Three conditions, three definitions. Everything in this area runs off a figure called the target balance, which 12 CFR 1024.17(b) defines as the estimated month-end balance just sufficient to cover the remaining disbursements for the year, taking account of the remaining scheduled payments and a cushion if there is one. Measured against that figure, the regulation defines a shortage as "an amount by which a current escrow account balance falls short of the target balance at the time of escrow analysis", a surplus as "an amount by which the current escrow account balance exceeds the target balance for the account", and a deficiency as "the amount of a negative balance in an escrow account." A shortage and a deficiency are therefore not degrees of the same thing. A shortage means the account has less than it should; a deficiency means it has less than nothing, which normally happens because the servicer advanced its own funds to pay a bill on time.
What the servicer may do about a surplus. If the analysis discloses a surplus, the servicer "shall, within 30 days from the date of the analysis, refund the surplus to the borrower if the surplus is greater than or equal to 50 dollars ($50)" (12 CFR 1024.17(f)(2)(i)). Below $50 it may refund the money or credit it against next year's escrow payments, at its option. This is one of the few consumer-protection rules that is genuinely automatic: no request is required.
What the servicer may do about a shortage, and why the asymmetry runs the borrower's way. Under 12 CFR 1024.17(f)(3) the answer depends on size. A shortage of less than one month's escrow account payment leaves the servicer three choices: allow it to exist and do nothing, require repayment within 30 days, or spread it over at least a 12-month period. A shortage of one month's escrow account payment or more leaves only two: allow it to exist, or spread it over at least 12 months. The 30-day demand is gone. The larger the shortfall, the fewer aggressive options the servicer has, which is the opposite of what most people expect and is the single most useful thing on this page for someone opening an alarming escrow statement.
What the servicer may do about a deficiency. The parallel rule at 12 CFR 1024.17(f)(4) looks similar and is not. Below one month's escrow payment the servicer may do nothing, require repayment within 30 days, or require "2 or more equal monthly payments". At one month or more it may do nothing or require two or more equal monthly payments. So the floor for a negative balance is two installments, not twelve, and a borrower who assumes the twelve-month protection carries across from the shortage rule will be wrong.
The condition attached to both. The surplus and deficiency provisions "apply if the borrower is current at the time of the escrow account analysis. A borrower is current if the servicer receives the borrower's payments within 30 days of the payment due date" (12 CFR 1024.17(f)(2)(ii) and (f)(4)(iii)). Where the borrower is not current, the loan documents govern instead: the servicer may retain a surplus in the account and may recover a deficiency on the contract's terms. That is a real cliff, and it is invisible on the statement itself.
The servicer still has to pay the bills. Whatever the account balance, 12 CFR 1024.17(k)(1) requires the servicer to make disbursements "in a timely manner, that is, on or before the deadline to avoid a penalty, as long as the borrower's payment is not more than 30 days overdue", and (k)(2) requires it to advance its own funds to do so on the same condition. An escrow account that is short is therefore the servicer's problem before it is the borrower's, which is why the advance and the resulting deficiency exist as separate concepts at all.
Aggregate accounting is mandatory. "All servicers must use the aggregate accounting method in conducting escrow account analyses" (12 CFR 1024.17(c)(4)), meaning the account is tested as a whole rather than item by item. Older single-item analysis, which computed sufficiency separately for taxes and for insurance, produced systematically larger balances and is no longer permitted.