"Elle" wrote
My house has a Rheem heat pump, installed in 1991 Model
#RPND-042C, with "Heater Kit RXPJ," according to its
nameplate. Temperatures are about 35-40 degrees F this
time of year where I am.

Often in the morning the heat pump blows cold air for a
half-hour (or more?).

Further update:
Last night I set the thermostat at 68 degrees F. Overnight
the outside temperature got down to about 38 degrees F. I
woke up about 5 AM to a freezing house and cold air blowing
from the registers. I waited 15 minutes, and cold air was
still blowing out.

At the thermostat's thermometer, the room temperature
reading was 62 degrees F. I then turned off the heat pump.
Freezing my rear off, I pondered it all. I let ten minutes
go by, set the thermostat at 63 F, and then turned the heat
pump back on. Cool air blew out for a minute or so, and then
warm air followed, and the house warmed. Over an hour or so,
I raised the thermostat setting to 71 F or so. The house
continued to warm, and all was well.

The same HVAC technician that came out a few weeks ago came
out again today. This time he determined that the reversing
valve was sticking. Obtaining a new valve will take a week
or so. Meanwhile he said if cool air was blowing out again
when I wanted heat, then switch the thermostat from heat to
cool, and then back, and listen for a "swish" sound. This
might jar the valve free and cause it to open(?) properly.

Any advice on helping ensure my house has heat in the days
remaining until the new valve arrives and the tech can
install it? I will continue with the "gentle touch" when
raising the thermostat setting. Perhaps the mechanical
features of the valve respond better to less extreme
electrical signals?

I know the basic function of the reversing valve. But I am
not sure whether it (1) throttles the refrigerant at all or;
(2) it's always wide open for heating mode, and wide open
for cooling mode, just directing the refrigerant flow in
opposite directions depending on the flow; or (3) switches
between the two directions when in heating mode, as a part
of the defrost feature?

I think understanding this might help my dealing with it in
the days before the repair. TIA for any light anyone can
shine on this. Meanwhile, I will google.