Low Pressure on the Hot Tap Only

First, find out what heats your water

Whether weak hot water is a fault depends entirely on the system. Find the one you have before anything else.

A gravity-fed cylinder has a copper tank in an airing cupboard and a cold water tank in the loft. The hot water pressure comes only from the height of that loft tank above the tap. Every metre of height gives about 0.1 bar. A loft tank two metres above the bathroom gives about 0.2 bar, while London mains cold is often 2 bar or more. On this system a hot tap that is much weaker than the cold is how it was designed, not a fault.

A combi has no tank. Mains water goes through the boiler and out of the tap, so the pressure should be close to the cold. It is a little lower because the water squeezes through the heat exchanger, and many combis have a flow restrictor so they are not asked for more water than they can heat.

An unvented cylinder is a sealed tank, usually with a set of valves on the cold inlet and a small expansion vessel beside it. It runs at mains pressure through a pressure reducing valve, and a well set up one gives hot and cold at nearly the same pressure.

One hot tap or all of them?

This is the second split, and it saves most people a call-out.

If only one hot tap is weak, the fault is at that tap. Unscrew the aerator on the spout and rinse out the grit and scale. Under the basin or sink, look for a small isolating valve on the hot pipe with a screwdriver slot. If the slot is not in line with the pipe, it is part closed. Turn it in line. A worn ceramic cartridge in the tap itself, or a kinked flexible tap tail, are the other causes, and both are part of a normal leaking tap repair visit.

If every hot tap is weak and the cold is fine, the fault is at the source: the boiler, the cylinder or the pipe between them.

• Check the hot pressure against the cold at the same tap, fully open, into a bucket for ten seconds each. A big difference on a combi or unvented cylinder is a fault.
• Check the loft tank on a gravity system, if you can reach it safely on boards: it should be most of the way full. A low tank means the float valve is stuck or the mains feed to it is restricted.
• Look at any valve on the cold feed pipe leaving the bottom of the loft tank, or on the pipe into a combi. Handles should be fully open.
• Shower only: descale the head and try the bath tap, since a scaled head can cut the flow noticeably on its own.

Do not touch the valves, tundish or expansion vessel on an unvented cylinder. They are safety devices. Work on unvented hot water systems is covered by Building Regulations Part G, section G3, and should be done by someone qualified for it.

Why a combi gives weak hot water

On a combi, the hot water passes through a filter, a flow sensor and a plate heat exchanger before it reaches the tap. Each can restrict it.

A blocked inlet filter is the first thing an engineer checks. Most combis have a small mesh filter on the cold water inlet, often built into the isolating valve or the flow sensor housing. After mains work in the street, grit and scale clog it quickly, and hot water pressure falls while the cold, which does not go through the boiler, stays strong.

Scale in the plate heat exchanger is next. The narrow passages in the plates scale up in hard water, which slows the flow and makes the water hotter than usual at the same time. Weak and very hot together is a strong sign.

A flow restrictor or a flow sensor that has scaled up can hold the flow down further than intended. A few boilers are also set up with a restrictor sized for a smaller output, which is worth checking if the combi was replaced recently.

Low mains pressure affects everything, but it shows most on a combi, because the boiler needs a minimum flow to run. If the cold is also weak, read our guide to low water pressure first.

Why a cylinder gives weak hot water

On a gravity cylinder, the classic causes of a hot supply that has got weaker over time are a stuck float valve leaving the loft tank low, an airlock in the hot pipes after the system has been drained, a partly closed gate valve on the cold feed to the cylinder, and sludge or scale at the bottom of the cold feed. An airlock in the pipes often gives spluttering and banging as well as weak flow.

If the hot has always been weak on a gravity system and you want more, the answer is a shower pump or a system change, not a repair. A shower pump on a gravity system draws from the stored hot and cold, which is the normal way to fit one. Check the notification rules first, though: under the Water Supply (Water Fittings) Regulations 1999, any pump or booster drawing more than 12 litres a minute that is connected directly or indirectly to a supply pipe must be notified to the water company, and it needs the company’s consent before it is fitted.

On an unvented cylinder, weak hot with strong cold usually means a blocked strainer in the inlet control group, a failing pressure reducing valve, or the cold being taken off before the reducing valve while the hot is after it. Our guide to pressure reducing valve problems explains what the valve does. An engineer can clean the strainer and check the valve with a gauge.

What the engineer checks

The engineer measures static and flowing pressure at the hot and cold taps with a gauge and a flow cup, so the fault is found by numbers rather than guesswork. On a combi they isolate the water, clean or replace the inlet filter and check the flow sensor, then measure flow and temperature to decide whether the plate heat exchanger is scaled.

On a gravity system they check the loft tank level and float valve, clear airlocks, and check each gate valve is open. On an unvented cylinder, they check the strainer, pressure reducing valve and expansion vessel charge. Gas work is carried out by Gas Safe registered engineers.

The visit is the £60 call-out, and the repair price is agreed with you on site before any work starts. If the hot water has dropped from strong to weak on a combi, book it under no hot water repair, since the same visit covers poor flow.

• Combi inlet filter cleaned: usually one of the smaller jobs
• Loft tank float valve replaced: typically about £80 to £180 in the UK market
• Pressure reducing valve on an unvented cylinder replaced: typically about £150 to £300
• Plate heat exchanger replaced on a combi: typically about £300 to £600