Furnace High Limit Broken in Ravenswood?

I had a call over the weekend in Ravenswood where the customer told me that the high limit was going off on the furnace.  He also explained that he replaced the limit the day before and now the unit is flashing the code that said there was a limit fault.

Unfortunately I was not able to make it over to this customers home until much later that evening and he needed this fixed right away.  It seemed that he knew a little about the subject and was not afraid to get his hands dirty, so I walked him through some troubleshooting over the phone to resolve the problem.  Here is a list of possible issues a furnace can have if the circuit board is giving you a code for high limit failure:

  1. The filter is dirty.  It is a good idea to change your filter every month for better air quality, better home heating.
  2. The limit is actually broken.  This is what the customer had thought in this case originally, and when he changed the limit the problem occurred again a few hours later.
  3. The evaporator coil is clogged.  If the filter was dirty and even after replacement the problem was still happening this could be a very good place to look.  Without an inspection camera it is hard to see if this is actually clogged.  If you can open the side of the plenum and lift up the coil to look that would be the easiest way to check.  Otherwise you need to pull the blower motor out from the bottom part of the furnace and shine a light up through the heat exchanger and see the coil this way.  This is not the easiest way to do it, but trust me it works.
  4. Crack in heat exchanger.  If there is a crack in the heat exchanger most likely the roll out limits will pop and need to be manually reset.  If you have to keep reseting them the problem is probably a bad heat exchanger and would need to be replaced.

This is a basic list of problems that could be happening if the main limit in the furnace is going off.  After I went through the list with the customer none of these things were to blame.  When he called me back and told be the coil was a block of ice i said “ah ha”.  The air conditioning and the heating were running both at the same time.  This froze the coil not allowing air to pass through thus overheating the furnace and shutting off.

A frozen coil was not on my list for things to check for because generally the heating and cooling will never be on at the same time.  What had happened was the wiring was bad within the walls and was passing power to the contactor on the air conditioning unit.  When that happened the A/C unit ran constantly and freezing the refrigerant in the unit.

After the wire repair by the customer we had him up and running.  I was glad to be able to help out the customer in a time of need when I was not able to make it over myself to resolve the problem.   Please feel free to contact at any time and I would be happy to give some free advice.  773-234-4575

Chicago Heating Repair
3526 S. Paulina St. ChicagoIL60609 USA 
 • 773-234-4575

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