Mr. Heater Big Maxx board blew up - what next

[allinon72]

Been noticing a faint electrical burn smell in the garage…well I found it after going out in an unexpectedly cold garage. As you can see from the pic, the line voltage terminal had a catastrophic failure (terminal was where the hole is), but my question is why. I have normal 120v from the wall outlet, connections in the main electrical box are fine. Is it possible that one of the electrical motors or transformer in the unit backfed somehow? Or maybe the board got some condensation on it? My fear is replacing the board only to have it burn up again for an external reason.

 

[allinon72]

Well at least it appears the boards are readily available on Amazon for $50-75

 

[PoorUB]

Possible bad motor, but probably a bad solder joint. You could "hot wire" the blower motor and inducer motor and check amp draw, if you have an amp meter.

 

[Bert_]

Had some sort of loose connection that got hot.

With enough heat and time the board will turn to carbon and become conductive. Once you get enough carbon tracking it arcs across and you get what you have in your picture.

 

[finn]

Another vote for a loose or bad connection, resulting in a high resistance joint.

 

[allinon72]

Line voltage from the junction box actually hits this factory crimp connection first to distribute 120 to the transformer and to the board. Not sure if this is cause or effect.

 

[58Yeoman]

When I first installed my Mr. Heater Big Maxx years ago, it turned on but the propane tank was valved off. Something on the board fried, but I don't remember what it was. It was under warranty, so I called and told them the problem and they sent me a new board. I think I still have the old one, but as I said I don't know what blew on it.

Another problem with mine that they may have already have fixed is the 'overfire' that shuts the furnace down with the upper limit switch near the top of the burner. The vertical burner wings aren't solid so as to allow the flame to jump to the burner above it for lighting. The problem is the top burner wasn't pinched so the flame would come out and get it too hot at the top and set off the overfire switch. The fix was to remove the burner and pinch the wing to prevent that. I did that a few years ago, and it raised its ugly head a couple weeks ago. It didn't shut down the furnace, but the blower kept running after the furnace shut down for the day. I removed that burner and hammered it closed again, then crimped it over so it was completely closed and couldn't spread apart again. Something to keep in mind.

 

[mm08822]

It could be that either relay's internal contact (black or white cubes) has been pitting due to many openings of the circuit. Eventually, getting worse and worse, generating more heat in either direction from the contacts, into/along the board until more components were included in the high heat further causing their deterioration.

Get a new board, install it, and do forensics on the old. If you can clean up the old board and solder in a new cube relay, it may become a viable spare. Traces may have been destroyed on the underside of the board. (Science project! )

 

[allinon72]

It could be that either relay's internal contact (black or white cubes) has been pitting due to many openings of the circuit. Eventually, getting worse and worse, generating more heat in either direction from the contacts, into/along the board until more components were included in the high heat further causing their deterioration.

Get a new board, install it, and do forensics on the old. If you can clean up the old board and solder in a new cube relay, it may become a viable spare. Traces may have been destroyed on the underside of the board. (Science project! )

I don’t think this one’s fixable haha

 

[mm08822]

I don’t think this one’s fixable haha

probably not......unless you were able to solder tails directly to relay instead. Myself, I would want to know if the relay caused the problem or was solely from the board's spade connection. Either way, it becomes and inspection point with the new board going forward.

 

[mreisner]

I know a few years ago they had some defective ones. I would call Mr Heater with your serial number and see what they say, they might give you the parts to fix it. They did for my buddy but that was a few years ago.

 

[Bert_]

Line voltage from the junction box actually hits this factory crimp connection first to distribute 120 to the transformer and to the board. Not sure if this is cause or effect.

Just another loose connection. It's the same type of failure but not related to the one on the board

 

[allinon72]

Hot wired inducer motor and blower motor, both tested good. Don’t have an amp clamp with me but I feel like there would be other signs if they were pulling an abnormal load. Went thru and fixed all the damaged wiring and got rid of the crimp splices. If nothing else, I learned more about how this heater works.

 

[whateg01]

... Don’t have an amp clamp with me but I feel like there would be other signs if they were pulling an abnormal load.

Like what? Smoke?

 

[PCustoms]

Like what? Smoke?

Gaping hole in the circuit board?

 

[whateg01]

Gaping hole in the circuit board?

I thought he jumped straight to the motor and heating elements

 

[fitter30]

What does it feed? Fan motor? Could be grounded or shorted or the wiring if motor doesn't stink.

 

[allinon72]

Like what? Smoke?

Noise or smell. I plan on doing an amp draw as well.

 

[allinon72]

Like what? Smoke?

