Why the Federal Reserve’s Balance Sheet Runoff Still Matters for Mortgage Rates

When the Federal Reserve stops raising its benchmark rate, why do mortgage rates so often refuse to follow the pause? The answer sits partly in a place most borrowers never look: the Fed’s own investment portfolio, and the slow process of shrinking it.

The Fed’s policy rate gets the headlines, but a 30-year fixed mortgage is not priced off it directly. It is priced off the market for long-term bonds, and the central bank is one of the largest players in that market. Whether it is buying, holding or letting those bonds run off changes what lenders must charge to move a loan.

How a Bond Portfolio Ends Up in Your Mortgage Rate

Most home loans do not stay on a bank’s books. They are bundled into mortgage-backed securities and sold to investors, and the yield those investors demand sets the rate a lender can offer a borrower. The 10-year Treasury yield works as the benchmark, and the mortgage rate sits above it by a margin called the spread. That spread pays investors for risks a Treasury does not carry, chiefly that homeowners may repay early when rates fall.

During the pandemic years, the Fed bought Treasuries and mortgage bonds in enormous volume, pushing its balance sheet to nearly $9 trillion. Its buying held down yields and kept the spread tight. Quantitative tightening reversed the arrangement. Instead of reinvesting proceeds when securities matured or were paid down, the Fed let them roll off, which left private investors to absorb supply the central bank once soaked up.

Why Mortgage Bonds Shrink Slower Than Anyone Planned

Treasuries have fixed maturity dates, so their runoff is predictable. Mortgage-backed securities shrink only when homeowners make principal payments, refinance or sell. The Fed sets a monthly cap on how much of its mortgage holdings may roll off without reinvestment, but a cap is only a ceiling. If borrowers are not paying loans off, the portfolio cannot shrink faster, whatever the cap says.

That is exactly what happened. Millions of homeowners locked in low rates in the years before the tightening cycle, and they have little reason to sell or refinance. Home turnover has been unusually low, so principal repayments on the older, low-coupon mortgage pools have trickled in. The Fed’s mortgage holdings have therefore declined far more slowly than its Treasury holdings.

This creates a loop. The Fed holds a large block of low-coupon bonds, and rates stay high enough that few owners of those loans give them up. Because the loans stay in place, the bonds stay on the Fed’s books and the runoff drags. The portfolio is not shrinking on the Fed’s schedule. It is shrinking on the schedule of the housing market.

The central bank has said it wants to end up holding mostly Treasuries, which implies the mortgage bonds must eventually leave the portfolio. Outright sales would be a sharp departure from simply letting bonds run off, and nothing about the current pace suggests the process will end soon. A buyer in the market today is still negotiating a rate shaped by a portfolio that has barely begun to unwind on the mortgage side.

The Spread Is Where the Pressure Shows

The clearest evidence of the balance sheet’s influence is the spread between the 10-year Treasury and the average 30-year mortgage rate. In the years when the Fed was a dominant buyer, that gap was comparatively narrow. Once the central bank stepped back and rate volatility jumped, the gap widened noticeably. Investors holding mortgage bonds now face greater uncertainty about how long loans will last, and they charge for it.

This matters because the Treasury yield and the spread can move independently. A borrower can watch the 10-year yield drop and still see little relief, because the spread absorbs part of the decline. Anyone waiting for the Fed’s rate cuts to deliver cheaper mortgages is betting on only one of the two pieces.

What Borrowers and Buyers Should Watch

The pace of runoff matters less than what the Fed does with the money it receives. If proceeds from maturing mortgage bonds are reinvested into Treasury bills, as the central bank has signaled it prefers, the Fed is not coming back as a buyer of mortgage securities. The private market keeps carrying that supply, and the spread has little reason to return to its earlier tightness on the Fed’s account alone.

Other forces act on the spread too. Volatility in Treasury yields raises it, and calmer markets lower it. Bank demand for mortgage bonds and the willingness of large investors to hold them also play a role. When rate swings quiet down, prepayment risk becomes easier to price, and the spread can narrow even while the Fed’s holdings stay large. That is why the balance sheet is one lever among several rather than a switch.

For a household, the arithmetic is concrete. On a $400,000 loan, a quarter-point difference between 6.5 percent and 6.25 percent changes the monthly principal-and-interest payment by roughly $65. Spread movements of that size are small enough to ignore in a headline and large enough to decide whether a particular house fits a budget.