Home AC question, fan not turning on automatically

[Citation]

I know this isn't garage related but I thought I would ask the AC experts here hoping this would save me some digging.

Short version:
Old gas central air system. Fan seems to work correct fine on heat cycle but does not turn on when I try the fan only (no heat or AC) nor when I use the AC. Fan does turn on when I use the manual setting on the thermostatic fan switch mounted in the air flow between the heat exchanger and the AC evaporator.

Long story:

My home central air system is an OLD ThermoPride gas furnace + AC on the output. I'm guessing the AC was added later as it's not built into the ThermoPride's large housing. I bought this place about a year back so I can't tell you about the history of the unit. Yes, it's old and I am tempted to replace it. A warranty was thrown in when we bought the house.

Anyway, last fall (ie when we would use the heat, not the AC) the system wasn't working so we called out the service guys. I don't remember the exact problem but the first attempted fix was replacing the Honeywell fan relay. That didn't fix things so they replaced the fan's thermostatic control that sits above the furnace but below the AC evaporator. I believe that device is basically meant to delay turning on the fan until the furnace heat exchanger warms up a bit and perhaps more importantly, keep the fan running until air temperatures drop again.

Fast forward to this spring. Time to turn the AC on. Nothing but a tripped 120V breaker. (Note the actual AC compressor is on a separate 240V line). Call the AC guys and they replace the motor. After the repair I had two problems, 1, basically no air flow when the fan was on. 2, the only way to get the fan to turn on was use the manual bypass feature of the Honeywell thermostatic control. The AC compressor does run without the fan (the coil quickly ices over). Since I don't recall this fan problem last winter today I tried turning on the heat. It seemed to work just fine. Burner started, once the temp rose a bit the fan turned on. Turn the heat off and the fan shut down after the temp dropped back down.

My guess is the guy who worked on the system last fall didn't actually wire the replacement fan relay correctly and the system is (again my guess) only turning on when the thermostatic switch says go, either manually or due to hot air. I don't know where the normal fan "ON" signal comes from (thermostat, AC compressor, and/or some part of the furnace system?). The fan does not turn on when I set the thermostat to FAN ONLY. I'm guessing that thermostatic switch is a back up to make sure the heat exchanger is cooled off after the thermostat tells the heat to turn off. I assume the fan relay (or what ever it is that takes a signal to turn on the fan) should be able to take more than one input.

Any suggestions what I should look for? I'm quite comfortable working on things like this in general but I've spent almost no time working on home HVACs so I really don't know how they are set up.

Thanks!

 

[Citation]

Follow up, just looked again. My green wire from the T-stat control is connected to nothing. Since my Nest thermostat (they came with the house) said that should be the fan signal, where should it normally go?

 

[mrobins297aaa]

the fan starts from the "G" terminal on the thermostat. when you switch the fan to the on setting on the thermostat, internally the thermostat connects the "R" terminal with the "G" terminal, which is usually the green wire. Inside the furnace the green wire would connect to the "G" terminal on the circuit board if you have one, if you only have a relay it should connect to one side of the coil of the relay (which may be labeled with a "G") with the other side of the coil going to ground or "C" terminal on the transformer

 

[dogdog]

yea don't go by "Green" wire, go for the G label wire.... the guy hooking it up might not necessary using Green as G. you say the fan works in Heat mode just not AC right... the　Ｇ　ｎｏｒｍａｌｌｙ　ｉｓ　ｔｈｅ　ｓａｍｅ　ｆｏｒ　ｂｏｔｈ unless you have some weird setup．．．．　ｏｎ　ｔｈｏｓｅ　ｆｏｒｃｅｄ　ａｉｒ　ｓｙｓｔｅｍｓ．．．　Ｍａｙｂｅ　ｉｔ　ｉｓ　ａ　ｓｅｔｔｉｎｇ　ｏｎ　ｙｏｕｒ　Ｎｅｓｔ　ｃｏｎｔｒｏｌｌｅｒ / Honeywell controller?

 

[Citation]

I found a wiring diagram.
http://www.thermopride.com/wp-content/uploads/MG-300.pdf
After playing with the system I'm more confused than before. The bottom of page 14 is my system. I haven't pulled the junction box cover (see the diagram) so I don't know if all those wires are connected correctly. The is no terminal block, just a rats nest of wires. It always all visible wires are correctly run except the green to the relay coil. When I connected that wire the circuit breaker tripped when I used the thermostat to turn on the fan. However, the fan-limit switch operates the fan correctly when I use heat (after resetting the breaker of course). So now I need to figure out why the fan relay trips the breaker but the fan limit switch does not. I hope this isn't some type of inadvertent ground situation.

 

[mrobins297aaa]

sounds like your shorted to ground on the 120v side of that relay, check the 120v wires that the relay energizes one maybe touching ground somewhere.
If the short was on the 24v side you would have smoked the transformer.
post some pic

 

[nsula_country]

Some split systems have 2 control transformers. One for heat, one for AC. On those, RC and RH should not have a jumper. If 1 control transformer, jump RC and RH.

I have seen this before. Both transformers fight each other until one burns out (if unfused). Then only one function will work (Heat or AC).

CT

 

[Citation]

Well I've definitely found the problem. It might have been the problem I've had since I bought the house.

