A B737-400 flight crew reported loss of cabin pressure climbing through FL200. They declared an emergency and descended; regained control of the cabin when the procedure was run; and returned to departure airport.

Narrative

We were climbing to FL230. At approximately FL200 we had the 'cabin alt/config' warning horn sound. We donned our O2 masks and proceeded with our memory items. The Captain leveled off at FL210; called Center; declared an emergency and asked for lower altitude and a turn to deviate around weather in the area. I ran the QRH procedures. The Controller didn't seem to understand the nature of our emergency and tried to only clear us down to FL180 (there was a lot of traffic in the sector). We were then cleared down to 11;000 FT and given a turn direct [to departure airport]. The cabin altitude made it up to 13;000 FT and we were able to descend before it climbed any higher. The QRH has you go straight to manual AC or DC which I did and closed the outflow valve enough to start the cabin back down. In the middle of all this I sent a message to [operations] and advised them of the situation and that we had declared an emergency; the Captain notified the Flight Attendant's what was going on and that we were returning to [departure airport]. We discussed what had happened and the fact that we never got an auto fail light and that maybe the Standby mode would work. I switched it to Standby and then the controller worked normally. We then briefed the approach and ran the overweight landing Decent/Approach check list. The approach was made through heavy weather (moderate turbulence and heavy rain). We weighed 124;000 LBS and did a flaps 30 landing. Touchdown was at approximately 100 FPM. The Captain did an outstanding job!