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Discussion Starter · #1 ·
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. If you have it set to AUTO and the thermostat is calling for cooling, you get fan.

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.
3. In the event that I cant locate a replacement FTDR, I am going to get a generic 24vAC time delay relay and wire the coil between W and common, and the contacts between R and G. That's basically what the stock FTDR does, except the stock relay disconnects G from the fan contactor when the fan is engaged through the heat relay. Why is this? I dont see a potential for backfeed, maybe I'm missing it?

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. A friend suggested it could also be the inductor fan. 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?
 

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Discussion Starter · #2 ·
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.
 

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Call a damn hvac tech. You see me replumbing houses? No! Because im not a plumber and I know it.
all your gonna do is tear something up or get someone hurt. i was in the electrical trade before i took two years of trade school to work on hvac....theres a big difference between runnin conduit, installing xformers and panel boards doing branch circuits and controls for hvac.....thats why your ****ing it up.
 

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Discussion Starter · #4 ·
I've had 3 techs out. The first two wouldnt even touch the units as they were tool old and they were unfamiliar to them. The third fixed some leaks, recharged the compressors, but couldnt figure out why the fan relay wasnt working. He spent about an hour on it.

When I finally got on the roof with a schematic and wiring diagram myself, I fixed it in about 10 minutes.

The unit that's buzzing needs the gas valve replaced. I dont mess with gas. I'll have the same tech back out to replace it.
 
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