Page 1 of 5 12345 LastLast
Results 1 to 13 of 54
  1. #1
    Join Date
    Sep 2012
    Posts
    373

    What's the proper cfm for a 2 ton system?

    Hello. It's me again. I have a question about the blower cfm of our upstairs system.

    Our upstairs system is a 2012 Amana Distinctions (Goodman, of course) SSX14 2 ton A/C unit matched up with a 2012 Lennox SL280v gas furnace. The A/C was installed back in early April and the furnace was installed last July. The upstairs is 1400 sq. ft. I am wondering what the cfm is suppose to be in cool mode. I know it's kinda too late to be talking about the A/C unit when A/C season for us is over, well whatever. The system is running in about 883 cfms and sometimes the unit short cycles. If the A/C has been on for 7 1/2 minutes, the blower speed will increase to about 1005 cfm. This is what the tech told us during our yearly tune-up. The evaporator coil is a CAPF3636A6 which is a 3 ton.

    The blower ramping option is set to the option A (factory setting) where it would run at a very low speed (50%) when the system starts up and after 30 seconds on running on this speed the blower will ramp up to 82% cfm. If the cooling demand is not met within 7 minutes then the blower will ramp up to 100% of the cfm setting. When the cooling demand is met the blower will ramp back down to 50% for about 30 seconds then ramp down to off. I got this information from the specifications/installer's manual on Lennox's website. This is also what the tech told me.

    So how much cfms should a 2 ton system have? Is this indoor coil and A/C unit the proper match up? Sorry about the long post. Thanks in advance.

  2. #2
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,322
    Hard to tell from here. Regardless of what the blower is set at, we have no idea of what your actual air flow is. Even if the cfm is set at 1050, the duct system may not be designed to deliver that amount of air flow. A two ton system should be putting out about 800 cfm. maximum.

  3. #3
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by wahoo View Post
    Hard to tell from here. Regardless of what the blower is set at, we have no idea of what your actual air flow is. Even if the cfm is set at 1050, the duct system may not be designed to deliver that amount of air flow. A two ton system should be putting out about 800 cfm. maximum.
    The ductwork is all 8 inch. I should have said that in my first post. Hope this helps.

  4. #4
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,482
    The ramping of the blower is good for de-humidification. I'm lost on the 883 cfm-1,005 cfm.

    Normally, it should start around (I would guess) 150-200 cfm, then go to (maybe) 300-350 and finish around (no more than) 400 cfm. Note: I'm not a tech and have not stayed at a Holiday Inn (lately).

    The ramping is in the furnace is timed. A better thermostat would control the airflow differently.

  5. #5
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by George2 View Post
    The ramping of the blower is good for de-humidification. I'm lost on the 883 cfm-1,005 cfm.

    Normally, it should start around (I would guess) 150-200 cfm, then go to (maybe) 300-350 and finish around (no more than) 400 cfm. Note: I'm not a tech and have not stayed at a Holiday Inn (lately).

    The ramping is in the furnace is timed. A better thermostat would control the airflow differently.
    I don't exactly if the cfm is at that setting, though I believe it's around that. Did you check Lennox.com to see the airflow settings in the specifications sheet? I could be wrong you never know. We plan to upgrade our thermostats on our first floor and upstairs systems to Honeywell Prestige 2.0 IAQs so that we can control the temperature over the internet.

  6. #6
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,350
    There's no one "proper" cfm for a 2 ton system. Nominally, 2 tons is 800 CFM @ 400 cfm/ton. But some building loads dictate slightly higher or lower airflow depending on the latent and sensible capacity the conditioned space requires. For instance a home in Florida would be better suited at 350 cfm/ton for improved humidity removal, whereas a system cooling a server room may be configured for 450 cfm/ton to obtain greater sensible capacity.

    The 070 size SL280V has a 3 ton blower. The factory cool speed setting is HIGH which corresponds to a 3 ton outdoor unit. The cfm range on that particular furnace is 645 cfm - 1372 cfm for cooling. What cfm your furnace is actually delivering depends on whether they wired the compressor to "stage 1" or "stage 2" cooling on the furnace board as well as whether they adjusted the dipswitches from the factory default (there are 4 speed settings as well as an adjustment). By what the technician said, it sounds like they may have hooked up the compressor to Y1 (stage 1) and left the blower speed on default. 883 cfm is HIGH speed for "stage 1" cooling (even though you don't have a 2-stage outdoor unit). If the enhanced ramp-up profile is enabled, the blower will ramp up as you describe to this speed (100%), but it won't go higher.

    I hope this is clear to you. See attached literature (pg. 15) for more info.

    ehb_sl280uh_icomfort_1207_210600_011.pdf

  7. #7
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by RyanHughes View Post
    There's no one "proper" cfm for a 2 ton system. Nominally, 2 tons is 800 CFM @ 400 cfm/ton. But some building loads dictate slightly higher or lower airflow depending on the latent and sensible capacity the conditioned space requires. For instance a home in Florida would be better suited at 350 cfm/ton for improved humidity removal, whereas a system cooling a server room may be configured for 450 cfm/ton to obtain greater sensible capacity.

