ACH is an electronic network for financial transactions in the United States. Deposits via ACH are available with Plaid and withdrawals via ACH are also available. You can make an online banking purchase using the Buy Crypto widget on the website and Kraken app. 

If you send an ACH transfer to Kraken via any other funding option besides Plaid it will be returned back to you after 5+ business days. We have little control over this automatic return process and it is largely up to your bank as to when they recredit the funds to your bank account. If your bank advises you to make a bank transfer using ACH, you should not carry this out.

As such, attempting to deposit using ACH, other than via Plaid, will result in significant delays.