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Old 04-25-2017, 08:45 PM   #1
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Attic Supply Plenum replacement/Reconfig advice


Hello,

I have learned a lot here and appreciate everyone who contributes. I'm looking to have an AC Tech to come out and replace my Attic duct board supply plenum because of mold and the butchered installation that was done 2yrs ago. What led me to inspect the coil/supply plenum is the diff in temps from the left 12" Flex supply line (56F) to the master bed and bath from the right 10" Flex supply (59F) that supplies a bed room, Hall/Foyer and bath room which the rooms are always warmer.

I found mold in the supply plenum and the transition they made is not insulated. I want to get an idea of what should be done so when they get up there I will know if they are in the ball park.

Equipment:
York P3HUA12N04801B 80% Natural Gas Furnace
Inside coil ICP N Type 2.5 Ton TXV END4X30L17A1 Aprox 65ft of line set from condenser. Correctly offset to left as per manual
Outside condenser Heil 2.5Ton HSA630GKA100 10 Degree Sub cooling
14" main Flex return in hallway
8" Flex return from Master Bed

The unit has 4 Flex supply runs to cover 1500sq ft upstairs. 8", 9", 10" and 12". All Flex supply lines feed the triangle pie boxes then flex taps off to feed individual areas.

I think the unit should have been horizontally installed from the beginning as you can see the supply is almost touching the roof.

Should I insist on an insulated metal supply that extends to the left to the next supporting truss then reroute flex lines or replace duct board like current config?

I think the mold may be due to the coil being so close to the supply lines. Thanks for any advice you can provide.
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Attic Supply Plenum replacement/Reconfig advice-ac-attic-front-view-temps-.jpg   Attic Supply Plenum replacement/Reconfig advice-ac-attic-front-view-next-truss.jpg   Attic Supply Plenum replacement/Reconfig advice-inside-supply-duct-board-2.jpg   Attic Supply Plenum replacement/Reconfig advice-inside-supply-duct-board.jpg  

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Old 04-26-2017, 05:15 AM   #2
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Re: Attic Supply Plenum replacement/Reconfig advice


Have the case coil re installed properly, instead of being offset like it is.

Extending the plenum may or may not help. You don't have enough return with only 1-14" flex, and 1-8" flex. Flex duct does not move the same amount of air, at the same pressure, as the same size of metal duct.

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Old 04-26-2017, 07:27 AM   #3
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Re: Attic Supply Plenum replacement/Reconfig advice


Thanks for the reply. The coil case is actually level. It's the plenum box duct board that is all jacked up. They put a large metal pan on top of the coil and set the duct board plenum inside of it and didn't bother to cut it square.

To remedy this, I was thinking to replace the duct board on top of the coil with a metal plenum and extend it to the left about 80". This would allow me to reroute the 4 flex or install metal trunk lines away from above the coil and not have so many turns.

I performed some static pressure tests:
TSP bottom of furnace to below coil= .75" (spec is .5")
Return before filter= .25" (seems a little high hence more return?)
Filter= .15"
Duct board plenum in box above coil (supply)= .19"
Coil= .25" wet which according to coil spec sheet the unit is producing approx. 1000 to 1100cfm. The unit is not e
excessively loud but I can hear it running.
Blower compartment fan area= .35".

I live in Va. (humid) and was thinking of dropping the cooling from high to Med setting to help with dehumidification. I know I will loose some capacity but I think it's currently oversized for the 1300sq upstairs area.

How much more return should I have? Will changing the 8" to 10" line get me in the ball park?

Below is a pic of the internally insulated return plenum I installed. Originally they had the 14" flex connected straight to the side of the Furnace and the 8" connected to the back of the furnace. Really?
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Old 04-26-2017, 04:03 PM   #4
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Re: Attic Supply Plenum replacement/Reconfig advice


Look at your case coil, see how the left side of it over hangs the furnace, while the right side is flush. That is what I am talking about.

Increasing to 10" is okay, if your bedroom needs more return. other wise, it is better to increase the 14 to a 16, and increase return grille size.

You have an N coil. A 4" transition to expand from the furnace opening to the case coil opening should have been made.

