gain is only about 2 degrees more than the shortest run, meanwhile the SHORTEST run is gaining 5 degrees being less than 10 ft from the AHU. ???? From a layman's perspective, the logical conclusion seems that something near the AHU is introducing about 5f gain into the entire system.

Using the temp rise formula

deltaT = kw X 3413 /(CFM X 1.08)

Assuming something like 1 kw for the motor and 1400 CFM for airflow

deltaT = 2.26° rise due to fan motor heat.
• 07-18-2013, 10:43 PM
SonicExplorer
The measurements taken at the AHU were taken directly ABOVE the AHU, so any heat generated by the fan is already largely reflected in the readings.
• 07-30-2013, 05:24 AM
az10101
This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

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• 07-30-2013, 05:57 AM
beenthere
az10101, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary or ask questions of the OP here.

You can find the rules for posting and qualifications here.

Further infractions may result in loss of posting privileges.
• 07-30-2013, 06:28 AM
az10101
This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise, commentary, or ask questions of the OP here.

You can find the rules for posting and qualifications here.

Further infractions may result in loss of posting privileges.
• 07-30-2013, 06:38 AM
beenthere
• 07-30-2013, 10:05 PM
SonicExplorer
New finding that I need to test again tomorrow, but it seems something else additional (or related) may be going on...

The measurements I had taken for the purpose of this whole discussion was after the system had been running during a cycle for 10+ minutes or so. However, today I tested (just by feel) the air coming out of various ducts within the first minute or so of a cycle starting. And what I found is that the air coming from the registers via ducts that run directly from the supply plenum which the AHU is connected to seem to very quickly begin blowing cool air. Not cold, but cool and certainly not warm or hot. HOWEVER, all the other ducts on the far side of the house blow hot air initially and then warm air for a good minute or two before starting to cool. Now, all those particular ducts have one thing in common, they connect to another juncture box that has a "main run" that connects to the supply plenum over the AHU. The extra length of those longer ducts is not much, not enough in my mind to possibly be making the difference I'm feeling between those that connect directly to the plenum over the AHU. To a layman like myself, this is telling me that maybe I should give a good once-over to that second juncture box? That box by the way is as high in the attic as any duct, and higher than the supply plenum box over the AHU. Is it possible the extra heat at that higher elevation may be at play as well? Do those boxes that are made out of duct board tend to be poor on keeping heat out, say worse than the R6 ducts themselves? If so, would it hurt to add some thin insulation on/around areas of that juncture box? That thing really should have probably been put on the floor of the attic. Why they left it elevated and with secondary runs coming off from above is beyond me.
• 07-31-2013, 12:09 AM
Shophound
Is that second junction box shaped like a triangle? If so, my condolences. If not, I am still sorry for your loss.

Air in motion within a duct can thermally stratify. I may have said this earlier in the thread but it bears repeating. The section of ducts that run warmer may have nothing to do with how short or long they are, but rather what kind of air they got from the main supply plenum. If it wasn't 55 degree air (more or less) that entered the secondary run, then it's a combination of coil bypass, gain through the insulation via a hot attic, and thermally stratified air not mixing completely as it heads toward the second junction box.

If you are eaten alive with curiosity, get a long temperature probe and start poking your ducts full of holes in various places to measure air temperature. Just patch the holes when you finish. If that sounds extreme for you to do yourself, it should. You would be better off getting a better duct design installed.
• 07-31-2013, 01:20 AM
SonicExplorer
Quote:

Originally Posted by Shophound
Is that second junction box shaped like a triangle? If so, my condolences. If not, I am still sorry for your loss.

Yes. Triangle shaped. One big duct entering and 4 smaller ones exiting (2 each on the other two sides) And, the box is almost twice as high as it needs to be. The top of the biggest duct connects at a point not much above the center. So I could wrap the entire upper half of the box in insulation if that might help. The top of the box is less than 10" from the roof decking.

Theoretically, I could alternatively saw off the top half and re-mastic the top back on so the extra box space is no longer there, and it would then also be further from the roof decking. That's more than I'd probably feel comfortable risking though.

If you think the insulation idea might help please let me know. I could also put a piece on top of the main supply plenum. As long as that isn't going to create some strange vapor/condensation effect of any sorts it would be easy enough to do. Or, I could get more duct board and put another layer or two in those areas.
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