Low Voltage Short - 6 Month Old Lennox RTU

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Marco Nunzio

Member
Location
Canada
Occupation
HVAC Tech
A service call that came in for me last Friday, a new Lennox RTU that was installed about 6 months ago, the thermostat was showing a blank screen (w/ a common wire connected).

Control board isn't fused, but the 600v to 24v transformer has a 4 amp resetable fuse built in and it was tripped.

Thoroughly inspected unit for any shorts and rub outs in wiring, nothing found. REMOVED THERMOSTAT WIRES FROM UNIT TERMINAL BLOCK, and left wires connected to thermostat downstairs. Checked for any continuity across all wires, no continuity. Checked each thermostat wire for any direct shorts to ground. No direct shorts found. Inspected thermostat wire about 20ft before it is no longer accessible and no shorts found. Reset fuse and JUMPED R-G, R-W, R-Y on UNIT TERMINAL BLOCK WITH THERMOSTAT WIRES DISCONNECTED and unit ran without issues. Reconnected thermostat wires but was not able to test unit from thermostat as tenants had left by time I was done on roof.

Today(Thursday), call comes in that same thermostat is blank again. Once again 4 amp fuse tripped on transformer. REMOVED ALL WIRES FROM UNIT AND THERMOSTAT DOWNSTAIRS. Checked and ensured no continuity across any wires and no continuity to ground. RECONNECTED THERMOSTAT WIRES BACK TO THERMOSTAT DOWNSTAIRS, but LEFT THERMOSTAT WIRES OFF OF THE UNIT TERMINAL BLOCK. Checked continuity across wires, and my R-G-Y wires had continuity. (With a blank screen)(on the previous Friday call doing the same exact test these wires had no continuity). PULLED THERMOSTAT FACE OFF and continuity across the 3 wires disappeared. Also checked other wires and had no continuity across R-W, R-C, W-C, Y-C, G-C, W-G with thermostat face on or off.

RECONNECTED THERMOSTAT WIRES TO ROOFTOP UNIT TERMINAL STRIP, and DISCONNECTED WIRES FROM DOWNSTAIRS THERMOSTAT. Reset fuse. Jumped out unit from thermostat wires downstairs, R-W and heat ran without issue. R-Y and cooling ran without issue. R-G and fan ran without issue.

One thing I didnt do which I will when I go back is measure amps on my 24v wire coming out of transformer with heat, cooling, and fan running their respective courses.

Anyways this leads me to believe the thermostat is bad and taking out transformer fuse by overloading the secondary side with a call for heat, and stuck closed contacts across R-Y?(due to the continuity across R-Y-G with a blank screen and wires disconnected from unit).
Or the thermostat board is malfunctioning and incorrectly closing the R-Y circuit randomley, and when a call for R-W comes in, the secondary side becomes overloaded blowing the 4 amp fuse, as the first call on Friday I didnt have continuity across the R-Y-G wires doing the same test.

I have not tried setting a call for heat with the thermostat I believe is defective as I don't want to try to blow the fuse on the transformer if I don't have too. I could wire in an extra inline 4 amp resetable fuse and then test it out but I don't want to have to go pick one up if I don't have too.

I think I am going to replace the thermostat and go from there. I have also confirmed the transformer is receiving proper 600v primary and outputting proper 24v secondary.

What are some thoughts?? For some reason I'm not confident the thermostat is the issue but the tests don't lie.

Anything I missed or can do??
Does my theory of overloading the secondary side make sense?
 

Beaches EE

Senior Member
Location
NE Florida
Occupation
Electrical Engineer / Facilities Manager
From your description it sounds like the thermostat is the cause. Was it replaced when the RTU was installed? Is the 4 A resettable fuse in the primary or secondary of the 600/24 V transformer? If in the secondary, it in the C line of the thermostat?
 
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