Mind Boggling No Heat

Old HVACr · November 29, 2016, 08:10:42 PM

A 14 years old builder model Carrier high with no HSI output out of the board ,no fault code ( Light stays on solid) and venter running constantly . Replaced whole furnace wire harness , board , HSI and transformer but still nothing , ground is good .
Any thoughts ?

PS : I figured it out at the end but just want to see what others will do !

Porcupinepuffer · November 29, 2016, 08:31:42 PM

I would have went to the customer and said, "well this 14 year old furnace is just too damn old and not worth throwing parts at". Lets replace the old POS with a new one.

When you say "still nothing" after all the replacing you did, do you mean it was still exhibiting the same symptoms?

Old HVACr · November 29, 2016, 10:56:42 PM

Well I could have said that to this repeat customer who I have known for as long as his furnace has been in his house since new from day one ( Builder installed ) but I also changed his HX two months ago since it blew so much CO at the vent pipe outside so it would have looked not proper to say such statement but just to let you know that even with a new furnace the same problem would have persisted !!!!!
Yes it was still exhibiting the same symptoms after putting in all those OEM carrier $$$$$ parts .

Admin · November 30, 2016, 08:08:44 AM

Well I'm curious now!

If the light is on solid I'm assuming there is no call for heat and no 24V coming from the thermostat. Although I have seen thermostats bleed voltage and cause the ventor to cycle on and off without a call for heat.

You mentioned replacing the heat exchanger previously. Perhaps a flame roll-out condition caused the ventor motor wiring to melt against a 120V supply wire, causing it to run continuously. But you mentioned replacing the wiring harness, so I'm not sure if that's a possibility.

What was the problem?

Old HVACr · November 30, 2016, 05:35:09 PM

The Stat was calling for heat then i disconnected stat wires and put a jumper wire between R and W but still no change in outcome !
Wires were not damaged nor was the roll-out switch but I just replaced the wire harness thinking there was a breakage somewhere but that didn't solve the problem . I could get the igniter to light up and burners come on and all is a go once but then when I make the furnace fail on either open pressure switch or turning gas off it will not recover on its own after reversing the intentional open circuit .
It turned out to be a weak Neutral connection inside the main electrical panel for the dedicated furnace circuit . The electrician did not push the bear copper wire in far enough on the neutral buzz bar back in 2002 and overtime it came off .There was mere 1/8 of an inch only in the neutral buzz bar and as soon as I pushed it back down ( Horizontal installed panel) all the way about 3/8s in then the problem on the furnace was solved ! It fired right up and recovered every time on its own when making it fail .

Admin · November 30, 2016, 08:31:56 PM

Good find! It's always best to test the power before replacing a board. You probably would have got voltage readings measuring from the furnace neutral to ground, if that panel connection was causing an open neutral.

Old HVACr · November 30, 2016, 08:49:01 PM

Actually now thinking about it I do remember ( with my scary photographic memory ) taking a reading between furnace neutral to ground and getting 124-125 volts !!! wow , you know back in the day before I gained my last 22 years of experience when I was in doubt about the power source I would hot wire the furnace to a good solid extension cord to eliminate that as a suspect but I have gotten old and slow so it takes me little longer to figure such hard to crack calls .
Thanks for the tip and will never forget that

Porcupinepuffer · December 01, 2016, 05:06:01 AM

Ha. I think I may have also overlooked the wiring by assuming if it had enough voltage and with proper polarity, it must not be an issue with the supply wires. I'll take note of this one

I've personally seen the effects of having a floating neutral inside the main panel. It's not pretty.

By any chance was this wired like many of the furnaces I remove where they marrette the ground directly to the furnace ground and don't actually ground it to the chassis of the furnace?

Old HVACr · December 01, 2016, 06:35:32 PM

Yeah this had to be one of the worst calls I have ever done and I tell ya those electricians who work on builder new homes re just down right butchers .
The incoming ground inside the furnace's junction box was grounded to the frame via a green ground screw