M80 forced air wood furnace problems switch?

Roxanne

New Member
Jun 9, 2020
6
New Brunswick
So might be a puzzling question here. I attached a video. Info... I have an M80 wood burning (and electric) forced air furnace. It makes a banging/ringing noise when trying to come on. Had furnace guy here 4 times, changed relays, and thermostat twice, checked wiring, and also had to replace fan motor cause it burned the copper coil/wires ruining fan motor. Other wires have also burned and melted, having to replace them. The thing is electric works just fine, comes on every time no problem. When wood is trying to come on it thrashes, rings and sounds like someone inside beating on it until it finally kicks in. I could not load full video to show this, file is to big. I am not trying to run wood and electric at same time, fire is hot. Unit is 24 yrs old. Wall thermostats are a couple years old but seem to work fine clicking on and off when temperature is turned up or down. Damper also is working fine. The only thing I have not changed is the transformer. Could it cause this. Do you think I got two seperate faulty thermostats? So frustrated. Ideas would be so appreciated. Furnace guys said they never heard this noise before and can't figure it out. Of course while they are here and a new fire is put on it kicks on and off fine, hence the video to show them. Thanks!
 

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begreen

Mooderator
Staff member
Nov 18, 2005
82,971
South Puget Sound, WA
The video loaded. Just takes a while. This sounds like something is not right with the blower squirrel cage.
Is this just when burning wood or does it also happen with electric operation? It sounds like something is rubbing at first (the scratching sound) and then gets shoved out of the way (the bang). I would pull the whole blower assembly and see if something has fallen down on the blower from the duct (insulation?). If at one point it jammed the blower, that would explain the burnt out motor.

Who makes this furnace? Blaze King?
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio
I would pull the whole blower assembly and see if something has fallen down on the blower
I agree...I think there is something loose banging around in the blower housing.
 

Roxanne

New Member
Jun 9, 2020
6
New Brunswick
It only does it when wood heat is on. Electric comes on and off no problem. It is a Hunter wood stove. Same as the blaze king or comfort valley M80.
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio

hobbyheater

Minister of Fire
Nov 14, 2011
1,134
Sounds like the bearings in the blower throwing the blower out of balance until it gets going fast enough to balance its self .
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio
I wonder if that could be the coils of the fan/limit switch rattling around? Like the classic Honeywell type switch...not sure why the wood heat would be any different that the electric coils though...unless maybe when the electric coils kick on, that directly starts the blower as the same time...but the fan/limit switch is used solely for the wood heat? Just a SWAG...
 

Roxanne

New Member
Jun 9, 2020
6
New Brunswick
I wonder if that could be the coils of the fan/limit switch rattling around? Like the classic Honeywell type switch...not sure why the wood heat would be any different that the electric coils though...unless maybe when the electric coils kick on, that directly starts the blower as the same time...but the fan/limit switch is used solely for the wood heat? Just a SWAG...
Yes fan/limit switch is just for wood. I was calling it a thermostat. It is Honeywell switch. The noise is coming from the area where blower and fan motor are but sounds like an electrical situation when fan limit switch kicks in. Trouble kicking in the blower? This is the second limit switch I replaced.
One question I asked in my post was if the transformer could be bad? Transforming the power from limit switch to blower? Has anyone ever had the transformer go and how would I know if it's bad?
 

BKVP

Minister of Fire
Maybe @BKVP has insight then?
I'm not certain if parts are interchangeable with the Hunter line. Our offices in Canada were a distributor for Hunter decades ago. I think OP will need to tear down and look at blower....
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio
One question I asked in my post was if the transformer could be bad? Transforming the power from limit switch to blower? Has anyone ever had the transformer go and how would I know if it's bad?
Blower runs on 120V AC...the transformer takes the incoming power down to 24V AC for the thermostat/control circuit...which I don't think will directly affects the blower...the 24V runs the intake damper if there is one.
I would pull the whole blower housing out and power it up...green to ground, white to white, black to whatever color speed tap wire is being used currently...with the blower sitting on the bench I bet the noise will become clear.
 

Roxanne

New Member
Jun 9, 2020
6
New Brunswick
Blower runs on 120V AC...the transformer takes the incoming power down to 24V AC for the thermostat/control circuit...which I don't think will directly affects the blower...the 24V runs the intake damper if there is one.
I would pull the whole blower housing out and power it up...green to ground, white to white, black to whatever color speed tap wire is being used currently...with the blower sitting on the bench I bet the noise will become clear.
Still does not make sense to me why blower has worked fine for last 3 months with electric only? Runs perfectly. Put fire on and within an hour or two of working fine, it starts with the tinging noise in fan limit switch and noise/bang in the blower/fan area as it tries to kick in. All wires have been traced already. Fan speed switch (for high and low speed) replaced, relays replaced, new fan motor, new fan limit switch. Thanks everyone. Guess it's back to the drawing board.
 

salecker

Minister of Fire
Aug 22, 2010
1,082
Northern Canada
Has the blower been removed and run on the bench?
 

brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio
At this point, your going to have to tear into this thing to find out what's going on...only makes sense to start with the part that is making the noise....take the whole blower out and run it on the bench is step one in my opinion.
I agree that it makes no sense that its fine when using the electric elements...so I doubt you are gonna solve this one with logical troubleshooting...time to tear it apart and look for the gremlin.
 
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salecker

Minister of Fire
Aug 22, 2010
1,082
Northern Canada
Also if you take the complete blower assembly out you will be able to see if there is anything under,behind,beside above,or just touching the blower assembly that could account for the noise if the blower checks out on the bench.
Also removing it will rule out a improper installation of the blower assembly.
"Just because the part is new doesn't mean it is 100%"saying that line because i have assumed a new part was good.
So don't assume anything when you are dealing with something that doesn't make sense.
 
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brenndatomu

Minister of Fire
Aug 21, 2013
5,062
NE Ohio
So don't assume anything when you are dealing with something that doesn't make sense.
Yes...back to 8th grade English class (man I hated that!)
When you break down the word assume, we find: ass-u-me...as in, make an... ==c