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Thread: Carrier RTU Inducer Issue

  1. #1
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    Carrier RTU Inducer Issue

    Greetings folks, name is Erik. Just signed up for the site. HVAC and electric service background. I encountered an issue with a Carrier TJF024 24 ton rtu yesterday. Friday afternoon I went to service this unit to get it ready for winter, ie checked blower belt, replaced all the filters etc. Went to fire it up and the DI motor was locked up. Checked for proper voltage, checked cap and all was right on the money there. Headed over to Carrier to pick up motor, housing, motor channel(mount) and got it together, made certain everything was sealed u tight with high temp rtv and fired it up. Well, DI motor came right on but no fire. Looked around to make sure there was nothing else left undone or anything else I might have missed and everything was good to go. 2-3 minutes, motor still running still no fire. Before dropping the call for heat and checking the pins at IGC board for proper voltages I covered the exhaust outlet with my hand and sure enough she fired right up. Motor obviously spinning at rpm higher than parameters of hall sensor. I have done dozens of these inducer motor replacements so I have not encountered a time when I haven't sealed the burner box cover properly nor the motor housing to burner box properly and caused a leak but just the same I ran some more rtv around the edge where the motor housing contacts the burner box. Still would not fire until I blocked the exhaust outlet. Stared to rain hard enough that I decided to wrap t up and go back today but it's still raining here so have not gone yet. The kicker here and what I have been thinking about since yesterday after I left the job is the fact that we installed a new heat exchanger in this unit last year. Had absolutely no issues all last fall and winter with it. We've replaced a ton of them and never had any issues but still was on my mind. I mention this because these motors will spin too high and fault if there is a large enough leak(as in hole or opening) If the motor was too slow due to a blockage somewhere is one thing, but too fast due to a possible leak has me a bit concerned. Once the rain abates I am going to head down and check the voltages on the board pins and the circuit resistor for any signs of burn. I suppose it's also possible the brand new motor hall sensor is bad but even though it does happen, I highly doubt that is the problem. Any input from you guys is greatly appreciated.

  2. #2
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    interesting, I wonder if the motor/sensor is correct for the unit? could be the wrong one for that exact model.

    Also, might wanna check the parts lists, it's possible that there should be an orifice at the fan inlet. were the inlets the same size on both old and new?

    are there any baffles in that exchanger tubes that could be missing?

    oh, and welcome to the site!! get your post count up and apply for PRO membership, all the fun happens in he closed sections of the site!

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  4. #3
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    Quote Originally Posted by 71CHOPS View Post
    interesting, I wonder if the motor/sensor is correct for the unit? could be the wrong one for that exact model.

    Also, might wanna check the parts lists, it's possible that there should be an orifice at the fan inlet. were the inlets the same size on both old and new?

    are there any baffles in that exchanger tubes that could be missing?

    oh, and welcome to the site!! get your post count up and apply for PRO membership, all the fun happens in he closed sections of the site!
    Good points 71CHOPS. One of the first things I wondered in fact is if there was something missing from the outlet/diverter on burner compartment door such as a baffle. The heat exchanger concerned me as well. It worked fine with no issues last season after we installed the new one and while sometimes people may be slow to admit a mistake was made I was still concerned in the back of my mind something was amiss with the HE, ie a baffle pate or something missing or askew. All that aside, the rain stopped and I just went over and checked my voltages at board sensor pins. With unit powered up I am getting -0- volts dc between 1 and 3 and bouncing around between 2 and 3 volts between pins 2 and 3. With sensor plugged in and burner alight those same readings changed to 4.5 volts between 1 and 3 and 16 volts between 2 and 3. I did not check new hall sensor but at this point I don't think I need to with those readings from the board itself. I'd say the board is the issue. The only other oddity I noticed when I got there today was the board flashing flame sensor fault code. It wasn't even calling for heat yet there it was flashing. Maybe another sign board is bad flashing that fault code. We always install new flame sensor, limit etc whenever we put in a new HE but moreso than even that, the board shouldn't have been flashing ANY code since it wasn't even calling for heat.

