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Douglas
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 2927
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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I have a Tempstar 5000 (around 20 years old) natural gas furnace.

Customer Question

I have a Tempstar 5000 (around 20 years old) natural gas furnace. The burner is having a cycling issue. When the thermostat calls for heat, the furnace operates as it should and heat through the duct works flows properly until the duct air tempature reaches appox. 110 to 115 degress F. The burner the kicks off and the blower fan continues to run until there is an approx drop of 5 degrees and the burner kicks back in. This repeats itself until the heating cycle is completed.
The system has a draft inducer fan and a vacuum limit switch. So I proceeded with some trouble shooting on my own. The draft inducer fan appears to work normally. I jumped the two wires connected to the vacuum switch and ran though a heating cycle to compare. The cycle completed normally and the duct tempature reached a tempature of 124.6 degrees with no drop off and turned off the burner once the termostat was satisfied and the air tempature was 81.6 degress when the fan shut down. To me this showed the fire box limit switch was working normally.

Is this cycling a normal condition? If not, do I have a failing vacuum switch? It is pussling as it closes for quite a while prior to opening and cycling. I have replace the vacuum tube as well. The Vacuum Switch is a TriDelta FS6441, HQ1005577. I have looked for this part locally but unable to find. I did find an online source for the part, but don't want to order if it might not be the cause of the cycling. Any suggestions would be appreciated.
Submitted: 2 years ago.
Category: HVAC
Expert:  Douglas replied 2 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

You really wouldnt want to condemn the pressure switch until you actually measured the vacuum and compared it to the setting of the pressure switch itself to make sure you dont have a slight flue blockage, slow inducer or something. That is to say, if it's a drop in vacuum, the pressure switch wont help.

You would use a manometer or a magnahelic to measure the vacuum and I would expect it stay about 0.2" above the setting of the switch or more throughout a run cycle.

the 5000 is a series, do you have an actual model number?

On some condensing furnaces, if the drain is slow, it can open the pressure switch.

Tempstar is an ICP product so you may find the part locally if you know of a Heil, Arcoair, ComfortMaker or Keepright. Since ICP was purchased by Carrier, you may find it at a Carrier distributor but I wouldnt try that first.

Customer: replied 2 years ago.

Sorry for the slight delay, I needed to get the actual model # XXXXX inside the unit.

 

It is NDGE 125PH02

NNN-NN-NNNN/p>

 

I had a similar problem years ago and called out a repair service. Ultra bad experiance. He diagnosed the problem as the burner limit switch, replaced with a universal 120/240 and didn't break the bridge to convert to 120 and blew componets in the unit. Replaced all blown parts to then diagnose the problem as the draft inducer fan.

 

The unit appeared to work normally for about a year and the same symtoms reappeared.

I found a replacement fan out of state in business travel and purchased it and stlll have it new in the box, The fan at this time that appeared to cause the problem was removed and inspected. Paint from high heater box temps had flaked off and clogged the vacuum port. I was able to reinstall the same fan and the unit appeared to work normally until late last season and the beginning of this. I do not have a manometer, nor did the orgininal technician use one.

Expert:  Douglas replied 2 years ago.
yeah, I am with you on bad experienced, I have to help technicians all the time and it seems many simply dont have the tools to do the job (nor the knowlege). Sometimes you just got to go at it your self.

Lets do this... lets do your test again, only this time, lets see if after a while, you remove your jumper if it shuts off. We can at least confirm your pressure switch opening is the issue.

Im not finding that model number (I find the NDGK which is about that vintage). Does your furnace vent with metal or PVC?

Does the inducer motor have a capacitor mounted on it?
Customer: replied 2 years ago.

Faster response this time. I am a retired plumber/pipefitter and taught apprenticeship for many years including HVAC so I some knowledge and a reasonable level of common sense. I did your experiment prior to my questions.

 

Yes, if vacuum motor is jumped the system operates normally. If I remove the jumper, the burner shuts down and the unit distribution fan continues to run. The burner will not re-ignite until I either re-jump the vacuum switch or reconnect the wiring to switch itself. Again, once attached to the switch the unit runs a significant time before starting periods of short cycling until heat demands have been reached. I'm puzzled why the vacuum switch holds for so long, the opens intermentantly. The fan is also pulling enough vacuum to actuate the switch. Why would it drop off after such a signicant timeframe?

 

I have a small air compressor and blew air through the fan vacuum port prior to startup and it was clear. I also blew air accross the expossed external fan blades to see if the fan turned freely without power. The fan spin like a top, though exact RPM is unknown.

Expert:  Douglas replied 2 years ago.
Ok... good job.

Did you confirm that model number? NDGE and was this a PVC vent pipe or a Metal Vent?
Customer: replied 2 years ago.

It could be NOGE 125PH02

 

The vent is metal. 4" outlet increasing to 5-6 inch sheet metal. Approximately 3 to 4 ft of 45 degree rise, then vertical through roof. There is no flue damper or other controls on the exhaust other than the flue exhaust fan in case this extra unasked for information helps.

Expert:  Douglas replied 2 years ago.
Ok, that rules out a drain.

Your cap on the roof is in good shape?
Customer: replied 2 years ago.
The rain cap is in good shape and no birds nests built in it. One of the first things I checked.
Expert:  Douglas replied 2 years ago.
Just making sure...You know how that is.

Seems like you've narrowed it down.

I still cant find that model but that's ok. Without a manometer its going to be a guess as long as you have (and you did) check the tube and make sure the port is open. There could be a pin hole in the diaphram in the pressure switch which allows it to loose ground over a period of time. I guess that next step is to replace the pressure switch unless you want to throw the new inducer on.

Again if you have a company called United Refrigeration around, they sell Arcoaire which is another ICP brand, they may have your pressure switch, otherwise online may be the best route.
Customer: replied 2 years ago.

Perhaps I can borrow a manometer. Did you say it should pull .20" of water column?

 

Though I'm sure you can not advise this, I'm thinking of operating the system temporarily in the jumped position until I can locate a manometer. The unit is located in my garage, not in the living space and I can keep an eye on the inducer fan to insure it continues to operate.

 

What about an adjustable vacuum switch available through WW Grainger. If all else fails, it could be set to a slightly lower closed setting and this might hold throughout the cycle.

Just some of my own thoughts. Perplexed!

Expert:  Douglas replied 2 years ago.
No, about 0.2 past the setting of the switch... So if the switch is set for say 1.2" then you would want to see 1.4". If it were set for 0.8, then you should see 1.0"

You dont want to jump it. Newer furnaces will not allow a second cycle until the switch actually opens because too many people were killed by techs jumping pressure switches. I wouldnt suggest an adjustable for less. There is alot of engineering that went into making the choice of pressure switches... You are right, I cant advise it.
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 2927
Experience: Manufacture Rep for Major Brand, Technical Trainer, NATE
Douglas and 8 other HVAC Specialists are ready to help you
Customer: replied 2 years ago.

Douglas,

 

Thank you for your time and advise!

Expert:  Douglas replied 2 years ago.
you are welcome.. Good luck and thanks.

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