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FOR SOLUTION LOOK TO END OF POST

My furnace's exact model is MMHB-077ABHR-06 ,Controller is UT-1018

I have a nordyne 7076700 gas furnace for a mobile home, and the blower never starts up when heater starts up.

Steps:

  1. gas is on, furnace power on, thermostat is on. ON/OFF/FAN Switch is set to ON
  2. i hear transformer hum
  3. i hear and see combustion motor work
  4. maybe 10-20 seconds pass
  5. furnace ignites
  6. a few clicks of relays
  7. after about 30s furnace turns off
  8. then repeats back to step 4, without blower ever turning on

Tests I have preformed

  1. gas is on
  2. transformer gives 24V
  3. Relay is new and operates properly
  4. when disconnected/cold, limit switch is close (~0 ohms)
  5. when disconnected/cold, fan switch is open ( inf resistance) ???Is this ok???
  6. control board has no blinking LEDs
  7. blower fan turns ON when ON/OFF/FAN is set to FAN
  8. when thermostat is set to lower than room temp, all operations stop
  9. when thermostat is set to higher than room temp, the steps above occur

Im not sure if the fan switch should be normally open or closed, maybe this is broke, they part is NORDYNE Part # 626337 https://www.amazon.com/626337-Nordyne-Furnace-Replacement-Switch/dp/B00FG9FNA2

i am cold, please help.

More Tests ive done

i tested a bunch of places with multi-meter voltage AC

  1. transformer voltage is stable
  2. none of the sensor/switches change state
  3. Relay does not switch (voltage stays low.)
  4. thermostat green to white wires show 24V when on 0.2V when off
  5. thermostat green to red, 24V with thermostat setting above or below
  6. turning fan on from thermostat works
  7. when thermostat is set to Heat ON and FAN ON, The fan blows constantly, but the furnace still cycles through the same process described above. flame going on and off.
  8. Turn off the gas input and controller led blink the proper error message

Another thing

this might be in my head, but in the past when the flames turned on I think the flames burning heating sound was louder, more forceful. but i might be making it up in my head.

Schematic

Schematic

Igniter/flame sensor

this is a photo of the igniter. as mentioned below also operates as the flame sensor. It looks pretty clean, but I'll give it a light cleaning before reinstalling it. What should be the measurement for the 'flame sensor' when the igniter(mode) is not active and the flame is on? igniter

Update

I read somewhere that taping the gas valve unit with a hammer might unjam some stuff in the gas valve unit. After i did that, the flame burns for about 3 min instead of the 30 second. This only occurs on the first loop. after that is burns for 30 seconds before resetting. But is is a noticeable change, and I'm not sure what it means. I could have just knocked some dust out.

Another update

Ive used my air compressor to blow and dust out of the burner assembly, igniter/flame sensor, flame chamber, and anything else i could easily. Then put the burning assembly/etc back. Also tightened some connectors that had a little more play than i liked.

The result. Now the burner goes for about 15min before resetting and continues to burn in 15min intervals. but the blower never goes on.

Solved (Solution)

Problem:

  1. small leak in Gas Valve

Symptoms:

  1. Furnace starts for a while, blower never goes on, then resets
  2. small smell of gas. not as strong as unlight stove or unlight pilot light
  3. furnace never gets hot as normal (forced blower on via thermostat, and heat on, only heats house a little bit)
  4. Burner doesn't sound as strong as normal

What I confirmed wasn't the problem

  1. all switches are normal
  2. blower works when forced via thermostat
  3. igniter/flame sensor is clean
  4. All voltages across component normal
  5. Relay works

What shows the problem.

  1. Used very soapy water (1 to 4 dish soap to water)
  2. Cover any possible leaky point.
  3. Find any bubbles, in my case a hex-screw thing on the bottom of furnace gas valve

Solution:

