Reset Limit switch - http://www.hvacmechanic.com/ Forums

Original message

Alex30093

24.98.216.198

"Reset Limit switch" , posted Mon 21 Nov 14:32:

I have a older Consolidated Industries #HDA080NH4R furnace that shows a code for the limit switch being open. I replaced the controller board, opened all registers, changed the filter. But it continues to blow lukewarm to cool air. Before the changes it was blowing cold air and the blower would never go off. So I am making progress but it is slow. I believe if I can reset the switch then I can find out if I still have a real problem and need a pro or if it is fixed.

Any help would be greatly appreciated.

Handyman want2b

[this message was edited by Alex30093 on Mon 21 Nov 23:19]

Replies:

homegnome

68.161.8.131

"Re(1):Reset Limit switch" , posted Sun 20 Jan 18:46

You have not said which limit switch is indicated as open. I am going to guess you have a Honeywell ST9141A1002 electronic fan timer (if so, download the following: http://customer.honeywell.com/techlit/pdf/69-0000s/69-0780.pdf If not, then yours might still be similar.) It has a primary limit circuit and burner limit circuit. The controller flashing code 1 indicates burner limit circuit open, while code 2 indicates primary limit circuit open. That is an abnormal condition (normal operation has both of these circuits closed). The instructions, pp. 4 & 5 of the download, indicate that when either of these circuits opens, the controller powers the circulating fan motor.

So it is understandable that you would have found the circulating fan running constantly. The controller did what it should do and replacing the controller may not have been needed.

With the limit switch open, you should not be getting heat (again, refer to pp. 4 & 5). The controller keeps the gas valve closed.

I recommend you locate the overlimit switch or switches. Check each to make sure that they are closed when cool (they are normally-closed limiters that open when too hot). Replace the bad one. Then confirm that the controller indicates normal operation. Confirm that the combustion air blower comes on and that the burners come on when the thermostat calls for heat.

moondog

24.118.198.197

"Re(1):Reset Limit switch" , posted Sun 4 Dec 08:35

Find the limit switch. Ohm it out. Depending on the type you may be able to replace it with a dial type. Do you know where the limit switch is? If it is the button type you may be able to unstick it by tapping it but once they stick, they should be replaced. Take a temp rise once you get it working.

moondog

24.118.198.197

"Re(2):Reset Limit switch" , posted Sun 4 Dec 08:39

Why do people say it's 'blowing cold air' or 'only blows cool air'

Why not? "There's fire in the hole!" or "There's no fire in the hole!"

If the burners are not on, what temperature can the air be? Maybe the temperature of the air going into the furnace?