Mira Thermostatic Mixer Shower Problems (Mira Select, Mira Advance)
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Post a job — we'll find you an engineer →Before working on a thermostatic mixer shower, isolate both hot and cold supplies at the service valves on the bar valve inlets. Do not rely on turning off at the boiler alone — stored hot water in the cylinder is still under pressure.
Mira thermostatic mixer showers are fitted in thousands of UK homes, and they're generally reliable — but when they go wrong, the problems are usually predictable and fixable without calling an engineer. The most common faults are a worn thermostatic cartridge (which controls temperature), blocked inlet filters (which restrict flow), or failed check valves. Hard water is often the culprit in many parts of the UK, because the wax element inside the cartridge can fur up and stop responding properly within just a few years. This guide walks through diagnosis and repair for both the exposed Mira Select bar valve and the concealed Mira Advance trim, and explains how to service the valve yourself if you're comfortable with basic plumbing work. If your shower is leaking from the valve body itself or you suspect a cracked cartridge housing, it's worth calling a qualified plumber to assess whether replacement is the safer option.
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Most likely cause & what to check
Identify your exact valve. The Mira Select is an exposed bar valve with separate flow and temperature controls. The Mira Advance is a concealed thermostatic valve (trim only visible) and shares cartridge design with the Select. Both use a Mira-proprietary thermostatic cartridge that is not interchangeable with other brands. The correct cartridge part number is printed on the cartridge body and listed at mirashowers.co.uk — search by product name or EAN barcode on the box. Cartridges are available from Plumb Center, dedicated shower parts suppliers such as Shower Spares (showerspares.co.uk), and direct from Mira.
Temperature control fault — water too hot, too cold, or no temperature control: this is almost always the thermostatic cartridge wax element failing. Close the service valves on both inlets (slotted screws on the valve body — turn a quarter turn to close). Remove the temperature handle by locating the grub screw under the handle or behind a small cap, and unscrew it with a 3 mm Allen key. Pull the handle off and remove the chrome shroud (usually a push-fit). The thermostatic cartridge is now visible — unscrew the retaining nut anti-clockwise (use a 25 mm or adjustable spanner). Pull the cartridge straight out and inspect the wax capsule at the front for damage or discolouration.
Fitting the replacement cartridge: note the orientation — there is usually a locating tab or flat that must align with a slot in the valve body. Fit a new O-ring (usually supplied with the cartridge) lightly greased with silicone grease. Do not use petroleum jelly (Vaseline) as it degrades rubber. Screw the retaining nut back down firmly but not over-torqued — the cartridge body is brass and the thread strips easily. Refit the handle temporarily, open the service valves slowly and test the temperature range. The Mira Select has a high-temperature limit stop (a small plastic stop behind the handle) that should be set to prevent scalding — Mira's instructions explain how to adjust this.
No flow or very low flow: check the inlet filters first. Mira bar valves have small mesh filter screens inside each inlet check valve. Unscrew the check valves from the inlet ports (they unscrew by hand or with a small spanner) and inspect the filters for debris or limescale. Clean with a limescale remover solution or replace the check valve cartridges entirely — Mira sells these in pairs as a service kit for around £8. Also check that the service valves are fully open (slot aligned with pipe direction). If flow is still low after cleaning, the problem is likely insufficient supply pressure — the Mira Select requires at least 0.5 bar dynamic pressure on both hot and cold feeds.
Leaking from the valve body: leaks from around the cartridge retaining nut usually indicate a worn O-ring on the cartridge itself. A leak from the inlet connections suggests the PTFE seal or compression fitting has failed. Isolate, disassemble, and replace the relevant O-ring or refit the inlet compression fitting with fresh PTFE tape. If the valve body is cracked (visible on concealed Advance valves after freezing or over-tightening), the full valve must be replaced — do not attempt to repair a cracked brass body.
Annual maintenance in hard-water areas: in areas above 200 mg/l hardness (TDS) — common in the East Midlands, East Anglia, and South East — the thermostatic cartridge wax element can fur up and fail within 3–5 years. Mira recommends annual inspection and filter cleaning. Using an inline scale inhibitor (such as a Harvey Twin cylinder softener or an Eddy electronic descaler on the supply) extends cartridge life significantly. For warranty support, Mira offers a five-year guarantee on the Select bar valve — register at mirashowers.co.uk or call 01242 221221.
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Frequently asked questions
Why is my Mira shower suddenly running too hot or too cold?
The thermostatic cartridge — a small component inside the valve — contains a wax element that responds to water temperature. When this wears out or gets damaged by limescale (especially in hard-water areas), it loses the ability to mix hot and cold water properly. Replacing the cartridge is usually the fix, and it's a straightforward DIY job if you have a spanner and a 3 mm Allen key.
Can I use a different brand's thermostatic cartridge in my Mira shower?
No — Mira cartridges are proprietary and not interchangeable with other shower brands. Using the wrong cartridge will not fit correctly and may leak or fail to control temperature. Always check the part number on the old cartridge or contact Mira directly (01242 221221) to confirm the correct replacement.
My shower has very low water pressure. Is it the valve?
Low flow is often caused by blocked inlet filters inside the check valves — these collect limescale and debris over time. Unscrew the check valves and clean the mesh screens, which usually solves the problem. If pressure is still poor after cleaning, check that the service valves are fully open; if both are open and pressure remains low, the issue is likely with your water supply, not the shower valve.
How often should I service a Mira thermostatic shower in a hard-water area?
In areas with hard water above 200 mg/l hardness — common in the East Midlands, East Anglia, and the South East — Mira recommends annual inspection and filter cleaning to prevent cartridge failure. A scale inhibitor installed on your main supply can extend cartridge life significantly and is worth considering if you've had repeat failures.