High Limit Swtich Open error - Goodman GMEC96 60K BTU furnace

[Poodle Head Mikey]

Have you checked the gas pressure at the burner to verify the actual heating input?

PHM
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ran the exact same test with a fiberglass 1" filter, with bypass open and water on:

Furnace ran for over an hour without tripping the limit.

Highest recorded supply plenum (3-4feet down the horizontal trunk from the coil) was 127F
highest recorded return plenum (before the filter) was 86F

So temp rise was 41F which was below the 45F rated rise at 0.5WC in the spec sheet.... in fact 41F is the temp rise at 0.2WC on that furnace in high heat.

however the temp just above the heat exchanger did continually rise over the hour from the 125F range up to a max at the end of the hour of 141.4F, not sure if this is normal?

So in summary it seems like I can run this system without tripping the limit with either the bypass taped off OR a fiberglass filter, but NOT with a merv8 pleated filter AND the bypass open. I'm surprised I need to go to these lengths with the temp rise numbers and seemly good airflow numbers i've gotten (less than 0.5WC TESP)

[pecmsg]

I doubt that .5 Static reading. If it was accurate you wouldn’t be tripping the limit.

[homerjaysimpson]

This seems like the only variable that hasn’t been accounted for. Considering the person who installed the furnace was incompetent, I’m concerned this might have been missed. Are furnace gas valves tuned out of the box or do they require adjustment in installation? Definitely not something I would check myself, but if it’s a possibility I can have someone come out and do it.

Have you guys seen newer high efficiency furnaces over firing Because of this? Wouldn’t this also give me higher temp rise than spec?

Have you checked the gas pressure at the burner to verify the actual heating input?

PHM
--------

[homerjaysimpson]

If this was much higher than 0.5 wouldn’t my temp rise be at least at spec for 0.5 (45f) or higher?

I doubt that .5 Static reading. If it was accurate you wouldn’t be tripping the limit.

[homerjaysimpson]

This seems like the only variable that hasn’t been accounted for. Considering the person who installed the furnace was incompetent, I’m concerned this might have been missed. Are furnace gas valves tuned out of the box or do they require adjustment in installation? Definitely not something I would check myself, but if it’s a possibility I can have someone come out and do it.

Have you guys seen newer high efficiency furnaces over firing Because of this? Wouldn’t this also give me higher temp rise than spec?

Any thoughts guys? If this is the case I'm going to have a technician come in and check gas pressures, burner output this week. Will have them do another static pressure test again and make sure the probes are in the direction of the airstream in spots without much turbulence. Have them replace the high limit as its been tripped too many times now.

Anything else you'd recommend making sure they are checking?

[pecmsg]

Any thoughts guys? If this is the case I'm going to have a technician come in and check gas pressures, burner output this week. Will have them do another static pressure test again and make sure the probes are in the direction of the airstream in spots without much turbulence. Have them replace the high limit as its been tripped too many times now.

Anything else you'd recommend making sure they are checking?

Why are you telling the tech what to do and how

[homerjaysimpson]

Why are you telling the tech what to do and how

Not telling, but making sure they do it. You just mentioned you don't believe the 0.5TESP measurement the technician took after installing the A-coil. Not looking to spend money for a guy to show up at my house an not do proper diagnostics like I have experienced repeatedly with "so called professionals" who can't resolve the issue. I'm pretty sure you've lectured me on this before.

Unless you have a technician in Toronto you know will do thorough diagnostics without any supervision, please send me their number! I probably would have never had to post on this forum if I knew him or her I'm not looking to be a DIYer here... (yes I've tried the site map, and there is no one local)

[pecmsg]

Static Pressure
TSEP
Gas Pressure
Temperature Rise

They all get used "Together"

[homerjaysimpson]

Static Pressure
TSEP
Gas Pressure
Temperature Rise

They all get used "Together"

thank you, really appreciated.

[BNME8EZ]

According to your 41 temp rise and 60k@ 95% you are moving just under 1300 CFM. Don't know where that falls in your specs, but I would think it would be close.

