ok pay close attention ! here we go i have been working on a gaspac for 2 days and itsw still messed up. This was a no heat call when i arrived the furnace was running fine i looked over the unit and couldnt see anything wrong, then it happend the fan shut off but the burners were still going ok bad fan limit or relay, i thought tested relay and it was sending power to the fan but no call for fan so i thought ok bad t-stat then fan came on and i said yep thats it went down and replaced the t-stat, and fan problem went away. ok check the unit one last time and get out of here..... ok looks good then i hear the burners shut off but inducer moter still going with ifm.
What i said to myself huh i checked all limit switches and safties all closed ok, now burners come back on and unit run good for 10 min then bam whole unit shuts down and in about 30-45 sec bam unit back on i check x-fmr, and all is good then 10 min later unit off again i had my meter on the x-fmr this time and we had 24v and a call for heat and all safties still closed hmmmm i say what the heckis going on. So called in for reenforcements and got 2 more techs out they both said it was the heat circuit board and fan circuit board, i say ok sounds good after 5 hrs i was beat and said ok. today i came back with boards and installed them and tested unit in heat 3x all ok checked in cooling no problems.... whew that was it
an hr latter customer calls back no heat .... ohhhh nooooo i finnish my call and head back.... ok same problem but i had been thinking it may have been the igition control board so i picked one up on my way back there and installed it tested unit and after some brain farts i and another tech got it running....Whew thats it as a 3rd tech showed up i was on my way out and bam lost 24 volts yet again here we go again i had to go but the other 2 techs spent another hr on it and still couldnt figure it out they replaced the high limit switch thinking it was weak that didnt do it they thought it may have been a high limit problem..... nope nothing has changed in the house prior to the problem occureing ( ie: nothing has been closed or opened vent wise and filter is ok split is 120 supply and 80 return. but my concern is the loss of 24v. for those who care this is a carrier
48NLT036-301BE ser# 1790C67579
we have replaced the following parts: heat C/B, the fan speed control C/B , the Ignition control board, and the pilot tube,high limit switch, ohhh and the 40va x-fmr.so now i want to thank anyone that may be able to help me figure this out in advance i have been in the field for a total of 10 yrs so im not new at this but this is just wierd ........ hellllllllp!