  5. #4
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    Welcome Erik. You sure sound like a "normal" Pro w/ your description of things. As 71CHOPS suggested, get your Post count up and get Pro Status.

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  7. #5
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    Quote Originally Posted by TechmanTerry View Post
    Welcome Erik. You sure sound like a "normal" Pro w/ your description of things. As 71CHOPS suggested, get your Post count up and get Pro Status.
    Will do and thanks for the compliment. I enjoy helping other techs out and receiving help as well whenever I feel stumped. Never too old to learn.

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    Quote Originally Posted by Erik52 View Post
    Will do and thanks for the compliment. I enjoy helping other techs out and receiving help as well whenever I feel stumped. Never too old to learn.
    WHAT AN ATTITUDE ! But (lol))for most around these here parts, that's called Plagiarism!!!!!

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  10. #7
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    Quote Originally Posted by TechmanTerry View Post
    WHAT AN ATTITUDE ! But (lol))for most around these here parts, that's called Plagiarism!!!!!
    ..guilty as charged

  11. #8
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    Good news turned to bad news. Went and got the board and will settle up with him tomorrow. Before I plugged hall wires back into board, I checked voltages at 3 pins and all was good. 10.8 volts between 1 and 3 pins and 22.5 volts between 2 and 3 pins. This time I tested hall sensor and it is good to go. Called for heat, motor came on...no fire. Blocked exhaust outlet, burner came on. This time I let it run until it heated up from 66 degrees to 70 degree set point and it remained running the entire time. Did not cut out. After t stat satisfied, I turned up temp 3 more degrees to start process over again, motor came on and this time burner fired up with no assistance from me. The odd detail here is the fact that even with new board I still am getting flame sensor fault code even with burner actually on. As stated previously, we installed a new exchanger just last season and it ran flawlessly all season. It had not been run in heat mode yet this season until I attempted it Friday and found inducer motor locked up. The other detail I hadn't mentioned is first go round when initially seeing draft motor locked up on Friday, I immediately spun the motor cooling fan and motor ramped up to speed and burner fired. This is with the old motor. I did not have to block exhaust outlet to get burner to come on. This issue has only started since installing new motor if you follow what I'm saying. That little detail somewhat allays my concerns that after only 1 heating season I've got a failed exchanger that was just installed last year. Frankly even last year with the old exchanger having holes in it all over the place I never had any issue with burner not coming on so obviously even with the number of holes in it, it did not over rpm. I try my best to look for simple things first and not assume the worst. That said, those are still assumptions but just the same, I definitely hope the issue here isn't an already failed exchanger. The only other places I can think of that are sucking air in would be burner box cover or the surface where draft motor mates to top of burner box. They did not have a new white gasket so I sealed that with high temp rtv and added some sealant to other spots I feet could be an issue yesterday even though I've never had to seal anywhere but on the bottom of housing around perimeter of screw holes in the past. I am not sure what to make of this situation at this point.

  12. #9
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    I left out that I called a buddy who is also an HVAC contractor in Kentucky and he had several boards at his shop and I went and got one from him.

  13. #10
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    I always clean the flame runners/check the alignment of them and check gas pressure..if the flame cannot be sensed, and itís in the flame, you could have pinched wire as well, corroded terminal...those mini wire brushes work great
    I always bypass the tstat at the RTU to save time...

    I always clean the rollout switch/gas valve terminals and check its function...make sure proper voltage is there..

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    Frown

    Quote Originally Posted by Unlimited1 View Post
    I always clean the flame runners/check the alignment of them and check gas pressure..if the flame cannot be sensed, and itís in the flame, you could have pinched wire as well, corroded terminal...those mini wire brushes work great
    I always bypass the tstat at the RTU to save time...

