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Low Pressure on the Hot Side Only

If cold is fine everywhere and hot is weak everywhere, the supply into the house is fine by definition. Something between the heater and the taps is not.

Top of a storage water heater with red-handled isolating valves on the inlet and outlet

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This is a useful symptom, because it rules a lot out. Water arrives cold and is split: some goes straight to the cold taps, the rest goes through the heater. If only the hot side is weak, everything before that split is working.

The usual causes

The heater's own shut-off valve

There is a valve on the cold inlet at the top of the heater. If it is part-closed, every hot tap in the house is weak. Same check as the main shut-off: a lever should lie along the pipe, a wheel should turn counter-clockwise to its stop.

Sediment in a tank heater

Minerals settle out of heated water and collect in the bottom of the tank. In hard-water areas this builds over years and restricts flow as well as wasting energy.

Pair of red-handled shut-off valves feeding braided flexible connectors

Old galvanized pipe on the hot side

Heat accelerates corrosion, so in a house with mixed plumbing the hot runs often close up before the cold ones do. If the house is old enough for galvanized supply pipe, this is a common pattern.

A failing mixing valve

Some systems have a thermostatic mixing valve to temper the output. When one begins to fail it restricts hot flow.

Copper pipe and fittings laid out with a soldering tool

What to check first

The inlet valve, because it is free. Then whether it is every hot tap or only one — one weak hot tap is a fixture problem, not a heater problem.