Limit switch / possible work around

I have a Heil 8DNL downdraft furnace. I noticed recently that it has been short cycling: burners on for about 2 minutes, cuts off suddenly with 4 blink error code (open limit switch), blower continues to run for about 45 seconds, then I guess it gets cool enough and the error code goes away and the burners eventually come back on.

I replaced the filter, removed and thoroughly cleaned the blower, but no improvement. So I had an HVAC guy look at it and he noticed two things. 1 - he got a "weird" electrical reading from the limit switch. 2 - if he removed the front panel that covers the blower (providing more return flow) the problem disappeared. So he recommended I first replace the limit switch, and if that didn't work I could add a grill onto the return duct directly above the filter.

I replaced the limit switch today, but no luck. Exact same problem.

Next step would be to install the grill but I have some hesitations. I took the blower panel off and it does prevent the limit switch from popping, but I noticed that the air coming out of the registers was significantly hotter than I'm used to.

Is it possible to have too much return airflow? Could adding the extra airflow prevent the limit switch from popping, but cause other problems, possible overheating elsewhere?

Maybe the hotter air from registers is a good thing and with more airflow the furnace is just more efficient. But I'm not sure and I don't want to risk fooling the limit switch and breaking something else or causing a fire.

By the way all vents are galvanized steel. Any advice is most appreciated. Thank you.

 

Welcome to the forums.

It is very common to find the return lines undersized for proper operation.
Without the blower door open the air should be very warm but not very strong. Opening the door should improve airflow.

You are not "fooling" the limit switch... the problem needs to be corrected as you don't want the plenum/heat exchanger to constantly be run that hot. Did you check and make sure all return grilles are unrestricted ? Also, if you shut down too many supply registers will also cause poor air circulation thru the heat exchanger.

You need to allow more air into the return line. If you cut a grill in make sure it's before the filter so that air is filtered.

 

It does seem to be an airflow issue, though nothing has changed so this could have potentially been happening for a long time.

I don't think I can put a grill on the return as the HVAC service guy recommended. I've since been told that since I have a natural draft water heater right next to the furnace, the grill could pull in the water heater's exhaust.

Not sure how else to get more return air without a lot of work.

 

You can put it anywhere in the return duct. How about moving away from the furnace ?

 

So another HVAC guy is telling me I need all new duct work. Supply and return - says they're all too small. Several thousand dollars. Can this be correct?

 

Yes , it's possible.

Hopefully the HVAC tech spent some time doing all the calculations to determine the requiremnt.

 

I added to the return flow by adding a 8 inch duct from a bedroom to the top of the plenum, but it did not help at all. This is really confusing to me because I previously ran a test... I removed the door to the blower housing and blocked off all but 25sqin. Doing this the furnace ran perfectly. No short cycling.

So why after add in 8 inch duct which is about 50 sqin no love?

I even removed the duct and just allowed an 8 inch hole in the plenum. Still no love.

Any ideas? Does this make sense?

 

I had the same problem with my Trane XE90. The noise dampeners that surround the fan had let loose from the glue that holds them and were being sucked into the squirrel cage thus suffocating my air flow causing it to overheat. Took forever to find this since they would fall back in place when the fan was off. Stupid design. I just took them out.