Combi Boiler Low Hot Water Pressure: Why the Taps Run Slowly

Combi boiler low hot water pressure usually comes from one of three places: a weak cold mains supply feeding the boiler, a boiler too small for the flow you are asking of it, or a restriction inside the boiler such as a scaled plate heat exchanger or a blocked filter. If the cold tap runs strongly but the hot tap at the same outlet trickles, the problem is almost certainly the boiler or its hot water side. If both are weak, start with the mains.

A combi heats water as it passes through, so it can only ever deliver what the mains pushes into it, at a rate its burner can heat. Knowing that makes the fault much easier to narrow down.

Is it the mains or the boiler? The two-minute test

Before anything else, work out which side of the boiler the problem sits on.

1. Turn on the cold kitchen tap fully and check its flow. It is fed straight off the mains on almost every home.
2. Turn it off and run the hot tap fully at the same sink.
3. Compare. If the hot runs at roughly the same strength as the cold, your combi is passing through what the mains gives it. If the cold is strong and the hot is much weaker, something on the hot side is restricting it.
4. Time a bucket. Use a container of known size. Time how long the cold tap takes to fill it, then the hot.

The water industry rule of thumb, set out by WaterSafe, is that the first tap in the house should fill a 4.5 litre (one gallon) container in about 30 seconds with everything else off. That works out at roughly 9 litres per minute. Much less than that from the cold tap and the mains is your limiting factor.

Weak mains supply: the ceiling on every combi

The water company’s guaranteed minimum is low. According to the Consumer Council for Water’s low pressure factsheet, the guaranteed standard is 7 metres head, or 0.7 bar, at the outside stop valve. Most companies aim higher, around 10 metres (1 bar), but a supply near the minimum will struggle to run a combi shower well.

The water company is responsible for the pipe up to the outside stop valve. Everything from there into your house, including the supply pipe under the garden, is the homeowner’s. Old lead or narrow galvanised supply pipes, a part-closed stopcock and hidden leaks all cut flow before the water ever reaches the boiler. Our guide on finding and testing your stopcock shows how to check that one easy thing first.

Why hot water flow drops in winter

Many combi owners notice weaker hot water in January. It is real, and it is physics. A combi can only add so much heat per second. The colder the incoming mains, the more it has to raise each litre to reach shower temperature, so the fewer litres per minute it can manage.

As a rough guide, each kilowatt can heat about 14 litres per minute by 1°C. A boiler giving 24kW to hot water can lift about 9.8 litres a minute by 35°C, but only about 8.6 litres a minute by 40°C. On a cold morning, when the mains is at its coldest, the boiler needs that bigger rise. Many combis have a flow regulator that slows the water deliberately so it still comes out hot, which you feel as lower pressure.

Manufacturers publish hot water flow at a stated temperature rise, often 35°C or 40°C, so check which figure you are comparing. Our combi boiler size guide explains how output matches to showers and bathrooms.

Blocked plate heat exchanger and other boiler-side causes

If the cold is strong but the hot is weak, the restriction is between the cold inlet on the boiler and the hot tap. The usual suspects are:

Scaled plate heat exchanger

Inside a combi, mains water flows through a small plate heat exchanger, a stack of thin plates with narrow channels between them. Hot heating water passes on one side and gives its heat to the tap water on the other. In hard water, limescale coats those channels. Flow drops, the water may surge hot and cold, and the boiler can be noisy when hot water runs. Our article on combi hot water fluctuating in temperature covers that related symptom.

Building Regulations guidance recognises the problem. As Fernox summarises Part L, in hard water areas above 200 ppm, the feed to a combi’s hot water circuit should be treated to reduce scale. Your water supplier publishes the hardness for your postcode, so it is worth checking that figure and whether a scale reducer is fitted on the cold inlet.

Blocked inlet filter

Many combis have a small mesh filter on the cold water inlet that catches grit and debris, especially after mains work in the street. A partly blocked filter cuts flow to the whole hot side.

Flow sensor or diverter valve problems

The boiler needs to detect water flowing before it fires for hot water. A sticking flow sensor or a faulty diverter valve can cause slow, lukewarm or interrupted hot water, and sometimes heating radiators getting warm when you run a tap.

All of these sit inside the boiler casing. Cleaning, descaling or replacing them needs the boiler isolated, drained down on the hot side and tested afterwards, so it is a job for a Gas Safe registered engineer.

Problems at the tap or shower

Before you book anyone, rule out the simple outlet problems:

• Tap aerators: unscrew the nozzle on the end of the spout and rinse out scale and grit.
• Shower heads: soak in descaler or vinegar to clear the holes.
• Shower hoses: kinked or collapsed inner hoses restrict flow.
• Thermostatic mixer cartridges: these have their own filters and can scale up, cutting the hot side only.
• Isolation valves: small screw valves under sinks and on shower feeds are sometimes left part closed after work.

If only one shower is weak and the kitchen hot tap is fine, the problem is almost always at that outlet. Our guide to a shower going cold when a tap is turned on explains how outlets compete on a combi.

When the boiler is simply too small

Sometimes nothing is broken. A smaller combi in a house with two bathrooms will always struggle when two people want hot water at the same time. If one tap runs well but two together are poor, and the boiler and mains both test fine, the fix is a bigger combi, better mains supply, or a cylinder system that stores hot water. That is a conversation for a survey, not a repair. Get a boiler quote and an engineer can measure your flow and recommend the right size.

Get your hot water flowing again

Run the tap test first. Both weak means the supply; hot only means the boiler; one outlet only means the tap or shower. Clean aerators and shower heads yourself, check the stopcock is fully open, and leave anything inside the boiler to an engineer.

Gasmaster’s Gas Safe registered engineers diagnose and repair combi hot water faults across Stoke-on-Trent, Staffordshire, Cheshire and Derbyshire. Visit our boiler repair page or contact us to book an engineer.

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