So if you have a fan relay and someone connects say one side of the relay to the white neutral wire and say the other side to the live hot wire what happens when the relay is turned on. That's what happens with my system.

I opened up the junction box inside the unit and looked at the rats nest of wires (vs the rats nest of low voltage wires outside the junction box). I don't know when it was done wrong and while I would like to blame the warranty company that serviced the unit last fall I suspect it might have been wrong from the word go.

Anyway, I would do it myself but I've already spent $75 (or possibly $150) to have these guys correct the problem so I might as well have them finish the job. At least I've got a new relay, fan-limit switch and fan motor out of the deal!

 

[dogdog]

But you mention that the fan works on heat cycle ? G=fan is the same for Heat and AC if it is a forced air unit.

 

[mrobins297aaa]

not on that system, the fan is started by the fan switch based on temperature. the thermostat does not start the fan for the heat cycle

 

[Citation]

not on that system, the fan is started by the fan switch based on temperature. the thermostat does not start the fan for the heat cycle

Exactly... not that I knew any of this a week ago. I just got tired of my system not working and started to look at the diagrams. This is why I think the real problem dates back to at least last fall. Last October the warranty company replaced the fan-limit switch and the fan relay (which wasn't bad). The fan-limit switch was probably bad given the age of the system. Near as I can tell it was installed in 1979. It's possible the system was wired correctly when I bought it but when they replaced the relay last fall they got the wires wrong.

Regardless, in the heat cycle the fan is controlled by the air temperature (hence the fan start-stop lags that of the burners), not a T-stat signal. The G-wire is only for AC and fan only operation. Does the G-wire control the fan on a typical electric heat system?

Again, not that I knew any of this last week before I searched for the info on the web.

 

[mrobins297aaa]

Exactly... not that I knew any of this a week ago. I just got tired of my system not working and started to look at the diagrams. This is why I think the real problem dates back to at least last fall. Last October the warranty company replaced the fan-limit switch and the fan relay (which wasn't bad). The fan-limit switch was probably bad given the age of the system. Near as I can tell it was installed in 1979. It's possible the system was wired correctly when I bought it but when they replaced the relay last fall they got the wires wrong.

Regardless, in the heat cycle the fan is controlled by the air temperature (hence the fan start-stop lags that of the burners), not a T-stat signal. The G-wire is only for AC and fan only operation. Does the G-wire control the fan on a typical electric heat system?

Again, not that I knew any of this last week before I searched for the info on the web.

yep you got it, the fan switch energized the low speed winding on a two speed motor, after that wire left the fan switch it was hook to the normally close side of the ac relay. so on a call for cooling the relay would take out the low speed wire from the fan switch and energize the high speed winding on the fan motor. that way it eliminated the possibility of having power to both windings at the same time.
I don't know about a electric heat system

 

[nsula_country]

Exactly... not that I knew any of this a week ago. I just got tired of my system not working and started to look at the diagrams. This is why I think the real problem dates back to at least last fall. Last October the warranty company replaced the fan-limit switch and the fan relay (which wasn't bad). The fan-limit switch was probably bad given the age of the system. Near as I can tell it was installed in 1979. It's possible the system was wired correctly when I bought it but when they replaced the relay last fall they got the wires wrong.

Regardless, in the heat cycle the fan is controlled by the air temperature (hence the fan start-stop lags that of the burners), not a T-stat signal. The G-wire is only for AC and fan only operation. Does the G-wire control the fan on a typical electric heat system?

Again, not that I knew any of this last week before I searched for the info on the web.

Yes. Heat pump and electric.

CT

 

[Citation]

Talked with the owner of the service company. Told him about the wiring issues and that I think the only real problem the system ever had was the fan limit switch. I was polite about it and he did seem concerned. He suggested putting in a claim for a new furnace.

Hmmm.

I would have to cover part of the cost, not sure how much. The upside is the old system is very big and my best guess is a 60% efficient system (1979, standing pilot light, connection draft flue). The fan motor is 1/3rd hp. I'm not sure how much a new system would cost. will not replace AC

If the warranty will really cover much of the cost perhaps I should jump. However I'm concerned that any warranty % coverage may be no more than typical negotiated discounts. I need to calculated/guess my power savings but beyond that any suggestions or warnings I should be aware of?

 

[Citation]

Just to close this out. Guy came by yesterday, verified the system was wired wrong when I showed him both how it was wired with a hand drawn diagram and the service manual diagram. He corrected the wiring and that was that. At least he can go back and help the techs learn something new. Well that and I leaned a bit about how my system works. The furnace will probably last another 30 years.

 

[LS6 Tommy]

But you mention that the fan works on heat cycle ? G=fan is the same for Heat and AC if it is a forced air unit.

Not really. The fan center (fan relay) is not controlled by the "G" circuit when the thermotat is set to "Heat". The low speed on the blower is fed from the Normally Closed contacts, then through the fan/limit switch and the high speed on the blower is fed from the Normally Open contacts which close when the thermostat is set to "Cool" or "Fan On". If the fan center line voltage side was wired as a dead short for the Normally Closed (high fan) you'd get the exact scenario with the blower only working on heat and the breaker tripping on a call for cooling.

Tommy