A/C start-up intermittent start up issue

Discussion in 'HVAC Heating & Cooling' started by jp102, Jul 11, 2011.

  1. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    I have a Rheem RGRM09 2 stage furnace with Carrier A/C unit. A couple of times this summer I've come home and the A/C compressor has been on, but the blower fan is not running. The system is iced up and when I shut the system down for ten minutes and turn it back on everything starts up and works ok. The system has an ECM blower motor with a power factor choke, not a start up cap.

    The furnace is about 9 months old, the A/C unit about 10 yrs. Never had a problem with a heat call this winter, this is the first summer with the new blower. Since day one, when the blower starts up (heat or cold) it rumbles for the first 30 or so seconds. Other thing that doesn't make too much sense to me is that the compressor and blower start at the same time. I would expect the blower to come on, get up to speed and then the compressor to kick. Same thing with the heat call; the exchanger fires at the same time the blower starts.

    If anyone has any thoughts on what might be the issue with the blower not starting, I'd greatly appreciate them. My initial reaction is to replace the power factor choke, but I really don't know much about them so I'm not sure if it would fix the problem.
  2. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    Welcome to Terry's Forums jp102,

    What is the Model of your Carrier A/C unit.

    You may want to try to put your Thermostat / Controller in Fan Continuous Run mode when it gets in that screwed up mode to further diagnose your problem, and see if you can manually control the blower.

    As far as the compressor and blower starting at the same time, That should not be a problem.


    Good Luck on your project.


    DonL
  3. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    DonL - thanks. Don't have the model of the carrier unit with me, I'll get that tonight. Good suggestion with the continuos run mode. The thermostat is an Aprilaire with a circlating mode (turns fans on every 20 minutes).
  4. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    The model number would help you greatly to get a better answer.

    Different Models and Different ECM blowers will ramp up differently, and some are programmable. Depends on the design.

    The thermostat Model also would help.

    Have a great day.


    DonL
    Last edited: Jul 11, 2011
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,143
    Location:
    New England
    On my variable speed blower, it starts up when the call for heat or cooling starts. The fan speed starts out slow, and ramps up over time and then ramps down after the call ends. how fast it goes depends on how long the call is as it changes based on time that it is running. Not sure how all brands work, but this is on my Trane stuff. Starting out slow helps in both heating and cooling, as a slow speed means the cold air in the duct during the winter doesn't blast you. in the summer, moving the air slower over the coil means it can dehumidify better. Ramping down at the end means you use all of the energy in the heating or cooling from the heat exchanger. A slow start also is much less obtrusive.
  6. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    Furnace model Rheem rgrm09
    AC model Carrier CD5AXW048
    Thermostat Aprilaire 8466

    Setup as 2 stage heat, 1 stage air. Not a variable speed furnace.

    Thanks in advance
  7. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    When you say "when I shut the system down for ten minutes and turn it back on everything starts up" are you using the thermostat to turn it off, or are you killing power to the unit ?

    Are you using the "CIRC" fan mode ?


    DonL
  8. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    I'm killing power to the unit. I did not try turning off/on with the thermostat. The system will actually start right away (blower and ac unit) if flip the power switch off/on, but it just won't cool as I suspect the lines are iced up. I waited the ten minutes for the lines to un-freeze. The thermostat was in "cooling" mode according to the thermostat display.

    Yes I'm using the CIRC fan mode.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,143
    Location:
    New England
    If things freeze up, you may need to call for service. The refrigerant levels may be off. The blower may be running, but if the evaporator is covered with ice, no air can get through. Also, make sure the filter is clean...reduced air flow can cause it to get too cold and freeze things as can an incorrect refrigerant level.
  10. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    Shut down by killing power. Did not try to use thermostat to shut it down. It will start up immediately (both blower and AC unit) if I kill and restore power however it won't start cooling as the AC lines are frozen up. Ten minutes was an approximation; one time it may have been fifteen, the other time 5. Basically just enough time to let everything defrost.

    I use the CIRC fan mode.
  11. Gary Beach

    Gary Beach HVACR tech

    Messages:
    4
    Location:
    Nixa, MO
    1st. If the unit is froze up, turn it off for about 24hrs. to be sure all the ice is cleared.
    check the air filters, change them is any doubts, use new ones in this case to be sure they are okay.
    If filters are not plugged up, and air flow is fine, call someone to check the charge on the unit.

    That cooling coil can be plugged with ice, or partially plugged. That will cause it to ice over again. Also a technician can not be sure of the charge if there is any ice on that coil left.
    Last edited: Jul 12, 2011
  12. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    I do believe that that is the problem that is causing it to ICE up, The Blower is not running, and is causing it to Ice up.

    jp102, I am still looking into the fan control system to see if I can help, or make a educated guess, as to why the blower is not starting.

    It could be something as simple as loose wire, or may could be a blower relay. Or even blower speed not fast enough.

    Did it ever operate correctly, after the Aprilaire 8466 was installed ?

    After you do restart it will it continue to operate normally, or does it ice up again ?


    Have a good day.


    DonL
  13. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    The reason the unit is freezing is because the blower is not coming on (or is prematurely shutting down) during a call for cooling. FIlters are ok, refrig levels ok. It has only happened three times during the summer (out of probably hundreds of call for cooling). AC has been on since late May and the three times I had the issue it was a fairly warm day and the failures have been weeks apart. Thermostat wiring is new and I have had no problem with the blower not coming on during the heating season so I suspect that wiring/loose connections are not the problem.

