[justplumducky]

Goodman model number: GM980703ANA 80%-er, 11-yrs old.
Serial number: 0410702469
Current high limit installed: L160-30F

Thought I had a high limit problem, installed a L165-30F (5° higher) per supply house recommendation (L160 wasn't in stock), but when I cycled furnace again a few times, I experienced intermittent flashing codes for open high limit and (alternately) a code for stuck open press switch. Then found flue pipe virtually 100% blocked (sandy-type gravel) at connection to chimney in basement (maybe 4-square inches still open on this 4-inch diameter flue pipe). Going back today to clear the blockage. Told them (yesterday) to not run the furnace till I return today to clear the blockage.

Is the the 5° higher limit spec OK? (no luck yet finding (online) the original spec for this high limit. This model number is not among the Goodman model numbers on HVACpartsshop.com.

The sensor side of the L160 that was installed before I changed it to the L165 is longer than the L165 substitution. Any problem with that? (why is it longer?) There was no insulation or barrier of any kind in/around the hole where the limit installs in cabinet face, right behind the gas valve (entirely open all around that hole). The pics below are not the actual switches, but effectively show the different lengths I'm talking about between the sensor side of the switches.

Where can I find the original limit switch spec for this model number?

 

[ben's plumbing]

is there a johnstone supply in your area.....they handle goodman...while I feel the 5 degree change won't harm a thing ..its always best to install the proper part ...

 

[justplumducky]

Oops, forgot to post the pics, and I assume the "longer" difference is just a non-critical design difference, at least in this case (no barrier or insulation around the hole interfering with the air's direct contact with the sensor part of the switch), but like you said Ben, I'll be installing the one that was there when I arrived (has been there for a long time), at least until I find out if the OEM switch spec is any different, thx.

The pics are not the actual switches, but effectively show the difference in sensor lengths of the actual switches.

 

[beenthere]

Your serial number is for a GMS80703ANA furnace. Which would be an 80% efficient furnace.

That limit looks like the aux limit that is mounted at/near the blower housing.

 

[justplumducky]

Your serial number is for a GMS80703ANA furnace. Which would be an 80% efficient furnace.

That limit looks like the aux limit that is mounted at/near the blower housing.

Looks like I mis-read the "S" to be a "9" (didn't get the label clean enough in that area). Also found the high limit spec on the label; it matched the actual limit originally installed.

You must be talking about the limit that's installed right into the blower housing snail shell. I was told that limit in that location was for an alternate downflow config of this furnace that it shouldn't affect the problem I was having with this furnace (going out on high limit). Its spec was L120 something I believe.

Actually, was still having this problem after installing a new high limit, but I was running it with both upper and lower cover panels/doors off. Doors back on, and no more ignition drop-outs (followed by re-ignition soon after). Consistently running without high limit interruption now (was flashing 4-times/”high limit stuck open”per label on furnace door). I also confirmed voltage was dropping out from one side of the high limit each time ignition would drop out.

I have condemned this furnace (with my partner) until an additional blockage I found beyond the flue connection is resolved by a qualified pro, because after clearing the blockage at the flue connection into the chimney space below the clay tile, I see blockage at the bottom of this clay tile, where flue gases enter the tile. Cannot see another clay tile, if there is one? Cannot tell if there are two tiles from looking at rooftop from the ground. Because of the size of the chimney (looking from the ground), there certainly could be two tiles. The tile that I can see via the flue is approx. 4 or 5” wide x 8” long maybe.

Because of the angle of the 3-foot (minimum) length of the flue itself, I can only see about half (maybe a bit more) of the open area at the bottom of the tile where gases enter the tile (the half farthest from me when looking at the tile thru the flue). I probably wouldn't be able to see that much of that area if I had not knocked off the bottom edge of the tile nearest me with my shop-vac extension wand. In other words, all of the area I can see is blocked. Maybe that area I can't see has to be open to let this furnace run without shutting down? I installed a new pressure switch per OEM -0.60 WC spec. This is, indeed, an 80%-er (70K Input / 56K Out Nat.Gas). Or is it possible there could be other damage to this chimney on the outside of the tile and gases are rising via an “elsewhere” channel thru this chimney?

Furnace cycles are 6-7 mins ON, 16-17 mins OFF, keeping room temp at 69°F.

