On Jun 17, 7:46*pm, wrote:
On Sunday, June 17, 2012 4:04:05 PM UTC-7, Robert Macy wrote:
Typical!

In AZ we have the Air Handler in the attic and the Heat Pump sitting
on some square beside the building.

Shut off the AC last winter, and when went to turn it on this one
time, nothing happened. Well, except the fan came on, but no big power
hum, no fan blade inside the heat pump running.

Just as a check I measured all the voltages coming from the the
thermostat out through to the unit, and they all seem correct. OFF no
voltage, ON 26 Vac, yet unit doesn't come on. *There is a distinct
power hum, but nothing happens.

I turned OFF the 220Vac going to it, and measured the voltages around
the huge relay right at the access and was disappointed to find almost
nothing there. I expected the relay[whatever it's for] to be powered.
Perhaps that's the key.

Inside the access panel lid is atype of schematic. One interesting
item caught my attention 'low pressure switch' so I thought perhaps
the unit has lost its charge and the swicth won't come on to prevent
damage when it's not properly charged. *But I don't know much about
HVAC units. *There are three access spigots for measuring/filling I
think all right under the access panel.

Anybody out there with a bit of tutorial to help? Like what pressure
range should be there, what to do if the pressure is too low, etc etc.

I've got two other identical, operating units to do comparisons to,
but it still helps to know WHERE to look.

I’m confused: You say that you “turned OFF the 220Vac going to it, and measured the voltages around the huge relay right at the access and was disappointed to find almost nothing there”
Why would you expect to find any voltage if you say you turned it off?- Hide quoted text -

- Show quoted text -

Agree. There should be 240V coming in to one side of the relay/
contactor
with the power turned on.
And there should be 240V on the load side of the contactor with the
power on and the
relay energized. If he's OK with safely measuring that, then I would
do that. If the
power hum is coming from the contactor when the thermostat turns it
on, that's
normal. Could also verify that the contactor pulls in by listening to
it while
someone else activates the thermostat. Just be aware that most
thermostats
have a 5 min lockout, where if it's been activated, then turned off,
it won't turn
on again for 5 mins to protect the compressor.