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  1. #1
    Join Date
    Apr 2013
    Posts
    7

    Buying parts for 35 year old Trane units (and confirm my troubleshooting)

    I am doing electrical and maintenance for a building my church owns. I've done all different types of electrical engineering before, from wiring huge buildings to vintage tube amplifiers, but this has been my first experience with HVAC. I've learned a lot, I feel fairly comfortable with the electrical side. I have a licensed HVAC tech that I call for all things freon-related, and so far we havent needed any gas repairs.

    3 of our units are original to the building, and we dont have the money to replace them. They run, but I am going to have to do a lot of maintenance.

    The paperwork that I have lists these units as TRANE SFCB 5 TON, the schematic is form 4-7422 A.

    One unit, when fan is set to AUTO, and the thermostat is calling for heat, the fan will not engage. If you set the fan to ON, the fan will engage and heat is produced.

    My reading of the schematic says the Fan Time Delay Relay is the culprit and needs to replace.

    1. Do the experts concur?
    2. Where can I buy such a relay, as inexpensively as possible? The goal is to spend as little on these units as possible, get through another two seasons, and hopefully replace them in a couple years.

    There is another identical unit that works perfectly, however when the thermostat is calling for heat, there is a very loud buzzing from inside the unit that resonates through the ducts. I am assuming one of the relays (possibly the same one) is going bad, but it has been too cold for me to crack the unit and confirm. Today was the first warm day and if this continues I will check it out Monday when I get back to the building. Is this a familiar problem to anyone?

    Thanks so much!

  2. #2
    Join Date
    Jan 2014
    Location
    New England
    Posts
    445
    You can buy a generic standard time delay anywhere, grainier.com would be one source, minimal cost item. or any HVAC supply house, will sell to trades people. Guessing it's 24 volts coil with at least one normally open contact. What happens when you run the A/C and the fan is in auto, does your blower fan start ?

  3. #3
    Join Date
    Apr 2013
    Posts
    7
    Quote Originally Posted by Bazooka Joey View Post
    You can buy a generic standard time delay anywhere, grainier.com would be one source, minimal cost item. or any HVAC supply house, will sell to trades people. Guessing it's 24 volts coil with at least one normally open contact. What happens when you run the A/C and the fan is in auto, does your blower fan start ?
    It does.

    I was talking to a friend who's an HVAC tech (in another state, sadly, or he'd be here helping) this morning and this is what he suggested:

    Get a generic time delay relay. Wire it between R, G, and W. Basically, connect the relay coil to R and W, and connect the contacts to R and G. I cant see any reason this wouldnt work.

    I found a generic 60 second 24vAC time delay relay and I'm going to give this a shot.

    Monday I'm going to open up the noisy unit and get some more information on that one. My friend suggested it could be the inducer fan.

  4. #4
    Join Date
    Apr 2013
    Posts
    7
    Slow moment: I need to wire the relay coil between W and common, not W and R.

  5. #5
    Join Date
    Apr 2013
    Posts
    7
    Looking at the schematic, the original FTDR has two sets of contacts: when engaged, it connects the fan contactor to 24vAC, and disconnects the fan contactor from G. Why does it need to be disconnected from G? Either the thermostat is set to auto, in which case G is dead, or the thermostat is set to on, in which case G is seeing the same 24vAC the fan contactor is already seeing.

  6. #6
    Join Date
    Apr 2013
    Posts
    7
    First warm day and I went up on the roof.

    The original FTDR was not hooked up correctly . . . there was a fair amount of hack wiring done inside the unit. Disconnected it and wired it back according to the wiring diagram. And . . . it worked. That's all it took.

    The bad news is the Fan Limit Control, which is supposed to connect to the FTDR, is . . . I would say disconnected, but there arent even remnants of wires going to it left to be reconnected. Someone disconnected it with a sawzall and a magic wand apparently. The unit functions fine without it, but I believe it needs to be there for safety purposes.

    The unit that was buzzing on call for heat has a bad gas valve. The buzzing is coming from the gas valve. I dont mess with gas. Once I obtain the part, my licensed tech will come replace it for me.

    Added ambient thermostats to all the 35 year old units with bad economizers (all of them). I had a scare this winter when someone kicked on the AC during -30 tempertures and it ran for a day or so before I discovered it. Now they wont turn on if it's below 55 outside.

  7. #7
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,473
    Do a proper repair or have it done. In two years the replacement money will still be a couple of years away, and any bandaid that you have done will have to last that much longer.

    Also, on most older gas furnaces (I'm not sure about your model without seeing it) the blower control also has a high temp safety limit built in, and if one part has failed it is very likely that the other has as well.

  8. #8
    Join Date
    Apr 2013
    Posts
    7
    Not sure which issue you're referring to-- going to have the gas valve replaced ASAP, I need to do some more digging before I can determine what needs to happen with the FLC i.e. does it still work but need new leads, does it need to be replaced, or is it missing entirely?

    The Fan Failure Limiter still works . . . for example, when the FTDR wasnt connected, if you engaged the heat in Auto, and the fan didnt come on, the fire would burn for a while and then shut off.

    My understanding is . . . the FFL is designed to shut off the gas if there's not enough airflow through the combustion chamber . . . the FLC is designed to turn the fan on if airflow is needed and the fan is not on from G or the FTDR.

    The Fan Limit Control is marked "MANUAL RESET" in big letters on the wiring diagram . . . I'm wondering if years past it tripped and whatever tech was servicing the unit at the time didnt know how to reset it . . . so they just removed it.

    But here's the oddest thing: on the other two identical units, the FLC connects to the FTDR. On this unit, there are no terminals on the FTDR for it to connect to.

    The unit is Trane SFCB-B503-HA Type 168-299-1-A Serial C80K-04752

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