Hi-limit thermal switch keeps tripping

petec88

Hi,

Want to ask on your thoughts on a 3 year old Goodman gas hot air furnace 80KBTU for a 1,000 sq ft apartment. The return is using a 16" vent with a 14" x 23" vent.

The hi-limit thermal switch keeps tripping after running for 5 to 8 minutes. When I remove the filter from the setup, the system seems to run fine without tripping the switch (ran for 20 minutes). As soon as I add a filter back in, it will run for 5 minutes (PFR 5 filter) to 8 minutes (PFR 2 filter) before it hitting the limit.

My question is would a filter of such low density really have that much effect on the system or is there something else I am missing.

Thanks in advance for your insight!

EBEBRATT-Ed

Your short on air flow by the sounds of it, It should run with the filter in it and not trip limit ever.

Could be undersized ducts,
fan motor wired to the wrong speed
duct outlets and or inlets restricted.

This is assuming the gas pressure and heat input is correct

petec88

Thanks for the feedback.

The ducts are 16" with a 14"24" opening which should provide at least 1,400CFM according to the charts. This 80K BTU furnace requires around 1,000 CFM per manual. Already checked the AC evaporator coils and pull the blower motor to check and clean the filter coils before the heat exchanger, both were clean (as expected for a 3 year old system). I checked all the vents and registers are currently opened.

Right now the fan is set to high speed for cooling and medium speed for heating. Should they be swapped?

pecmsg

1st 80K for 1000Sq Ft is HUGE! Unless there little if any insulation its way oversized. Too late now.
Should be good for X CFM's and measured are 2 different things.
How old is the furnace and A/C? If that system hasn't been cleaned in several years and the filter hasn't been changed regularly ill guess the blower wheel and evaporator coil are plugged causing your issues.
A simple filter is all that's need. The filter is to protect the equipment not clean the apartments air.

EBEBRATT-Ed

@petec88

Get the furnace running and use a thermometer in the supply duct near the furnace and measure the return temp at the same time. Let it run a few minuets to stabilize.

If you only have 1 thermometer put it in the supply and get the return temp off the thermostat room temp.

Check and make sure the blower wheel is turning in the right rotation. Make sure a plastic bag didn't get sucked into the blower

You have one or a combination of these three issues

Air flow is restricted
Furnace is over fired
Bad limit control

Subtract the return temp from the supply temp. This is your temperature rise. Look on the furnace nameplate or in the manual for the recommended temp rise. Usually around 50 to 70 degrees

JUGHNE

One issue reported on another posting was the blower motor capacitor MFD had dropped below the nameplate rating.
This could slow down the motor and cause overheating of the windings.
Even though only 3 years old, some capacitors fail this early.

Has this worked up until now?
In some cases the fire would go out on high limit, the blower runs to cool down and the fire comes back on. This could have gone on unnoticed for years. Have seen it happen and only when the high limit switch would not reset was it noticed.

petec88

I measured the delta between the two and its about 60-70 degrees difference. The sticker on the panel has a temp rise of 35-65. I measured it at the AC evaporator coil and it was giving me a reading of 140 while the intake is about 70. The max outlet air temp according to the panel is 170 so it should be under the limit. Does this indicate that it's the outgoing air ducts rather than incoming?

The blower seem to be working fine and blowing nicely. Quite honestly I never bother paying attention to it until recently so not sure if this has been a problem since day 1 or just started .

Tinman

If your Delta T is 70* and the acceptable range is 36-65*, your a delta T is unacceptable. 

EBEBRATT-Ed

If you measured the supply at 140 minus the return air at 70 then your temperature rise is 70 so your not moving enough air.

The high limit switch is telling you there is a problem and the air temp measurements confirm it.