The two comparisons that narrow it
Do these before anything else. Together they place the fault within one of four possibilities.
Hot versus cold, at the same tap. If both are weak, the problem is the supply to the house — the regulator, the main shutoff, or a leak. That is a different investigation entirely, covered in low water pressure. If only hot is weak, everything upstream of the heater is exonerated.
One fixture, or all of them. Weak hot at a single tap with good hot elsewhere means that fixture — its aerator, its shutoff valve, or its cartridge. Weak hot everywhere means the heater or the hot line leaving it.
Two questions, four outcomes. It costs nothing and it is what a plumber does first anyway.
Worth knowing
Test at a bathtub spout if you can. Tub spouts usually have no aerator and no flow restrictor, so they show you what the pipe is actually delivering rather than what a fixture is metering out.
One fixture only
The good outcome, and usually a ten-minute fix.
The aerator. On our water the screen at the end of the spout collects mineral scale, and hot-side deposits build faster because calcium comes out of solution when heated. Unscrew, soak in vinegar, rinse, refit.
The fixture's own shutoff valve under the sink, partially closed or corroded internally.
The mixing cartridge inside a single-handle faucet, scaled to the point where it restricts the hot side.
If cleaning the aerator restores it, that was the whole problem and there is nothing wrong with your heater.
Whole house, hot only: the heater
This is where the answer usually is, and there are three common causes.
Sediment. Calcium settles on the tank floor and accumulates. It occupies volume and, more relevantly here, can partly obstruct the outlet. The tell is often noise as well — popping or rumbling as the burner fires, which is water flashing to steam beneath the sediment layer.
The heat trap nipples at the tank's inlet and outlet. These contain small plastic balls or flaps that prevent convection losses. They fail, and a failed one can restrict outflow significantly. Inexpensive to replace and frequently overlooked.
Scale at the outlet or in the first section of hot pipe leaving the tank, which is the hottest point in the system and therefore where deposits form fastest.
On a tankless unit the equivalent is scale in the heat exchanger, which narrows the passages directly — see flushing a tankless heater.
Whole house, hot only: the line
If the heater checks out, the hot line itself is next.
Galvanized hot lines corrode internally and the bore narrows over decades. Hot lines go first because heat accelerates it. If the house has galvanized supply, this may be well advanced — and it is one of the clearer arguments for a repipe.
A partly closed valve on the hot side, often left that way after previous work.
A hot-side leak, including a slab leak. Hot lines fail first under slabs for the same reason they scale first — they move with every heating cycle. If weak hot pressure arrives with a higher bill, a warm patch on the floor, or water audible with everything off, treat it as a slab leak until ruled out.
That last combination is the one worth acting on quickly. Everything else on this page is an inconvenience; that one is doing damage while you decide.
The order worth working through
Compare hot to cold. Compare one fixture to the rest. Clean the aerator on the affected fixture. Check whether the heater has ever been flushed. Then look for the leak signs.
That sequence resolves most cases without a service call and tells us exactly where to begin when it does not.
The one thing not to do is jump to conclusions about the heater. “Low hot water pressure” gets a lot of heaters replaced that only needed a flush, a heat trap nipple, or an aerator cleaned in vinegar.