Mr. Heater Big Maxx Pressure Switch CAB Solution

[wdrumheller]

My Mr. Heater Big Maxx has been running perfectly for years. Two days ago it stopped working.

Symptom: Thermostat calls for heat, then the exhaust fan runs, and then 20 seconds later it turns off, and the LED is blinking three times over and over, which is the message for Exhaust Pressure Sensor malfunction.

When this error happens, the heater is saying "Hey, I can't detect that there is proper exhaust pressure or vacuum to show that your exhaust fan is running and the exhaust path is clear, so go figure it out so I don't burn the place down. The exhaust pressure is checked by a little switch that is connected to a clear rubber hose attached to the exhaust fan housing.

Solution:

1. There is a clear rubber hose that goes from the exhaust fan housing up to a flat-looking pressure switch that is about 3" x 3" x .5" thick with a limit switch and two wires coming out of the bottom. One red, and one brown. Locate the switch and take a picture of it and how it's wired so you don't forget.

2. Remove the brown and red wires from the switch. Then, have someone turn the thermostat to call for heat. When the fan comes on, touch the wires together for a good contact and wait. If the heater fires up, then you have a problem with the switch.... MAYBE. Remove the switch and the rubber hose from the exhaust fan end, leaving the hose connected to the switch. **** on the hose with your mouth gently to see if you can hear a click of the switch operating. If you can, then get your voltmeter out and check for continuity on the two terminals that had the red and brown wires attached. Normally the switch is "open". Then **** on the hose a little and see if you can get the switch to close. If it clicks and the switch closes, then the switch is good. It shouldn't take hardly any suction at all to get this switch to trip, like almost nothing.

3. If the switch is functioning perfectly, check the exhaust pipe for an obstruction causing back pressure or in MY case I hooked the rubber hose to the small hose port on the fan exhaust housing and tried to **** air through it or blow through it and that LITTLE ORIFICE where the hose connects to the exhaust fan housing WAS CLOGGED WITH SOOT! SO, I took a paperclip and ran the wire through that orifice and cleaned out the soot. Re-installed the hoses and covers and it works perfectly.

Good luck! These heaters are really great!!!!!

This is the first trouble I've had with any of the three that I own in the 4 years I've had them.

 

[naturalgas]

Nice! You figured it out yourself


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[ForceFed70]

Great write up - thanks!

 

[BoBiggles]

I tried the cleaning out the hose port on the exhaust housing and it worked! Mine was clogged with soot as well. Heater is working like new. Thank you so much!

 

[wdrumheller]

I tried the cleaning out the hose port on the exhaust housing and it worked! Mine was clogged with soot as well. Heater is working like new. Thank you so much!

This makes me so happy! So glad it worked for you, and you took the time to post about it. Glad to be helpful!

 

[Dustball]

My heater quit working earlier this month as well- same blinking code. I connected my Mityvac to the pressure switch and it started right up. It ended up being a clogged opening on the exhaust so I pushed a small drill bit through and opened it up.

 

[wdrumheller]

My heater quit working earlier this month as well- same blinking code. I connected my Mityvac to the pressure switch and it started right up. It ended up being a clogged opening on the exhaust so I pushed a small drill bit through and opened it up.

That vacuum switch doesn't need much vacuum, you can produce the required vacuum with your mouth. Hooking a mityvac to the switch has the potential to damage that delicate switch.

Glad you got your heater fixed.

 

[D45]

My Hot Dawg was shutting down also......the upper limit switch was tripping and turning the heater off

From my research Mr. Heater and Hot Dawn heaters are both affected by the "burner pinch"

Here was my fix:

https://www.garagejournal.com/forum/showthread.php?t=374715

 

[Brian Paddock]

3. If the switch is functioning perfectly, check the exhaust pipe for an obstruction causing back pressure or in MY case I hooked the rubber hose to the small hose port on the fan exhaust housing and tried to **** air through it or blow through it and that LITTLE ORIFICE where the hose connects to the exhaust fan housing WAS CLOGGED WITH SOOT! SO, I took a paperclip and ran the wire through that orifice and cleaned out the soot. Re-installed the hoses and covers and it works perfectly.

Any idea what is causing the soot to build up?


I'm having the same problem with the soot clogging the small port on the fan exhaust housing. My Big Maxx was trouble free for the first two years. Then I needed to unclog the port once a year, then twice a year and now its once a month I'm unclogging the port.

