Summary
"The shower's lost its pressure" is one of the top five bathroom call-outs and one of the most misquoted. The customer feels less water on their shoulders and reaches for the word pressure; the engineer needs to think in litres per minute, because an electric shower's output is set by two things — how much cold water the mains will push through it, and how much of that water the heating element can warm to shower temperature. Either one can fall, and the fixes are completely different. Replacing a shower unit to cure a part-closed stopcock is a £250 mistake; cleaning an inlet filter to cure a scaled heater can is a wasted visit.
The second confusion is seasonal. An instantaneous electric shower heats water as it passes through, so the flow it can deliver at a given temperature depends on how cold the incoming water is. Mains water in the UK is around 15–18°C in late summer and 5–8°C in January. At 8.5 kW the same shower that gave a decent 5 litres a minute in August gives barely 4 in January, and the thermostatic or flow control automatically throttles the water to hold the temperature. Customers describe this as "the pressure has dropped"; it is the shower doing its job. The remedy, if they want more, is more kilowatts, which brings in the electrician and the cable-size question.
This guide gives a measure-first diagnosis — flow at the hose outlet, flow with the head off, flow at a nearby cold tap, dynamic pressure at the inlet — a decision tree that separates showerhead, unit, supply pipework and mains, and the fixes for each including when the answer is a higher-rated unit or mains boosting. For a shower that has stopped altogether see electric shower not working; for a shower that runs hot then cold see shower runs cold.
Key Facts
- Minimum supply — most UK electric showers specify a minimum maintained (dynamic, with the shower running) pressure of 1.0 bar and a minimum flow of 8 litres per minute at the inlet; some lower-rated or "low pressure" models accept 0.7 bar. Maximum static pressure is typically 10 bar; above that a pressure-reducing valve is needed
- Water company standard — the reference level of service for mains pressure is 10 metres head (1 bar) at the boundary stopcock at a flow of 9 l/min; the statutory minimum under the Water Industry Act 1991 is 7 metres static head. Below the reference level the customer has a complaint with the water company, not with the shower
- Physics of flow — heat required = flow × temperature rise × 4.18 kJ/kg·K. An 8.5 kW shower raising water by 30°C (8°C mains to 38°C) delivers 8.5 ÷ (4.18 × 30) = 0.068 litres per second ≈ 4.1 l/min. At 10.5 kW it is 5.0 l/min; at 12.5 kW about 6.0 l/min. In summer with 16°C mains the same 8.5 kW unit gives about 5.5 l/min
- Head scaling — the single most common cause in hard-water areas (above 200 mg/l CaCO₃, which covers most of southern and eastern England); limescale closes the spray holes and the flow regulator in the head. Check first, costs nothing
- Inlet filter — a fine mesh strainer in the shower's inlet connection catches grit and scale; it blocks slowly over years and is the second most common cause. Cleaning it is a 20-minute job with the unit isolated
- Heater can scaling — scale builds on the element and inside the heater can, narrowing the waterway and causing the shower to run hotter at lower flow until the thermal cut-out trips; a can that has scaled to this point is replaced, not descaled
- Flow control and solenoid — a solenoid that does not open fully, or a flow adjuster at minimum, restricts the supply inside the unit
- Isolation valve — the service valve on the supply pipe (often a screwdriver-slot ball valve) is frequently left part-closed after other work; a quarter-turn valve must be fully in line with the pipe
- Stopcock — a failing or part-closed internal stopcock throttles the whole house; a stopcock that will not open fully, or whose jumper has detached, drops dynamic pressure at every outlet
- Supply pipework — an electric shower should be fed in 15mm copper or equivalent from the rising main, as directly as possible; long runs with multiple tees, old galvanised steel or lead pipe, or a shared 15mm supply to a whole bathroom all reduce dynamic flow
- Shared supply — if the flow collapses when a toilet refills or a tap runs elsewhere, the supply is undersized or shared; the shower should be teed off as close to the rising main as possible and ideally before any other outlet
- Tank-fed is wrong — an electric shower fed from a cold water storage cistern will only have the head of the cistern above it (a loft tank 3m above the shower gives 0.3 bar) and will never perform; instantaneous electric showers are designed for mains supply unless specifically a gravity-fed pumped model
- Pressure-reducing valves — a PRV fitted to protect an unvented cylinder or appliances may be set too low or have failed closed; check its downstream pressure
- Mains boosting — a pump drawing water directly from a supply pipe at more than 12 litres per minute requires notification to and consent from the water undertaker under the Water Supply (Water Fittings) Regulations 1999; a break tank and pump avoids the restriction. See mains water boosting
- Cable and circuit — upgrading from 8.5 kW to 10.5 kW means 45.7A at 230V and almost always a larger cable and protective device; 10.5 kW on 6mm² cable is a fire risk, not an upgrade. See shower circuit cable size calculator
- Part P — a new or upgraded shower circuit is notifiable work in England and Wales (new circuit, or alteration in a room containing a bath or shower)
- Pricing — descale head and clean filter £60–£100; isolation valve or stopcock replacement £80–£180; new like-for-like shower unit £150–£300 fitted; upgrade with new circuit £600–£1,200+ (see electric shower installation pricing guide)
Quick Reference Table
Diagnosed the problem? Create a repair quote in minutes with squote.
Try squote free →| Measurement | How to take it | Healthy result | Points to |
|---|---|---|---|
| Flow at shower head, shower running on cold | Jug and stopwatch, 30 seconds × 2 | ≥ 6 l/min cold on an 8.5 kW; manufacturer's figure | Below this → continue down the table |
| Flow with head removed, hose only | Same, head unscrewed | Noticeably higher than with head | Head scaled or flow regulator blocked |
| Flow at nearest cold tap (bath or basin) | Jug and stopwatch | ≥ 10–12 l/min typical mains | Low here too → supply/mains, not the shower |
| Flow at kitchen cold tap (first draw off rising main) | Jug and stopwatch | ≥ 12–15 l/min typical | Low here → stopcock, mains, or PRV |
| Dynamic pressure at shower inlet | Gauge on a tee or at the nearest cold outlet while shower runs | ≥ 1.0 bar maintained (or MI figure) | Below → supply pipework or mains |
| Static pressure at outside tap or washing machine valve | Gauge, nothing running | 2–5 bar typical; below 1 bar is a water company issue | Mains supply |
| Flow drop when toilet refills or tap runs | Run shower, flush WC | Minor drop only | Large drop → shared or undersized supply |
| Temperature at full flow in winter | Thermometer in spray | 38–40°C at the manufacturer's rated flow | Hot only at a trickle → scaled heater can, or rating too low for winter |
Detailed Guidance
Step 1: Measure the flow, then take the head off
Run the shower on its coldest setting (so the thermostatic control is not throttling for temperature) into a measuring jug for 30 seconds and double the figure. Note it. Now unscrew the head and repeat with the bare hose. If the flow with the head off is markedly higher — say 7 l/min against 4 — the head is the problem. Soak it in a proprietary descaler or a 50/50 white vinegar solution for an hour, pick the holes out with a pin, flush, and check the small rubber flow regulator in the head inlet is clear or remove it if the manufacturer permits. Replacement heads are £10–£30 and a low-scale design pays for itself in hard-water areas.
If the flow is poor with the head off as well, the restriction is upstream of the hose. Move on.
Step 2: Compare with another cold outlet
Before opening the shower, run the nearest cold tap — the basin or bath — and then the kitchen cold tap, which is usually the first draw off the rising main. Measure both.
- All three are poor — the problem is the house supply or the mains: stopcock, pressure-reducing valve, lead or galvanised supply pipe, or the water company. Go to Step 5.
- Kitchen is fine, bathroom outlets are poor — the pipework between the rising main and the bathroom is restricted or shared. Go to Step 4.
- Only the shower is poor — the fault is inside the shower or its own isolation valve and filter. Go to Step 3.
Ask also whether the shower collapses when the toilet refills. If it does, the shower is sharing a 15mm supply with the cistern and is teed off downstream of it — a layout problem rather than a fault, fixed by re-plumbing the shower feed directly from the rising main.
Step 3: Inside the shower: isolation valve, filter, solenoid, heater can
Isolate the electrical supply at the pull-cord and the consumer unit and prove dead before removing the cover — electric showers run on 32–50A circuits (see safe isolation procedure). Turn off the water isolation valve.
Isolation valve. With the cover off, check the slot or handle of the service valve is fully in line with the pipe. A valve knocked a quarter closed during a tiling job or a previous repair is a common find and a free fix.
Inlet filter. Undo the inlet connection and remove the mesh strainer. Rinse out grit and scale, or replace it (a few pounds). A heavily silted filter in a house with no other symptoms usually means a recent mains disturbance — water company works, a new stopcock — stirred up sediment.
Solenoid valve and flow adjuster. Some units have a manual flow adjuster that may have been set to minimum; others have a stepped flow selector. A solenoid that is sticking or has a weak coil may not open fully and will usually also give intermittent "no water" symptoms, covered in electric shower not working.
Heater can. If the shower only reaches a comfortable temperature at a trickle, runs hotter than expected at a given setting, or has started tripping its thermal cut-out, the heater can is scaled and the waterway through it has narrowed. Descaling a can in situ is not practical or reliable; the realistic repair is a replacement can (£50–£120, where available for the model) or, for units more than eight to ten years old, a new shower. In hard-water areas a scale inhibitor fitted to the shower's supply extends the life of the replacement.
Step 4: The supply pipework
Trace the shower's cold feed back to the rising main. The ideal is a dedicated 15mm copper feed teed off the rising main as the first or second branch, with a full-bore isolation valve. Common faults:
- Teed off the end of a bathroom cold run that already serves the basin, bath and WC — the shower gets what is left. Re-pipe the feed directly.
- Microbore or 10mm pipe used for part of the run — adequate for a basin, not for a shower at 8 l/min.
- Multiple compression tees and elbows in old pipework each adding resistance.
- Old galvanised steel internally corroded to half its bore, or lead supply pipe with a reduced bore — replace back to the stopcock in 15mm or 22mm copper or MDPE where underground.
- Push-fit fittings with the inner bore reduced — push-fit has a smaller internal diameter than copper; several in series on a marginal supply are noticeable.
- Pressure-reducing valve downstream of the stopcock set to 1.5 bar for an unvented cylinder, starving a shower that wanted 2 bar dynamic — check its setting and whether the shower can be fed upstream of it (the shower's own maximum, usually 10 bar static, allows this).
Step 5: Stopcock, mains and the water company
If every cold outlet in the house is poor, start at the internal stopcock. Open it fully, then close it a quarter turn (to stop it seizing open) and remeasure. A stopcock whose jumper has come adrift, or whose washer has swollen, can present as a house-wide drop; replacing it with a full-bore lever valve is £80–£180 and often transforms the flow. See stopcock types.
Next check the external boundary stopcock is fully open — contractors and meter fitters routinely leave them part closed. Then put a gauge on the outside tap or washing machine valve with nothing else running: a static pressure below 1 bar, or a dynamic pressure that collapses below 1 bar when one tap runs, is a mains problem. If the supply pipe from the boundary is old lead or ½" galvanised, replacing it with 25mm or 32mm MDPE is the fix and is usually the customer's cost from the boundary inward; many water companies will replace their side free or subsidised at the same time.
If the pipe is sound and pressure at the boundary is still below the 10 metres head at 9 l/min reference level, the customer should raise it with the water company, who must investigate and may be liable for a payment under the Guaranteed Standards Scheme if the pressure falls below the standard on two occasions. Record your readings for them — a dated photograph of the gauge is the most useful thing you can leave behind.
Step 6: When the shower is simply too small for the job
If the flow with the head off is at or above the manufacturer's rated figure for the measured inlet temperature, and the customer still finds it weak, the shower is working correctly and the answer is more power or a different type of shower:
- Higher kW unit — 9.5 or 10.5 kW gives 15–25% more flow at the same temperature than 8.5 kW. It needs the circuit checked: 10.5 kW draws about 46A and typically requires 10mm² cable and a 45A or 50A protective device depending on the cable's installation method and run length; see shower circuit cable size calculator and electric shower installation. This is notifiable under Part P and is an electrician's job.
- Thermostatic mixer shower fed from a combi or cylinder — delivers 10–15 l/min where the hot water system can supply it, and removes the kilowatt ceiling altogether; see electric shower vs power shower comparison.
- Mains boosting — where the mains itself is marginal, a break tank and pump set (or an accumulator) is the compliant route; a direct inline pump above 12 l/min needs the water undertaker's consent under the Water Fittings Regulations. See mains water boosting.
Explain the winter effect to the customer before they spend money: a 10.5 kW shower in January performs like an 8.5 kW shower in August. If they were happy in summer and unhappy in winter, the unit is not faulty.
Diagnostic decision tree
ELECTRIC SHOWER WEAK / "LOW PRESSURE"
│
├─ Measure flow at head, on COLD setting
│ └─ Remove head, remeasure
│ ├─ Much better with head off ──► descale / replace head
│ │ & flow regulator ──► DONE
│ └─ Still poor ──► continue
│
├─ Measure nearest cold tap AND kitchen cold tap
│ ├─ ALL poor ──► house supply / mains
│ │ ├─ Internal stopcock fully open? jumper intact? ──► replace with
│ │ │ full-bore lever valve
│ │ ├─ Boundary stopcock fully open?
│ │ ├─ PRV fitted? setting / failed? ──► adjust / replace / feed shower upstream
│ │ ├─ Lead / galvanised supply pipe? ──► replace in MDPE
│ │ └─ Static < 1 bar or dynamic < 1 bar at boundary ──► water company
│ │ (10 m head at 9 l/min reference level) ──► customer complaint
│ ├─ Kitchen fine, bathroom poor ──► bathroom pipework
│ │ ├─ Shower teed after WC/basin? drops when WC refills? ──► re-pipe
│ │ │ direct from rising main in 15 mm
│ │ ├─ 10 mm / microbore section? push-fit chain? ──► upsize in copper
│ │ └─ Corroded steel section? ──► replace
│ └─ Only shower poor ──► inside the unit (ISOLATE ELECTRICS FIRST)
│ ├─ Isolation valve fully open? ──► open
│ ├─ Inlet filter blocked? ──► clean / replace
│ ├─ Flow adjuster at minimum? ──► reset
│ ├─ Solenoid not opening fully? ──► replace solenoid
│ └─ Hot only at a trickle / TCO tripping? ──► heater can scaled
│ ──► replace can or unit; fit scale inhibitor
│
└─ Flow matches MI rating for inlet temperature?
└─ YES ──► not a fault. Options: higher kW (check cable, Part P),
mixer shower from combi/cylinder, or compliant mains boosting
Frequently Asked Questions
The shower was fine in the summer and is weak now it is winter. What has failed?
Probably nothing. An instantaneous shower heats water as it passes, so colder mains water in winter means the unit must slow the flow to reach the same temperature. An 8.5 kW shower that gave around 5.5 litres a minute with 16°C summer mains gives around 4 litres a minute with 8°C winter mains. Measure the flow on the cold setting: if it matches or exceeds the manufacturer's figure, the shower is working as designed. The cure for a customer who wants more in winter is a higher-rated unit, with the cable and circuit checked and the work notified under Part P.
Can I fit a pump to boost an electric shower?
Not an ordinary shower pump. Electric showers are mains-fed, and a pump drawing directly from the mains at more than 12 litres per minute requires notice to and consent from the water undertaker under the Water Supply (Water Fittings) Regulations 1999; most will refuse. The compliant options are a break tank and pump set or an accumulator vessel on the supply, which store water and release it at a stable pressure. Pumped electric showers exist for gravity (tank-fed) systems, but they are a specific product, not a standard shower with a pump added.
Is 1 bar enough for an electric shower?
Just. Most manufacturers specify 1.0 bar maintained (dynamic) pressure at the inlet with 8 litres a minute available, and a few models are rated down to 0.7 bar. The important word is maintained: a house with 3 bar static that drops to 0.8 bar as soon as the shower runs has an undersized supply pipe, not a pressure problem, and the readings at the shower will be poor however good the gauge looks with everything off.
Why does the shower go weak when someone flushes the toilet?
Because the shower and the cistern share a supply pipe and the cistern's fill valve is taking a share of a limited flow. The proper installation feeds the shower directly from the rising main in 15mm copper as one of the first branches, before any other bathroom outlet. Re-piping the feed is usually a half-day job and removes the problem; a flow-restricting fill valve on the WC is a cheaper partial fix.
How much should I charge to diagnose and fix a weak electric shower?
A call-out and diagnosis including descaling the head and cleaning the filter is typically £60–£100 and under an hour. Replacing a seized stopcock or a part-closed isolation valve is £80–£180. A like-for-like shower unit replacement is £150–£300 fitted, and re-piping the feed from the rising main is similar. An upgrade to a higher kW unit with a new circuit is a two-trade job at £600–£1,200+; see electric shower installation pricing guide. Always take the flow measurements first — they justify the quote and stop the customer paying for a shower when the stopcock was the fault.
Regulations & Standards
Water Supply (Water Fittings) Regulations 1999 — fittings quality and installation requirements; notification to the water undertaker for pumps drawing more than 12 l/min from a supply pipe and for certain other installations
Water Industry Act 1991, s.65 and the Water Supply and Sewerage Services (Customer Service Standards) Regulations 2008 (Guaranteed Standards Scheme) — statutory minimum pressure and the reference level of service (10 m head at 9 l/min at the boundary stopcock)
BS EN 60335-2-35:2016+A2:2021 — Household and similar electrical appliances: particular requirements for instantaneous water heaters (electric showers)
BS 7671:2018+A2:2022 — Requirements for Electrical Installations: shower circuit design, cable sizing, RCD protection in locations containing a bath or shower (Section 701)
Building Regulations 2010, Approved Document P (2013 edition) — notifiable electrical work: new circuits and alterations in rooms containing a bath or shower
Building Regulations 2010, Approved Document G (2015 edition incorporating 2016 amendments) — cold water supply and hot water safety
BS EN 806-2:2005 and BS 8558:2015 — Specifications for installations inside buildings conveying water for human consumption: design of supply pipework
BS EN 1717:2000 — Protection against pollution of potable water: backflow protection relevant to pumped and boosted supplies
Ofwat — Guaranteed Standards Scheme — minimum pressure standards and compensation
GOV.UK — Water Supply (Water Fittings) Regulations 1999 — fittings and notification requirements
WRAS — Water Regulations Advisory Scheme guidance — approved fittings and pump notification guidance
GOV.UK — Approved Document P: Electrical safety — notifiable work in bathrooms
Electrical Safety First — Electric showers — consumer and installer guidance on shower circuits
electric shower not working — no water, no heat and tripping faults on the same unit
shower runs cold — temperature loss on combi, cylinder and mixer showers
water pressure flow rate — pressure versus flow explained, with measurement methods
mains pressure testing — static and dynamic testing at the stopcock and outlets
mains water boosting — accumulators, break tanks and the 12 l/min notification rule
electric shower installation — circuit design and cable sizing for 8.5–10.5 kW units
shower circuit cable size calculator — cable and protective device sizing by kW and run length
electric shower installation pricing guide — replacement and upgrade pricing