I would redo that, and then probably do the extension like you said.
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Old 04-26-2017, 04:26 PM   #5
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Re: Attic Supply Plenum replacement/Reconfig advice


i would not cut the fan speed down much with a 2.5 ton a/c. i suspect you can go down to 900 cfm but not much lower.

if it's moving enough air, why re-do the plenum?

i would leave it alone and when the condenser fails, downsize then deal with any issues at the same time. you may only need 1.5 or 2 tons - as u said, probably oversized.
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Old 04-26-2017, 07:21 PM   #6
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Re: Attic Supply Plenum replacement/Reconfig advice


Quote:
Look at your case coil, see how the left side of it over hangs the furnace, while the right side is flush. That is what I am talking about
Ok, gotcha. I agree. At least they did follow the instructions that show if the coil is wider to offset to the left. Still not ideal. If I add a transition it will push the top of the duct board to the roof and make the 8" Flex going into the front impossible to connect because of the roof truss.

Quote:
if it's moving enough air, why re-do the plenum?
I'm trying to figure out why the right side 10" Flex supply that feeds the front side of the house bed rooms and foyer is about 3 degrees warmer then the left 12" Flex supply that feeds the back of the house Master Bed. I have not found any excessive duct leaks in this distribution.

Possible reasons:
1. Coil is offset to the left per instructions but is causing coil bypass on the right side.

2. No transition from furnace to coil causing excessive coil bypass

3. Insufficient charge. I just had my lower 4Ton unit serviced and asked them to check the upstairs 2.5Ton. We charged it to 10 degree Sub cooling per outside nameplate. It has about 65 feet line set with 25ft vertical run. I have read where the subcooling in this case may need to be increased for the pressure loss? It's only 2yrs old and during the hottest days of the summer it runs all day but otherwise has quick cycles causing higher humidity in the top floor.

4. Inadequate supply plenum causing air turbulence or flow issues

I really appreciate your input. Thanks

Last edited by snknby123; 04-26-2017 at 07:24 PM.
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Old 04-26-2017, 07:50 PM   #7
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Re: Attic Supply Plenum replacement/Reconfig advice


good chance that is causing an issue with uneven flow over each side of the coil.

when you got the charge checked i hope they checked low side pressure + superheat and not only subcool -> superheat tells you what's happening in the coil.

otherwise to my limited knowledge, all you know is that there's a full column of liquid going to the coil.

You don't know if the txv is under or over feeding the coil. either will change the subcool value.

or if it's properly regulating refrigerant flow but low airflow. (that would show up as low pressure but normal superheat, and I imagine, it would throw the high side pressure/temperature off with less heat being absorbed and the valve clamping down)
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Old 04-26-2017, 09:31 PM   #8
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Re: Attic Supply Plenum replacement/Reconfig advice


Ughh, No they didn't check SuperHeat.

so I guess I start with process of elimination, which would be the Superheat I guess, to see if that points to something.

What led me to have them check it was I noticed an odd noise from the liquid line. Not a steady stream noise. Sounded like it was flashing. Now it has a steady stream sound of refrigerant.

thanks

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Old 04-26-2017, 09:58 PM   #9
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Re: Attic Supply Plenum replacement/Reconfig advice


i don't know if under-feeding will cause temperature differences between sides, only the experienced pros would. it shouldn't because the coil is engineered with different smaller circuits, not one continuous run of pipe going through both sides.


they didn't hook a gauge to the low side, the insulated line?

check the temperature of it, it shouldn't be over 60f - ideally less, if the valve is working right it will vary a bit depending on the load on the indoor coil, getting colder as the return air temperature and humidity drop. It will maintain consistent superheat, but the low side pressure and hence boiling temp the superheat is referenced to will change, hence the variations.
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Old 04-27-2017, 02:50 AM   #10
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Re: Attic Supply Plenum replacement/Reconfig advice


With a 25 foot vertical rise, and 25 foot of horizontal run in the attic, you lose about 4 of subcooling.

Offsetting the coil can cause a supply located nearer the straight aligned side to be slightly warmer, then the side that is over hung on the furnace.
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Old 04-27-2017, 06:54 AM   #11
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Re: Attic Supply Plenum replacement/Reconfig advice


Quote:
With a 25 foot vertical rise, and 25 foot of horizontal run in the attic, you lose about 4 of subcooling
In these situations is it standard practice to offset the subcooling to compensate for the vertical runs?


