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Thread: Home AC 'Misfires' on Startup (sometimes)

  1. #1
    Join Date
    Aug 2017
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    5

    Default Home AC 'Misfires' on Startup (sometimes)

    My Equipment:
    Compressor: Rheem RAMA-048JAZ
    Air Handler: Rheem RBEA-21J11NUEAI
    Thermostat: Honeywell RTH111 (non-programable LCD unit)

    This system cools very well. Performance has not changed in years.

    Recently the ac is developing a strange intermittent problem.

    If the ac has been switched off for an hour or so....and I turn it on (by switching the thermostat to Cool and lowering the temperature selector below ambient) then...sometimes...just a click is heard from the air handler and a slight sound of the fan trying to start up, but only for a second. Then nothing! Five minutes later everything starts normally and the system cools fine.

    Occasionally, when the ac is on and cycling normally it will repeat this 'misfire' when starting up, but also run fine after a 5 minute delay.

    Any suggestions would be greatly appreciated.

  2. #2
    Join Date
    Feb 2012
    Location
    Lala land, OH
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    235

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    Where is this sound coming from? inside or outside in the condenser unit? Keep in mind there may be a slight delay when the condenser kicks on before the inside blower starts. (it allows the evaporator coil to cool off and not blow hot air). The condenser unit may shut down before the blower shuts off (since the evap coil is still cold.. free cool air)

    If the noise is coming from the indoor air handler, and the condenser unit starts up just fine when this happens....Try flipping the fan switch from auto to on(when the AC is off) If it doesnt come on at times, it's probably the blower's starter cap. It could also be a failing bearing (likely can't be rebuilt or oiled) or a bad blower relay.

    If the issue is outside, there are 2 components that will come on.. The compressor itself and a fan. Are they both coming on? Both also have starter caps and will hum when they go bad.

    If nothing comes on at all. It's probably a control board issue (cooling relay). There is a delay built into your t stat and probably the control board to prevent the system from starting up right away after a shut down.
    Last edited by Mr T; 08-07-2017 at 06:50 PM.

  3. #3
    Join Date
    Aug 2017
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    Mr T,

    I hear two sounds. The first is from the thermostat and it simply sounds like a relay click. The second, simultaneous, sound comes from the air handler and it sounds like the fan starting up but only for about 1 second. There is no noise or action from the outside compressor that I can detect during this one second.

    Then, there is silence and no action until the ac starts normally, by itself, after a five minute delay without me doing anything.

    I am aware of the 5 Minute delay built into the ac to prevent starting the compressor for at least 5 min if it has been 'recently' running. My problem 'acts as if' something has 'invoked' the 5 Minute delay even though the compressor has not run for a very long time.


    Thanks for your help.

  4. #4
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    Aug 2017
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    Mr T,

    I replied to you but my reply did not show up. Maybe my computer had a 'Misfire' too!

    The thermostat makes sound like a relay clicking and displays 'Cool On'. At the same time the air handler makes a very short (about a second) sound that is like the fan starting up. But then nothing. No Fan and no noise from the outside compressor. Wait 5 minutes and everything starts and cycles normally.

    I know about the 5 minute delay built into the ac system to prevent starting the compressor if it has run recently. In my case nothing has recently run. It's as if the 5 minute is being activated in error, sometimes.

    Hope this helps....

  5. #5
    Join Date
    Feb 2012
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    Lala land, OH
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    First thing i would try is turning the AC to off and flip the fan switch from auto to on. Let it run a minute, then turn it off, then turn it back on, then back off. Repeat this a few times in a row and in similar intervals that cause the problem to occur. IF that doesn't cause any issue then run the AC until it finishes a cycle, turn it to off once it shuts down. Wait a similar time that often triggers the issue then turn the fan on. If both of these don't cause the issue then your blower is likely ok.

