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I have two Intertherm M1MB 056A BW heaters in need of help: 1: One is flashing five flashes. The label indicates that this would be a reversal of L1 and L2 polarity or no power at L1 (limit switch). It does nothing else. Since nobody has done anything that might reverse the polarity, I am assuming that it is a lack of power at L1. Where is the limit switch and what might cause a lack of power to it? Is this something I can fix myself? 2: The second one starts its cycle in a normal manner except that the combustion chamber fan is getting quite noisy and probably needs replacement. It continues until the flame detector light goes on and immediately shuts down. It will then do nothing until the power is turned off and back on. I can see that the electric igniter is working properly. Having two heaters is nice, except when they both fail at the same time. Given that I am relatively handy and have some experience with electronic circuits, are these problems I can fix myself or is it time to call the repairman? Thanks for your help.

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Yes there are somethings you can check your self...
On the first are getting power to the board but the board thinks that the hot (L1) and the neutral (N) are reversed or there is not a good ground to the furnace...Use an electrical meter to make sure that the black wire is the "hot" wire and that it goes through the switch and is hooked to the L1 terminal of the board...also make sure that your furnace is properly grounded and also that the electrical panel is grounded properly...this is very important..the L1 does not mean limit... it is referring to the terminal on the main board...
Check page 38 of this for the wire schematic and the note for the five flashes there...

On the second one... the furnace is not staying lit because it is not proving that you have ignition...
It could be a bad ground issue also...check that the flame is hitting the ignitor when it lights...the ignitor is also the flame sensor...if the burner ground is bad to the board it will not sense the ignition...a bad furnace ground will also do that...
Also a bad ignito or a very dirty ignitor can also cause that very careful if trying to clean the ignitor as they are very fragile and can break easily...

I sounds very possible that you have a bad grounding problem and that is the cuase of both problems...they are just reacting to it differently... did both problems crop up at the same time???

Comment back with any questions...


Posted on Mar 26, 2011


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