I don't recall if they had a line on the low side. They were going to "beer can cold" the charge until I walked up and said we need to dial it in using Subcool method. He slowly added refrigerant while monitoring the liquid line temp and pressure until we met the name plate 10 degree subcool.

I went upstairs and checked the closest register temp and after 15min of running and it didn't change from the 59 degree temp I took the previous days. But it did stop the gurgling noise in the liquid line in the attic. Now it's a steady stream sound.

I'm gonna check the coil spec sheet and see what it says about extended and vertical runs.

thanks again
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Old 04-27-2017, 12:24 PM   #12
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Re: Attic Supply Plenum replacement/Reconfig advice


well, sounds like it was under-charged regardless.

but the low side temp should have been checked before correcting it.

and funny thing is, the tx valve will mask it unless it's really low and open up to compensate, the tech will just feel the line, see that it's cold and think it's okay.

just think about the number of people who get ripped off, paying good money for things being done improperly; this would never be tolerated in any other profession.
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Old 04-27-2017, 04:04 PM   #13
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Re: Attic Supply Plenum replacement/Reconfig advice


Quote:
Originally Posted by snknby123 View Post
In these situations is it standard practice to offset the subcooling to compensate for the vertical runs?
Its suppose to be done, but many techs don't know to do it.

Long horizontal runs in a hot attic are suppose to be insulated. Yes, the liquid line is what I am talking about. But often you can get a way without doing it.
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Old 05-11-2017, 08:51 AM   #14
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Re: Attic Supply Plenum replacement/Reconfig advice


Thanks for the feedback.

Over the 12yrs I have lived here I have had prob 10 different HVAC contractors come out. With the knowledge I have now from you good people, unfortunately, I couldn't recommend even one of them. Not one has ever mentioned the word air flow, static pressure etc. I have spent thousands of dollars over this time period and have crappy performing systems and HIGH electric bills. I'm totally fed up at being at the mercy of these hacks. I know there are good people out there. I just haven't found them.

This has driven me to purchase my own equipment to baseline both my split systems. I bought the Testo Bluetooth pressure gauges and temp probes.

Enclosed are the results from yesterday's pressure readings on this 2.5T unit. I figured out target SH at 9. OAT was only 70 degrees and the house was not under heavy load. %RH upstairs was 45%. WB at the return was 57. Temp delta (supply/Return) has always been low at 13-14 degrees. The first 5 to 10min of run time the SH was very high (>25) but everything seemed to level out after about 15min. The charge on the nameplate doesn't have liquid line pressure below 251psi so it doesn't seem like it's optimal conditions to charge?

High SH and Low Subcool is telling me undercharge as long as airflow is good. Is 3 degrees low on Subcool enough to cause my temp delta issue? My coil is just not getting cold enough to me.

Update: I spoke with a Co. yesterday and requested to speak with there best tech. He actually seemed very sharp, asking about my static pressure, return size and filter area etc. I have him scheduled to come out when it warms up and the house has a decent load.
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Old 05-11-2017, 10:22 AM   #15
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Re: Attic Supply Plenum replacement/Reconfig advice


take this with a grain of salt with the lack of real world experience:

the target sh is for fixed orifices only. your txv will maintain consistent sh. just my unqualified opinion - unless it's out of whack, like over 20 or below 5 and i think you're good.

i don't think subcool of 6 will cause issues as long as you have a full column of liquid at the coil. If refrigerant is flashing off in the lineset, i suspect you'll see a temperature drop in the liquid line by the time it reaches the indoor coil.

because when liquid flashes off due to pressure drop it draws heat from the remaining liquid to "boil".

The low side sat temp looks good at 39f. (possible it's a little low - 13+ seer units may run at higher suction temp, but than again your return air is cool)

And if you had low airflow you would have a high split across the coil, assuming everything else is working right.

There are charts you can search for that tell you what the supply temp should be at a certain wetbulb and drybulb return.

Assuming everything else is working properly, for better dehumidification you can cut the fan speed to get the target drybulb supply temp, re-checking pressures after.

But without actually knowing the cfm and wetbulb temps, you can't determine capacity so it's still a crap shoot setting fan speed.

btu/hr = cfm x 4.5 x (supply drybulb - return) (not 100% on the correction factor)

will be negative since it's removing heat.

You'll have better luck with more of a load on the coil i suspect.

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