    I always clean the rollout switch/gas valve terminals and check its function...make sure proper voltage is there..
    I appreciate your input Unlimited1. I have been on the phone to Carrier tech support 3 times and unfortunately the last convo revolved around the support rep suggesting trying yet another inducer motor(as though the new motor I just installed was somehow bad) Ok, I'm running out of options so today I went ahead and did it even though I knew that isn't the problem. Sure enough, I returned motor we just installed and exchanged it, reinstalled, and same deal. Today I yet again removed combustion box, inducer, and burner assembly and this time I switch circulation fan to 'ON' and felt around and felt a little air leakage between exchanger flange and cabinet on lower left side nearest the first exch. tube. I sealed that edge up with foil tape. Now no air whatsoever near that burner. Buttoned it all back up and still same thing. Would not fire until I blocked motor outlet and you can distinctly hear the motor speed ramp up and it then sparks and starts. That informs my mind that there may be a blockage of some sort in heat exchanger. But how on earth can that be? We just installed the exchanger in October of 2018 and had -0- issues until the first start of this year. Did something crawl into a burner tube and die and block air flow? I mean I sincerely don't know what to think at this point. I cnnot remember in 25 years of working in the field ever having a situation this insidious. The inducer fault code on the igc is based on inducer time-on and after several minutes of running where it does not fire the burner, you get a 6 flash code - "Inducer Fault". I still wonder however if it is something in the board and while I do not subscribe to the 'hang parts on it til it works" method of repairing an issue, I am getting close to doing just that. That's how frustrated this beast has me at this point. I am a professional and above all thorough service tech but I must admit at this point, I am at a serious loss as to what to think. Even the gentleman at my local Carrier tech support is stumped and he is in fact a guy with plenty of field experience. My next step in this fiasco is to try and get him out to look at it this Monday. That's all I can do at this point. I'll send you guys my picture so you can hang me up as the guy with the worst luck, lol.

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    *As an aside, an issue came up in my convo with Carrier tech support that revoved around baffle in the heat exchanger tube outlets. The original factory exchangers had baffles in them(the pucks they put in outlet openings), the replacement exchangers do not. They are a redesign and in place of baffles the tubes are pinched down just in from the outlet. I mention this because someone suggested to me that we were given the wrong heat exchanger or that the one we installed should have baffles in the tubes. From what I know, the necked down tube near outlet IS acting as a baffle of sorts.

  17. #13
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    What is the model of the Unit you are working on. Inducer fault (trigger)would be the 3 pin/wire sensor on the back of the motor that plugs into the board...It codes when it senses no movement, the new inducer should have come with it....it detects the motor running, Not by length of time that Iím aware of.....with both icp and carriers...

    .I also mentioned the gas gas pressure and flame runners but you didnít say what it was And how clean the runners are...you said it would fire if the inducer motor exhaust was partially blocked...But Iím seeing a normal S.O.O with spark and start(?) What is start? Just a few second ignition?

    Thats making me think you donít have the volume of gas needed..Sounds stupid but I always check gas flow at the union 1st with troubleshooting and Iíve seen bad gas regulators and partially closed gas valves (if you have them)

    So as a review...Sequence of operation, inducer runs (checks sensor) So no problem there, (Make sure the board terminals are connected properly and clean...)
    good Strong spark (?) Iím guessing because itís a new board...What is the spark gap set at( and clean?) Spark not grounding against the rail if zip tied to it..?

    Then the gas valve kicks in( 24v there?) hear the click of the solenoid? Smell of gas?(you will also smell it at the inducer exhaust... Gas supply confirmed using manometer at gas valve?

    The flame runs across to the flame sensor unit should stay running..

    I have replaced plenty of bad flame sensors as well..

    Is there another unit there you can check the connections with and possibly swap a part for testing????

    Most Of the 50 Commercial buildings I am at have 24-35 RTUs There , so Iíve done it a few times if I get frustrated or just exhausted..Random splits are there also...

    Food for thought, I have been to buildings where the gas company has shut off the gas ...it takes a purge Of gas and a few restarts to get the unit running
    Last edited by Unlimited1; 10-23-2020 at 11:47 PM.

