M1MB070A AW Weak Flame/Lockout due to failed ignition

[spenceman3000]

I have an Intertherm M1MB 070A AW furnace in my mobile home. I have had a few technicians come look at it and the problem is still happening and I am in desperate need of help.

So, my furnace ignites the flame looks alright, then after the blower kicks on the flame seems to struggle(goes weak) and eventually goes out. My furnace will then (after a few tries) lock itself out due to failed ignition.

So far the technicians have moved the flame sensor closer to the flame, knocked all the ice off of the chimney stack, checked the pressure after the gas valve and a various few other things. The flame is still struggling though. It seems to work fine for about a day after we take everything apart, but then eventually the same problem will start to re-occur.

The only possible explanation I can come to is that the gas valve is slowly failing and might not fully open sometimes, or there is something stuck in the exhaust area causing too much back pressure and snuffing the flame out.

Any insight would be highly appreciated, I have been struggling with this for over a week now.

Thank you very much!

[DaleM]

A few things. Did anyone check to see if there was a web in the main burner oriface? Did anyone pull the termination cap to see if any rust was built up on the flue baffle? Is there any evidence on the top of the furnace that looks like you have a roof leak?

[spenceman3000]

When we pulled the burner assembly they checked out the orifice, but never detached it from the assembly or specifically cleaned it out. Should I detach it and blow some air through. I kept mentioning the orifice to the techs but they didn't seem to think it could be the problem.

Didn't pull the termination cap. Just knocked off all the ice on/in/around it.

No evidence of a roof leak as far as I can tell.

Could it possibly be a gas valve that is slowly failing? This type of furnace has "low-fire" and "high fire" steps. Could it be struggling to move past the "low-fire" stage, if so, how could I tell?

Again, many many thanks for the help.

[DaleM]

Low fire is caused by 2 things, either you're not getting enough gas through to the main burner or you're not getting enough fresh air for proper combustion. A blockage in the oriface or a blockage in the flue assembly.

You have to remove the oriface to check for a spider web. Pull the valve, remove the oriface, look in tube with a flashlight. If the gas valve has a straight tube it's easy, if a 90 degree tube then you have to remove the 90 as well. You should have a clear view all the way to the valve outlet where there is a visible screen. Remove any and all debris, I usually use a Q-tip. DO NOT USE COMPRESSED AIR any backflow will destroy the diaphrams in the control valve.

The roof leak thing isn't necessarily a leak. Condensation is formed as part of the combustion process. That condensation is what forms the ice on the roof vent. That condensation also causes the vent pipe to rust and degrade over years of use. I have seen rust slake off and form a barrier so thick that the roof vent pipe actually held water, I wet vacced a quart out of one. Another problem could be holes in the inner flue pipe which will cause recirculation of flue gasses, not enough oxygen for proper combustion. Usually you get the rust blockage prior to the holes in the pipe.

Start with the oriface and work from there.

[spenceman3000]

Thanks, I will check when I'm home and see if it solves the problem.

Much appreciated.

[spenceman3000]

We took the cap off the chimney and found that it was really rusted and bits had even broken off.

They replaced that part becuase they thought that exhaust gases were getting from the exhaust into the intake and recycling back to the burner causing bad combustion.

After reaplcing the whole chimney section though, the problem was still occuring. He narrowed it down to the fact that the rust has plugged the heat exchanger and is now saying I have to replace the whole furnace.

Is this plausible if we have tried all other trouble shooting problems?

[DaleM]

Get a shop vac with a couple extensions, no end nozzle. Get up on the roof and pull the top cap. Drop the vac wand down the inner tube of the roof vent and vacuum out the rust, it's blocking flue gases and they cannot exit. Looking down in there with a flashlight you will only see what looks like a plate, that's a top baffle. There's a gap around it, roughly 1/8" to 3/16" that allows flue gases out. That gap is blocked by rust, move the vac tube around the perimeter, that will get you going. Be sure to tape the extension tubes together, you don't want to lose one and not be able to reach it. Furnace doesn't necessarily need to be replaced......yet.