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2 Nd stage of t stats failure

I recently went to do a service call on a install on a 7.5 ton carrier pac ,unknown to myself the call was for not enough cooling in the after noon .i arrived all seemed well stat satisfied .check unit had to adjust indoor fan motor mount and belt tension ,cycled unit on and lowered stat to bring on second stage after waiting nothing went through normal bs and finally jump3d 2 Nd stage all good ,being I was just filling in advised by buddy of the issue and he stated that like every year the second stage on the stats fails. Now please take in mind that the stat wires where crap so years ago we ran new 18 gauge stat wire but the issue keeps happening every year or so ,and it’s indifferent to weather it’s a white Rodger or Honeywell stat .he also stated that it only seems to be this unit since we installed about 5 other units at the same building and not a issue but there trane roof tops . On another note we had issues w the older 30 year old carrier. I checked all I could chk and will suggest re ohming wires for possible short but I find it funny that it will work correctly for up to 9 months then go south .manual economize so that’s out of the pic ,my parter blames it on crap mim t stats .has anyone else had issues like this ,unit was replaced about 2 years ago .peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating


  • ratioratio Member Posts: 2,489
    Take a look at how exactly the old ones failed. Take amps of Y2 when it's energized. Maybe a marginal contactor is drawling just sightly more current than the relay can carry. If the contactor were chattering (or even cycling on say the low pressure switch) from time to time it'd be drawing inrush current a lot more often than it should, again resulting in an overload that would slowly eat away at the contacts or even overheat the plastic of the relay housing.
  • HVACNUTHVACNUT Member Posts: 3,412
    You went through the normal BS and got nothing? What about the abnormal parts of the 2nd stage circuit? Is the whole control circuit 24v or are there isolation relays and line volt contractors? Break out the schematic and follow the yellow brick road. Or climb the ladder.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,091
    edited August 2019
    try a relay. Have second stage pull in a rib relay to start 2d stage. Maybe too much amp draw or to much voltage drop.

    I had this happen but the unit I was working on had (1) 2 speed Bristol compressor with high speed, low speed and tie contactors. I would work fine on low speed but on high it pulled in high speed and tie contactors and the stat wire couldn't handle both. Put relays in the unit so the only voltage /current going to the stat and back was rib relay coil
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