Bar Valve Shower Problems (Concealed & Exposed Bar Valves)
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Post a job — we'll find you an engineer →Close both hot and cold service valves on the bar valve inlets before disassembly. Do not attempt to work on a bar valve without isolating both supplies — cross-flow between hot and cold under pressure can cause scalding.
Bar valve showers are the workhorse of UK bathrooms — reliable, convenient, and found in thousands of homes with gravity-fed or pressurised systems. But when the temperature won't hold steady, the water runs scalding hot, or flow drops to a trickle, it's usually one of three culprits: a worn thermostatic cartridge, a failed check valve, or a blocked inlet filter. Most of these faults are straightforward to diagnose and repair yourself in under an hour, provided you isolate the water supplies safely first. This guide walks through the most common bar valve problems and how to fix them, with practical steps for both exposed chrome valves and concealed in-wall installations. If the valve body itself is leaking behind the wall or you're unsure about isolation, it's worth calling a Gas Safe registered engineer to prevent water damage to your home.
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Most likely cause & what to check
Locate and close the service valves. On a correctly installed bar valve, there should be a service valve on each supply pipe (hot and cold) within easy reach of the bar valve — usually visible as a slotted brass fitting on each inlet connection. Turn each a quarter-turn clockwise to close. If there are no service valves, you will need to turn off water at the cold cistern gate valve (for a gravity system) or at the main stopcock and drain the hot cylinder (for a vented system), or at the boiler isolator for a combi. The absence of service valves is a non-compliant installation — consider fitting them when the valve is open for service.
Temperature not controlling or fixed temperature: the thermostatic cartridge is almost certainly at fault. On a typical bar valve (Triton, Mira, Bristan, Aqualisa, Grohe, Hansgrohe) the thermostatic cartridge sits behind the left-hand (temperature) handle. Remove the handle by finding the grub screw — usually accessed through a hole in the handle or under a push-fit cap. Allen key sizes vary by brand: Triton and Mira typically use 3 mm, Grohe and Hansgrohe use 4–5 mm. Pull off the handle and shroud, then unscrew the cartridge retaining nut anti-clockwise. Note the cartridge orientation before removing — take a photo.
Sourcing the correct cartridge: bar valve cartridges are brand- and sometimes generation-specific. A Mira Select cartridge will not fit a Grohe Grohtherm. Identify the brand and model from the valve body markings, then search at showerspares.co.uk or the manufacturer's website. Supplying the old cartridge to a specialist merchant (Plumb Center, City Plumbing, Wolseley) is the most reliable way to get a match. Prices range from £15 (Bristan, Triton) to £90+ (Hansgrohe, Grohe). Fit the new cartridge with fresh silicone grease on the O-rings, ensure the locating tab aligns, and torque the retaining nut to hand-tight plus a quarter turn.
Water too hot even at the coldest setting — check-valve failure: all WRAS-compliant bar valves have integral check valves (non-return valves) on each inlet, required by UK Water Regulations (WRAS category 2 protection, Schedule 2). When a check valve fails, hot water can back-feed into the cold inlet and vice versa, causing loss of temperature control. The check valves are usually small cartridges inside each inlet adaptor — remove the inlet connections and extract them. Check the rubber seats for cracks or distortion. Replacements are cheap (£5–£15 per pair) and widely available — always replace both together.
Leaking from the valve body: leaks from around the cartridge nut indicate a worn cartridge O-ring. Replace the O-ring (measure with callipers and match to a BS1806 size from any plumbers' merchant or from showerspares.co.uk). Leaks from the inlet connections on an exposed bar valve are usually a failed fibre washer inside the inlet adaptor — isolate, unscrew the adaptor, replace the washer. On concealed bar valves where the body is in the wall, access is via the trimplate; leaks behind the wall require opening the wall and should be attended to promptly to prevent structural damage.
Low flow from the bar valve — filter blockage: most bar valves have a mesh filter screen inside each inlet check valve. After pipework disturbance (new installation, or after nearby pipework work), debris can lodge in these filters within days. Remove the inlet check valves and inspect the screens — rinse under the tap or replace. Also check that both inlet service valves are fully open (not just partially open, which is a common post-installation error). Low dynamic pressure on either supply (below 0.5 bar for gravity systems) will impair thermostatic performance — consider a shower pump if your gravity pressure is inadequate.
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Frequently asked questions
Why has my bar valve suddenly started running only hot water?
This almost always points to a failed check valve (non-return valve) inside one of the inlet adaptors, allowing hot water to back-feed into the cold supply. The fix is straightforward: isolate both supplies, remove the inlet connections, extract and inspect the check valve cartridges inside, and replace both together — they're inexpensive and widely stocked by any plumbers' merchant.
My shower temperature control isn't working — is it worth replacing or should I buy a new valve?
Replacing just the thermostatic cartridge is almost always the answer and costs £15–£90 depending on your brand. The cartridge is the wear item, not the whole valve, so provided the body isn't leaking or damaged, a new cartridge will restore full temperature control within minutes.
What's the difference between concealed and exposed bar valves, and do they need different fixes?
Concealed valves sit behind the wall with only the handles and trimplate visible; exposed valves are mounted on the surface. Both use the same cartridge and check-valve system, but concealed valve leaks are harder to access — you'll need to open the wall — so they should be attended to quickly to prevent damp and structural damage.
How do I know if my bar valve is actually faulty or if it's a pressure problem?
Low pressure usually affects both temperature control and flow together, whilst a faulty cartridge typically causes temperature to stick or swing wildly. Check that both inlet service valves are fully open, then run the cold tap elsewhere — if pressure is weak throughout the house, it's a system issue; if only the shower is affected, the valve itself is likely the problem.
Do I need to turn off water at the main stopcock to work on my bar valve?
Not if your bar valve has service valves (isolator taps) on each inlet — a quarter-turn clockwise on each one isolates the shower safely. If there are no service valves, you'll need to isolate at the main stopcock (and drain the hot cylinder if it's vented), which makes this a good time to fit service valves for future convenience.
Why is my shower flow very weak even though water pressure elsewhere is normal?
The inlet filters inside the bar valve are almost certainly blocked with debris — common after new pipework installation or system disturbance. Remove and rinse the filter screens inside each inlet check valve; if they're damaged, replace them. Also check that both service valves are fully open, not just cracked open by mistake.