Miller mobile home lack of combustion air?

[Leconte]

Called out and unit on lockout for failed ignition, She had another tech out there and same old bullshit, "you need a new furnace, bad heat exchanger". Cycle through and after about 15 seconds the flame starts to weaken eventually drops off flame sense. I Disconnected combustion air intake above the draft fan, and it fires and runs fine. I plan on climbing on the roof in the morning and pulling apart the vent to check for a blockage. I don't have much time on mobile home units, so looking for any second thoughts.

[rundawg]

I Disconnected combustion air intake above the draft fan, and it fires and runs fine.

Wouldn't that be the Flue/Vent pipe, not combustion air?

I'm guessing an 80% furnace?

Can you please post a model number of this unit?

Is a pressure switch opening?

The more info you provide, the more help you will receive.

Thanks.

[oldguy1949]

Called out and unit on lockout for failed ignition, She had another tech out there and same old bullshit, "you need a new furnace, bad heat exchanger". Cycle through and after about 15 seconds the flame starts to weaken eventually drops off flame sense. I Disconnected combustion air intake above the draft fan, and it fires and runs fine. I plan on climbing on the roof in the morning and pulling apart the vent to check for a blockage. I don't have much time on mobile home units, so looking for any second thoughts.

Roofers are notorious about tearing off the double pipe intake/vent & replacing with single jack. I probably fixed a dozen or more thru the years.

[Leconte]

yeah, sorry about that, 1m1mb077aaw, its 80%, from what i can tell the vent is concentric. exhaust is inside, and intake is outside. pressure switch closes and stays closed until flame loss.
its combustion air, its a forced draft system, so the draft motor is before the burner and pushes the intake air into the combustion chamber then out the flue.

[rundawg]

yeah, sorry about that, 1m1mb077aaw, its 80%, from what i can tell the vent is concentric. exhaust is inside, and intake is outside. pressure switch closes and stays closed until flame loss.
its combustion air, its a forced draft system, so the draft motor is before the burner and pushes the intake air into the combustion chamber then out the flue.

O.K., now that I have the model number, your explanation makes sense.

Some info on this system:

To prevent pressure switch faults from being treated as flame losses, the control on this unit checks to see if the pressure switch has been open momentarily within 2 seconds prior to the flame loss.

If the pressure switch was open any time within the 2 seconds prior to a flame loss, the control assumes the flame loss was due to the open pressure switch.

Any fault code lights?

[Leconte]

yeah, Lockout due to failed ignition is the code , it has a strong flame at first, then watching it through the window, it starts to wave, and the flame sense LED starts blinking, when the flame lifts completely off the flame sensor/ignitor it drops out as expected. continuity on the pressure switch stays the entire time. I believe it was 5 cycles before locking out.

[beenthere]

Any snow covering the roof? If so, some could have been drawn into the combustion air intake, and may have refroze into ice.

[Leconte]

Resolved! climbed on up and checked the venting, all seemed fine. Replaced the combustion blower as it was the last thing i could think of affecting the draft, Ran it and sat around for 20 eating lunch, Turns out the combustion motor was turning just fast enough to close the pressure switch reliably, but not to keep up with the required intake. amps were fine on the motor, and the pressure switch was closing as rated. I followed up about an hour ago and she's still running well.

[rundawg]

Resolved! climbed on up and checked the venting, all seemed fine. Replaced the combustion blower as it was the last thing i could think of affecting the draft, Ran it and sat around for 20 eating lunch, Turns out the combustion motor was turning just fast enough to close the pressure switch reliably, but not to keep up with the required intake. amps were fine on the motor, and the pressure switch was closing as rated. I followed up about an hour ago and she's still running well.

Thanks for the update.

[kangaroogod]

My crystal ball says you will be back because the roof jack is rotted out. It will recirculate exhaust back into the intake when holes appear in the inner pipe. It will do exactly as you described
The best way to check this is to put your combustion analyser in the intake before the inducer. Fire the furnace and if your o2 drops your roof jack is junk.

'The more you know, the more you realize you don't know'

[Leconte]

My crystal ball says you will be back because the roof jack is rotted out. It will recirculate exhaust back into the intake when holes appear in the inner pipe. It will do exactly as you described
The best way to check this is to put your combustion analyser in the intake before the inducer. Fire the furnace and if your o2 drops your roof jack is junk.

'The more you know, the more you realize you don't know'

thanks for the info, Ill be contacting her monday to follow up with the combustion analysis.

[rundawg]

thanks for the info, Ill be contacting her monday to follow up with the combustion analysis.

Question: Did you happen to take a before and after reading of the pressure differential of the pressure switch?

It might be nice to at least know the reading after you replace the motor, so you have "Data Point" for future reference.

[Leconte]

Question: Did you happen to take a before and after reading of the pressure differential of the pressure switch?

It might be nice to at least know the reading after you replace the motor, so you have "Data Point" for future reference.

I did not, Ill get the reading while I'm there for the combustion analysis, I still have the old one as well, so i may toss that on just to get the before reading, just to see how much it's increased. In hindsight, I definitely should have done that already. A reading showing a drastic increase would be reassuring.

[rundawg]

I did not, Ill get the reading while I'm there for the combustion analysis, I still have the old one as well, so i may toss that on just to get the before reading, just to see how much it's increased. In hindsight, I definitely should have done that already. A reading showing a drastic increase would be reassuring.

If I am not mistaken, the differential needs to be greater than -0.30" wc for the switch to close.

[Juan Madera]

Most mobile 80% units will work without the inducer as it is there (in my opinion from observation), to provide a draft in the correct direction. Once the flame is established, the natural and balanced draft is enough for the furnace to operate.

In the older units, the pilot would provide the correct draft.

[Leconte]

Following up, the O2 level dropped from 20 to 19 after firing, is that enough to suspect holes in the stack, or is that what you'd expect from the close distance of intake and exhaust on these?

[rundawg]

Following up, the O2 level dropped from 20 to 19 after firing, is that enough to suspect holes in the stack, or is that what you'd expect from the close distance of intake and exhaust on these?

Did you drill a hole in the flue pipe about 12” above the unit for the test?

The atmosphere is made up of 21% oxygen, and your analyzer should read that on every start up before you insert it in the flue.

The O2 will continue to drop for a short while after the unit starts, but if you are still reading 19% after the burners have been operating for over 1 minute, your analyzer is not working correctly.

I would expect to see something at least down around 12% or less.

[Leconte]

Did you drill a hole in the flue pipe about 12” above the unit for the test?

The atmosphere is made up of 21% oxygen, and your analyzer should read that on every start up before you insert it in the flue.

The O2 will continue to drop for a short while after the unit starts, but if you are still reading 19% after the burners have been operating for over 1 minute, your analyzer is not working correctly.

I would expect to see something at least down around 12% or less.

I did the test in the combustion air intake before the draft, as kangaroogod suggested, to see if the intake was pulling in exhaust through rotted venting. As for the reading of 20, it was jumping around a bit before I went into the house, may be time for a new one.

[rundawg]

I did the test in the combustion air intake before the draft, as kangaroogod suggested, to see if the intake was pulling in exhaust through rotted venting. As for the reading of 20, it was jumping around a bit before I went into the house, may be time for a new one.

I miss understood what you were testing. I don't think that small of a drop is an indication of a rotted vent, but I have no data to prove it one way or the other.

What model analyzer do you have, and when was the last time you changed the O2 sensor?

[kangaroogod]

Every time i have encountered recirculation i have seen 02 to drop several percentage points. Although you should sit there indefinitely.

'The more you know, the more you realize you don't know'