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<p class="MsoNormal">Just installed a RUUD modulating furnace with an in-house
electronic control tailored for a specialty light commercial application. The
control takes 24vac from the furnace via a 24vac to 12vdc power supply. Firing
rate is determined by the control and communicated to the furnace (V terminal)
using the same PWM signal that the Rheem modulating thermostats produce. Done
this before and it works like a charm. But… this time I’m running into an issue
where the negative side of the control’s DC circuit (0V) and the furnace common
are not on the same potential. In order for the furnace to see the V signal the
0V and common must be tied together to close the circuit, but… with 17v between
them that blows the fuse. Tested the signal and it’s correct – I’m measuring
+9.5vdc (during the on-cycle) in relation to 0V but I’m reading negative 7.5vdc
in relation to C. Everything else works but the furnace doesn’t see the PWM
signal and doesn’t modulate. Must be a simple miss-wiring. Any ideas?<span style="mso-spacerun: yes">  </span>Verified polarity on the 120v single phase
(dedicated) circuit feeding the furnace.</p>
 
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