Noise or smell. I plan on doing an amp draw as well. I just jumped power direct to each motor individually and let it run for a bit.

 

[Junkman]

I bought a Mr Heater about 15 years ago, and it never worked correctly. They kept sending me parts to repair it, and it never worked properly again. It had problems from day one, and I call it a turd. It is still hooked up on the ceiling, but it hasn't been used. The spare parts are still on top of the furnace, and I don't care about them anymore. Let the new homeowner figure it out.

 

[Hooked]

I bought a Mr Heater about 15 years ago, and it never worked correctly. They kept sending me parts to repair it, and it never worked properly again. It had problems from day one, and I call it a turd. It is still hooked up on the ceiling, but it hasn't been used. The spare parts are still on top of the furnace, and I don't care about them anymore. Let the new homeowner figure it out.

Bought one about same time frame. Worked first winter/few months but never again.

 

[pbon]

I am at 8 years with a Big Maxx 50k. 800 SF walls and ceiling but not floor insulated, attached on part of one wall. No issues. It is bigger than I probably need, but was $258 on sale and recovers quickly after the garage doors have been open. In the Northeast. I keep it around 60 and turn it up to the mid 60s if I am in there working, but it can go as hot as I want and even heat the second floor of if I open the door at the top of the stairs.

 

[allinon72]

What would you guys consider as a short cycle for a 30k heater? I would consider it oversized for my 21x25 2 car garage. In average winter temps it runs 6-10 minutes at a time. Wondering if the possibility of short cycling contributed to board failure.

 

[Bert_]

What would you guys consider as a short cycle for a 30k heater? I would consider it oversized for my 21x25 2 car garage. In average winter temps it runs 6-10 minutes at a time. Wondering if the possibility of short cycling contributed to board failure.

6 - 10 minutes isn't too bad. Even if it was that's not what fried the board.

 

[PoorUB]

What would you guys consider as a short cycle for a 30k heater? I would consider it oversized for my 21x25 2 car garage. In average winter temps it runs 6-10 minutes at a time. Wondering if the possibility of short cycling contributed to board failure.

How long is it off between cycles?

You can buy a thermostat with a wider on and off headband so it fits up at 70F and shuts down a 72F, and repeats.

 

[American Locomotive]

I really wouldn't think too much of it. The motors should all be thermally protected, and a spade terminal like that should be able to handle some current. The electronics in most homeowner HVAC equipment (and a disturbing amount of commercial equipment, too) are the lowest quality garbage imaginable. Cheap, low quality PCBs, low quality components and low quality soldering. Why use a nice high quality relay for $3.50, when you can use a whatever a "HF JQX-1" is for $1.50? The slightly toasty crimp should be indication that their QC is pretty low, as well. No crimp should be getting hotter than the wires themselves.

 

[allinon72]

How long is it off between cycles?

You can buy a thermostat with a wider on and off headband so it fits up at 70F and shuts down a 72F, and repeats.

I just installed a WiFi thermostat yesterday so my sample size is small. But today it has cycled on 26 times so far for a total of 2hours and 42 minutes, set at 62 with outside temps in the 40s and 50s today. Seems like a lot for how warm it was outside today. I can change the threshold on this new thermostat so I may play with that.

 

[Bert_]

I just installed a WiFi thermostat yesterday so my sample size is small. But today it has cycled on 26 times so far for a total of 2hours and 42 minutes, set at 62 with outside temps in the 40s and 50s today. Seems like a lot for how warm it was outside today. I can change the threshold on this new thermostat so I may play with that.

Sounds pretty reasonable. If you want to keep it at 62 it's going to have to run to keep it that warm.

Total run time isn't going to change by adjusting the deadband.

 

[PoorUB]

I just installed a WiFi thermostat yesterday so my sample size is small. But today it has cycled on 26 times so far for a total of 2hours and 42 minutes, set at 62 with outside temps in the 40s and 50s today. Seems like a lot for how warm it was outside today. I can change the threshold on this new thermostat so I may play with that.

I don't know where you live, but cycles at 50F are going yo be mush shorter than cycles ay -20F!
Most likely the heater is oversized, but not much you can do about it considering you have 30k BTU already.

For a decent insulated garage 30K BTU will heat 1,000 sqft. Yours is half that, so yes, probably oversized.

I have turned down the gas pressure or put in smaller burner nozzles to reduce the BTU output, but it isn't an approved modification.

 

[pbon]

I have a Big Maxx 50k and 800SF, similarly oversized. The cycles are short unless the doors have been opened. Mostly insulated and heated full time to 60 plus. 8 years no issues. I don’t count the cycles, run time or calculate the natural gas cost.