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tranagan7 · Dec 27, 2009, 06:52 PM

Hi,my unit just act up about 10 days ago,while run on heat it keep running the only way to stop it is to shut down the main switch . Thermostat is useless at this time no matter what position setting fan still running ac won't come on the same to heat.Just change the circuit board still won't help , there're no voltage on the board thermostat terminal . I saw a fire burn mark on the wire with fuselink to the limit switch but my friend check and he said that it has power through.Should I call a contractor ? Thank's

KC13 · Dec 27, 2009, 07:07 PM

Check the primary limit switch. If open, it may reset with a few taps from the butt of a screwdriver between the terminals. If the fuse link looks burned, double-check it.

tranagan7 · Dec 27, 2009, 07:22 PM

My friend checked the limit switch and he said that it's OK.

KC13 · Dec 27, 2009, 07:31 PM

With power applied, check for 24v between "SEC-1" & "SEC-2" on board. If not present, transformer has failed.

tranagan7 · Dec 27, 2009, 07:41 PM

I forgot to give the unit model , it's a 48NLT roof top unit . The old board have the S1 to 5 on the right and N1 to 5 on the left , no fuse on it .

tranagan7 · Dec 27, 2009, 07:46 PM

Also the board number is HH84AA017

KC13 · Dec 27, 2009, 07:49 PM

Here's a link to the manual. http://www.xpedio.carrier.com/idc/gr.../48nt-20si.pdf Page 21 is a troubleshooting guide.

tranagan7 · Dec 27, 2009, 07:58 PM

I'll try tomorrow after work.

tranagan7 · Dec 28, 2009, 01:51 PM

Double check the fusible link and that's the problem . Thank's KC13.

KC13 · Dec 29, 2009, 04:47 AM

The fusible link was the SYMPTOM. Why it failed is the actual problem. Flame roll-out is the most common cause.

tranagan7 · Dec 29, 2009, 01:36 PM

Still wait for part , when install the F L , I will check and clean the flame sensor replace if need.

KC13 · Dec 29, 2009, 04:31 PM

How old is the unit? If the fusible link failed due to a faulty heat exchanger, operating the unit with it bypassed is dangerous. A replacement link will fail in short order as well.

tranagan7 · Dec 30, 2009, 11:31 AM

20 years old and no the unit is not operate at this time , I just bypassed the F l to make sure it's not the board problem . Will inspect the heat exchanger . Thank's for the advise.

tranagan7 · Dec 31, 2009, 02:31 PM

Heat exchanger look good , flame sonsor look a bit dirty , just clean it , still wait for the F-L .

tranagan7 · Jan 7, 2010, 08:29 PM

Install new fusible link , work fine now

tranagan7 · Jan 18, 2010, 07:53 PM

Just finish repair my unit from won't stop running by replace the fuseible link and it work fine for a week or so until now come another problem . Last night just come back in town , turn on the heater all I hear is the humming sound from the draft inducer motor and nothing come on ,today I turn on the unit , draft inducer come on , the ignitor clicking for about 30 seconds then stop , no gas hissing sound at all , no fire . Give me some idea . Thank's

tranagan7 · Jan 18, 2010, 07:56 PM

I forgot my unit is HVAC Carrier model 48NLT036300 rooftop unit.

wmproop · Jan 18, 2010, 09:02 PM

Is it doing anything? How about when you set tstat to fan on, does the blower then blow cold air,, if not have you checked the breaker

tranagan7 · Jan 23, 2010, 12:30 PM

The fan come on when turn on the tstat , ac also on when turn to cool , breaker OK , I think the flame sensor or pilot ignitor maybe the problem . Found new part online reasonable price , will replace the pilot burner assembly since it's too old anyway .

wmproop · Jan 23, 2010, 01:12 PM

In many/most cases you can clean flame senser and not have to replace also pilot assembly