Heat is Off - Goodman MES80

SlowYourRoll

Hello friends, the heat is off at my girlfriend's house. Gas fired furnace. Goodman brand, model MES80. The control board has an LED that flashes to indicate the error, and it's flashing 4 times, so I took a pic of the manual's description (see below), but basically it sounds like it's related to an insufficient amount of air blowing over the heat exchanger, which it determines by temperature limit switches. The burners aren't turning on, but I'm guessing that's because it is running and failing this check prior to ignition.

I replaced the air filter attached to the furnace and the two air filters on the return vents on the first and second floor. I couldn't find a return vent in the basement (it's a finished basement with heat vents). I thought there'd be a return vent for the basement but I'm not a pro, so maybe that's normal? Anyway, the house has brand new filters and still no heat.

My girlfriend mentioned that a previous tech said the blower was improperly sized for the vents in the house, and so the blower speed had to be adjusted to keep something from overheating (I'm guessing the heat exchanger?).

It's also really dirty inside the furnace (she hadn't changed the air filters in the 2 years she's lived here), so I'm wondering if maybe a coating of dust is interfering with the temp sensors that the limit switch uses.

I'm pretty handy in general but not an HVAC pro. We've got a few calls out to pros in the area to see if anyone can fix it tonight. In the meantime I wanted to see if there's anything I didn't think of that's worth a try.

SlowYourRoll

CORRECTION: my girlfriend read the post and wants everyone to know she's only lived here one year

pecmsg

That system is filthy!
the blower wheel is cacked in dust. 

It needs to be taken out and throughly cleaned. 

Jamie Hall

Are either of the limit switches manual reset, or one shot? Locate them -- should be in the manual -- and see.

realliveplumber

The single wall smoke pipe is way too close to combustibles.

SlowYourRoll

Thanks Jamie! I think they are automatic reset. Here's what the manual says... 

SlowYourRoll

SlowYourRoll

pecmsg said:

That system is filthy!
the blower wheel is cacked in dust. 

It needs to be taken out and throughly cleaned. 

Yeah it's really dirty. Is cleaning it something a homeowner can do? Or is that best left to the pros?

Jamie Hall

You can do a lot on the cleaning yourself. I'd use a shopvac, but even your normal vacuum will do.

On the limits -- if it is telling you that one or the other is open -- but since they are automatic reset, that tells you that one or the other of them is lying to you. It may just be a bad connection -- they happen -- or it could be that it may need replacement.

pecmsg

That blower wheel need to be removed and washed. Can you do it, I don’t know. 

EBEBRATT-Ed

@SlowYourRoll

It sounds like a limit is open. Get a piece of scrap wire and look at the wiring diagram trace the wires and try jumping the limits one at a time.

You may be able to find which limit is open. Then you will know what to replace. Do not leave it jumped or run it that way you could have a fire. The furnace needs service asap.