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  1. #1
    Join Date
    Jul 2012
    Location
    Central Illinois
    Posts
    5

    Confused Is this system NOT putting out proper BTU?!?

    I installed a new Am. standard 2 ton 410A system, single stage 65k BTU firnace and cased txv coil. It runs non stop and rarely satisfies the stat on a 975 sf ranch home with no basement. Check out these measurements, any clues?

    Ambient air temp: 99 deg
    Suction 150 PSIG. 68 degrees with 13 degree superheat
    Liquid 377 PSIG. 101 degrees with 12 degree subcool

    Compressor drawing 13 amps

    Dry bulb temps: return 77.6 deg
    Supply 61.8 deg
    Wet bulb temps: return 68.5 deg
    Supply 61.3 deg (all averages!)

    What the heck am I missing?! Is there WAY less insulation in the walls or is this unit just not producing?? Oh, and these temps are with the fan speed set to LOW! Replaced the TXV and factory said we are moving too much air over the coil lol!? Really? Too much return air? Any help will be greatly appreciated!

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,960
    Quote Originally Posted by Thardwick View Post
    I installed a new Am. standard 2 ton 410A system, single stage 65k BTU furnace and cased txv coil. It runs non stop and rarely satisfies the stat on a 975 sf ranch home with no basement. Check out these measurements, any clues?

    Ambient air temp: 99 deg
    Suction 150 PSIG. 68 degrees with 13 degree superheat
    Liquid 377 PSIG. 101 (?)degrees(?) with 12 degree subcool

    Compressor drawing 13 amps (around 8-amps for those conditions; what is its amp Rating?)

    Dry bulb temps: return 77.6 deg
    Supply 61.8 deg
    Wet bulb temps: return 68.5 deg
    Supply 61.3 deg (all averages!)


    What the heck am I missing?! Is there WAY less insulation in the walls or is this unit just not producing?? Oh, and these temps are with the fan speed set to LOW! Replaced the TXV and factory said we are moving too much air over the coil lol!? Really? Too much return air? Any help will be greatly appreciated!
    Okay, 377-psig condensing pressure is 112.5-F; not 101-F(?). What was the temp of the liquid line?
    Therefore 112.5-F CT with an (?)89-F(?) liquid line is 23.5-F subcooling which would be way too high; indicating an overcharge. (This could be be wrong, as I don't know how you figured the subcooling???)

    Well, the indoor humidity seems to be around 63.5%.

    Okay, @ sea level, using the SA/RA wet bulb temps & my text's enthalpy table; 5.14 enthalpy change:
    @ 700-cfm I get 16,191-Btuh
    @ 800-cfm = 18,504-Btuh
    @900-cfm = 20,817-Btuh
    This a 2-Ton 13-SEER System; @80-F indoors; 63.5% Relative Humidity, @800-CFM the unit should be delivering around 23,000-Btuh

    I believe the system may be only 'delivering' around 700-CFM airflow (needs to be checked), plus there may be excessive losses in the duct system & the unit has to have the proper CFM before it can be charged properly.
    Last edited by udarrell; 07-24-2012 at 12:33 AM. Reason: Clarity problems...Made an error 5.14 enthalpy change

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,875
    Quote Originally Posted by udarrell View Post
    Okay, 377-psig condensing pressure is 112.5-F; not 101-F(?). What was the temp of the liquid line?
    Therefore 112.5-F CT with an (?)89-F(?) liquid line is 23.5-F subcooling which would be way too high; indicating an overcharge. (This could be be wrong, as I don't know how you figured the subcooling???)


    He's posting line temp, not sat temp.
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  4. #4
    Join Date
    Jul 2012
    Location
    Central Illinois
    Posts
    5

    Confused

    The temps I listed are the line temps.

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    4,960
    Quote Originally Posted by beenthere View Post
    He's posting line temp, not sat temp.
    Thanks, that resolves that misread by me I thought he was listing the CST.

    Also, I read the 61.3 as 61.8 (vision problems, the 3 looked like an 8; it was late last night & the lighting was not good enough; so that changes the Btuh.

    Therefore, the enthalpy difference is 5.24 which raises the Btuh a little.

    Okay, @ sea level, using the SA/RA wet bulb temps & my text's enthalpy table; 5.24 enthalpy change:
    @ 700-cfm I get 16,506-Btuh
    @ 800-cfm = 19,440-Btuh
    @900-cfm = 21,222-Btuh

    They need to know close to what the CFM of airflow is through the coil.

    Example: a 2-Ton 13-SEER System; @95-F outdoor-temp; @80-F DB, 71-WB indoors or 64% Relative Humidity, @800-CFM the unit should be delivering around +24,000-Btuh; 397-psig = 112.8-CST minus 10-F subcooling is a 102.8-F liquid-line.

    What is the subcooling-temp supposed to be for that system?
    Last edited by udarrell; 07-24-2012 at 10:55 AM. Reason: 5.24 enthalpy change...

  6. #6
    Join Date
    Jul 2012
    Location
    Central Illinois
    Posts
    5
    It's a 15 SEER
    Recomended subcooling temp is 10 +/- 3 degrees

  7. #7
    Join Date
    May 2008
    Location
    Myrtle Creek. Oregon
    Posts
    182
    Whats your TD across the evaporater?
    a stupid question is a question you wont to ask, but don't

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,960
    Quote Originally Posted by cboe View Post
    Whats your TD across the evaporator?
    I installed a new Am. standard 2 ton [15-SEER] 410A system, single stage 65k BTU furnace and cased TXV coil. It runs non stop and rarely satisfies the stat on a 975 sf ranch home with no basement. Check out these measurements, any clues?

