Draft Inducer Vs. Pressure Switch Vs.? - HVAC - Contractor Talk

Draft Inducer Vs. Pressure Switch Vs.?

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Old 03-09-2005, 09:39 AM   #1
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Draft Inducer Vs. Pressure Switch Vs.?

I have a 12 year old Bryant 90 Plus gas furnace. Earlier this year I had to have the draft inducer fan blade replaced (it had cracked and was making a rattling sound; only the plastic fan blade was replaced, not the inducer motor). The furnace was working fine until a recent cold snap when I began noticing that it was taking longer to get the house up to temperature after recovering from my low overnight and daytime set-back thermostat settings. I also noted that the inducer seemed to be running much longer than usual before the furnace would ignite.

With information obtained by searching various internet forums, I performed the following tests: removed & cleaned flame sensor w/steel wool; inspected ignitor for cracks; checked PVC vent pipes for restrictions; checked condensate drain hose for blockage; checked pressure switch and draft inducer tubes for blockage; sucked on pressure switch tube and heard it ‘click’; ran control board self test (all relays operating properly). I also eliminated the flame roll-out sensor as a possibility since it appears to be a ‘manual reset’ type and has never been tripped. I then checked the voltage at the “ON” terminal of the pressure switch:

(1) With the furnace operating normally, I get a steady 26+ volts shortly after the inducer starts, then the ignitor glows, the burners ignite, and within 2 minutes the blower starts.

(2) During those times when the inducer runs but the furnace will not ignite, the voltage fluctuates rapidly between 0 and 26+ volts, accompanied by an audible clicking of the pressure switch (this is true whether I probe the switch itself or the appropriate test port on the control board). If I jump the pressure switch terminals for 10-15 seconds during this period, the ignitor will glow and the burners will ignite.

(3) Sometimes the burners will shut off after the furnace has been running for a while, even though the t-stat is not satisfied. The inducer continues to run, however, and when I check the voltage at the pressure switch it shows a steady 0 volts. If I switch off the power for 15 minutes, the system will usually start normally and may or may not continue to run until the t-stat is satisfied. BTW, the inducer motor gets very hot— almost too hot to touch.

This past weekend when temperatures moderated a bit, the furnace ran fine. Now we’re having another cold snap and the system is once again misbehaving. It’s as if the problem occurs only when the system is really being taxed, as when it runs for extended periods during recovery from low set-back temperatures. I suspect one of the following components but don’t know how I can confirm or rule them out:

(1) Inducer motor— is it possible that the motor is overheating and not spinning at sufficient rpm to create or maintain a sufficient draft?
(2) Pressure switch (is it normal for the voltage to fluctuate as described above)?
(3) Control board?
(4) The only other thing I can think of is that the high limit switch is being tripped— maybe the airflow thru the furnace is marginal and only during periods of extended running does the switch open; other times when the furnace cools down between cycles, the switch doesn’t trip.

Thanks for reading this very long post! Any help or suggestions are greatly appreciated.
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Old 03-10-2005, 03:13 PM   #2
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Re: Draft Inducer Vs. Pressure Switch Vs.?

i can betcha that when the inducer was put back on, it was not installed correctly and there is a pressure leak from around the mounting, or the blower wheel is incorrect or loose, thus resulting in an intermittent p-switch on and off to the burner problem. it is the pressure switch that commands via the furnace module the burner to come on and off. the furnace module might have a diagnostic blink sequence to help trouble shoot the problem. see what the sequence is and match it to the chart on the furnace door. your system may be too old to have this technology. the fluctuation of voltage is due to the switch itself opening and closing. an open switch will read voltage, and a closed one will read 0. which limit is tripping? the main burner limit, or other thermistor type ones found along the inside? if it is the main limit this is usually due to low airflow or overfiring the burner. if the flue inducer is not pulling fast enough then the furnace may rise in temp due to the thermal energy backing up resulting in the hi limit tripping. if the other thermistor type limits are going off or the manually resettable rollout switch is going off, then you have a flame, or heat roll back condition that can be caused by the flue inducer not pulling right. bottom line...get that guy back in and have him do it right


Last edited by NickTech; 03-10-2005 at 03:22 PM.
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Old 03-11-2005, 02:13 PM   #3
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Re: Draft Inducer Vs. Pressure Switch Vs.?

NickTech, thanks very much for your reply. My system doesn't have a control board with the diagnostic led's, so I’ve been relying on this forum for diagnostic help! I’ve more or less come to the same conclusion as you have…the pressure switch is doing its job but the draft inducer is having trouble creating or maintain a sufficient draft.

Wednesday night I pulled out the inducer assembly to make sure the wheel wasn't slipping and to check the rubber gasket that seals the inducer halves together. The wheel wasn't slipping but I noticed that there was a lot of end play on the shaft-- nearly 1/4" I'd say-- and there were signs that the wheel was rubbing on the housing (although this was more likely from the original wheel after it cracked). The rubber gasket on the housing looked ok. I know the wheel is the correct part because I was right there when it was replaced-- in fact I helped the guy remove the old wheel, and it took a bit of persuading to take it off. I’m wondering if the bearings in the motor are shot-- either from old age, or from running too long with a cracked, wobbling, out of balance wheel, or from all the tugging and pulling on the shaft when we tried to get the old wheel off. Maybe this is causing the motor to overheat and slow down just enough to either trip the pressure switch or allow the furnace temp to rise enough to trip the high limit switch.

In any case, since all signs seem to point to the inducer, I took a gamble and ordered a new motor…I’ll post the results on Monday.

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Old 03-15-2005, 08:29 AM   #4
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Re: Draft Inducer Vs. Pressure Switch Vs.?

CONCLUSION. Well, I went ahead and replaced the inducer motor since that seemed like the most logical place to start. Plus, since it was running hot and the shaft had an excessive amount of end play, I figured it would fail eventually anyway, so it seemed like a good investment even if it didn’t fix the problem. (By the way, the new motor has an external fan attached to the shaft, which tells me that overheating was an issue with the original design). With the new motor installed, the furnace fired right up and for the first time in over a week, the furnace behaved normally. The system cycled on/off all day long, never needing the occasional 15 minute “rest period” like before, and I assumed the problem was solved.

The next morning, however, I awoke to find the inducer running continuously without the furnace lighting. Once again, the problem had surfaced during “recovery” from my low (62deg.) overnight t-stat setting. So now I started re-checking the things I had already checked at least twice before…but this time when I checked the condensate drain hose it was filled with water. I removed the hose, drained the water, and the furnace started. It turns out that the trap was partially clogged with gunk. The hose would eventually drain (explaining why I never saw it filled with water when I checked it before), but I suppose during those times when the furnace ran for long periods, the water wasn’t draining fast enough, and the hose backed up with water, which in turn caused the pressure switch to open. The furnace has been fine for a total of four “recovery periods” since cleaning the trap, so hopefully the problem is solved.

Thanks again to NickTech…
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