[dspiffy]

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?

 

[beenthere]

Trane distributor is who you need to gt it from.

Energizing the G terminal the way you said. Will run the A/C when the heat is on.

 

[dspiffy]

Trane distributor is who you need to gt it from.

Energizing the G terminal the way you said. Will run the A/C when the heat is on.

How? I'm not connecting anything new to Y.

 

[beenthere]

Back feeding threw the thermostat.

 

[dspiffy]

I'm not following.

Right now, when the thermostat is calling for heat, it energizes W. I get fire but no fan. If I set to "ON", G is also energized, I get fan and heat. What I propose would energize both W and G in AUTO. As long as the thermostat is set to HEAT, Y is disconnected at the thermostat, and I cant trace any path inside the unit that links G to Y.

In order for Y to be engaged with W, there would have to be some path from G to Y that conducts when the thermostat is set to "heat" and "auto". What am I missing?

 

[beenthere]

The internal operation of the thermostat. When you use the thermostat, it opens the internal interlock between Y and G.

 

[dspiffy]

In HEAT mode? And it would have to be only in AUTO, otherwise any time you set the fan to ON you would have cooling.

I have a few thermostats apart and they seem to completely disconnect Y when not set to COOL. But they're not the same as the ones we're currently using. I'm going to get a model number, and look for a schematic, and maybe pull one apart. They're very basic HEAT/OFF/COOL AUTO/ON with a digital temperature read. No programming, no battery, just 4 wires.

 

[beenthere]

Once you follow the internal schematic, you'll understand. But get the diagram of the one you are using.

Of course, you could just get a new digital thermostat, and set it to run teh fan when it calls for heat.

 

[flashheatingand]

Possible to install a 7" cam stat?

 

[beenthere]

Possible to install a 7" cam stat?

Would have to alter the line voltage wiring.

 

[dspiffy]

Looking for a schematic . . . I believe all our thermostats are Honeywell T8400

 

[flashheatingand]

Would have to alter the line voltage wiring.

Could use just 24v circuit. One set of contacts would be r to g, The other set of contacts would be the r to w. Or could be the w to w circuit.

 

[beenthere]

R to G will bring on the A/C. When its done at the furnace.

The T8400 is a low end thermostat.

 

[flashheatingand]

In that case, replace the stat as well. Trane distributors get awfully proud of their gear.

 

[dspiffy]

R to G will bring on the A/C. When its done at the furnace.

The T8400 is a low end thermostat.

Anyone have a schematic for them?

This winter we were renovating a wing of the building, and there was no thermostat when there was no drywall. I think I shorted R G and W together so we had heat while we were working.

When we bought the building it had a myriad of different thermostats, some new and programmable, some as old as the oldest units. There were also a bunch of MasterTrol damper systems that were mostly defunct. I removed all but two of the MasterTrol systems (only one is connected and engaged, the other I plan to work on this summer) and replaced the rest of the thermostats with T8400s for uniformity.

 

[beenthere]

Its a thermostat that has been obsoleted for a long time. May be hard to find an internal schematic for it.

 

[dspiffy]

Still going to look for a stock replacement.

In the meantime, I ordered this:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=271178129307

It's DPDT so I can use it as normally open to connect G, and normally closed to disconnect Y.

 

[dspiffy]

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.

 

[beenthere]

You'll need to get that high limit replaced and wired back in ASAP. It keeps the place from burning down from the gas valve not closing or the fan not working.

 

[dspiffy]

I'm wondering just how many years it's been without it. Only me and my tech have been in these units since we owned the building. Prior to us buying it, it sat vacant for almost 10 years. It's a little scary to think about, especially given all the other things that have malfunctioned (and been hack wired) with these units.

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.

So in order for anyone to notice the missing/disconnected FLC, there would have to be no fan call from the thermostat OR the FTDR, AND the FFL would have to have failed to cut the gas off. Correct?

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. If that's true, I wonder what caused it to trip.