Furnace shutting down for overtemp

[ebasista]

I've got a Goodman 92% 60,000 BTU input natural gas furnace. It looks like it is kicking out at times for overtemp so it has a hard time maintaining temperature.

Temperature rise in the ducts is approximately 45 degrees F. Input 70F output 115F so it sounds like the overtemp sensor is active (verified a kickout with multimeter once.

I am getting approximately 825 CFM into the furnace, does that seem awfully low? Took a look at the coil and I don't see any buildup along the interior of the A coil to restrict airflow, fan blades don't look horribly filthy.

Any thoughts?

 

[Jla74]

just a thought but the filter isn't clogged is it. Take it out and see if changes the condition.

 

[ebasista]

I wish that was the case, I changed filters yesterday.

I measured windspeed thru the intake at 10mph on 13.5" diameter duct to give me 825 CFM. I do not see any obstructions along the coil or outlet vents.

Should I expect more CFM? I plan to measure the actual temp that the high limit cuts out at (should be 200 per the PN).

 

[zmaxmotorsports]

Picture of cold air return?

 

[Stuff]

What blink code are you seeing?

 

[dw1]

I've got a Goodman 92% 60,000 BTU input natural gas furnace. It looks like it is kicking out at times for overtemp so it has a hard time maintaining temperature.

Temperature rise in the ducts is approximately 45 degrees F. Input 70F output 115F so it sounds like the overtemp sensor is active (verified a kickout with multimeter once.

I am getting approximately 825 CFM into the furnace, does that seem awfully low? Took a look at the coil and I don't see any buildup along the interior of the A coil to restrict airflow, fan blades don't look horribly filthy.

Any thoughts?

How old is your furnace? My dad had furnace problems last year (he's a retired electrician) so I drove the 70 miles, he said was bad Hi temp Limit, he had a replacement, I changed it out, all was well for a few days, it did it again, next trip I replaced the fan relay, thought it was good to go, the blower quit running again while I was there, I pulled the blower assembly, the cheap plastic capacitor was cracked down the side of it, occasionally the blower would quit running, then fault out on O.T. A new $6.00 capacitor and its ran good ever since. Might be worth pulling your blower assembly and check it out.

 

[zmaxmotorsports]

How old is your furnace? My dad had furnace problems last year (he's a retired electrician) so I drove the 70 miles, he said was bad Hi temp Limit, he had a replacement, I changed it out, all was well for a few days, it did it again, next trip I replaced the fan relay, thought it was good to go, the blower quit running again while I was there, I pulled the blower assembly, the cheap plastic capacitor was cracked down the side of it, occasionally the blower would quit running, then fault out on O.T. A new $6.00 capacitor and its ran good ever since. Might be worth pulling your blower assembly and check it out.

 

[Brian_WK]

I've got a Goodman 92% 60,000 BTU input natural gas furnace. It looks like it is kicking out at times for overtemp so it has a hard time maintaining temperature.

Temperature rise in the ducts is approximately 45 degrees F. Input 70F output 115F so it sounds like the overtemp sensor is active (verified a kickout with multimeter once.

I am getting approximately 825 CFM into the furnace, does that seem awfully low? Took a look at the coil and I don't see any buildup along the interior of the A coil to restrict airflow, fan blades don't look horribly filthy.

Any thoughts?

That temp rise should be fine there is a label on the inside of the furnace cover usually on one side or the other that will have BTU, Model #, Serial #, temp rise parameters, max temp, etc.

I'm with checking that the fan isn't quitting or that the limit weak or bad.

Most furnaces when they trip on high limit shut the flames down but keep the blower going.

Sounds like your on the right track though.

Brian

 

[normie]

Capacitor

 

[theoldwizard1]

I have the same problem with a Carrier furnace that is installed horizontally in a crawl space. The hot air "backs up" at the exhaust plenum because there are 2 - 90° turns. On super cold days (<0F) with high winds, it will kick off so often that the house temp is in the low to mid 60s.

The only possiblities are lower gas pressure so that you are making less heat (most regulators have very little range) or a bigger/higher speed fan.

In my case I have a third option. Bust out the floor. Dig down deep enough (4-5') to pour a new floor and install the furnace vertically.

Never, NEVER build a house with a horizontal HVAC plant ! More expensive, less choices.

 

[ebasista]

I've confirmed that it is shutting down for overtemp in the heat exchanger. I did this by leaving a multimeter on the switch and sure enough it eventually showed an open circuit at the switch and shut the furnace off.

For the time being I increased fan speed by swapping to the cooling tap on the motor (was on the med-high setting) and got a 15% increase in airflow (11.5mph vs 10)

I plan to take the motor and cage out Saturday AM and clean it all off, may get a new motor though.

Trying to get a thermocouple to verify the internal temp on the switch that cuts out.

Does anyone think 850CFM on a 60,000 BTU furnace is adequate?

 

[gdh33]

It is mainly about your outlet temperature and delta T. Delta T should be stamped on unit or in instruction manual. Where did you take your outlet temperature reading? Anyway to test the temperature right beside Overtemp Switch? Temperatures sound normal to me, which mean flows sound good.

 

[ebasista]

Bought a K type thermocouple/multimeter to measure temps instead of just an IR thermometer.

In front of the intake duct the air would be 72 and the register (middle of register) would read ~125 after a few minutes (kept slowly climbing).

 

[zmaxmotorsports]

Still waiting for a picture of the furnace and the return.

 

[ebasista]

Pictures tomorrow when I check the flux capacitor oops I mean blower assembly

 

[Stuff]

850 CFM should be fine. It's all about the temp rise. Here's a sample Goodman chart for various blower speeds of a 60K unit.

 

[sms1974]

850 CFM should be fine, your A/C coil might be clean but there is a secondary heat exchanger (looks like a car radiator ) directly above the blower that might be plugged. to many variables to be able to say exactly what without pics or more info.

 

[bonneyman]

Don't mess around with an overheating furnace. Could cause a fire. Time to call a contractor you trust and have him go through the unit.

 

[Iroc-Z]

Mine was doing the same thing last night. Same brand. It would short cycle and blink code four. Furnace filter was bent into the finance. Removed the filter. No other problems. Filter doesn't look dirty and is only three weeks old. Getting new filter tomorrow.

 

[theoldwizard1]

Don't mess around with an overheating furnace. Could cause a fire.

Highly UNLIKELY ! It is design to shut itself down and that part IS WORKING !