Mr Heater Big Maxx problem again

[radio-active]

I know this has been discussed at length here previously, but here I go again.

MHU80 -- Mr Heater 80K LP garage furnace. It's too big for my space, but it usually works. My exhaust is 6' long, going horizontal thru a side wall with a "T" type termination. There is one 45 degree elbow right at the heater outlet. The tubing has the proper slope.

However I have an ongoing, recurring problem. I am now on my 4th pressure switch. The unit will run for anywhere from hours to weeks after replacing the switch following a failure and a "3 flash" error code.

Sometimes I can get it to work again for a while simply by sucking on the hose a few times. But this is temporary and eventually I replace the switch. This last one lasted only 7 days. This turns out to be a big problem for me as I am 1500 miles away during part of the heating season.

I've checked the flue, I've cleared the exhaust fan port (never find anything), I replaced the hose. I have NOT yet replaced the fan or the gasket.

The support people at Mr. Heater (Enerco) have been of very little help, other than asking me to throw parts at it and reminding me it is out of warranty.

I'll entertain any suggestions, even off-the-wall ones. I'm getting desperate.

 

[PCustoms]

It's obviously not the switch.

Check your flue termination, are you getting wind blowing back into the unit?

 

[radio-active]

If it was a wind problem, wouldn't it return to working condition when the wind stopped?
The exhaust exits to the south and is pretty well sheltered. I've seen this heater working properly during a blizzard.

 

[PoorUB]

I worked in HVAC for years. I see pressure switches "settle in" after running a while. I suspect that something in your heater is a bit off and a new switch works fine until the spring relaxes a bit and perhaps the pressure falls off a bit.
Have you checked operating pressure at the pressure switch while the heater is running? I would be that it is running right a the pressure switch setting. It should have a bit cushion". If the switch setting is .7 inches, the heater will probably run at 1" or better, .7" is the failure point. I don't know what pressure the Mr Heater switch setting is, look on the switch, it will have the spec.

I would get, or make a manometer and verify the pressure at the pressure switch. There might be something wrong with your venting, or perhaps a weak inducer motor, possibly a leak on the heat exchanger, possibly a few other things that affect the pressure.

 

[radio-active]

I suspect you are right. I do have a manometer that I used to set the gas pressure when I converted the unit to LP. I have not tried to measure the fan pressure yet -- but that's a great idea.

I'm pretty sure my venting is solid, I think the inducer fan is too... but I'll check both of those out when I return next month. Thank you for the suggestions. These are the best ideas I've heard yet.

The switch spec is .4" . Mr. Heater also sells a switch that trips at .22". I'll get one of those on order. Thanks.