  1. Seal the leaky area
Zef the Tinker
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  • lets review the electric schematic find and post – Ruskes Nov 24 '22 at 03:12
  • Fan is working, the signal from thermostat turns it on, so the really is working – Ruskes Nov 24 '22 at 03:31
  • here is something for you to read https://www.ohthelovelythings.com/home-gear/nordyne-furnace-troubleshooting/ – Ruskes Nov 24 '22 at 03:37
  • Small possibility an issue with the furnace turning on the fan - Turn off the furnace, turn FAN to ON via the thermostat, turn on the furnace, fan should start right away - see if it goes through the ignition, etc. and whether it stays on or cycles. If it stays on then it may be an issue of the furnace not properly turning on the fan but with it already on everything works. If it does not stay on then sounds more like flame sensor or similar. – manassehkatz-Moving 2 Codidact Nov 24 '22 at 05:20
  • I added some more tests ive done and the schematic. I have no flame sensor. I think ive covered all your comments – Zef the Tinker Nov 24 '22 at 14:25
  • I take it that the furnace's behavior is the same for both positions (OFF and ON) of the fan switch on the furnace? – ThreePhaseEel Nov 24 '22 at 17:50
  • the OFF position turns the whole thing off, ON position turns on the furnace and it goes through the steps described above. FAN position turns on the blower fan only, so the heating part of the furnace does nothing. – Zef the Tinker Nov 24 '22 at 18:29
  • "none of the sensor/switches change state" - pressure sensor on the inducer blower should go from open to closed. If it isn't the flame sensor, then the next trip out is an unsatisfied exhaust. I've never known a furnace to trip a roll-out sensor. There isn't much else if the blower, the gas valve, and the igniter all work. On some furnaces the igniter is the flame sensor. Clean it with 3000 grit sand paper. – Mazura Nov 25 '22 at 05:32
  • A clean sensor or not, if the *flames burning heating sound was louder* and it's being intermittently starved for supply, then it's doing what it's supposed to. Any change to the (gas) stove's performance; any change in color? – Mazura Nov 25 '22 at 05:37
  • My unit doesn't have a pressure sensor, my gas stove looks normal flame height and color normal. Ive added an update to my status, see above. – Zef the Tinker Nov 26 '22 at 17:47
  • A hot surface igniter wasn't what I was expecting to see if it's also the FS. But w/e; [it *is* both](https://www.starsupplyusa.com/products/igniter-fm-903110): banging on the gas valve makes it work? Bad gas valve or under-powered transformer. - *"combustion motor with centrifugal switch"* : PSI and PSO closes when it runs, right? That's the signal from the inducer blower running. – Mazura Nov 26 '22 at 20:52
  • ... Which is a horrible way to do it. A fan spinning doesn't mean there's *not* a bird's nest stuck in the pipe. Also, *is there* a bird's nest stuck in the pipe? – Mazura Nov 26 '22 at 20:59
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    @ZeftheTinker -- post your solution as an answer and I'll +1 it – ThreePhaseEel Dec 02 '22 at 04:50
  • I'm glad you got this working! If you'll spend a moment taking the [tour], you'll note that putting the _answer_ in the box labeled _question_ and changing the title is _not_ the way we roll here. Please copy/paste your "solution" section down below into the box labeled "Your Answer", remove it from the question (it's not a question, after all) then click the check mark next to that answer as soon as the system will allow it. – FreeMan Dec 02 '22 at 15:10

1 Answers1

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You did great work so far.

Based on that I would say it is the "Flame Sensor" not working.

Steps:

gas is on, furnace power on, thermostat is on. ON/OFF/FAN > > Switch is set to ON

i hear transformer hum

i hear and see combustion motor work

maybe 10-20 seconds pass

furnace ignites >>> here is where the flame sensor goes on to report flame is on

a few clicks of relays

after about 30s furnace turns off (missing flame sensor report)

then repeats back to step 4, without blower ever turning on

It looks like this, held by one screw. Take it out and clean it, and try again, if not replace it.

sensor

source https://daycon.com/product/nordyne-flame-sensor/

Ruskes
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  • I dont seem to have one or those. Ive attached a schematic. the is no flame sensor, and nothing attached to FSI on controller. and i dont see one on the furnace – Zef the Tinker Nov 24 '22 at 14:05
  • @ZeftheTinker you do have a flame sensor, you just did not find it. For orientation, it has a single wire connected. https://www.youtube.com/watch?v=w5w4QMcCNUM – Ruskes Nov 24 '22 at 17:57
  • I am certain i dont have one, there is nothing like it on the furnace, or the schematic – Zef the Tinker Nov 24 '22 at 18:24
  • @ZeftheTinker OK, if you say so, then I give up – Ruskes Nov 24 '22 at 18:45
  • +1. "**The ignitor is also the flame sensor**." https://www.mobilehomerepair.com/phpbb/viewtopic.php?t=2693 @ZeftheTinker – Mazura Nov 25 '22 at 05:26
  • thanks, i attached a photo of the ignitor/flame sensor. It looks clean, but ill clean it a bit. what should be the measurement and value for a working flame sensor? – Zef the Tinker Nov 26 '22 at 16:40