    Ambient air temp: 99 deg
    Suction 150 PSIG. 68 degrees with 13 degree superheat
    Liquid 377 PSIG. [Liquid-Line] 101 degrees with 12 degree subcool

    Compressor drawing 13 amps

    *Dry bulb temps: return 77.6 deg
    *Supply 61.8 deg
    Temp-drop or split: 15.8-F
    So, what is the indoor airflow CFM & the percent of relative humidity?


    Wet bulb temps: return 68.5 deg
    Supply 61.3 deg (all averages!)
    Is that the temp-drop U wanted?

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,316
    Quote Originally Posted by Thardwick View Post
    I installed a new Am. standard 2 ton 410A system, single stage 65k BTU firnace and cased txv coil. It runs non stop and rarely satisfies the stat on a 975 sf ranch home with no basement. Check out these measurements, any clues?

    Ambient air temp: 99 deg
    Suction 150 PSIG. 68 degrees with 13 degree superheat
    Liquid 377 PSIG. 101 degrees with 12 degree subcool

    Compressor drawing 13 amps

    Dry bulb temps: return 77.6 deg
    Supply 61.8 deg
    Wet bulb temps: return 68.5 deg
    Supply 61.3 deg (all averages!)

    What the heck am I missing?! Is there WAY less insulation in the walls or is this unit just not producing?? Oh, and these temps are with the fan speed set to LOW! Replaced the TXV and factory said we are moving too much air over the coil lol!? Really? Too much return air? Any help will be greatly appreciated!
    What was the outdoor ambient air temp when you recorded the above?

    At 150 psig, your saturated vapor temp is running around 53 degrees. This means the evaporator coil temperature is probably about five or more degrees warmer than that. Not cold enough to get the house air dew point temperature below 55 degrees, which is what you want (and which is why it would be good if more techs measured dew point...right away you'd get some idea something's wrong somewhere).

    How to explain the system's meager performance? Look around. Where is the air handler located? In an attic? And if so, is it pulling in hot air from the attic through duct leakage, or a door on the air handler not sealing correctly?

    Are the ducts also in the attic? If so, any of them leaking? Supply duct leaks will cause a house to go negative pressure to outdoors and draw in hot, humid air through the building envelope. Return duct leaks will put too much load on the system. Combine them together and you have double trouble, since supply quantities will almost always outstrip return leak quantities, due to the sheer surface area of supply ducting vs. return ducting.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    Join Date
    Jul 2012
    Location
    Central Illinois
    Posts
    5
    Sorry I forgot to list a couple things. Outdoor temp was 99 degrees at time of these readings. Also low speed on the unit is 1050cfm. Nothing but minor air loss in the supply ducts and the return in in the attic. For troubleshooting we have even closed off the attic return and are using a temp central return until we resolve this install. It's an American standard 95 furnace, coil, and 15 SEER matching system.

  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    Quote Originally Posted by Shophound View Post
    What was the outdoor ambient air temp when you recorded the above?

    At 150 psig, your saturated vapor temp is running around 53 degrees. This means the evaporator coil temperature is probably about five or more degrees warmer than that. Not cold enough to get the house air dew point temperature below 55 degrees, which is what you want (and which is why it would be good if more techs measured dew point...right away you'd get some idea something's wrong somewhere).

    How to explain the system's meager performance? Look around. Where is the air handler located? In an attic? And if so, is it pulling in hot air from the attic through duct leakage, or a door on the air handler not sealing correctly?

    Are the ducts also in the attic? If so, any of them leaking? Supply duct leaks will cause a house to go negative pressure to outdoors and draw in hot, humid air through the building envelope.

    Return duct leaks will put too much load on the system. Combine them together and you have double trouble, since supply quantities will almost always outstrip return leak quantities, due to the sheer surface area of supply ducting vs. return ducting.
    An absolute bullseye impact point as usual Shophound.

    In long times past we had over capacity compressors & no oversized coils, many times even under a heavy heatload conditions those larger capacity compressors would pull those evaporators down to around 64 to 66-psig, that's a 37-F & 38-F saturation temp-coil, if U add 5-F to the coil that will be well below the dew point &, when sized right, many were way oversized also to the ductwork design,they did the job on high humidity situations.

    That is why many up-size a half ton or so & don't go with the oversized indoor coils, plus with a TXV metering device, going to 350-or even less CFM per-ton of cooling, so the coil gets enough below the dew point.

  12. #12
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,316
    Quote Originally Posted by Thardwick View Post
    Sorry I forgot to list a couple things. Outdoor temp was 99 degrees at time of these readings. Also low speed on the unit is 1050cfm. Nothing but minor air loss in the supply ducts and the return in in the attic. For troubleshooting we have even closed off the attic return and are using a temp central return until we resolve this install. It's an American standard 95 furnace, coil, and 15 SEER matching system.
    Is the air handler also in the attic? Being a 95 furnace, I'm thinking closet installation inside the house, since you stated in your OP there is no basement. No crawl space, either...house is slab on grade?

    "Minor air loss"...how did you reach that conclusion?

    Are all the supply boots sealed to the ceiling drywall?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Jul 2012
    Location
    Central Illinois
    Posts
    5
    It's in a utility room and on a crawl space. We inspected EACH run and trunk... No, the boots at the floor aren't caulked and not every joint is taped but there are no holes in the ducts or runs. I know, not very specific sorry. We are a carrier dealer. However, with the location of the furnace our width was VERY limited. The homeowner has to squeeze between the furnace and dryer to do laundry and at the time of furnace install carrier didn't have a design with flue pipes thru the top, now they do! I think an "M" coil would be much better? Again tho, width is a factor. We did find today that they are siding the house. It's an older ranch style and there is virtually NO insulation in the walls! I think we may have found our problem? I still don't think this system is performing the way it was designed to...

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