    My current thoughts are that it could be one of the following:

    1. Bug in the firmware or some other issue with the thermostat related to the CIRC feature.
    2. Bug in the furnace firmware related to the call for cooling during a fan ON
    3. Blower motor or power factor module
    4. Power brownout/surge preventing blower from starting.

    #1 in my mind is the leading candidate, #2 doubt it and #3 probably not as I would have had some heating call issues or more cooling issues. #4 not sure of as I would think a blower on signal would continue and start the motor throughout the cycle.

    Next time it fails I will check the blower lead at the thermostat during the failure and trace it through the furnace, however I'd like to fix the problem before it happens again so I don't freeze my AC unit and have to replace it. I am surprised that the control board of the furnace allows for a cooling cycle to occur without seeing some feedback from the ECM motor.

    Any other thoughts? Am I correct in my thinking of the above items?

    Thanks again
  14. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    DonL - thanks. Some of the replies are getting out of sync so bear with me if it sounds like I'm repeating myself.

    Yes it has operated correctly with the Aprilaire 8466. The 8466 was installed with the furnace and the system has operated from Oct-April last year on heat, May to present on AC. Many, many calls for AC, only failed three times. Always in CIRC mode.

    After I resart, the system continues to operate normally and does not ice up.
  15. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    Did you set the dip switches on your Furnace controller to match the AC unit size ?

    It sounds like your blower may be running slower than needed for your A coil size.
    Then it ices up, if it has to run longer for higher outdoor temps.

    Can you here the blower change speeds when you go from "AUTO" to "ON" ?
    If not than your T-stat may have a wire in the wrong place, not letting the blower change speeds.

    It looks like the blower runs at 4 different speeds.


    DonL
  16. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    DonL - I have SW1-5 ans SW1-6 set to ON/OFF (profile B) which gives 1600 CFM HI, 1200 CFM low which I believe matches the 4 ton AC unit specs.

    Yes I can hear the blower changing speeds.

    Question - when the AC frezes up, does or should the blower shutdown? Should the entire system shutdown when it freezes?
  17. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    It should be set for 1600, on cooling.

    And the whole system would shut down, not just the blower.

    You may want to set for 1600. Then run the t-stat on auto.


    DonL
    Last edited: Jul 13, 2011
  18. jp102

    jp102 New Member

    Messages:
    20
    Location:
    Michigan
    Thanks. Maybe I'm chasing the wrong problem. I've been trying to figure out why the blower isn't starting, maybe I should be figuring out why the AC unit isn't shutting down when it freezes. Obviously I need to figure out why it freezes, but that might be another issue. Can you share any thoughts on the shutdown circuit of the system if it detects a freeze?
  19. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,327
    Location:
    Houston, TX
    The blower could be running and it could be hard to tell, because when it freezes up there is no place for the air to go.

    But for a 4 ton then you want to run the blower at least 1600, or it could Ice up.

    As for the shutdown, I am not sure if that part works on the air side.
    Could only be for heating.

    With it being two different brands, that is hard to tell, if the controller even knows that it is iced up.

    Wish I could have the answer, but I am not for certain.


    I would start by upping the air flow, and your problem may be solved.


    Enjoy your day.


    DonL
  20. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I have the same 9 month old Ruud (same model number and equipment as Rheem). Check your flows on the ECM blower by reading the blinking red light in the little view port on the air handler (one blink = 100 CFM per p. 54 of manual.) This isn't the actual flow but represents the target speed call by the board. Mine is also paired with a 4 ton AC (new Rheem two stage AC.)

    The manual states on p. 47 that the default for these is max flow, profile A, which is 2000/1500 cfm. That is how mine was set up IIRC (I've got notes on dip switch positions...but have misplaced them.) I've since dialed back with the optimization switch set (which is noted wrong in the manual per my recollection and testing--the manual is confused and confusing in this section) so that it is running about 1800/1300 to better dehumidify on low stage. I'll take a look at this tonight once we get back below 95 F. I might try setting it to B to see if any problems appear. To do that I'll have to reset the optimization switches to default.

    My furnace also tends to start up a little noisily, haven't noticed the blower making the rumble in summer. I'll take a closer listen tonight. I believe the rumble in winter is from the furnace first firing up as I recall. Something to do with the sealed combustion and ID set up, maybe some baffle rattle. It actually became more noticeable when I had some of the side ports on the furnace blocked off (they weren't originally, but should have been.) There could be some sort of "critical speed" vibration on the blower at the lowest speeds, I suspected that. Since it doesn't stay in that mode more than a few seconds I haven't tried to follow it up.

    At any rate, mine seems to be doing pretty well at cooling so far...while the old one was having trouble and approaching the point of siezing up.

    Note that the default "FAN" circ. setting is very low for the blower if in the "low" setting (around 500-600 CFM I think.) I've reset mine to "high." Our 90k units are not in table for this on page 48. Even "high" is not that high--something like 1200 CFM I think I measured. So if this is being used for the blower speed when the 4 ton is running, it could be having trouble freezing up from low air flow. That could be the case if there is some sort of miswiring.
Similar Threads: start-up intermittent
Forum Title Date
HVAC Heating & Cooling Oil Furnace - intermittent problem Feb 25, 2011
HVAC Heating & Cooling Intermittent ignition on W-M boiler, Beckett AF oil burner Dec 2, 2008

Share This Page