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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