FROM Boeing QRH 737 6 June 2003:
If both pack trip off.....
At level off:
AIRSPEED ..........290 MINIMUM
[ Flight deck and cabin temp may increase rapidly below 290 kts]
PRESSURIZATION MODE SELECTOR.................MAN DC
OUTFLOW VALVE SWITCH...............................FULL OPEN
[Increases airplane ventilation]
IF Flight Deck And Cabin Temperatures Are Excessively Warm:
Flight Deck Door ............................OPEN
CABIN LIGHTING.............................DIM
WINDOW SHADES...........................CLOSED
GALLEY POWER ...............................OFF
As you can see,no mention regarding pressure differential ,or pack valves in hot position.My guess it's,if we're not talking about different planes,then you have some old qrh.Sorry if i'm wrong,no disrespect intended.They must have corrected a wrong information.
Brgds Alex