Low Hot Water Pressure? Why It Happens and How to Improve It

First, work out what is actually weak

Two minutes of checking tells you which of four quite different problems you have, and saves paying someone to find out.

Open the kitchen cold tap fully, then the kitchen hot tap. Then do the same in the bathroom. Then try the shower. Note which ones are weak and which are fine.

• Everything weak, hot and cold, everywhere: a supply problem. Check the stopcock is fully open, then look at the incoming main.
• Cold strong, hot weak at every outlet: the hot water system itself, covered below.
• Only one tap or shower weak: that outlet, usually scale, a clogged filter or a half closed isolation valve.
• Fine on its own, collapses when two outlets run: a system at the limit of what it can deliver, or a restriction in a shared pipe or valve.

Vented cylinder: why gravity gives you weak hot water

If you have a copper cylinder in an airing cupboard and a cold water tank in the loft, your hot water is gravity fed. The pressure at any tap depends only on the vertical distance between the water level in that tank and the tap. Every metre of height gives roughly a tenth of a bar. A tank a couple of metres above a first floor shower gives you about 0.2 bar, which is a trickle next to a mains fed cold tap.

That is not a fault. It is how the system was designed, and it explains the usual complaints: a weak shower upstairs, a mixer tap that runs cold because the mains cold side overpowers the hot, and a top floor or loft bathroom that is almost unusable because it sits barely below the tank.

Before blaming the design, rule out the things that make a vented system weaker than it should be. Check the loft tank is actually full and the ball valve is refilling it. Check the gate valve on the cold feed from the tank to the cylinder is fully open. And if the flow has got worse recently, particularly after work on the pipes, an airlock can partly block the hot feed and tends to show itself as spluttering at the taps.

How to increase hot water pressure on a vented system

There are three realistic options, and they suit different houses.

A shower pump boosts the flow from the stored water to one shower or, with a larger pump, to the whole hot and cold supply from the tanks. It is the cheapest fix for a weak shower on a gravity system. The pump must draw from the stored water, never directly from the incoming mains, and it needs an adequate cold tank and a properly sized feed from the cylinder to work without cavitating or drawing air.

Raising the loft tank gains a little, because every extra metre of height adds roughly a tenth of a bar, but in most houses there is simply not enough headroom to make it worth the work.

Replacing the vented cylinder with an unvented one gives mains pressure hot water at every outlet, with no tanks in the loft and no pumps. It is the proper fix when the mains supply is good enough, and an unvented cylinder installation is the job we do most. Whether your mains can support it is a simple test you can do yourself with a bucket, explained in our unvented vs vented cylinder guide.

Do not fit a pump to a system without checking what it is connected to. A pump on the wrong supply, or on an unvented cylinder, can breach the water regulations and damage the pump or the cylinder.

Unvented cylinder: weak hot water is a fault

An unvented cylinder is fed straight from the mains through a pressure reducing valve, so the hot water should arrive at close to the same pressure as the cold. If the cold is strong and the hot is noticeably weaker, something on the hot side is restricting it.

The first suspect is the strainer in the inlet group on the cold feed. It is a small mesh that catches grit and scale before they reach the valves, and if it has not been cleaned for a few years it can choke the flow into the cylinder. It is cleaned as part of an annual unvented cylinder service, and a surprising number of weak hot water calls end with nothing more than that.

The second is the pressure reducing valve itself. When the valve scales up, it can hold a normal reading with the taps closed and then fail to open properly under flow, so the pressure collapses as soon as you run a bath. Our guide to the pressure reducing valve explains how that shows up.

Less often, an isolating valve on the cold feed has been left part closed after work, or the cold supply was balanced so that the hot side gets a restricted share. In every case the fix involves the unvented safety set, so it is work for someone qualified on unvented systems rather than a DIY adjustment.

Combi boiler: low hot water flow

A combi heats water as it passes through, and it can only heat so many litres a minute. Open two hot taps and each gets roughly half. In winter, when the incoming water is colder, many combis deliberately slow the flow to keep the temperature up, which is normal behaviour and not a fault.

What is not normal is a combi that gives a weak flow of hot water from a single tap when the cold is strong. The usual causes are a blocked filter on the cold inlet to the boiler, a scaled plate heat exchanger, or a diverter valve that is not moving fully. Our no hot water from combi boiler guide covers the checks, and the boiler itself should only be opened by a Gas Safe registered engineer.

Only one tap or shower is weak

When the rest of the house is fine, the cause is almost always at the outlet. London's hard water leaves scale in shower heads, tap aerators and the filters inside mixer and thermostatic shower valves, and it builds slowly enough that people do not notice how far the flow has dropped.

Unscrew and descale the shower head or tap aerator. Check the small isolation valves on the pipes feeding the tap or shower, the ones with a screwdriver slot: the slot should run along the pipe when the valve is open. On a thermostatic shower, the inlet filters and the cartridge scale up in the same way, and a cartridge that is failing often shows as a weak or lukewarm shower. Our no hot water in shower guide goes through that in detail.

Hot water pressure suddenly dropped

A sudden drop, rather than a gradual one, narrows things down. If both hot and cold dropped together, check whether neighbours are affected, because it may be a burst or works on the main in the street. If only the hot dropped, think about what changed: work on the pipes, a new tap, a drained and refilled cylinder, or a valve someone turned and did not turn back.

On an unvented cylinder, a sudden drop in hot water pressure along with a dripping tundish is worth a call, because it can mean the inlet valves are failing. On any system, a drop in pressure with a damp patch, a running meter with all taps closed, or a hissing sound somewhere in the house can mean water is escaping, and that is a leak to find before it is a pressure problem to fix.

When to call us

If the checks above point at the cylinder, the inlet valves or the boiler, or you want to know whether an unvented cylinder would work in your home, call us or send an enquiry. A photo of the cylinder, its data badge if it has one, and the valves on the cold feed tells us most of what we need. We have 20 engineers across London and hot water systems are our core work. Our rate is on the pricing page and agreed before anyone attends.