Im having a problem with my furnace that no one can seem to diagnose. Can some of you techs out there take a look at this and give me an idea of what you think might be the problem? This is a Bryant gas-fired, induced combustion furnace.
This is the order that things happen:
1. Furnace is working properly. Its 45 degrees outside and the furnace only has to stay on for short periods of time to keep the house temp. at the thermostat setting.
2. Temperature outside drops to 30 degrees overnight so now the furnace has to stay on for longer periods of time to keep the desired house temperature.
3. Sometimes the furnace will shut down before temperature on thermostat is reached. It shuts down in this order:
a. inducer motor shuts off
b. furnace flame goes out
c. main blower shuts off after a couple of minutes
4. Furnace attemps to restart in this order:
a. inducer motor starts
b. Im assuming pressure valve closes because I have tested it and it seems to work properly.
c. pilot sometimes lights and sometimes doesnt.
d. inducer motor stops. Sometimes the pilot will stay on when the inducer motor stops, other times it
goes out.
e. inducer motor starts after a few seconds and the whole process is repeated.
f. eventually the furnace will light. Sometimes it takes up to 15 of these cycles for the furnace to light.
g. after the furnace lights, sometimes it will stay on for only 15 seconds and other times for 15
minutes other times in between.
5. If I shut the furnace off for a half hour or so and restart it, it will start and run fine. At some point, I
believe something gets to hot and shuts down the furnace, and the whole cycle starts repeating.
6. My schematic shows 1 Limit Switch and 1 Auxilliary Limit Switch, both are manual reset.
7. Ive replaced the Standing Pilot, the Inducer Motor circuit board and the Blower Motor board.
Does anyone have any ideas what could be causing the problem? Could the gas valve be causing this problem?
Thanks for any help.
Is this a high efficiency or mid efficiency furnace? Metal venting or plastic venting?
I'm assuming it's a high efficiency furnace with an intermittent pilot.
The fact that the inducer motor stops before the burners is odd. Normal sequence would keep the inducer motor running abit after the burners have shut down, to completely exhaust the flue products. There is a primary limit control that is of the automatic reset type, if the furnace is overheating it would interupt the 24V circuit, causing the heat to stop. You may have an airflow problem, plugged acoil, filter ect that causes this switch to open. Usually the primary limit resets itself after the furnace has cooled down.
The intermittent igniton problems you're experiencing might be caused by a pressure switch that will not stay closed. Get the tech to use a manometer to test the venting.
What do you mean, the pilot sometimes lights? Does this mean it's trying to spark but just won't light? It's uncommon to have an intermittent gas valve problem. Usually it's the board that has the intermittent problems, but you've already replaced that. If the inducer motor stops, but there is a pilot, there's a problem. When the inducer motor stops, the pressure switch should open, and the board should not be trying to light the burners or pilot.
I suspect your furnace is accient. My guess is that the secondary heat exchanger is plugged internally. After the furnace has been running for awhile the condensation builds up but isin't draining fast enough through the secondary heat exchanger, which causes the pressure switch to open.
How old is the furnace? Do you have a model number? Is this propane or natural gas?
You've had this problem for nearly a year now? Do you live near Ottawa?
This is a Bryant model #376B. Its a pretty simple Downflow, Gas-Fired, Induced-Combustion Furnace. Its about 20 years old. All the limit switches are manual reset so that isnt the trouble. Ill check that heat exchanger and see what happens. Thanks.
Yes, Ive had this problem for over a year. The weather started getting warmer when I was working on this in the spring so I wasnt a problem. I live about 30 miles NW of Chicago.
Another possibility could be a plugged condensate drain. After the furnace runs for a while the condensate builds up and plugs the venting. When this happens pull the plastic/rubber tubing off the bottom of the ventor housing and see if water comes out. If yes then this is the problem.
Another thought, when you said the inducer motor shuts down before the burners do, there's a possibility the inducer motor is overheating, and shutting off on its overload protection. It would make sense that your problem is happening after the furnace has been running for a longer period of time.
Im not sure if this motor has overload protection. I dont think its the motor because as soon as the motor shuts off it starts right back up. I believe the relay is shutting off the motor.
I had a tech come and look at the furnace. The result was that he has no idea what the problem is. He couldnt help me.
After the tech left I bypassed all the limit switches, disconnected the pressure valve, took the hose off the pressure valve and disconnected the pilot and ignitor. After I did this the inducer motor does the exact same thing, it cycles on and off. The only difference is the furnace doesnt light because everything is disconnected. Whats making the inducer motor shut off for a second or two?
It sounds like a power robbing thermostat. Sometimes voltage leaks down the W wire and the board energizes the ventor motor.
Disconnect W wire at the furnace board and see if the cycling stops. Replace the thermostat or install an isolation relay to correct this problem. I've also used a Honeywell 1000-ohm resistor part #4074FAB to solve the problem. It's the easiest and cheapest way, you wire the resistor between the W and C terminal at the furnace board.
Ive bypassed the thermostat and jumpered the thermostat wires at the board and the motor still cycles.
Well my guess would be a faulty board, but you've replaced both boards...
Double check the polarity on the 120V that comes to the furnace. You should measure 0V to ground from the neutral wire and 120V to ground from the line wire.
