Without my brand new inducer motor coming on at all ever when power is turned on my gas furnace starts,igniter starts andlights pilot and then burners light and then big flame blowback like its getting too much gas far too soon...all this happens in 5 seconds or less after power to furnace is turned on.....What is going on here? Inducer motor coming on should be at start of firing sequence but never comes on yet furnace fires up albeit with blowback??

  • I should add to above that i replaced the Inducer Delay Relay recently too as well as the Honeywell Ignition Board too. – jackodylan Oct 21 '17 at 19:57
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    bad pressure switch? Its stuck on, what is the furnace brand model? Something is bypassed or stuck "on" – noybman Oct 21 '17 at 20:26
  • It is a Olsen Duomatic Gas Furnace which little or no info exists on the internet unfornately...it has a Honeywell Ignitor Board and Honeywell Gas Valve....I bought new inducer motor and found that as opposed to my old one that died the new one has a valve to which the pressure switch would connect into ....the old inducer did not and the pressure switch was connected to the furnace itself.....but connecting pressure switch to new inducer or connecting it as before to the furnace results in the same result.....replaced the honeywell ignition board and the thermocouple and the IDR as well.. – jackodylan Oct 21 '17 at 22:09
  • Also the new inducer motor has a capacitator attached to it which the old inducer motor did not have - but all the specs are the same with new inducer vis a vis the old motor. – jackodylan Oct 21 '17 at 22:10
  • Also possibly important is that the furnace was installed upside down to be a downdraft furnace as it heats rooms below the attic where it is in. – jackodylan Oct 21 '17 at 22:13

I will agree that something is miswired. The inducer motor is the first thing to come on. The inducer motor is actually sucking the gas through the heat exchanger. Without the sucking the gas just goes where it likes. The gas shouldn’t flow unless the draft proving switch closes. Maybe you got the wires mixed up. The inducer fan should also come on immediately so there is a problem with where it is getting its power. Without knowing which boards and relays you are talking about it is hard to give a more difinitive answer.


Sorry, No offense on first response.
These furnaces operate on a very complicated and sophisticated control sequence. It in driven by a printed circuit board in which it is impossible to trace the circuitry in the field. It evolves an extremely flammable and combustive gas. I have done this very type of repair for years. So I say again with one minor addition, do your due diligence, research and find a competent individual. They are out there. Get some help. I'm behind you 100%. Wish I was there to help.

  • The joke portion: How does the furnace Evolve the flammable gas? I suppose one could consider a flame an evolution, others might consider it a death or transformation. The serious part, this is more-so a clarification, very few things are made such that they are impossible to troubleshoot in the field. Moreso, most HVAC technicians will work to the LRU vs. board level troubleshooting, and this is usually sufficient. But a knowledgeable technician can perform circuit level diagnosis and repairs if they so choose to do. – noybman Oct 25 '17 at 2:17
  • To the OP, I believe the point @PaulLogan is driving at is the same I have done in the comments: The path for success I believe you are looking for is to do MORE than "shot gunning" parts which it seems you have been doing so far. To do this, ideally you want a schematic, solder iron, dvom, and component level knowledge of how resistors, caps, relays, logic gates, transistors, and general silicon based components work and how they are tested. In many cases you have to remove them from a circuit to do this testing. So it is likely you hire a tech, and they may call for a new control board – noybman Oct 25 '17 at 2:27
  • Trust me I am very well aware that what I face is technical knowledge beyond mine and my current skills. I am even more aware of the fact that I lack the financial resources to put in a new furnace or to pay the near equivalent of a HVAC guy to fix this mess. With a Canadian winter only days away I am acutely cognitive of the kamikaze nature of this problem I face.I have a fast learning curve but not that fast and lack key tools.A HVAC guy would tell me to replace the furnace & have to pay him 300$ for that info, here they dont touch electrical issues of furnaces, just tell u to replace. – jackodylan Oct 25 '17 at 16:09
  • So in light of that financial situ & lacking skills &tools to test components one by one....I have to at least assume that the components are fine as I replaced the relays with new ones, replaced the Honeywell ignition board, thermostat and the inducer motor & go with problem being electrical/wiring....there are 3 limits on the schematic...burner, auxilliary, and primary..but on furnace see only 2,,,,the limits on the burners, and one on the furnace manifold below the burners.....is that the primary limit? and auxilliary limit on the blower motor fan? – jackodylan Oct 25 '17 at 16:31

More information is needed. The board should not start the ignition sequence if a pressure switch is closed. That's how potentially dangerous explosions are avoided. The first action is ventor motor, pressure switch closes. Next, igniter then gas valve, if flame is not sensed within .7 seconds (usually) the gas valve will close. The flame sensing circuit uses impossible (almost) to fool flame rectification to prove safe ignition. The board sends out an alternating current to the burner faces and the flame only allows half of the sinewave to pass through. So if you can simulate a 60 cycle DC half wave 20mV signal you can trick the flame proving circuit. But for the board to even get there with a pressure switch stuck closed is impossible. Unless the wrong board is installed. Or is it an early high efficiency model, like 1980's? They had some unsafe ignition sequences that could lead to a small explosion. Not a home wrecking explosion, just a furnace wrecking explosion. You really shouldn't be fiddling around with your furnace. It'll probably cost you more this way then to have a good tech come out. We do exist.

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