Extra thermostats


HVAC at this house was installed by the owner/designer's brother in law in 1987 (long now out of business) and we knew the wiring was a real rat's nest with nothing marked and no schematic . . . . Extra features appear to be:
1. The electric heat component was designed for heating only, thus has no connectors for AC 2. I forget where the AC power was taken from, but the system required an extra non-standard jumper at the thermostat, between Y and RC. 3. We noticed years ago a metal bulb, the size of half a pencil, on the outside wall under the deck, but we cannot see where it may be connected inside to anything (the entry point obscured by ductwork.) It looks like a temperature sensor but we cannot guess what it does. 4. Fastened to the fuse panel, which happens to be near the outdoor compressor, was an extra thermostat (marked PENN, Johnson Controls, model A19ABC-24) with ground and two wires running through the wall, presumably to the compressor, with ALARM painted onto the cover. There is no alarm indicator (and if there is a sound alarm it is invisibly small and never sounded in the last 17 years.)
Recent failure was (1) temperature error at the thermostat in the main hall (overreading to indicate 28 or 29C late in the day, when indoor temperature was about 24C) on two consecutive evenings; (2) then the AC simply failed: (3) If restarted from all Off, AC would run for 3 min., then stop.
The repairman fiddled for a long time, trying to chart the wiring: and only then spotted #4, disconnected it, and the AC now runs OK.
Is there any imaginable reason for ## 3 or 4, even on a system where we must "steal" power from a heat-only furnace to run the AC compressor?
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Don Phillipson
Carlsbad Springs
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I was no where near there! Honest!
(Toner tracer set like the phone guys use, may help. I've used one, on occasion.)
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Christopher A. Young
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