You dont need deca to watch recordings from your HD receiver..
DECA is DirecTV's system of ethernet over coax so the various receivers and DVRs talk to each other. Whole home, or "MRV" (Multi Room Viewing) uses DECA as the approved and supported technology to implement/accomplish MRV.
The H/HR24 products have DECA built in. H21/H22 receivers and HR20/21/22/23 DVRs require an external DECA adapter in order to incorporate them into an existing (or new) DECA infrastructure.
If you already have a DVR, you can add either an HD receiver (H2x) or an HD DVR (HR2x) to your system for multi-room. If you add another DVR, then recordings from both/all can be viewed throughout the system. If you add a receiver, it will be able to view recordings from all DVRs on your system.