HVAC unit not working - What now?


  #1  
Old 02-14-03, 01:43 PM
123rj123
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HVAC unit not working - What now?

Equipment:
HVAC Unit - Rooftop - Lennox GSC20H-311-75-3P
Thermostat - MapleChase 0960. I think this is equivalent to the current Robertshaw 9600.

OK, so here it goes.

Woke up cold a few days ago. Checked thermostat. Read 42 but set for 56. Everything was set properly. Tried Fan and A/C but nothing would start. Replaced batteries and the same thing. Went to check breaker, it was fine. Tripped it once for good measure, still nothing. Grabbed multimeter, 127 on both poles.

Climbed up on the roof (not having any idea about what I was going to do) and found another junction box on the unit. Pulled red fuse junction box thingy and found two fuses in it. Check for power with multimeter, still 127 on both. Continuity test of fuses showed fuses were fine.

Back down into house. Pull faceplate on thermostat. The thermostat has 5 terminals, RC-RH-Y-W-G. Using the multimeter on DC, I seem to get nothing on any of the terminals, using AC I get a 4.3 reading on all of them. What should the readings be? AC or DC? If this is proper, then what do I begin to check next?

Thanks in advance for anyones help. I just want to make sure nothing relatively simple is wrong before calling a repair person. I am pretty technically adept with things assuming I can get some initial direction from someone.

Thanks again,
rj
 
  #2  
Old 02-14-03, 04:37 PM
fjrachel
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Voltage: 127 vac to ground? Or 127 vac from L1 to L2?
T'stat voltage: 24 vac between Rh and W at sub-base.
 
  #3  
Old 02-14-03, 06:15 PM
123rj123
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Voltage: 127 to ground correct.

T'Stat: Ok, so what you are saying is that when the tstat should try to start the Heating portion of the HVAC unit I should have 24 vac across Rh and W. If I don't?? Bad tstat??

Thanks again, we really appreciate it.

rj
 
  #4  
Old 02-14-03, 07:33 PM
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123rj123:

The correct way to test 220 volt power is between the two legs.
Measuring the voltage to ground can still give a reading if one of the legs was open.

The thermostat you have only has one side of the 24 volt line. Testing for voltage at the thermostat is also not the correct way to test the control voltage.

The easiest thing to do would be to jumper Rc/Rh to W.
This should energize the heat.
If it does nothing you can then check for voltage at the furnace.
You should have 24 volts between R and C.
Then check C and W.
If there is voltage at C and W, the thermostat is doing its thing.
From there you don't say what fuel you use???
 
  #5  
Old 02-14-03, 09:03 PM
123rj123
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Thank you. I will check and report back. The heater is natural gas.

Thanks for all your patience.
rj
 
  #6  
Old 02-15-03, 09:36 AM
H
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120volt or 208V??

Is the rooftop unit 120volt or 208/230V IAW the dataplate? 120V to ground is OK on 120Volt circuits only, but you could still have an open neutral, check L1 to L2 for the voltage reaching the unit,.. I only check power to ground to test for ensuring lines are dead before I touch things, even after the disconnect is off...It's a good habit to get used to.. I also carry a hotstick,, available at Home desparate for about $30.00 by Greenlee( trade quality..) if it gets near a hot lead it lights and buzzes. When on the rooftop, jump RH or R to W and the unit should start, it might have a 5 minute time delay.,,If not , check the filters and belts, fanmotors rotate freely..hit reset buttons (ie high limit) I think Lennox has a high pressure limit that is a manual reset...if when you push it in, it clicks, you'll know it tripped, then you need to find out why...
 
  #7  
Old 02-18-03, 02:59 PM
123rj123
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FIXED THANKS!!

Well the rain finally stopped yesterday and I was able to get out and test some of these suggestions. The first one worked!!

I was testing the 220 incorrectly. I was testing it to ground instead of across L1 and L2. Learn something new all the time. It was showing 0.00 across for the HVAC breaker. Checked the dryer breaker and it showed 248-ish. Plugged the HVAC wiring into the dryer breaker (both were 30 amp) and viola, HEAT!! Replace the HVAC breaker and all is well.

This provokes a couple more questions.
1. How common is it that a breaker would go bad? This breaker has never tripped during the two years we have lived here.
2. If this isn't a common thing, should I continue investigating to find the true cause of the problem or just wait it out till next time??

Thanks again. This is a great site. I am going to go and click-thru one of the advertisers and buy something!! THANKS!!

rj
 
  #8  
Old 02-18-03, 05:42 PM
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rj:

There is a possibility that the breaker is not bad.
Although not common, it is possible for one leg to trip and not move the handle.
To check for this I would forcefully turn the breaker on and off a couple of times, kinda like flicking it.
This is sometimes all it takes to reset it.
 
 

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