 

[wdrumheller]

My Hot Dawg was shutting down also......the upper limit switch was tripping and turning the heater off

From my research Mr. Heater and Hot Dawn heaters are both affected by the "burner pinch"

Here was my fix:

https://www.garagejournal.com/forum/showthread.php?t=374715

Thank you, what a long ordeal to figure out your problem!

 

[wdrumheller]

Any idea what is causing the soot to build up?


I'm having the same problem with the soot clogging the small port on the fan exhaust housing. My Big Maxx was trouble free for the first two years. Then I needed to unclog the port once a year, then twice a year and now its once a month I'm unclogging the port.

I've had the HVAC furnace guy out here once when the heaters were installed to make sure that the pressure was perfect coming into the heaters through the regulators. The regulators needed to be adjusted. Too much fuel and you'll have a rich mixture that will leave soot.

I think you've got too much fuel or fuel pressure. That is only my opinion, but I"d say that it's a likely culprit.

 

[LS6 Tommy]

Any idea what is causing the soot to build up?


I'm having the same problem with the soot clogging the small port on the fan exhaust housing. My Big Maxx was trouble free for the first two years. Then I needed to unclog the port once a year, then twice a year and now its once a month I'm unclogging the port.

^^This^^

wdrumheller says it's been "running perfectly". If you have soot buildup, I guarantee you it is not. You're running rich. Check your gas pressure.

Tommy

 

[LS6 Tommy]

Never mind. I didn't get to your last post before I replied.

I gotta start forcing myself to read ALL the posts in a thread...

Tommy

 

[Brian Paddock]

I think you've got too much fuel or fuel pressure. That is only my opinion, but I"d say that it's a likely culprit.

Checked pressure and it was 4.7 inwg. Turned it down to 4. I hope that does the trick. Thanks!

 

[Notch1988]

Bringing this back from the dead as it applies to my situation.

How far into the little port should you be able to clean? I put a wire through and it stops dead about an inch in. Not sure if I'm plugged off inside or if this is normal.

 

[Brian Paddock]

Bringing this back from the dead as it applies to my situation.

How far into the little port should you be able to clean? I put a wire through and it stops dead about an inch in. Not sure if I'm plugged off inside or if this is normal.

Use a 5/64 drill bit. It should go in about 3-4 inches.

 

[Notch1988]

Use a 5/64 drill bit. It should go in about 3-4 inches.

Thanks. She was completely crusted over, had to use a hammer and tap the drill bit through the crud (drill wouldn't fit in there). Solved the problem though.

 

[Thadraperyfallz]

Read through this thread and did all of the troubleshooting, great information but I'm still having an issue. I checked the tube and everything, no obstructions switch appears to be working and getting a good draft. The weird thing is when I touch the wires together it still won't fire. 3 blinks... any other ideas? Thanks in advance.

 

[D45]

Burner pinch mod?

https://www.garagejournal.com/forum/showthread.php?t=374715

 

[tenthirtytwo]

My Mr. Heater Big Maxx has been running perfectly for years. Two days ago it stopped working.

Symptom: Thermostat calls for heat, then the exhaust fan runs, and then 20 seconds later it turns off, and the LED is blinking three times over and over, which is the message for Exhaust Pressure Sensor malfunction.

When this error happens, the heater is saying "Hey, I can't detect that there is proper exhaust pressure or vacuum to show that your exhaust fan is running and the exhaust path is clear, so go figure it out so I don't burn the place down. The exhaust pressure is checked by a little switch that is connected to a clear rubber hose attached to the exhaust fan housing.

Solution:

1. There is a clear rubber hose that goes from the exhaust fan housing up to a flat-looking pressure switch that is about 3" x 3" x .5" thick with a limit switch and two wires coming out of the bottom. One red, and one brown. Locate the switch and take a picture of it and how it's wired so you don't forget.

2. Remove the brown and red wires from the switch. Then, have someone turn the thermostat to call for heat. When the fan comes on, touch the wires together for a good contact and wait. If the heater fires up, then you have a problem with the switch.... MAYBE. Remove the switch and the rubber hose from the exhaust fan end, leaving the hose connected to the switch. **** on the hose with your mouth gently to see if you can hear a click of the switch operating. If you can, then get your voltmeter out and check for continuity on the two terminals that had the red and brown wires attached. Normally the switch is "open". Then **** on the hose a little and see if you can get the switch to close. If it clicks and the switch closes, then the switch is good. It shouldn't take hardly any suction at all to get this switch to trip, like almost nothing.

