Can You Have a Mixer Shower With a Combi Boiler?

Why a mixer shower suits a combi

A mixer shower blends hot and cold inside a valve on the wall. It works best when both supplies arrive at similar pressure, because the valve can then hold the temperature steady with small adjustments. On a combi, the hot side is simply mains water that has passed through the boiler, so it is at almost the same pressure as the cold. That balance is exactly what a thermostatic valve wants.

On an older vented system the opposite is true: the hot comes from a cylinder fed by a tank in the loft at low pressure, while the cold is often at mains pressure, and the valve spends its time fighting the difference. That is one of the reasons people who move from a tank fed system to a combi notice their shower improves.

How much hot water can your combi make?

A combi heats water as it flows through, so its hot water output is limited by its size. A useful rule of thumb: divide the boiler's hot water output in kilowatts by about two and a half, and you get roughly the litres per minute it can raise by thirty five degrees. That rise is about what it takes to turn a cold winter main into a hot shower.

So a boiler with a larger hot water rating gives you noticeably more flow at a good temperature than a smaller one. In summer, when the cold main arrives warmer, the same boiler gives you more. In a cold snap it gives you less, which is why a shower that is fine in August can feel weak in February.

The hot water rating is on the boiler's data badge and in the manual. It is often higher than the heating rating, so check the right figure.

Your mains flow sets the ceiling

However large the boiler, it cannot deliver more water than comes in from the street. The figure that matters is flow in litres per minute, not just pressure. A supply can show good pressure on a gauge with every tap shut and still collapse to a trickle when you open two outlets.

A simple test is to time how long the kitchen cold tap takes to fill a bucket of known size, fully open. If flow is poor, the cause may be an old supply pipe, a partly closed stopcock, or a pressure reducing valve that has failed. Our guide to low hot water pressure explains how to tell the difference.

Choosing the right shower head and valve

Every combi has a minimum flow rate below which it will not fire, stated in its manual. A very low flow shower head, a scaled head or a nearly closed isolating valve can push the flow below that point, and the shower then runs hot and cold as the boiler starts and stops.

• Pick a thermostatic valve rated for high pressure or all pressures, so it can hold temperature when someone uses a tap elsewhere.
• Match the shower head to what your combi can supply. A large rainfall head wants more hot water than many combis produce.
• Avoid pushing the flow below the boiler's minimum firing rate with an over restrictive eco head, or the boiler may cut out and run cold.
• Make sure the valve has working check valves on both inlets, which stop hot and cold crossing into each other.
• Keep the pipe runs short and at the right size so flow is not lost between the boiler and the valve.

Can you add a pump to a combi shower?

No. A combi is fed straight from the mains, and the water regulations do not allow a pump to draw directly from the mains supply to boost it without the water company's consent. Shower pumps are designed for tank fed systems with a cold water cistern above them, and a combi has no tank to draw from.

If the shower is weak on a combi, the fix is upstream: improve the mains flow if it is the limit, service or repair the boiler if its hot water output has dropped, or choose a head that suits what you have. If the demand simply outgrows a combi, the usual answer is stored hot water.

If your mixer shower suddenly runs cold while the kitchen tap is hot, the problem is usually a check valve or cartridge in the shower, not the boiler. Our page on no hot water in the shower goes through that test step by step.

Two bathrooms on one combi

A combi can only make so many litres a minute of hot water, and every open hot tap shares them. One shower running on its own will be fine. Open the kitchen hot tap and the thermostatic valve protects you from a scalding or freezing change, but the shower will lose flow. Run two showers at once and both will be poor.

For a flat or a small house with one bathroom, a combi and a mixer shower is usually the simplest and best arrangement. Where two showers are used at the same time most mornings, a system boiler with an unvented cylinder makes more sense. We compare the two properly in our guide to a combi or system boiler with a cylinder.

Swapping an electric shower for a mixer

Many London flats have an electric shower because the previous hot water system could not feed a mixer properly. With a combi in place, changing to a mixer is often worthwhile, because the combi will usually give you more flow than an electric unit. It does mean running a hot supply to the shower position and fitting a new valve, and on a tiled wall that usually means some retiling.

Before you commit, check the combi's hot water rating and your mains flow as described above. If you are also thinking about the wider system, our comparison of an instant water heater or hot water cylinder covers why electric instant units are limited in the first place.

Get your combi and mains checked first

A quick visit to measure mains flow and check the combi's hot water output takes the guesswork out of choosing a valve and a head. If the boiler has lost output through scale or a failing part, that is worth fixing before the new shower goes in, and if you are thinking of a boiler replacement at the same time, the shower should be part of that conversation.

Call 020 4652 5111 to book. Our rates are on the pricing page and are agreed before we attend.