Air Handler Fan Start Is Sporadic


  #1  
Old 01-19-03, 09:16 PM
here2do
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Thumbs down Air Handler Fan Start Is Sporadic

I've checked current postings and still wasn't sure, so here's the situation:

About 3 weeks ago, my fiance's air handler fan stopped working properly. When you turn the unit on, you can hear the outside compressor come on but the air handler, in the attic, doesn't always come on. As a matter of fact, it rarely comes on at all now.

The unit is a Trane and is about 10 years old. On the outside of the blower case there is this info: 1/2 hp fan motor, FLA of 3.3 amp, 220-230 volts (and maybe 1 or 2 other things, but I left the paper with the info over at my fiance's house).

Within the last 6 months, she's had the heating element, and thermostat replaced, coils cleaned, and freon added (about $750 in all). From the little I understand about how these systems work, I summized that the problem might be the start capacitor. However, I don't know how to verify this, and if it is the capacitor will I be able to replace it myself. When I went up in the attic to get the info of the panel, I shut the power off, open the panel and looked inside. Basically, all I could see was the blower fan and the fan housing. I turned the fan to see if it would move freely. It did.

Since she's already shelled out $750 on this thing, I'm gonna start training for my Do It Yourself degree (which I'll need very shortly, since we're also in the market for a new home).

Any advice would be greatly appreciated.

PS: one of the pieces of paper I forgot to bring home was a schematic pulled from the fan housing.
 
  #2  
Old 01-20-03, 05:48 AM
GregH's Avatar
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here2do:

As a place to start you could go to the air handler and while watching the fan have someone turn on the system and see what happens.
If it's a fan problem you will either have a relay kick in with the fan having a hard time running, or the relay itself may not be cutting in consistantly.

Let us know what you find.
 
  #3  
Old 01-20-03, 08:08 AM
bigjohn
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Well, how about a discussion on capacitors for your first lesson? In HVAC units, there are 2 types of motor capacitors. One is a START type and the other is a RUN type. Start capacitors are intended to assist a motor in starting and are only in the circuit when the motor starts. As the motor comes up to full speed on startup, the start capacitor and start winding are taken out of the circuit either by a relay or a centrifagul switch inside the motor depending on the application. Start capacitors are used in applications where the motor needs a high starting torque. For example, on residential a/c units with TXV metering devices, the compressor motor usually needs a higher starting torque because the system pressures do not equalize on shutdown or at least not rapidly. Start capacitors have hard plastic cases with a fiberboard looking top, but they do not have a provision to dissapate heat which is why they can only be in the circuit for a short time. [seconds] Start capacitors are wired in series with the motors' start winding. Run capacitors are used on lower starting torque motors to shift the phase angle between the start and run windings which provides a lower starting torque than capacitor start motors but enough to start the motor. Run capacitors usually have a metal case and the case encloses the whole thing. [some run caps have a plastic case but it's not as hard or shiny the plastic cases used on start caps. As with the metal type case, the case encloses the whole thing IOW- the top and the case are the same material] Since run capacitors are in the circuit any time the motor is running, they are filled with oil to dissapate heat. The capacitance rating is called a farad, and in the HVAC industry, we use capacitors rated in microfarads. The symbol is uF or sometimes you'll see mfd. Capacitors also have voltage ratings; the symbol is vac. In residential HVAC units, you'll generally encounter three types of motors: Capacitor Start/Iduction Run which has only a start capacitor, Permanent Split Capacitor which has only a run capacitor and Capacitor Start/Capcitor Run which has both. The vast majority of motors in residential HVAC units are PSC. All this brings us to your motor. Quite often a motor will not start or draws excessive current because of a bad capapcitor. Capcitors are cheap and a simple replacement may solve the problem. When replacing run capacitors, there are 2 rules. The uF or mfd rating can be + or - 10% and the vac rating has to be equal or greater. Since that motor will probably use a 7.5 or 10 uF capacitor, there isn't much wiggle room on the capacitance, so you just buy one with the same value. In the HVAC industry, there are only 2 vac ratings namely 370 and 440. Since the vac rating has to be equal or greater, most service guys just stock all 440 vac caps on their trucks. Now the problem with the init is going to be that either it isn't getting power to the fan motor in the first place due to possibly a bad sequencer or fan relay, or power is getting to the motor and it can't/won't start because of a bad cpacitor or maybe the motor bearings are worn out. You'll first need to determine whether your getting power to the motor by testing for voltage when the motor is supposed to start. No power?- trace the wires back and see if they go to a fan relay or sequencer and see if you have power in but not out. However, if you hear the motor humming like it's trying to start, try replacing the capacitor before condeming the motor. The capacitor may be mounted on the blower housing or it may be up in the control panel. The ratings will be printed on the capacitor case. Any chance you can post the wiring diagram? That will help us guide you to where to check the voltage. if you wind having to replace the motor, use our third rule: new motor, new capacitor. Don't assume a new motor will use the same rated cap as the old one. Check the nameplate on the new motor.
 
 

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