3. If the switch is functioning perfectly, check the exhaust pipe for an obstruction causing back pressure or in MY case I hooked the rubber hose to the small hose port on the fan exhaust housing and tried to **** air through it or blow through it and that LITTLE ORIFICE where the hose connects to the exhaust fan housing WAS CLOGGED WITH SOOT! SO, I took a paperclip and ran the wire through that orifice and cleaned out the soot. Re-installed the hoses and covers and it works perfectly.

Good luck! These heaters are really great!!!!!

This is the first trouble I've had with any of the three that I own in the 4 years I've had them.

Thanks for sharing the solution. I’ve had my 50,000 BTU Big Maxx running for 2 years with no issues, then all of the sudden…no ignition!

Used a 5/64 drill bit to clean the orifice and she fired right up. Next step is to have my plumber buddy come by and measure the gas pressure, then adjust it down to try and mitigate the soot. I’m guessing that if it keeps clogging after getting the gas pressure dialed in, I probably have a bad burner tube that is causing the soot?

Thanks again!

 

[4x4Pete]

Thanks for sharing the solution. I’ve had my 50,000 BTU Big Maxx running for 2 years with no issues, then all of the sudden…no ignition!

Used a 5/64 drill bit to clean the orifice and she fired right up. Next step is to have my plumber buddy come by and measure the gas pressure, then adjust it down to try and mitigate the soot. I’m guessing that if it keeps clogging after getting the gas pressure dialed in, I probably have a bad burner tube that is causing the soot?

Thanks again!

Possibly. It isn't burning properly if it's natural gas. Propane soots much easier than natural gas but still shouldn't unless it's not burning correctly. More than likely a bad heat exchanger rather than a bad burner. Your plumber buddy should know when he has a look at it.

 

[jedohrer]

My Mr. Heater Big Maxx has been running perfectly for years. Two days ago it stopped working.

Symptom: Thermostat calls for heat, then the exhaust fan runs, and then 20 seconds later it turns off, and the LED is blinking three times over and over, which is the message for Exhaust Pressure Sensor malfunction.

When this error happens, the heater is saying "Hey, I can't detect that there is proper exhaust pressure or vacuum to show that your exhaust fan is running and the exhaust path is clear, so go figure it out so I don't burn the place down. The exhaust pressure is checked by a little switch that is connected to a clear rubber hose attached to the exhaust fan housing.

Solution:

1. There is a clear rubber hose that goes from the exhaust fan housing up to a flat-looking pressure switch that is about 3" x 3" x .5" thick with a limit switch and two wires coming out of the bottom. One red, and one brown. Locate the switch and take a picture of it and how it's wired so you don't forget.

2. Remove the brown and red wires from the switch. Then, have someone turn the thermostat to call for heat. When the fan comes on, touch the wires together for a good contact and wait. If the heater fires up, then you have a problem with the switch.... MAYBE. Remove the switch and the rubber hose from the exhaust fan end, leaving the hose connected to the switch. **** on the hose with your mouth gently to see if you can hear a click of the switch operating. If you can, then get your voltmeter out and check for continuity on the two terminals that had the red and brown wires attached. Normally the switch is "open". Then **** on the hose a little and see if you can get the switch to close. If it clicks and the switch closes, then the switch is good. It shouldn't take hardly any suction at all to get this switch to trip, like almost nothing.

3. If the switch is functioning perfectly, check the exhaust pipe for an obstruction causing back pressure or in MY case I hooked the rubber hose to the small hose port on the fan exhaust housing and tried to **** air through it or blow through it and that LITTLE ORIFICE where the hose connects to the exhaust fan housing WAS CLOGGED WITH SOOT! SO, I took a paperclip and ran the wire through that orifice and cleaned out the soot. Re-installed the hoses and covers and it works perfectly.

Good luck! These heaters are really great!!!!!

This is the first trouble I've had with any of the three that I own in the 4 years I've had them.

Had the same problem glad you posted the fix on it saved me a lot of headaches trying to figure it out Thanks your a good man