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ptmoy · Jan 15, 2006, 01:36 PM

I have a Bryant 395C furnace. The blower won't come on even when thermostat fan switch is set to "on." I've verified that I'm getting 24V AC between terminals marked G and C on the furnace. Blower has 3 wires going into circuit board on furnace. These are colored red, black, and white; red is connected to a terminal marked "LO," black connected to "HI," white to "COM." I measure no voltage across both red/white and black/white. I think the blower is not getting any power. Please help.

wheatnbarley24 · Jan 15, 2006, 03:42 PM

If you have voltage across G and C on the furnace and aren't getting any power to the motor the relay on the control board is bad. Unforunatly you can't replace just the relay. The whole board has to be replaced

labman · Jan 15, 2006, 06:36 PM

Yeah, likely it will take a new board to fix the problem. Usually the black and red wires connect to different relays. A little unlikely they both went bad. Before replacing the board, jumper 120V to the black and white wire. Show respect to the 120 volts. If the blower runs, then check it to the red and white. If the blower runs, replace the board. May or may not be both relays, but boards are beyond most DIY. The techs don't even mess with them.

Just carefully note where everything plugs in and any jumpers. I paid $300 for a board when a cheap little relay went bad on mine.

ptmoy · Jan 15, 2006, 06:50 PM

I suspect it's relay-related also. When I apply power to the blower (24v on terminal G), I hear the blower relay click, I can actually hear the blower trying to start after this click. Then almost immediately, I hear a second (softer sounding) click, then the blower stops. I measured voltage to the blower immediately after the first click, then no voltage after the second click.

There are two blower-related relays on the board: "blower motor relay" and "blower motor speed change relay." Each relay has 3 terminals labeled COM, NO, and NC. Is there any way to test these relays by measuring across these terminals somehow using a volt meter?

labman · Jan 15, 2006, 07:39 PM

N is normally, and O and C are open and closed. On ohms, with no power, the com and NC should be connected. With power to the coil, the com and NO should be connected. On a circuit board, the coils may be 12 volts DC rather than the 24 volts from the thermostat. Usually the coil voltage is marked on the relay.

ptmoy · Jan 15, 2006, 07:58 PM

I jumped 120V to the black and red wires as you suggested in your first reply. Blower comes on in both cases.

I'm a bit confused by your second reply. On the blower motor relay, I measured 120v across COM and NO regardless of whether there's 24V on terminal G. If there's 24V on G, COM/NC measures 24V; otherwise, COM/NC measures 12V. Does this make sense?

Sounds like I should just replace the board. Do you think there's anything simple I can do on the board, such as changing a fuse or hitting some reset switch to fix this (although I don't see either on this board)?

wheatnbarley24 · Jan 15, 2006, 08:21 PM

Normally if there is a fuse on a board it is for the 24v transformer. Since your are getting 24v at g and c the fuse is good. I am more than sure that you need to replace the board.

ptmoy · Jan 15, 2006, 08:51 PM

Thank you Wheatnbarley24 and Labman for your help. I've ordered a new circuit board on the Web.

I had 3 consecutive short power outages this morning. Furnace was working fine last night. Circuit board may have been damaged by a voltage spike. Labman, you mentioned on another thread that you wired a varistor in your line to protect against voltage spikes. How exactly do you do this? Where can one purchase a varistor?

labman · Jan 15, 2006, 09:34 PM

''I'm a bit confused by your second reply. On the blower motor relay, I measured 120v across COM and NO regardless of whether there's 24V on terminal G. If there's 24V on G, COM/NC measures 24V; otherwise, COM/NC measures 12V. Does this make sense?''