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  19. #14
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    The unit is a 48TJF024-5A1AA. I tested the Hall sensor on the motor. With sensor plugged into board I got -0- volts on 1/2 turn of motor and 8.72 on second half of turn. Unplugged from board I got 10.6 volts between pins 1 and 3 and 22.5 volts between 2 and 3. All satisfactory. Something that crossed my mind this evening is the line voltage coming into the unit. I checked that early on and it is running at 207.6 volts leg to leg. The initial issue was a bad inducer motor. At the risk of repeating myself, I went to do the yearly pre-winter service and when I tried to start the unit in Heat mode the inducer motor got ugly. It was locked up. Pulled the panel, checked voltage to motor, checked cap and saw that motor was bad. Replaced motor, housing and motor mount bracket, sealed all up tight with high temp rtv and tried to fire unit. Called for heat, inducer motor spun up but no spark and no gas valve. I waited a minute or 2 and then blocked inducer outlet with my hand and then I got ignition and burner fired. I let it cycle(2 degrees) and once stat satisfied and circ fan finally stopped I kicked it up several degrees to start cycle again and that time it DID fire. Tried that again and it didn't fire on next attempt until I once again blocked exhaust outlet of inducer motor. You can distinctly hear ind motor speed ramp up when blocking outlet.(increasing back pressure) The combustion box and inducer housing are sealed up tight so no excess air is leaking into it there at all. The key for me anyway is that blocking outlet of motor causes motor speed to ramp up. If it needs to ramp up to fire then obviodsly something is causing it to bog down when not assisting it in this way. That has to be a blockage of some sort not an air leak. An air leak I woud think woud tend to ram motor speed up, not down and blocking the outlet would not then make it fire. The replacement inducer motor is a Fasco rated at 208/230 .48 amps and 3450 rpm. The only thing I can think of that I have not done and need to do is take an amp reading on the motor.

  20. #15
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    What is your gas pressure after the gas valve? Measured at the output of the gas valve...
    You donít need to use the tstat just jumper it out at the RTU..
    You said you have 208v at the unit. What is the high voltage terminal/wires Onto, on The low voltage transformer...? If itís on 230v move it over to 208v and retest..also make sure the high limit has a good connection and the connections are clean...

  21. #16
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    Incoming is 7, Manifold pressure is 3.0. 3.5 is optimum but to get to proper temp rise we set it at 3. Not a gas pressure issue however. This is an airflow issue. We're not even getting to spark or gas valve activation until I block the inducer motor outlet with my hand. Let's say the board was bad. In that case, blocking off outlet of motor would not satisfy requirements and get spark then gas valve to come on. On a cold start(call for heat) this inducer will start right up and keep running with no spark or gas valve until I block outlet and increasing motor speed which speaks more to a blockage somewhere than an air leak at least in my mind. I have also moved the location of the inducer fan wheel, up, down and in the middle where it mounts on the shaft all to no avail. And yes, motor is spinning in right direction and the new wheel is exactly the same as original with same number of blades. Believe me, I counted them.

  22. #17
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    Was there an air flow block off reduction/choke plate behind the old inducer and not transferred over?

  23. #18
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    What's your replacement inducer motor number.
    You don't have a 24 ton unit.
    Attached Images Attached Images  
    Last edited by VTP99; 10-24-2020 at 04:27 AM.

  24. #19
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    he might have a bad inducer motor out the box

    HX was replaced last year and ran fine until motor locked up

    I think your chasing your ass in circles because of a bad motor

  25. #20
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    2 things - Number one, there is no baffle in the mouth/outlet of inducer housing. It simply dumps straight into the hood on cabinet door. which is also how original was. Just the same I asked Carrier same question and they said no baffle there. Number 2 - Even though it was the correct motor matching the exact specs of the old one, Carrier thought maybe there was something wrong with the new motor so they gave me another one and I installed it. Same thing, no fire without help. I also tweaked where the fan was located on the shaft, up-down-in the middle. No effect at all. Voltage going to motor is good as well. As previously stated, early on in this I checked 3 pins on board that the Hall sensor connects to and voltages there are fine as well. Now, that said, I still wonder if there is anything else about the board itself that could cause this.

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