Short cycling limit switch?

Artbibi

I have a Intertherm G4RA072C-12 furnace that has been short cycling. It runs fine for about 5 to 10 minutes, lifting the house temperature 2 degrees, before having to cycle again. I have also noticed sometimes the blower fan (Nordyne 902131 1/3 HP) seems to make a noise starting up like it is struggling to get up to speed?

I have changed the air filter, and cleaned the flame sensor. Thermostat is good. I have narrowed it down to the Limit Switch and/or the blower motor condenser.

I cannot find a new replacement for the limit switch – if I am right, the model is 62636000. I found a Nordyne 626509R with the same L195-20 notation. Would this be the right replacement?

Have yet to look at the condenser as it is difficult to reach. Anyone know the part number for that?

Any other ideas?

Thanks

EdTheHeaterMan

If the limit is the reason the burner stops, and the fan continues to operate then you have an air flow problem. the test is to use a multi meter and set if to measure 120 VAC and place the leads on the two limit switch terminals as shown in the photo.

When the burner stops and the fan continues to run. check to see if the meter reads any voltage (24 V on gas or 120 V on oil heat). The reading of any voltage indicated the limit switch opened. If the burner stops and the meter reads zero, then the limit is not the reason the burner stopped.

pecmsg

Make sure ALL the supply vents are wide open and nothing blocking them. 

Artbibi

pecmsg said:

Make sure ALL the supply vents are wide open and nothing blocking them. 

pecmsg said:

Make sure ALL the supply vents are wide open and nothing blocking them. 

Already checked. Thanks 

Artbibi

EdTheHeaterMan said:

If the limit is the reason the burner stops, and the fan continues to operate then you have an air flow problem. the test is to use a multi meter and set if to measure 120 VAC and place the leads on the two limit switch terminals as shown in the photo. When the burner stops and the fan continues to run. check to see if the meter reads any voltage (24 V on gas or 120 V on oil heat). The reading of any voltage indicated the limit switch opened. If the burner stops and the meter reads zero, then the limit is not the reason the burner stopped.

Great instructions! This will be my next step.

EBEBRATT-Ed

That limit switch opens at 195 and resets at 20 degrees lower...175

That is way too hot for a furnace to run. You need to find out why you are not getting enough air flow. As far as the blower goes it could be a bad motor or bad capacitor. (capacitor most likely). You will have to pull the old one out and discharge it then order or pick up a new one

Artbibi

Tested the limit switch. With burners and blower running, 0v, burners kicked off and it read 26v. Blower continued til cooler. Next photos are the inside of the burners. USB snake camera works great. 

Artbibi

Was able to see the condenser for the blower motor and figured out how to pull it out (I think). I'll save that until I get more feedback here.

HVACNUT

Artbibi said:

Was able to see the condenser for the blower motor and figured out how to pull it out (I think). I'll save that until I get more feedback here.

I think you mean capacitor. 
Do you have a probe thermometer to check the rated temperature rise?

Artbibi

HVACNUT said:

Artbibi said:

Was able to see the condenser for the blower motor and figured out how to pull it out (I think). I'll save that until I get more feedback here.

I think you mean capacitor. 
Do you have a probe thermometer to check the rated temperature rise?

Unfortunately no. Will see if I can get my hands on one.

EdTheHeaterMan

Do you have a meat thermometer for checking food temperature? that is a probe thermometer.

This sounds like there is not enough air flow across the heat exchanger to keep the furnace from overheating. You have a furnace that is too big, or a duct system that is too small, or there is something keeping the fan from moving enough air across the heat exchanger. Look at the fan blades to see if they are full of dirt from years of operation.

Check the return duct to see if the opening is big enough. See if someone installed a filter in the ductwork and in the filter grill and in the return opening of the furnace. You only need one good filter, I have seen furnaces with a filter in the return opening and a filter in the filter grill. Two filters in the same air flow is too restrictive. And if you only change the one at the filter grill and the one in the furnace return opening get forgotten, then that can become a major restriction.

You would not believe some of the dumb stuff I saw out there

Artbibi

EdTheHeaterMan said:

Do you have a meat thermometer for checking food temperature? that is a probe thermometer. This sounds like there is not enough air flow across the heat exchanger to keep the furnace from overheating. You have a furnace that is too big, or a duct system that is too small, or there is something keeping the fan from moving enough air across the heat exchanger. Look at the fan blades to see if they are full of dirt from years of operation. Check the return duct to see if the opening is big enough. See if someone installed a filter in the ductwork and in the filter grill and in the return opening of the furnace. You only need one good filter, I have seen furnaces with a filter in the return opening and a filter in the filter grill. Two filters in the same air flow is too restrictive. And if you only change the one at the filter grill and the one in the furnace return opening get forgotten, then that can become a major restriction. You would not believe some of the dumb stuff I saw out there

I have a meat thermometer, not digital, but it should work well enough. 

I in fact do have light filters on return ducts. Have for years and keep them clean. Either haven't had issues until now or maybe just didn't notice?

I did see quite a bit of dust and dirt when snooping around the blower fan with the scope camera. I'll do my best to get in there and vac without pulling the whole blower out, and maybe do a more complete job in the spring. Any problems with just vacuuming it out, other than not sucking loose any wires?

Thanks again 

hot_rod

You could remove all the filters and give it a try for a bit. That would eliminate that potentials restriction.

rick in Alaska

Noisy motor, and an overheat tells me you have a bad motor. Most likely you need to replace it.
Rick