Furnace Troubleshooting Help

Discussion in 'HVAC Heating & Cooling' started by mdr, Jan 21, 2010.

  1. mdr

    mdr New Member

    Nov 8, 2008
    Hi all,

    Yesterday, our furnace (Ruud Silhouette) stopped working properly. I've been trying to diagnose the problem and can use some advice.

    It's not freezing here (but is raining...), so I'm willing to do some troubleshooting prior to sucking it up and calling a furnace repair guy. The big challenge is that the furnace is not easily accessible - It's in a 24" crawlspace in the center of the house with access from the outside - each visit is a trip into the rain, through a 18"x20" scuttle and then a 30' GI Joe crawl. Accessing the control board, etc. is very difficult. So before I pull things (e.g. transformer, circuit board, igniter) out to test, I'd like to have some idea where I'm going.

    Symptoms: both main blower and draft inducer blower run whenever there is power to the furnace. No flame. If the furnace power is cut and then turned on, both blowers start up simultaneously.

    Proper start (heater) sequence is:
    a) heat call from thermostat
    b) inducer draft blower starts and runs for 30 seconds
    c) igniter warms up for 30 seconds
    d) gas - main burner on
    e) main blower on

    Checked thermostat, works ok.
    However, no 24v on "red" thermostat wire - the power wire from the furnace controller
    and no 24v at the controller output on the furnace.

    Circuit board getting power and indicating "OK" via an LED, whatever that's worth.

    Haven't checked igniter.

    The lack of 24v suggests xformer problem, but there's power to the circuit board. Any ideas?

  2. drick

    drick In the Trades

    May 16, 2008
    I'm not an expert but;

    1. Its probably not the igniter. If it were the furnace would start up normally with the draft blower, than power up the igniter ( which is dead but the furnace wouldn't necessarily know this yet), then open the gas valve. After a couple of seconds the furnace would realize that there was no flame because the photo eye would not see anything and cut the gas. This triggers an error code on the red led.

    2. The circuit board is probably powered by 120 volts and has its own step down circuitry separate from that of the tstat . I suspect the circuit board is supposed to generating the 24 volts for the tstat but no longer is. If thats the case you are looking at a new control board. Still its probably worth a look to see if there is a separate 24 xformer off of the incoming 120v line. If there is you may be in luck.

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  4. nhmaster

    nhmaster Master Plumber

    Jul 30, 2008
    Tech. Instructor
    S. Maine
    Without the proper diagnostic equipment there is almost no point in trying to troubleshoot the equipment.
  5. therinnaiguy

    therinnaiguy New Member

    Jan 3, 2008
    On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

    What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).
  6. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    Based on your description of the jobsite, if I were servicing this, I would take in a control board, an igniter, and a flashight to inspect the electrical connections including the system ground.

    Do you have an led diagnostic signal on that model? What is the code?
  7. mdr

    mdr New Member

    Nov 8, 2008
    Thanks all for your input. Further diagnosis (with an HVAC contractor) suggests cracked heat exchanger. Not sure why I didn't receive 24v at the thermostat, but that is most likely a design feature for a roll out sensor trip. The 24v comes back up on power reset.

    As I was looking into the burner section, found a large crack on the inducer blower motor housing, running from the motor mount boss to one of the bosses which mount the assembly to the air handler. The housing is some form of composite and apparently is prone to cracking. (Later furnaces and replacement parts use a SS housing.) As an intermediate fixed, I used a high-temp RTV to seal the crack and continued to diagnose.

    After this repair, the furnace continued to display the same symptoms - heater call, draft inducer startup, igniter heat, burner light, run 3 minutes, and then shut down flame. (Apparently the furnace fail safe in this situation is continue to run both the draft inducer and the main blower to cool down the furnace.) The upper roll-out sensor keeps tripping. I switched the two roll-out sensors (top to bottom) and the upper location still continued to trip. Detailed examination of the burner housing shows excessive heat (oxidation residue on burner housing).

    After this, I gave up and called in an expert. He pointed out that though the burner didn't show a dramatic roll-out flash on ignition, a portion of the flame was blowing back out of the combustion area, hence the roll-out trip. He suggested that furnace was running v. hot and that the flame problem was most likely due to a cracked heat exchanger. The furnace is older than I thought (when we bought the house last year, was led to believe the furnace was 10 years old, but in fact it's 18 y.o.) Our bad for not explicitly checking the S/N date code.

    Given that any repairs will be expensive - expensive repair parts, significant labor due to access limitations, we'll replace the furnace. The old furnace is an 80%, but given that we can get a 90+% for the same price factoring in the tax credit, we'll probably go with that.

    Thanks again,

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