Furnace utility room (10' x 10') and living rooms have been checked and show zero CO levels with furnace running continuously (no OFF cycles – turned set temp up) over a 45-minute period. Was given the key to this home – no occupants present.

 

[beenthere]

Chimney could have air leaks in it further reducing draft on the furnace. Or the tile could be broken enough that the flue gasses are passing through the chimney joints outside.

 

[justplumducky]

I mentioned in this thread that with doors OFF, furnace went out consistently on high limit code, which I verified with voltage check (presence of) on each side of high limit. Voltage drops out off one side, accompanied by 4-flashes. Blower door ON, runs normally.

So if I understand it correctly, Doors ON, blower is at higher amp draw & higher speed, because it has to work harder to pull thru the restricted Return path and across high limit switch.

Doors OFF, unrestricted air flow available to blower wheel decreases the load on PSC motor (load on PSC motor determines amp draw), and blower wheel slows down with decreased amp draw. Less air moved across high limit. Result: 4 flashes.

Fan blade and motor characteristics are different vs. blower wheel.

 

[beenthere]

PSC blower.

Doors off, amp draw is higher. Speed is slower, due to attempting to work too hard, moving all of that free air.

 

[justplumducky]

PSC blower.

Doors off, amp draw is higher. Speed is slower, due to attempting to work too hard, moving all of that free air.

Oh yeah, sure thing! Was just testing you :biggrin2:

 

[yuri]

Riddle me this Batman.:vs_smirk:

How come that does not happen with a belt drive?

 

[justplumducky]

Riddle me this Batman.:vs_smirk:

How come that does not happen with a belt drive?

Lol, was gonna go googling, but you'd figure out that I was gone too long and cheated, no doubt, so I'm guessing maybe that it's because a belt drive is probably not a PSC motor, which has different load characteristics? Or the indirect linkage of the blower wheel to the motor has something to do with it.

 

[yuri]

Has a start winding with a centrifugal switch.
:vs_bulb:
Also the fan blades on a d/d are forward curved a lot more and closer spaced and have different characteristics.

 

[justplumducky]

How much is a Chimney Sweep going to charge to clear the blockage in the chimney in question/this post? Guess that would depend on how much blockage, location of chimney.

 

[yuri]

Thats a big 10-4.

Probably it should have a liner AND you need to find out why the debris got in there or it will happen again.

 

[justplumducky]

Thats a big 10-4.

Probably it should have a liner AND you need to find out why the debris got in there or it will happen again.

The overwhelming majority of it is fine, soft & damp (partial caking, loosens easily), dark red clay/ brick-colored. Natural brick deterioration apparently. But maybe there's additional debris of other types above that part of it ... higher up than I can see, above the flue connection to chimney I mean.

 

[beenthere]

The overwhelming majority of it is fine, soft & damp (partial caking, loosens easily), dark red clay/ brick-colored. Natural brick deterioration apparently. But maybe there's additional debris of other types above that part of it ... higher up than I can see, above the flue connection to chimney I mean.

It is probably suppose to have a dilution/masonry kit on it.

How big is the chimney internally in sq in.

 

[justplumducky]

The only thing I can tell you about internal size is that the tile I was looking at via the flue is 4-5" x 8" approximately. Can't tell if it has two tiles, but looking up at it from outside, it's certainly big enough for two. Maybe all chimneys have two tiles?

But since you're talking about "brick " now, maybe the tile size is no help.

There was a flue connection to this same tile (not far above the furnace flue), from a gas water heater that was replaced with an electric water heater, if that means anything.

 

[beenthere]

Most likely a non code compliant chimney.

 

[yuri]

Yeah and now YOU or your partner may be liable if you don't get it upgraded to the proper code or liner. If you find a unsafe condition like that which can produce CO or be life threatening you cannot just walk away and say what the he*ll that is the way it has been for years. NO Grandfather clause for existing non code meeting chimneys once you touch it. People have been killed from collapsed or plugged chimneys. Had a VERY bad one years ago where I am in a rooming house. Several deaths and a couple people now in comas.

If the owner refuses to get it fixed I call the gas co and get it red tagged.

 

[justplumducky]

Can't tell you the age of the home, but the city (not my city) was founded in 1791, and the home is located in the heart of the city. Block construction with an addition on top. Pic below.

Thx for the warning. Back in just a few...