    I did not run your model numbers, but most units are 'dumb' or open loop units. The thermostat sends a call for AC to your air handler's control board. It sends a 24V signal to your condenser unit and power to the blower via relays. (A call for heat on a fuel burning furnace is very different...it is a closed loop/controlled system) In most systems it never knows if the condenser and blower actually started and ran. Some very modern (and very expensive) units can tell... I don't think you have one. If your thermostat starts the AC and there's a issue with the condenser or the blower the other will usually just run until it's noticed or the thermostat senses the temp is low enough to shut it back off. You have not described this situation....so, this kinda-sorta rules out the condenser and blower units....in a way

    If there is a short in your low voltage control system (signal wire to the AC, wires to the T-stat and some circuits on the control board), it can cause some issues like yours. For example, if the coil on your condenser unit's contactor is damaged and causing it to draw alot more current then normal, the power supply in your air handler can only produce enough current before it will start to drop it's output voltage (due to ohm's law). When this voltage gets too low the control board wont function any more and the whole system will think you had a brief power outage... the system will reset and is designed to go into it's lockout period.. The buzz you hear could be a relay trying to fire and doesn't have enough current.. at that point it will become a buzzer. Most thermostats have batteries in them, so you won't ever see anything unusual. The relays in your thermostat that you hear click are triggered by the internal batteries. Some systems have fuses, some dont. At some point, something will give and you will be without heat or ac. A failing or burnt relay on the control board can also cause this. (not uncommon). So can a damaged control wire (lawn mower/weed trimmer got the wire outdoors), or a staple cutting through the insualation.

    Troubleshooting this can be a nightmare. If you feel comfortable doing this. TAKE LOTS OF PICTURES, LABEL EVERYTHING!!! Turn power off to the air handler and find where the wires to your AC condenser connect to it. It should be just 2 wires. If you have more then 2 that are connected, stop and let us know. Disconnect and insulate one of these wires so it can't touch anything. Put the covers back on and fire up the AC. The inside blower should start but the outside unit wont. If this happens then you have a issue with your wiring to the condenser or a issue with the condenser. If things still don't work it's a issue in the air handler and probably time for a service call unless you want to gamble with non-returnable parts. Keep us posted on your progress...

  6. #6
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    Also, check the obvious.. Fresh batteries in your thermostat, check for loose wires in your thermostat. Some newer ones have push connections for the wires.. Make sure you have bare copper under them and not a piece of insulation. Check the same wires on the control board of your air handler.

  7. #7
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    Mr T,

    Thank you, for your many good suggestions. I will get to work diagnosing the problem and get back to you with my results. I have an electrical engineering background, so I am comfortable working on these sort of things. I'm just not a HVAC expert.

    Before I posted here I asked for help on a HVAC forum. I asked the same simple question and was met with no response. They told me to contact a 'service provider'.

    It is comforting to post to a forum that has members willing to share their expertise.

    Thanks again.

  8. #8
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    Lala land, OH
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    Why didnt you say that.... In that case... Look for excessive current draw 24VAC control circuits.

    You can also (usually) manually activate the AC contactor and see if the fan and compressor come on. They are often (somewhat) open frame so watch what you touch. I still think your issue is on the 24V side.

    I think I know what board you asked on. They usually aren't helpful unless you are a contractor or in the industry. We are a little bit of everything here. Feel free to chime in if you see any thing on here that you are knowledgable in.

  9. #9
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    Mr T,

    Update on my 'Misfire':

    Seems that the problem was the thermostat!

    About a week ago I bought a new thermostat (same kind...non-programmable Honeywell). Installed and let it run. It has been a week now and I have not had one 'Misfire' on my ac startup!

    BTW, this new thermostat (and the one I replaced) do have a setting for a 5 minute delay. It was enabled on the old one and I set up the new one the same way.

    I do not know, for sure, if my air handler also has a 5 minute delay built into it. Is there any way to check this? All the paperwork I have on the Rheem air handler makes no mention of a delay on restart.

    There seems, to me, to be two possible reasons for the 'Misfire' I was getting:

    1. The relay in the old (7 years) thermostat was 'chattering' slightly on startup, causing the air handler to detect ON-OFF-ON, so that it then implemented a 5 minute delay.

    OR

    2. The thermostat simply was malfunctioning and inserting a delay sometimes when it really was not necessary.

    Thanks again for your help....

    BTW, the first thing I did, before posting on this site, was to change the batteries on the old thermostat....

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