How can I find out if my induced draft blower is bad?


Hi have a Rheem 80 furnace dating to 1983 (schematics attached). Right after the call for heat (WH = 24VAC), the INDUCED DRAFT BLOWER RELAY should turn on. But occasionally I see only 27VAC on it instead of the 110VAC. The relay is fine and I already replaced it (will verify that it's shorted and connected after the call for heat). When the Furnace does turn on, it's after a few minutes and I see 140VAC on the Induced Draft Blower terminals of the Relay.

q how can i find out if my induced draft blower is bad
q how can i find out if my induced draft blower is bad

This is the close up of the area. At terminal 1 of the Relay I should see 110VAC but sometimes I only see 27VAC. This is with a brand new relay.

  8 answers
  • Robyn Garner Robyn Garner on Feb 11, 2021,a%20humming%20or%20whirring%20sound.

    You can try resetting the blower motor

    "First, turn off the power supply to the furnace by turning off the breaker in the circuit box. Locate the reset button on the furnace and press it to reset the furnace. It is usually located inside the blower compartment on the side of the blower motor. Be cautious as the housing may be hot."

    Checked to be sure you don't have a clogged air filter.

    Meanwhile, your furnace is 38 years old and the expected lifespan is 20 yrs. Don't be surprised if you've gotten to the point where "it's one thing after another".

  • Betsy Betsy on Feb 11, 2021

    Hi Neil: One of the first indications of a bad draft inducer motor is likely to be the sound it makes. A failing inducer motor may create a noise shortly after a heating cycle begins. This could be a tapping noise, or a humming or whirring sound. The average cost to replace a draft inducer motor on gas furnace is between $245 to $465 and up, depending on the person installing it. Is it possible that it's only the igniter that's bad? That's what goes bad on my furnace every 5 years or so. It will try to start several times and then start. You might get a replacement part at, if you think you can fix this yourself.

    Good luck

  • William William on Feb 11, 2021

    First we would check if there is a pressure switch connected to the inducer motor. If there is disconnect it. Then use jumper wires and run the inducer directly from an outlet. If it runs it's fine. Reconnect the pressure switch if there is one and run the inducer and check if the switch opens. Also some inducer fans have an inline fuse. Then work back to the connections, relays, and power transformer. Basically a process of troubleshooting and elimination.

  • Cheryl A Cheryl A on Feb 12, 2021

    Rheem should have a website that can help you out. They will have the parts or tell you what you need to do. Good luck.

  • Neil Neil on Feb 20, 2021

    Wow, what great responses. Thank you all for taking the time to respond. I do appreciate it. I was offline because I lost the link to this page and the system was working better after I replaced the capacitor. But now it's gone back to its old ways. :(

    In answer to some of the questions: Yes there's a Negative Pressure Switch (see below). I'm going to try jumpering it and see if that just solve the problem (then remove it of course).

    I couldn't see a reset button in the schematic. NOr did I see one physically. If you can point it out I can track the wires back to it.

    I hear no obvious sounds of the IDB dying but I'm going to replace the IDBlower anyway because I'd bought one last year thinking that eventually, the old blower will die. But even if it fixes it (and I'll let everyone know if it does), I'm still puzzled by a combination of two things.

    As I mentioned when the IDB does not turn on, it shows a drop of about 100 VAC across the relay. This could mean the IDB is just sucking current and dropping it across the relay (I'll check for heat also).

    But what throws me for a loop is that when I suck on the Negative Pressure Switch tube (faking the suction) the IDB kicks in and the voltage at the IDB relay terminal to ground goes back to the normal 120VAC.

    I've looked at the circuit 10 times and can't figure out why the NPS would affect the IDB. I don't see any where that these 2 circuits intersect.

    Any suggestions? See picture. I always feel that unless I understand why the fix worked, it's going to come back again. As I say: Problems that go a way by themselves will come back by themselves.

    So the key question is: Why does the Negative Pressure Switch switching over, cause the ID Blower to turn on and run and stop having a 100 volt drop across the relay.

    If someone can shed some light on this, I would be most appreciative.



  • Neil Neil on Feb 20, 2021

    So I replaced the IDBlower. The old one was really rusted but still working fine. But I'm glad I replaced it. I'm not sure if anything changed. The drop across the relay still happens and the IDB does not kick in. But if I start and stop it a few times it looks like it does kick in eventually.

    I also did the following tests.

    The relay when open is actually open. So that's good.

    When closed it's about 2 ohms.

    But that means when the relay is closed but the Blower is not on, it's dropping 113V which means it's sinking 60Amps+ in the Relay. I can't believe that. The relay is rated at 35RLA. The Relay would burn open or fuse closed. I forgot to feel the side of the relay. I've got to look at it some more maybe I measured something incorrectly. Maybe I have the wrong ground/common.

    I also (as suggested by someone) shorted the NPS so it was always enabled. It worked (I'll have to try it a few more times to make sure it was consistent).

    So now the question is really: why does the relay not allow the full AC power to flow to the blower and why does the NPS make it work.

    For kicks I'm going to replace the NPS (those seem expensive). This one is a FS4122-75 AKA 42-31582-03 ...anyone know of a cheaper alternative? Or if someone can point me to a spec sheet for that, I can figure out a working alternative.

    Any suggestions on figuring this out would be welcome. Thanks

    Oh someone asked about replacing the furnace. I would do it, but this is the wrong time to do that financially :)

  • Neil Neil on Feb 21, 2021

    On the NPS what I need is a replacement for the FS4122-75 or 42-31582-03 (I can't find that part on the web except for another used one on EBAY and I don't want to go from my old used unit to someone else's. So I've been looking for a Negative Pressure switch SPDT(it has 3 poles) but must be negative. I figure anything from -0.1 to -0.4" WC. I can't seem to locate anything. I found a bunch in that range but they were just SPST (2 poles).

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