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Old 05-11-2017, 04:29 PM   #16
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Re: Attic Supply Plenum replacement/Reconfig advice


With only 6F of SC. You have flashing liquid flashing to vapor at the TXV. Your losing 3 to 4F of SC on your vertical rise height alone. head/liquid line pressure is way low for an outdoor temp of 73.

Your losing another 3PSIG for the length of your line set. So by the time the liquid has reached the TXV, you have lost 5F of SC. And that doesn't take into account any heat gain that the line set has on warm/hot days.

Since you have a TXV, ignore the target SH.

Your unit should be fully stabilized after running somewhere between 15 and 20 minutes of run time.

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Old 05-15-2017, 07:45 AM   #17
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Re: Attic Supply Plenum replacement/Reconfig advice


Quote:
With only 6F of SC. You have flashing liquid flashing to vapor at the TXV. Your losing 3 to 4F of SC on your vertical rise height alone. head/liquid line pressure is way low for an outdoor temp of 73.
In your opinion what are my options here? The nameplate has a spec of 10 SC with +-3 degrees. So I want to be on the high side of the spec of 13 degrees SC I would think to help compensate for the vertical rise?

I also found the TXV bulb was insulated but the end of the buld/insulation was open not sealed to the suction line. I added some insulation and sealed the end. Not sure if this was causing an issue or not.

Thanks everyone for your input.
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Old 05-15-2017, 04:58 PM   #18
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Re: Attic Supply Plenum replacement/Reconfig advice


Well, if your going to have a few warm days this week like my area. When its 80 plus outside, get your SC up to 10 first. Let system continue to run for another 15 minutes. Check liquid line temp at condenser and evap coil, they should be the same temp. If warmer in the attic, add charge to get to 13 SC.
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Old 05-19-2017, 09:20 PM   #19
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Re: Attic Supply Plenum replacement/Reconfig advice


It warmed up so I have some updated info. I have a Tech coming out Tuesday next week to hopefully dial this in. With my recent coil temp readings I discovered that the majority of my temp differences from my ducts is heat infiltration into the flex duct from the attic. I'm losing about 2 to 3 degrees depending on time of day and OAT.

Conditions during test:
I moved Fan speed to "Med"
2 month old Aprilaire 2410 4" MERV 8 filter
TESP=.55 WC" Furnace data sheet puts this between 938-994CFM
Coil Pressure drop (wet)= .2 WC" Coil data sheet puts this at 900-975CFM
I think volume of airflow can be ruled out. If I use 950CFM that puts me at 380CFM per ton.

OAT= 87.5F
Attic Temp= 105F
Condenser discharge temp 97F. Puts it right at 10F for a 16SEER. Maybe low?
Indoor DB= 73.2F with 49.7% RH measured in return before coil
Indoor WB= 60.9 measured in return before coil
Coil discharge DB = 57.1F 65.4% RH (in plenum above coil)
Coil discharge WB =50.2F (in plenum above coil) From the Enthalpy chart the DB and WB puts me at 389CFM/Ton if my calculations are correct.

Here are the pressure readings: Now with a decent load it looks like I'm in the ballpark as far as charge but I think I'm on the low end of the spec. I'm hoping the Tech can add some refrigerant to get my coil discharge temp closer to or below 50F as my humidity always seems to be hovering in the 50% to 55% range. Not much condensate comes from the drain. Just a steady drip..drip..drip

I did notice that my suction temp leaving right at the evap coil would cycle from 55.5- 58 degrees. SC seemed to mimic this as well. Not sure whats going on here. I forgot to get the liquid line temp at the Evap!
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Last edited by snknby123; 05-19-2017 at 09:36 PM.
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Old 05-19-2017, 09:30 PM   #20
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Re: Attic Supply Plenum replacement/Reconfig advice


Just want to add a FYI on these adapters. For the Novice like me it makes it so easy to connect my Testo Bluetooth gauges with almost zero refrigerant loss.
Screw it on to the service port, screw on your hose or Testo device and then just screw in the thumb screw to depress the schrader valve in the service port. Barely had a hiss from the high side when removed. It pleased me greatly.
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Old 05-19-2017, 10:50 PM   #21
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Re: Attic Supply Plenum replacement/Reconfig advice


well, this says it should be closer to 53 to 54 according to this: http://markbeiser.com/HVAC/documents/msc/TEET.jpg

maybe it's heat gain from the plenum.

but then i calculated btu per hour at 950cfm and got 28770 btu/hr which is pretty good for a 2.5 ton. maybe a tad bit low if it's under 90 to 95f outside.

If you look at product specs, you'll see that the ratings aren't a perfect 12 000 btu per ton any more. 2.5 ton can rate out at 28k and the split should reflect that.

so you probably just had too much airflow with nothing else wrong. possible you have too much heat gain through ducts but the machine itself is working fine.
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Last edited by user_12345a; 05-19-2017 at 10:58 PM.
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Old 05-20-2017, 07:51 AM   #22
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Re: Attic Supply Plenum replacement/Reconfig advice


Quote:
well, this says it should be closer to 53 to 54 according to this: http://markbeiser.com/HVAC/documents/msc/TEET.jpg
Agreed.

Reading up on the subject about high humidity problems and the general consensus seems to be to get the coil temp down to 50 or less for optimal moisture removal. My original 2.5T always had a steady stream from the drain. This unit is just drip, drip drip.

For comparison, the bottom floor 4Ton unit suction line at the coil is much lower at 48-49F and has a steady stream of water from the drain. Keeps RH on the main floor between 45 and 49%.

Hopefully the Tech will have something up his sleeve. Thanks!!!
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Old 05-20-2017, 12:34 PM   #23
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Re: Attic Supply Plenum replacement/Reconfig advice


R410A performance rating is at a 45F evap coil. R22 was at 40F.

The temp wing you saw was indicative of a low charge.
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Old 05-20-2017, 01:52 PM   #24
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Re: Attic Supply Plenum replacement/Reconfig advice


this is a long shot - but if your unit has an adjustable txv maybe your tech can get the superheat down from 15 (maybe in the 7 to 10 range), because once all the liquid is gone the refrigerant rapidly warms up.

Could help with dehumidification.

but this would only apply if the charge is right to begin with and it's getting full liquid.

It may also be worth looking into adding a humidistat in the return. There could be a spdt relay where an even lower speed is connected to normally close terminal and on humidity fall, the relay gets energized and brings on high speed. ...and you get the best of both worlds.

at 350 cfm per ton you can go down to 875cfm.

Worse dehumidification is the dirty secrete of high-seer the manufacturers will never talk about. The evaporator is bigger relative to compressor capacity so the coil runs at a higher temp.

The bigger coil may take longer to get down to temperature and holds a lot of water before condensate can start running down the drain.

...and if you have to turn the fan speed down or set it at a lower temperature to be comfortable, any savings evaporate.

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