ACH does not ship cash from one vault to another. It is a batch messaging network. Your bank, or the other bank, sends a file of credits and debits to an operator — the Federal Reserve or The Clearing House. The operator sorts the file, delivers the items, and settles by moving balances in the banks’ own accounts.
“Send money” is the consumer phrase. The mechanism is two ledgers and a clearing file. One bank writes that it owes you more, or less. The other bank writes the opposite. The operator exists so those two books do not drift.
A credit pushes. A debit pulls.
A credit is a push. Direct deposit of a paycheck, a Social Security payment, or a tax refund is the usual case. You already handed the originator a routing number and an account number. That originator sends a file to its bank — the originating depository financial institution, or ODFI — with an amount and a settlement date. The ODFI batches many originators together and forwards the file. The operator tells your bank — the receiving depository financial institution, or RDFI — to credit your ledger on that date.
A debit is a pull. The mortgage servicer or the electric company you authorized is the usual case. The biller is the originator this time. Its bank sends a file that tells your bank to take the money and send it the other way.
Nacha writes the operating rules. It does not run the sort. The two national operators do that. If the sending bank and the receiving bank sit on different operators, the operators exchange the item. Those interoperator items still settle on Reserve Bank books.
The file waits for a window
ACH is store-and-forward. Items sit until a processing window. Same-day ACH exists: a credit or a debit can settle on the banking day it is sent if it meets the operator deadline and stays under the per-payment cap of one million dollars (as of August 21, 2026). Ordinary items settle the next banking day. Some credits can be dated two banking days out. Debits are not allowed more than one banking day into the future.
The network processes for most of a business day and settles four times a day. Settlement still needs the Federal Reserve’s settlement service to be open. That service is closed on weekends, on federal holidays, and overnight on business days from 6:30 p.m. to 7:30 a.m. Eastern.
This is not Fedwire. It is not a card authorization. It is not FedNow or The Clearing House’s RTP network. Those rails have different cutoffs, different reversal habits, and different participation lists. Naming them does not mean they are on your account.
The number in the app is a ledger
“Available” is your bank’s decision about what it will let you spend. It is not a crate of currency with your name on it. Some institutions show a payroll credit before the operator has finished settlement, advancing their own funds. A pending debit can reduce availability before the item posts. A credit that looks posted can still come back if the originator sent a duplicate or the account numbers were wrong.
The same routing and account numbers that set up legitimate direct deposit can also launch an unauthorized debit. For a consumer account, federal electronic-fund-transfer rules generally treat a debit you did not authorize as an error you can dispute if you tell the institution in time. You can revoke an ACH authorization. Separately, you can place a stop-payment order. Neither step cancels the underlying bill.
Deadlines, return windows, and same-day caps change. Confirm the current rule with the deposit agreement and the agency or network that issues it.