    The 070 size SL280V has a 3 ton blower. The factory cool speed setting is HIGH which corresponds to a 3 ton outdoor unit. The cfm range on that particular furnace is 645 cfm - 1372 cfm for cooling. What cfm your furnace is actually delivering depends on whether they wired the compressor to "stage 1" or "stage 2" cooling on the furnace board as well as whether they adjusted the dipswitches from the factory default (there are 4 speed settings as well as an adjustment). By what the technician said, it sounds like they may have hooked up the compressor to Y1 (stage 1) and left the blower speed on default. 883 cfm is HIGH speed for "stage 1" cooling (even though you don't have a 2-stage outdoor unit). If the enhanced ramp-up profile is enabled, the blower will ramp up as you describe to this speed (100%), but it won't go higher.

    I hope this is clear to you. See attached literature (pg. 15) for more info.

    ehb_sl280uh_icomfort_1207_210600_011.pdf
    Thanks. I'm also wondering why the system is short cycling (only on days that are not too hot).

  8. #8
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,350
    Quote Originally Posted by 545GAlady View Post
    Thanks. I'm also wondering why the system is short cycling (only on days that are not too hot).
    What kind of thermostat do you have now? Some have a cycles per hour setting that can be adjusted (lowered) to increase average runtimes. 3 is a good setting. Decreasing the airflow a bit can also help increase runtimes and possibly improve comfort, but there are lots of things to consider when setting airflow. How short are the cycles? Runtimes are going to be shorter on milder days.

  9. #9
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by RyanHughes View Post
    What kind of thermostat do you have now? Some have a cycles per hour setting that can be adjusted (lowered) to increase average runtimes. 3 is a good setting. Decreasing the airflow a bit can also help increase runtimes and possibly improve comfort, but there are lots of things to consider when setting airflow. How short are the cycles?
    Thanks. The thermostat is a Honeywell RTH6450D. The cycles usually range from 5 minutes to 9 minutes. Except on hot days the cycles are like 30 mins to 1 hour. Sometimes 2 hours depending how hot it is.

  10. #10
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,350
    Quote Originally Posted by 545GAlady View Post
    Thanks. The thermostat is a Honeywell RTH6450D. The cycles usually range from 5 minutes to 9 minutes. Except on hot days the cycles are like 30 mins to 1 hour.
    Glancing through the installation guide, looks like there's a heating cycles per hour setting but no cooling cycles per hour setting. Go figure. You might find that the Prestige thermostat with CPH set appropriately allows for better (longer) runtimes. 5-9 minutes is short. It takes at least that time for the system to reach its peak efficiency and capacity. Surprised the runtimes are that short even with the ramp-up profile providing low airflow. 10-15 minutes is more ideal on mild days and likely achievable with a better thermostat controlling to a particular CPH during partial-load conditions.

  11. #11
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by RyanHughes View Post
    Glancing through the installation guide, looks like there's a heating cycles per hour setting but no cooling cycles per hour setting. Go figure. You might find that the Prestige thermostat with CPH set appropriately allows for better (longer) runtimes. 5-9 minutes is short. It takes at least that time for the system to reach its peak efficiency and capacity. Surprised the runtimes are that short even with the ramp-up profile providing low airflow. 10-15 minutes is more ideal on mild days and likely achievable with a better thermostat controlling to a particular CPH during partial-load conditions.
    Thanks for the info. Also sometimes the air never feels "very cold". Mostly on warmer days. The air is pretty cold, but not as cold as our first floor system and basement system. On hot days we lose that "cold feeling". Is that suppose to happen?

  12. #12
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,350
    Quote Originally Posted by 545GAlady View Post
    Thanks for the info. Also sometimes the air never feels "very cold". Mostly on warmer days. The air is pretty cold, but not as cold as our first floor system and basement system. On hot days we lose that "cold feeling". Is that suppose to happen?
    Not too surprising for an attic system. On very hot days, attic temperatures can easily reach 130+ deg. You'll have some capacity loss in the supply ductwork depending on how well its sealed and insulated. Same for return ductwork -- could be drawing in some hot attic air. You can check to make sure you have a good temperature drop across supply and return at the furnace/coil -- if charged properly chances are the temp drop is reasonable. But the air is probably picking up some heat before it reaches the registers. Best thing to do is make sure the attic ductwork is properly insulated and sealed.

  13. #13
    Join Date
    Sep 2012
    Posts
    373
    Quote Originally Posted by RyanHughes View Post
    Not too surprising for an attic system. On very hot days, attic temperatures can easily reach 130+ deg. You'll have some capacity loss in the supply ductwork depending on how well its sealed and insulated. Same for return ductwork -- could be drawing in some hot attic air. You can check to make sure you have a good temperature drop across supply and return at the furnace/coil -- if charged properly chances are the temp drop is reasonable. But the air is probably picking up some heat before it reaches the registers.
    I forgot to mention that we have a solar attic fan and it keeps the temperatures down usually in the 115 deg range in the attic.

Page 1 of 5 12345 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event