NACHA rules don't limit check conversions to checks on demand accounts. Money Market Deposit Account (MMDA) checks are eligible, for example. However, the BOC rules do state that HELOC checks are not eligible. As Buzz notes, if the HELOC checks are not in "business check" format, with encoding in the auxiliary on-us field of the MICR line (normally where business check serial numbers are encoded), there is often no way for the payee business to know of their special status.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8