If the low voltage wiring is disconnected at the furnace, then there is no 24V that should be energizing the relay. If the ventor runs, then it can only be the board as far as I see it.
Don't jumper anything, simply remove the W wire at the furnace. Does the cycling stop?
When I disconnect the w wire the inducer motor shuts off.
QuoteIt sounds like a power robbing thermostat. Sometimes voltage leaks down the W wire and the board energizes the ventor motor.
Disconnect W wire at the furnace board and see if the cycling stops. Replace the thermostat or install an isolation relay to correct this problem. I've also used a Honeywell 1000-ohm resistor part #4074FAB to solve the problem. It's the easiest and cheapest way, you wire the resistor between the W and C terminal at the furnace board.
It definitely sounds like a thermostat problem is causing the ventor motor to cycle. Your first post suggests otherwise though. The description about the pilot staying lit after the ventor motor has stopped has me scratching my head. It's possible there are multiple problems though. I would change the thermostat and see what happens.
If possible post a picture of the manual reset primary limit switch. Most limits are automatic reset. I'd just like to see what we're dealing with.
I have replaced the thermostat and bypassed the thermostat and nothing changes. Ive have completely bypassed the limit switches and the motor still cycles. Maybe I can take a video of whats going on and post it so you guys can see and hear whats happening.
Supply the venter motor with 120v from a different source and see if it cycles, at the same time put your volt meter on the terminals that supply the venter motor. Run the furnace through a cycle. Let me know the results. THe wiring should go directly from the board to the venter (IDFan ) . If all is well there run a new t-stat wire temporarily to the side of the furnace, There could be a problem with the wire inside the wall. If you have access t oa manomiter put a tee in the vacuum line run the furnace and see if the reading is above the pressure switch setting, it shoould be marked on the switch. If that switch drops out for a milisecond you will see the same problem as you discribed.
Quote from: Zzz on December 25, 2008, 10:53:45 AM
Supply the venter motor with 120v from a different source and see if it cycles, at the same time put your volt meter on the terminals that supply the venter motor.
I
like it... that makes a lot of sense.
QuoteIf that switch drops out for a milisecond you will see the same problem as you discribed.
On my (mom's) Day & Night-badged 376BAW036075, the pressure switch (actually the microswitch on it) was dropping for at least that long, which would kill the burners, yet the inducer motor kept on going just fine.
Original Poster wrote:
QuoteIve replaced the Standing Pilot, the Inducer Motor circuit board and the Blower Motor board.
[and]
I bypassed all the limit switches, disconnected the pressure valve, took the hose off the pressure valve and disconnected the pilot and ignitor. After I did this the inducer motor does the exact same thing, it cycles on and off. The only difference is the furnace doesnt light because everything is disconnected. Whats making the inducer motor shut off for a second or two?
[and]
I have replaced the thermostat and bypassed the thermostat and nothing changes. Ive have completely bypassed the limit switches and the motor still cycles.
Before going further, let me note that i am not an HVAC professional. I am a skilled troubleshooter who fixes all sorts of things, and so far has a decently good track record with the family's HVAC gear (natural gas forced-air furnaces, mainly). I have spent vast amounts of time with Mom's horizontal-mount 376B in the last several weeks, found this thread, and have some thoughts on it.
The results of Zzz's recommended direct-power to inducer motor test would be good to know (hopefully the OP will come back at some point). Other than dirty connectors, there really are not that many electrical modes of failure for the inducer motor not already addressed, and a direct connection would eliminate a lot.
So far, no one has brought up the possibility of the inducer motor mechanically hanging up. I would have assumed that the sagging inducer motor grommets (at least for the horizontal mount) which can lead to the inducer motor stalling which appears to lead to the blower-wheel-end bearing seizing would be common and well-known... it is at the house i grew up in! Found that failure in 1994: inducer motor not running. Bearings seized and wheel was rubbing the housing and could not have turned anyway. Circuit traces burned open. Burner and main blower would run, yet not for long, since the safeties would shut down the burner.
Back in '94, checked with the OEM (Lord), and the weight of the motor assembly exceeds the load rating of the rubber grommet isolators. Lord had a stiffer rubber version, yet nothing in that form factor which was designed to hold that 4 lb. assembly. Mom has the stiffer version in her furnace now, which is holding up better, yet still sags with time and heat. I have to rotate them (spin them in their mounts), not unlike rotating car tires, every few years.
It is not inconceivable (at least to me) that the inducer motor blower could be almost at the point of mechanically hanging up, possibly varying with temperature, possibly hanging now and then for a second or two.
Anyone interested in the full story, or who perhaps wants some possibly entertaining reading of a non-expert doing HVAC work, may wish to go to the pages i just put up, starting from 376B Furnace Fun (http://siber-sonic.com/HVAC/376B/). One linked page discusses this inducer motor issue. The other discusses my more recent adventure with a combination of user-induced failure and a very funky microswitch. I am thinking in OP's case that the microswitch on the pressure switch assembly may be quenching the burner at times, though not once did it even cause the inducer motor to slow down or blip.
Hopefully, something here was or will be helpful to someone. If not that, hopefully at least a pleasant diversion.