Go Back  DoItYourself.com Community Forums > Heating, Cooling, Air Conditioning, Ventilation and T-Stat Controls > Gas and Oil Home Heating Furnaces
Reload this Page >

Pressure switch is closed with Inducer off, tried several experiments. What next

Pressure switch is closed with Inducer off, tried several experiments. What next

Reply

  #1  
Old 12-15-12, 02:41 PM
Member
Thread Starter
Join Date: Dec 2012
Location: USA
Posts: 1
Received 0 Votes on 0 Posts
Pressure switch is closed with Inducer off, tried several experiments. What next

I have a Gibson, GL1RA, forced air furnace. It is about 7 to 8 years old. For the last year, I had intermittent problem when it would not light up. When I turned off the power fully and turned on, it generally cleared the problem. But now, it does not solve the problem. Now when I turned on the thermostat, it turns on the inducer motor, then turns on the igniter. Igniter glows bright orange. But the gas does not light up. Then the igniter shuts down. After some time the cycle repeats. At the end, the control board light begins to blink. It is three blinks which per furnace document means "Pressure switch is closed with Inducer off". Note that inducer motor is running all this time.
I have done the following tests
1. With furnace off (i.e., power on but keeping thermostat below room temperature), the voltage across the pressure switch leads is around 4.5 volts.
2. With Furnace on, and both wires going to pressure switch taken out, the voltage between the two wires is around 24 volts. (I can't easily measure voltage when both wires are connected to the pressure switch and furnace on).
3. I disconnected both leads to the pressure switch. Then turned on the furnace. Control board immediately started blinking with 2 blinks meaning "Pressure switch is open with inducer on". When I shorted the two leads (I guess this is same as forcing the pressure switch to close), igniter came on but gas did not light. Then eventually ended up with light blinking three times.
4. I disconnected pressure switch hose and blew air to ensure that it is not plugged. It was easier to suck air then to blow into it.


What do I do next?
 
Sponsored Links
  #2  
Old 12-15-12, 10:38 PM
user 10's Avatar
Member
Join Date: Mar 2004
Location: NA
Posts: 1,916
Received 24 Votes on 19 Posts
You already did most of the necessary troubleshooting. At a minimum the pressure switch needs to be replaced.

With the power off, check the pressure switch for continuity. Open is normal when the furnace isn't running; near zero resistance indicates that the switch is stuck closed.

As a general rule, never bypass a pressure switch with the gas on...

With the gas to the furnace off (so it doesn't light off without a proper draft), turn on the furnace and bypass the pressure switch a a second after the draft inducer fan comes on.

If the board is good, it will give a "flame not sensed" (or something along those lines) error code shortly after the igniter glows.

It's 24 volts so you won't get shocked - just don't allow any disconnected terminal to touch the chassis as to prevent the low voltage fuse from blowing.
 
  #3  
Old 12-16-12, 12:35 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 52,608
Received 339 Votes on 317 Posts
Almost sounds like a control module issue. The module gave you two blinks as it should when the pressure switch was disconnected. You tried connecting it, ignitor activated and then three blinks. You should have gotten the 3 blinks before the ignitor activated. Once the ignitor activated it should have continued with the cycle.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: