Two showers, one cylinder, and only one of them works
The clearest demonstration of this whole subject sits in thousands of ordinary two-storey houses. Downstairs cloakroom shower: fine. Upstairs en-suite, same cylinder, same cistern, fitted on the same day: a warm dribble that goes cold when someone runs a tap. Nothing is faulty. The two outlets are simply at different heights below the same body of stored water, and on a gravity system height is the entire power supply.
Four product families are sold to solve that and they ask completely different questions of the house. A manual or thermostatic mixer asks what pressure and flow arrive at its two inlets. An electric shower asks nothing of the hot side and everything of the cold main and the consumer unit. A pumped mixer asks whether the system is open to atmosphere, because if it is not, the pump is neither lawful nor physically sensible. A digital shower is a mixer with a remote valve, the same underlying demands and a socket. The showroom sells all four in one row, as though the choice were about finish.
What follows turns the label on the box into readings taken in the house: the head or pressure at the outlet, the flow the branch pipe will pass, the kilowatts available to heat water instantaneously, and the litres available from storage. Every product family fails on at least one of those, and no brochure tells you which.
Five arrangements, and what each one hands a shower valve
Before anything is measured, name the system. It is a two-minute inspection and it eliminates most of the catalogue on its own: look for a cold water storage cistern in the loft, an open vent pipe rising over it, an expansion vessel and a tundish beside the cylinder, or no cylinder at all.
The distinction that matters is not gas against electric. It is whether the hot side is open to atmosphere or sealed against the main. An open system has a cistern feeding the cylinder and a vent pipe over it; its pressure is set by the height of the water and it can lawfully be pumped. A sealed system — unvented cylinder or combi — is at mains pressure throughout, and there is nothing in it to pump from without pulling on the undertaker's main, which is a different job with a different rulebook. Almost every disappointing shower purchase crosses that line without knowing it exists.
One row below deserves separate mention because it is now common: a cylinder heated by an air source heat pump. It plumbs like an unvented cylinder but usually stores at 50 to 55 °C rather than 60, with a periodic pasteurisation cycle. That lower store temperature changes how many litres a shower consumes per minute, which is dealt with further down and catches out anyone sizing a cylinder from an old habit.
| Arrangement | Hot side pressure | What limits the shower | May a shower pump be fitted |
|---|---|---|---|
| Vented cylinder, cistern in the loft | Height of stored water above the outlet | Head, then friction in the branch | Yes — this is what shower pumps are built for |
| Unvented cylinder, sealed | Incoming main, less losses | Stored litres, then the main's flow | No — nothing to pump from without boosting the main |
| Combi boiler, no storage | Incoming main, less losses | The boiler's hot water kilowatts | No — a pump downstream adds nothing the burner can heat |
| Thermal store or heat bank | Incoming main through a plate or coil | Heat exchanger output and store temperature | No, on the mains-pressure secondary side |
| Heat pump cylinder | Incoming main, less losses | Stored litres at 50–55 °C, then slow reheat | No — sealed, and the store is the real limit |
On a gravity system the head is a tape measure job
Makers of gravity-rated mixers quote a minimum head measured from the base of the cold water storage cistern down to the shower outlet — not the loft floor, not the ceiling, and not the water surface, which flatters the figure and disappears as the cistern draws down. Take it vertically with a tape and write down where both ends were.
The number is usually worse than expected. A cistern sitting directly on the loft joists of a standard two-storey house has its base roughly 2.7 m above first-floor level; an overhead outlet at 2.05 m leaves 0.65 m of head, which is 0.064 bar or about 0.92 psi. That is below the minimum working pressure printed on most gravity mixers and far below anything with a thermostatic cartridge in it.
The same house measured at the ground-floor shower room tells a different story. The cistern base is now 5.4 m above that floor, the outlet at 2.05 m, the head 3.35 m — 0.329 bar, or 4.77 psi. Five times the pressure, one storey down, same cylinder. It is why a plumber's first question about a poor shower is which floor it is on, and why raising the cistern onto a stand is so effective: half a metre takes the upstairs example from 0.65 m to 1.15 m and lifts the pressure by three quarters.
That figure decides two things at once: whether the valve operates at all, which is a threshold, and how much flow it passes, which is a curve. A valve can meet its stated minimum and still deliver four litres a minute, because the minimum is where the mechanism functions rather than where it performs. Both are published in the installation instructions rather than the catalogue, and the flow-against-head table there is what decides the purchase.
Convert the vertical distance you just measured into the pressure unit the valve's paperwork uses. Most US-market fittings state a minimum in psi, and a British loft measured in metres has to be translated before the two can be compared at all.
The vertical height of the water column, in feet.
Static pressure
43.35 psi
1 ft of water at 4 °C = 62.428 lb/ft³ ÷ 144 in²/ft² ≈ 0.4335 psi. Equivalently, 2.31 ft per psi. Assumes clean water at 4 °C. For any other fluid, multiply by its specific gravity.
- Head entered
- 100 ft
- Pressure
- 43.35 psi
- Metric equivalent
- 43.35 psi
- Bar
- 2.99 bar
They open the calculator with your figures already in it
Feet of Head to PSI Calculator: 43.35 psi — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 43.35 psi — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is static head only. Friction loss through pipe, fittings, valves and the meter, and the residual pressure a fixture, shower or sprinkler head needs to work, are all excluded. Total dynamic head is static head plus those two, and sizing a pump on this figure alone is how a system ends up short at the top outlet.
- The factor assumes clean water at 4 °C and the result does not scale for any other fluid. Glycol, brine, fuel oil and hot circulating water all differ in density, so multiply by specific gravity yourself — a strong glycol charge reads low here, hot heating water reads high.
- It describes a column at rest. Velocity head, a pump running near shut-off, thermal expansion in a closed circuit and water hammer on valve closure can all push real system pressure well above this figure, and it is the peak, not the static value, that splits a fitting or lifts a relief valve.
- Only the water column's own pressure is counted. Fill pressure in a sealed heating circuit, an expansion vessel's charge, a booster set's outlet pressure or gas pressure above the water in a closed tank all add on top of this number and none of them are in it.
- This is not a pressure rating check and not a pump selection. It says nothing about the pressure class of the pipe, fittings, cylinder or appliance at the bottom of the lift, and nothing about NPSH available at a pump suction — cavitation is governed by suction conditions, not by the discharge head being converted here.
Pumps are catalogued in feet of head; gauges and fixtures are rated in psi. Converting between them is the moment a pump curve becomes a check you can carry out with a gauge, and it is where a system that looked adequate on paper turns out to be 15 psi short at the top floor.
A minimum pressure written for a market that is not yours
Shower fittings are traded internationally and their specifications are not. A British gravity mixer is rated in bar, commonly a tenth of a bar as the operating minimum and two tenths for a thermostatic model, while a mains-pressure thermostatic valve typically wants a full bar maintained while running. A North American valve tested to ASSE 1016 quotes psi instead, and its flow rating is measured at 80 psi — 5.52 bar, higher than most British houses ever see. Comparing a valve rated at 20 psi with one rated at 1.0 bar means noticing that the first wants 1.38 bar and is the more demanding of the two, not the less.
Flow figures cross markets badly for the same reason. The United States has capped showerhead flow at 2.5 gal/min measured at 80 psi since the Energy Policy Act of 1992; the EPA's WaterSense specification sets 2.0, and several states legislate 1.8. Those are 9.5, 7.6 and 6.8 L/min — but only at that test pressure. A 1.8 gal/min head on a 0.064 bar gravity supply is not a water-saving head, it is a restriction sitting on a supply that had nothing to spare. Read every flow rating together with the pressure it was measured at and treat the two as one number.
Put the bar figure from a European valve's instructions in and read it against the psi on a North American one, or against the pressure gauge you borrowed. Same physical quantity, two markets, and the comparison is only meaningful once they are in the same unit.
The pressure in bar.
Pressure
58.02 psi
1 bar = 14.5038 psi, from 1 bar = 100,000 Pa exactly. One bar lifts water roughly 10.2 m — useful when comparing a pressure rating against a static head.
- Conversion factor applied
- 14.5 psi per bar
They open the calculator with your figures already in it
Bar to PSI Calculator: 58.02 psi — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 58.02 psi — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The conversion has no way to know whether the number you typed is gauge or absolute. The two differ by about one atmosphere, roughly 14.7 psi, so on a low-pressure water or gas system converting the wrong one is a larger error than anything else on the page.
- Pressure is not flow. An appliance minimum is normally a pressure and a flow rate together, and whether the flow actually arrives depends on pipe bore, run length, fittings and height, none of which this calculation touches.
- Static and dynamic pressure are not distinguished. A standing reading with nothing running is higher than what a fitting sees under flow, and transients such as water hammer or thermal expansion in a closed system go above both. Whichever figure you enter is simply converted.
- Converting a rating is not checking one. No safety factor and no test-pressure multiplier is applied, and nothing is derated for temperature: plastic pipe, seals and hoses hold materially less hot than cold.
- Negative gauge pressure is outside the input range. The field floors at zero, so a vacuum reading such as -0.8 barg cannot be converted here.
Equipment rated in bar meeting gauges and fittings marked in psi is routine wherever European plant lands on an American site, or vice versa. Multiply by 14.5 and the job is done, though the shortcut of multiplying by fifteen overstates by three percent and should stay out of anything involving a safety margin. Two useful anchors go with this conversion. A compressor's tank pressure and its regulated output are different figures, and tools are specified against the latter. And one bar corresponds to about 10.2 metres of water head, which is the relationship that lets a pump duty, a static lift and a component's pressure rating all be compared on the same terms.
Twelve metres of branch spends the head before the shower sees it
Head measured at the outlet with nothing running is the whole budget. Friction between the cylinder and the valve is the first claim on it, and on a low-pressure system that claim routinely exceeds the entire budget. This is where a shower that satisfies every published minimum on paper still fails.
Take an unremarkable run: twelve metres from the cylinder in the airing cupboard, along a joist void and up a stud wall to the valve. The Hazen-Williams loss over it, at three flows a shower might ask for, is below. The exponent is what makes it interesting — loss rises with flow to the power 1.852, so the pipe is generous at six litres a minute and closed at twelve.
The 15 mm plastic row changes minds. A 15 mm barrier pipe is not a 15 mm copper tube: the wall is far thicker, the bore is around 11.5 mm rather than 13.6, and every push-fit joint carries an insert that narrows it further. At nine litres a minute that run wants 2.76 m of head, and the upstairs gravity shower measured above has 0.65 m in total. The pipe alone has defeated the system four times over, and no valve, head or brand changes that.
Read the other way, the same table is the fix. Twenty-two millimetre copper takes the nine-litre loss from 1.59 m to 0.23 m, a factor of nearly seven, which is why the trade habit on gravity work is 22 mm as far as the valve and 15 mm only at the last connection. Note what the table excludes: straight pipe only. Elbows, tees, stop valves and the shower's own hose all add equivalent length, and on a short run with many bends the fittings contribute as much as the pipe.
| Branch pipe | Bore | C | At 6 L/min | At 9 L/min | At 12 L/min |
|---|---|---|---|---|---|
| 15 mm copper | 13.6 mm | 130 | 0.75 m | 1.59 m | 2.71 m |
| 15 mm plastic barrier pipe | 11.5 mm | 150 | 1.30 m | 2.76 m | 4.70 m |
| 22 mm copper | 20.2 mm | 130 | 0.11 m | 0.23 m | 0.39 m |
| 22 mm plastic barrier pipe | 17.0 mm | 150 | 0.19 m | 0.41 m | 0.70 m |
Run your own branch: the measured length, the real internal bore of the pipe that is actually there, and the flow the shower is being asked for. If the answer is larger than the head you measured with the tape, the shower cannot work and nothing on the shelf will make it.
The total length of the pipe run.
The design flow rate through the pipe.
The pipe material's Hazen-Williams roughness coefficient.
The pipe's actual internal (bore) diameter.
Friction head loss
17.96 ft
They open the calculator with your figures already in it
Domestic Water Pipe Friction Loss Calculator (Hazen-Williams): 17.96 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Hazen-Williams is an empirical fit to cold water in turbulent flow, roughly 40 to 75 °F (4 to 24 °C) at velocities in the 2 to 10 ft/s (0.6 to 3 m/s) band. Outside that it drifts: glycol-charged loops, hot recirculating mains and low-flow trickles are Darcy-Weisbach problems, where viscosity enters the calculation instead of being absorbed into a single C value. The formula still returns a number for any of them; it is just not the loss you will measure.
- Head loss is one term of the pressure the fixture actually sees. What is available is the incoming main, less the static lift to the outlet, less this friction, less the fittings, and less every device in the line — a meter, a backflow preventer, a filter and a softener can take 20 to 30 psi (207 kPa) between them. A run whose friction loss looks comfortable here can still open at a shower with nothing left.
- Velocity is not reported, and velocity is often what sizes the pipe rather than head loss. Copper is normally held near 8 ft/s cold and 5 ft/s or less on hot recirculating lines, because water moving faster erodes the inside of elbows and tees over the years and makes the pipe audible in the wall. A small pipe on a short run can pass a friction check comfortably and still be the wrong size.
Sizing for the bathroom, not for the shower on its own
A branch sized for one shower and a branch sized for the room the shower is in are different pipes, and the difference decides whether the temperature holds when the basin tap opens. A single shower counted at two water supply fixture units resolves to a minimum bore of about 12.3 mm at a 2.4 m/s velocity ceiling, which 15 mm copper clears comfortably. The same calculation for a private bathroom group at six fixture units gives 16.2 mm, which 15 mm copper does not clear and 22 mm does.
That is the arithmetic behind the commonest retrofit complaint: fine on test, unusable once the household is awake. The pipe was sized for the fixture rather than for the fixtures. Where the branch also feeds a bath — a single sustained draw that no diversity assumption smooths away — the group figure is the only honest input.
Fixture unit values are a code question, not a physics question. The International Plumbing Code and the Uniform Plumbing Code publish different tables; BS EN 806-3 does the equivalent job through loading units on a different curve, with BS 8558 carrying design flow rates for individual fittings underneath it. Use whichever your work is signed off against, and where two answers straddle a size take the larger — the difference on a domestic branch is trivial against opening the wall again.
Enter the fixture units for everything the branch serves, not just the shower. This sizes on velocity from a diversified flow and says so plainly: it does not compute the pressure available at the outlet, which is the previous section's job, and both have to pass.
The sum of water supply fixture unit values for every fixture served by this pipe segment.
A calibration constant that scales the square-root approximation to your building's fixture mix.
The maximum velocity allowed in the pipe to limit noise and water hammer risk.
The pressure available where the supply enters the building.
The length of pipe from the point of supply to the furthest fixture, following the run.
How much extra length is added to represent the elbows, tees and valves in the run.
The height of the highest fixture above the point of supply.
The pressure the metering assembly loses at your design flow.
The flow pressure the furthest fixture needs at its inlet to work properly.
Which Hazen-Williams roughness coefficient the friction calculation uses.
Minimum pipe diameter
0.862 in
Velocity governs this run. The diversified demand would fit through a smaller bore on pressure alone, but sustained velocity above the ceiling erodes the pipe and is audible through the building, so the velocity limit is the binding one and the pressure budget has room to spare.
- Estimated peak demand
- 14.31 GPM
- Diameter set by the velocity ceiling
- 0.86 in
- Diameter set by the pressure budget
- 0.77 in
- Pressure lost to static lift
- 58,286.8 Pa
- Pressure lost to friction at this diameter
- 122,120.36 Pa
- Pressure left at the furthest fixture
- 198,803.83 Pa
- Equivalent length used for friction
- 147 ft
They open the calculator with your figures already in it
Domestic Water Pipe Sizing by Fixture Unit Calculator: 0.8616 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.862 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Fixture-unit methods work because fixtures are used intermittently and rarely together. That diversity is the whole basis of the sizing, and it is why the pipe is far smaller than the sum of the fixture flows would suggest.
- Hunter's curve is old, and its age biases this in one direction. It was derived in the 1940s from fixtures using several times the water modern ones do, and it is well documented as oversizing systems built with low-flow fittings — the demand factor is the handle for that, and lowering it is a judgement with consequences rather than a correction.
- Excludes continuous-demand loads such as irrigation and hose bibs, which do not benefit from diversity and are added at their full flow.
- The friction calculation assumes one diameter for the whole run. A real system steps down as branches leave it, so the true loss is somewhere between this figure and the loss of a system sized entirely at the smallest branch — this is the screening answer, and a segment-by-segment calculation is the design one.
- Fitting losses are an allowance, not a count. Each elbow, tee and valve has its own equivalent length and a push-fit insert reduces the bore as well, so the percentage is a stand-in for a takeoff nobody has done. On a run with many tight bends it will understate the loss.
- The meter figure is yours to supply and this page cannot check it. Meter loss rises steeply as flow approaches the meter's rating and a backflow preventer or pressure-reducing valve often costs more than the meter itself; entering a nominal figure where the real assembly loses far more is the quiet way this calculation goes wrong.
- Velocity limits govern independently of flow: sustained velocities above roughly 2.4 m/s (8 ft/s) cause erosion and noise regardless of what the fixture units allow.
- The result is an internal diameter, not a pipe size. Nominal designations are not bores — copper, PEX and CPVC of the same nominal size have materially different internal diameters, and a push-fit system loses more at every joint — so take the next size up whose actual bore meets this figure in the material you are using.
What a pump is worth, and the systems it may not touch
On a vented system a shower pump is transformative, and the reason is worth seeing in the right units. A unit sold as 1.5 bar stands in for a cistern 15.3 m above the outlet — a five-storey column of water where the loft was giving two thirds of a metre. Two cautions come with the rating: it is generally a maximum delivered at low flow, so the maker's duty curve says what actually arrives at nine or twelve litres a minute, and the pump adds to what it is given rather than replacing it. On the end of that 15 mm plastic branch, much of the gain vanishes into friction that did not exist at the old flow.
Type matters as much as rating. A twin-impeller pump boosts hot and cold together, which keeps a manual mixer balanced; a single-impeller pump boosts one side and unbalances any valve not rated for unequal supplies. A positive-head pump starts on flow and needs a stated minimum of water above the outlet, commonly around 600 mm and always model-specific; a negative-head or universal pump carries its own sensing arrangement and starts with little or no head at all. Fitting a positive-head unit under a cistern barely above the shower is the classic false economy: cheaper, and it will not start.
The hot connection ruins otherwise correct installations. Drawing the feed from the vent pipe or a tee near the cylinder crown pulls air into the impeller, and an air-locked pump is loud, hot and short-lived. Take it from a Surrey, Warix or Essex flange so the draw comes from stored water below the air, and mount the pump low, on anti-vibration feet, with flexible connections either side.
Then the prohibition the whole purchase turns on. A shower pump may not be fitted to a combi, an unvented cylinder or the mains-pressure side of a thermal store. With no vented reservoir to draw from it would pull directly on the undertaker's supply pipe, and in England and Wales the Water Supply (Water Fittings) Regulations 1999 make a pump drawing more than twelve litres a minute from a supply pipe, directly or indirectly, notifiable before installation, with the undertaker entitled to refuse; Scotland and Northern Ireland have equivalent provisions. Beyond the paperwork it does not work, because a combi's flow is capped by its burner rather than by pressure. Where a mains-pressure system genuinely lacks flow the answer is an accumulator, a break tank with a booster set, or a larger service pipe — a whole-house problem, not a shower purchase.
Turn a pump's pressure rating into the column of water it is standing in for. Doing it in head rather than bar makes the comparison with the tape measure reading direct, and makes it obvious how much of the gain the branch pipe is about to take back.
The water pressure in PSI.
Equivalent feet of head
115.3 ft of head
They open the calculator with your figures already in it
PSI to Feet of Head Calculator: 115 ft of head — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Head is counted from wherever the gauge is, not from the ground. A basement tank reading 50 psi converts to 115 ft, but a fixture 25 ft above that gauge has already spent 10.8 psi of it before a tap opens and sits at about 39 psi at rest. Read the pressure and the point you care about as a pair, or the converted figure describes a place nobody is showering in.
- A well or booster system does not hold one pressure, so a single converted figure describes one moment of the cycle. A 40/60 switch runs everything between 92 and 138 ft of head, and a sprinkler zone or top-floor shower that performs at cut-out can fade badly just before the pump restarts. Convert the cut-in pressure, not the cut-out one, when what you want is the worst the system will do.
Kilowatts are litres a minute wearing a different coat
Anything that heats water as it passes — a combi's hot water circuit, an electric shower, an instantaneous heater — obeys one equation, and that equation settles most shower arguments. Raising water by one kelvin at one litre a minute takes 0.0698 kW. Multiply by the flow you want and the rise you need, and the required output falls out with no room for opinion.
The rise is where seasons enter. Mains water arrives at perhaps 18 °C in August and 5 °C in February, so a 40 °C shower needs a 22 K rise in one and 35 K in the other. That difference alone explains the electric shower that was acceptable when it was fitted in July and unbearable by Christmas: the appliance did not change, the water did.
Read the table as arithmetic, not as a product ranking. A 9.5 kW electric shower delivers under four litres a minute in winter — barely a gallon a minute, well under even the 1.8 gal/min ceiling the strictest US states impose, so the flow cap never binds, and about half what a mixer on a decent supply gives. That is not a defect; it is what 9.5 kW does to cold water, and the only cures are more kilowatts or a smaller rise.
The combi rows carry a caveat in the other direction. Makers quote hot water performance at a stated rise, frequently 30 K, which is why a 30 kW combi is advertised at around 14 L/min and gives 12.3 in February. The summer figures are a ceiling rather than a promise, since most combis carry an internal restrictor and all are limited by what the main passes. And the decisive row is the one that is not there: two showers at nine litres a minute each in winter needs 44 kW of instantaneous output, which no domestic combi provides. Two simultaneous showers is a storage problem, and no combi upgrade solves it.
| Heat source | Winter, 35 K rise | Summer, 22 K rise |
|---|---|---|
| 8.5 kW electric shower | 3.5 L/min (0.9 gal/min) | 5.5 L/min (1.5 gal/min) |
| 9.5 kW electric shower | 3.9 L/min (1.0 gal/min) | 6.2 L/min (1.6 gal/min) |
| 10.5 kW electric shower | 4.3 L/min (1.1 gal/min) | 6.8 L/min (1.8 gal/min) |
| 24 kW combi hot water output | 9.8 L/min (2.6 gal/min) | 15.6 L/min (4.1 gal/min) |
| 30 kW combi hot water output | 12.3 L/min (3.2 gal/min) | 19.5 L/min (5.2 gal/min) |
| 35 kW combi hot water output | 14.3 L/min (3.8 gal/min) | 22.8 L/min (6.0 gal/min) |
Enter the simultaneous flow the household actually wants and the winter mains temperature rather than the summer one, and the required output is the figure to hold every instantaneous appliance against. Size on February and the shower disappoints nobody in August.
Total flow of the fixtures used at once.
Cold mains temperature in winter.
Temperature at the outlet.
Heater output required
25.35 kW
Exact thermodynamics for the stated flow and rise. Real units are rated at a specific rise, so compare against the manufacturer's flow-at-rise table rather than a headline kW.
- Temperature rise
- 32 K
- BTU/h
- 86,508.62 BTU/h
- Flow in US GPM
- 3 GPM
- Flow in gallons per hour
- 180 GPH
They open the calculator with your figures already in it
Tankless Water Heater Sizing Calculator: 25.35 kW — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 25.35 kW — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Gas units need adequate supply pipe and flue; an undersized gas run limits output regardless of the appliance rating.
- Electric units of this size often need a dedicated high-current supply that many domestic boards cannot provide.
- Assumes no simultaneous demand beyond what you entered. A tankless unit does not run out of hot water, but it does run out of flow.
The electric shower's other supply
An electric shower is chosen twice — once as a plumbing appliance and once as a load on the consumer unit — and the second choice is what turns a like-for-like replacement into a project. At 230 V the design current is about 37 A for an 8.5 kW unit, 41 A for a 9.5 and 46 A for a 10.5. The step between two adjacent model sizes frequently crosses a cable size once the real installation method is applied.
Cable and protective device follow from BS 7671 with the installation reference method, grouping, ambient temperature and any thermal insulation the cable passes through all applied; Appendix 4 carries the tables, and a cable buried in loft insulation is derated hard enough to change the answer. Section 701, covering locations containing a bath or shower, brings 30 mA residual current protection and the rules on zones and supplementary bonding, and the unit needs a double-pole means of isolation — conventionally a ceiling pull-cord inside the room or a switch outside the door. None of it appears on the packaging.
North American readers will find this largely academic, since individual point-of-use electric showers are effectively absent from that market. The equivalent is a whole-house electric tankless unit, and at 27 kW on 240 V that is roughly 112 A of connected load, typically split across several double-pole circuits, with the appliance governed by NFPA 70 Article 422 and the branch circuits by Article 210. The physics above is identical; what differs is that the load becomes a service capacity question rather than a circuit question.
How many showers are actually in the cylinder
Stored systems fail differently from instantaneous ones. They do not throttle the flow — they deliver exactly what the valve asks for and then run out, which is a more annoying failure and a much easier one to predict. The prediction has two parts: how much stored water one shower consumes, and how much of the cylinder is genuinely usable.
A shower blends stored hot with incoming cold, so the stored litres per minute depend on the store temperature. At 40 °C delivered from a 60 °C store with 10 °C cold, the hot fraction is 0.60 and a nine-litre shower draws 5.4 litres a minute of stored water. Drop the store to 50 °C, normal for a heat pump cylinder, and the fraction rises to 0.75: the same shower now eats 6.8 litres a minute of a store that took far longer to heat. That is why a cylinder sized on a boiler-era habit comes up short after a heat pump retrofit.
Then subtract what you cannot have. A cylinder does not give up its nameplate volume at usable temperature — the thermocline arrives and the outlet cools well before the tank is empty. Treat 70 to 80 per cent as a planning fraction and confirm it against the maker's own peak-draw or first-hour figure. On a 150 litre cylinder at 60 °C that is around 120 usable litres, or about 2.8 eight-minute showers; at a 50 °C store, 2.2. Whether that is enough is a household question, and it should be asked out loud before a cylinder is ordered.
The calculator below is a starting bracket and honest about being one: a household-size rule of thumb in US gallons, built around a storage heater at a typical American set point, knowing nothing about your store temperature, your shower flow or whether two bathrooms get used at once. Take its range, convert it, then correct it with the table — the correction is frequently a size.
| Store temperature | Hot fraction | Stored litres per minute | Per 8-minute shower | Showers from 120 usable litres |
|---|---|---|---|---|
| 60 °C | 0.60 | 5.4 L/min | 43 L | 2.8 |
| 55 °C | 0.67 | 6.0 L/min | 48 L | 2.5 |
| 50 °C | 0.75 | 6.8 L/min | 54 L | 2.2 |
| 45 °C | 0.86 | 7.7 L/min | 62 L | 1.9 |
Use this to get the household-size bracket on the page, then argue with it using the table above. A cylinder chosen from occupancy alone, with no store temperature and no shower flow behind it, is the commonest reason a new bathroom runs cold in its second year.
How many people regularly use hot water in the home.
Recommended tank size
57.5 gallons (recommended)
This is a rough rule-of-thumb range. Homes with multiple bathrooms used simultaneously, soaking tubs, or high-flow fixtures should size toward the higher end or consider a tankless system sized by flow rate instead.
- Recommended range, low
- 50 gal
- Recommended range, high
- 65 gal
They open the calculator with your figures already in it
Water Heater Sizing Calculator: 57.5 gallons (recommended) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Sizes stored volume only — the heat source is not in the model. A gas, electric-resistance and heat-pump tank of the same nominal size recover at very different rates, and an electric or heat-pump cylinder usually needs more volume than these ranges give to cover the same back-to-back demand.
- Headcount stands in for peak-hour demand rather than measuring it. No fixture count, bathroom count, simultaneous-use scenario, shower flow rate or duration, and no bath or soaking tub enters the calculation, so two houses with the same number of occupants and completely different fixtures get the same answer.
- No water temperatures are entered. Incoming mains temperature falls in winter, storage set point and thermostatic blending change how much usable hot water a given tank delivers, and stratification breaks down before a tank is nominally empty — this is nameplate volume, not what you will draw at temperature.
- The bands stop at five occupants and describe a conventional storage heater. A household of ten returns the same range as a household of five, and heat-pump cylinders, solar-preheated stores and instantaneous units are sized on different grounds entirely.
- This is not an installation or compliance check. Tank size says nothing about the relief and expansion arrangements a stored hot-water system requires, scald protection at outlets, legionella control in storage and distribution, or whether the gas, flue or electrical supply can serve the appliance — those come from the code in force where the work is done and from the installer competence that code demands.
The hour between the first shower and the second
Storage capacity answers how many showers back to back. Recovery answers how long until the next one, and on a morning with three people and one bathroom that is the number that governs the household. Reheating 150 litres from 10 to 60 °C takes 8.7 kWh into the water, and the time depends entirely on what delivers it: about 35 minutes from a 15 kW boiler coil, about 87 from a 6 kW heat pump into the coil, and about 174 from a 3 kW immersion heater.
That spread is the case for matching the heat source to the household's habits rather than to the cylinder's label. A boiler-fed indirect coil recovers fast enough that the cylinder can be modest. A heat pump recovers slowly, and at its worst efficiency at the top of the range, so the same household needs a larger store and a real reheat schedule. An immersion heater as sole source is a fallback, not a plan; used as one it decides when people can shower.
Read the calculator with its assumption visible. It applies a 0.75 recovery efficiency typical of a flued gas storage heater, so for an immersion element — where essentially all the input reaches the water — its figure runs about a third longer than the physics. That is a sound upper bound rather than a wrong answer, and an upper bound is the right thing to plan a morning around. For a boiler-fed coil the limit is usually the coil's own heat transfer rating rather than the boiler's output, so take that from the cylinder's data plate.
Enter the cylinder volume, the winter cold feed temperature, the store temperature you actually intend to hold and the real heat input at the coil or element. The answer is how long the second person waits, which is the number households care about and specifications rarely state.
The rated storage capacity of your water heater, in the unit shown.
The temperature of the cold water feeding the tank.
The thermostat setting on the water heater.
The heater's rated power input, from its data plate, in the unit shown.
How much of the input energy actually ends up in the water.
Recovery time
58.5 minutes
This estimates recovery from a fully-drained tank using the nameplate input rating — real-world recovery is somewhat slower due to standby losses and burner cycling.
- Recovery rate
- 0.86 gal/min
They open the calculator with your figures already in it
Water Heater Recovery Time Calculator: 58.48 minutes — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- This is the time to reheat the entire tank contents from your incoming temperature up to the thermostat setting. It is not the time until you have usable hot water again — a tank stratifies, so the top of the cylinder is back up to temperature well before the whole volume is.
- It assumes nothing is drawn off while the tank recovers. Any continuing draw replaces heated water with cold and pushes the time out; a draw that outruns the recovery rate means the tank never catches up at all.
- Nameplate input is taken at face value, with no allowance for gas derating at altitude, an electric element running below its rated supply voltage, or scale on the element and fouling on the flue side of an older heater. Most residential dual-element electric heaters energise only one element at a time, so enter one element's rating rather than the sum of both.
- Recovery efficiency stops at 1.0, so this cannot represent a heat pump water heater, which moves more heat than it draws in electricity and whose output falls as the surrounding air gets colder. For an indirect coil the figure you enter is the heat reaching the coil — a boiler that cannot sustain that output, or a coil undersized for it, recovers more slowly than this predicts.
- Nothing here checks the target temperature for safety. Stored water temperature is a trade-off between scald risk at the outlet and controlling bacterial growth in the tank, and this calculator will model any setting in its range without comment — set it against the requirements that apply to your installation, and fit a thermostatic mixing valve where outlet temperature has to be limited.
What the valve is approved to do when the temperature moves
Two things move under a running shower: the pressure at one inlet, and the temperature of the hot supply. Valves are approved against one or both, and the difference is not a marketing distinction. A pressure-balancing valve — ASSE 1016 Type P — senses a change in the ratio of inlet pressures and closes the other side down to match, so a flushing toilet no longer scalds anyone; it does nothing about a cylinder running cold. A thermostatic valve, Type T, holds the outlet against both, and Type TP combines them. The International Plumbing Code at Section 424 and the International Residential Code at Section P2708 require a listed valve of one of these types on individual showers, capped at 120 °F.
The European route to the same protection is a different pair of documents, and matching the valve to the supply is where installations go wrong. BS EN 1111 covers thermostatic mixing valves at PN 10, meaning systems at mains-type pressure; BS EN 1287 covers low pressure thermostatic mixing valves, and that is the standard belonging on a gravity system. Putting the higher-pressure valve on a 0.064 bar loft supply asks it to do something it was never tested to do. Where the setting must be verified and maintained, the TMV2 and TMV3 schemes apply, with BS 7942 covering Type 3 valves in care establishments.
In England and Wales, Approved Document G adds a requirement easy to miss because it does not concern the shower. Requirement G3 limits the hot water supply to a bath in a new dwelling to 48 °C, normally through an in-line blending valve, so a bath-shower mixer can bring that provision with it. G3 also governs unvented hot water storage: the competent-person regime, and the discharge from the temperature and pressure relief valve through a tundish to a safe visible point. Fitting an unvented cylinder to feed a new shower is notifiable work, not a preference.
Last, the backflow point flexible hoses create. Under Schedule 2 of the Water Supply (Water Fittings) Regulations 1999, a shower head on a hose able to reach below the spill-over level of a bath, basin or bidet is a contamination risk; the answer is either a restraint making that physically impossible or check valve protection on the supplies to the mixer, with BS EN 1717 setting out the principles. Five minutes at first fix, expensive afterwards.
The half hour that settles it
Everything above collapses into one visit with a tape, a jug, a watch and a pressure gauge. Take these in order and the product decision makes itself before anybody opens a catalogue.
- Identify the system: cistern and vent pipe, expansion vessel and tundish, or a combi on the wall. This alone rules a pump in or out.
- On a gravity system, tape the vertical distance from the cistern base to the shower outlet and note where you held both ends.
- On a mains-fed system, take static pressure at the nearest cold outlet with everything shut, then flow into a measured jug against a watch with the tap fully open.
- Read the data plate: the combi's hot water kilowatt rating and minimum inlet pressure, or the cylinder's volume, coil rating and store temperature.
- Trace the branch that will feed the valve — length, material, bore, fittings — and establish whether anything else tees off it.
- Get the winter cold water temperature for the area rather than the temperature on the day, because that is what sizes anything instantaneous.
- If an electric shower is a candidate, look at the consumer unit, the existing circuit and the route a new cable would take before pricing the appliance.
- Take the valve's minimum maintained pressure and flow-against-head table from its installation instructions, not the catalogue, and hold your readings against it.
What to have written down before the shower is ordered
Seven measurements and two data plates. With them the product family chooses itself and the argument is about finish. Without them, the box in the hall is a guess.
- System type, named out loud — Vented, unvented, combi, thermal store or heat pump cylinder — the one fact that decides whether a pump is even lawful.
- Head from cistern base to shower outlet — Vertical, with a tape, on gravity systems. Metres converted to the unit the valve's paperwork is written in.
- Static pressure and measured flow at the cold outlet — On any mains-fed system. Pressure standing still and flow into a jug against a watch, recorded as a pair.
- Branch pipe length, material and true internal bore — Fifteen millimetre plastic and fifteen millimetre copper are different pipes, and the bore is what the friction calculation uses.
- Hot water kilowatts, or cylinder litres and coil rating — From the data plate. The first sizes an instantaneous shower, the second sizes how many showers happen before it runs out.
- Winter mains water temperature for the area — Not the temperature on the day of the survey. February sizes the appliance; August only flatters it.
- The valve's own minimum maintained pressure — Copied from the installation instructions, with the flow-against-head table beside it, and compared to the readings above.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.