[homerjaysimpson]

According to your 41 temp rise and 60k@ 95% you are moving just under 1300 CFM. Don't know where that falls in your specs, but I would think it would be close.

in the furnace spec chart (GMEC960603BN): https://www.goodmanmfg.com/pdfviewer....pdf?view=true


That seem well above the airflow you would see at 0.5"WC...closer to 0.2-0.3". Something isn't adding up! Is my return air temp just too hot period? I mean I assume any furnace would overheat if you fed it 100F+ air in the return, mine was well into the mid 80s when it was hitting limit in recent testing.

[comfortdoc]

Is there an HVAC service technician involved in this testing? If there is you need a new one. If there is not, hire one. All the information needed to figure this out is in the manufacturer's documentation. Hire a technician that you don't have to tell how to test and adjust this unit properly.

Typically 80° is max return temp.

[homerjaysimpson]

Is there an HVAC service technician involved in this testing? If there is you need a new one. If there is not, hire one. All the information needed to figure this out is in the manufacturer's documentation. Hire a technician that you don't have to tell how to test and adjust this unit properly.

Typically 80° is max return temp.

any idea on how to find one? Seems like finding a unicorn here (Toronto). Technicians here don't even bring a manometer into the house when they show up to diagnose unless told...

[BNME8EZ]

any idea on how to find one? Seems like finding a unicorn here (Toronto). Technicians here don't even bring a manometer into the house when they show up to diagnose unless told...

I don't carry one in either until I see that I need it. That goes for numerous other tools, but they are on the truck if I need them.

[homerjaysimpson]

so for anyone's curiosity, i had a technician come today and run all his tests. he gave me 0.52TESP (0.31 return including a merv 8 4" filter and 0.21 supply) and told me the incoming gas pressure was fine at 7"WC and the high and low were both tuned at the upper limit of the ranges at 2.2" low and 3.8" high.

He also came with a new limit switch from goodman in case. Interesting thing was the new switch is 170F vs the 150F in the furnace. I was kind of worried about the idea of increasing the high limit based on some comments early in this thread. But either way he tuned down the high heat burner a bit to the lower end of the range at 3.3" and did his measurements and tested it without tripping the limit. He said he could change the limit switch if I want and that he would check to see if there was a service bulletin as to why goodman switched to 170F.

[BNME8EZ]

Sounds like you finally got a good tech on site.

[kdean1]

IMO, a manometer is an essential tool and it should be on every technician's van. There's no way to know static pressure or have a clue as to air flow without one.

I'm glad your furnace is working now but I don't see the results of a combustion analysis. Changing gas pressure can have unintended results if combustion has not been tested. Gas pressure is one of several essential measurements but it is by no means the only one. Did he clock the gas meter to see if the furnace is close to its capacity?

[homerjaysimpson]

IMO, a manometer is an essential tool and it should be on every technician's van. There's no way to know static pressure or have a clue as to air flow without one.

I'm glad your furnace is working now but I don't see the results of a combustion analysis. Changing gas pressure can have unintended results if combustion has not been tested. Gas pressure is one of several essential measurements but it is by no means the only one. Did he clock the gas meter to see if the furnace is close to its capacity?

He did mention he checked burner performance at the new gas pressure... I hope that is what he meant by this. But he also said that he kept it within the acceptable manufacturer range of 3.2"-3.8" at high fire which i verified in the installation manual.

[kdean1]

He did mention he checked burner performance at the new gas pressure... I hope that is what he meant by this. But he also said that he kept it within the acceptable manufacturer range of 3.2"-3.8" at high fire which i verified in the installation manual.

Gas pressure is a ballpark. Combustion analysis is a measure of safety and performance not possible from gas pressure alone.

[homerjaysimpson]

Gas pressure is a ballpark. Combustion analysis is a measure of safety and performance not possible from gas pressure alone.

Was told CO and O2 were well within spec for furnace but Goodman gives no guidelines.