Showers: Types, Pumps and Balanced Supplies (Level 2 Hot Water Guide With 16-Question Mock Test)

Showers get a learning outcome of their own in this unit, worth 3 questions, and they turn up again in the components outcome. They are also the fitting customers complain about more than any other — and almost every complaint comes down to one idea: balance.

For the rest of the unit see the hot water revision guide.

The three types of shower

The specification names three, and they differ by where the hot water comes from.

Gravity

Hot from the cylinder, cold from the cold water storage cistern, mixed at a shower valve. Both supplies come from the same cistern in the loft, so both are at the same pressure — which is exactly what a mixer needs.

The limit is head. Pressure comes only from the height of the cistern above the shower head, so a loft cistern and a ground-floor shower give a good one, while a cistern barely above the bathroom ceiling gives a dribble. A minimum of 1 metre of head is generally needed for a gravity shower to work at all.

Pumped

A gravity system with a booster pump added, to overcome exactly that problem. Covered in full below.

Electric

Takes cold water only, straight off the mains, and heats it instantaneously as it passes through. There is no hot supply and no cylinder involved at all.

That makes it independent of the hot water system, which is its great advantage — it works when the cylinder is empty and it can be fitted where there is no hot supply. The trade-off is flow rate: it can only heat so much water per second, so an electric shower delivers less water than a good pumped one, and delivers less still in winter when the incoming mains is colder.

The four installation arrangements

The chapter sets out four ways a shower valve is supplied, and the difference between them is where the two supplies come from and whether they are balanced.

Why balanced supplies matter

This is the criterion behind most of the marks, and it is worth understanding rather than memorising.

A mixer valve blends two streams. If one arrives at a much higher pressure than the other, it dominates — and the temperature at the head is set by whichever supply is winning rather than by where you set the control.

The consequence everybody has experienced: somebody flushes a WC, the cold pressure drops for a moment, and the shower runs hot. On a badly unbalanced system that is not just unpleasant, it is a scalding risk, and it is why thermostatic valves exist.

Balance means equal pressure and equal flow rate on both supplies, and the way to get it is to take both from the same source:

• Gravity system — hot via the cylinder and cold direct, both from the same cistern. Balanced by design.
• Unvented system — both effectively at mains pressure. But the cylinder runs slightly below mains, so the cold for the shower must be taken after the pressure-reducing valve and before the single check valve. That is what makes the two match, and getting it wrong is a classic fault.
• Combi — hot at mains pressure but limited in flow, cold at full mains. Genuinely unbalanced, which is why a combi shower needs a thermostatic or pressure-balancing valve.

One rule that follows: never take the cold for a shower from the mains and the hot from a cistern-fed cylinder. That is the worst case — two or three bar against a metre of head — and the cold will win every time.

Shower pumps

Two types, and the difference is where they sit relative to the mixing valve.

Twin impeller inlet pump

Fitted before the valve. One electric motor drives two impellers, one boosting the hot supply and one the cold, so both arrive at the valve boosted and still balanced. The water is mixed after the pump.

Single impeller outlet pump

Fitted after the valve. The water is mixed first at cistern pressure, then a single impeller boosts the mixed water to the head. Common with concealed valves and large "deluge" heads.

Head requirements

A pump changes the head rules:

• Without a pump, a gravity shower needs about 1 metre of head.
• With a pump, that 1 metre is no longer needed — but a minimum of 150mm is, because the flow switch that starts the pump has to be lifted by flowing water.

Positive head means water reaches the shower under gravity on its own; the pump senses the flow and starts automatically to boost it. Negative head means the cistern is below the shower outlet, so nothing flows at all and no flow switch can be lifted. That needs a specific negative head pump, started another way — typically a pull-cord switch.

The rule that gets tested

Shower pumps may not be fitted to a mains cold water supply. The Water Supply (Water Fittings) Regulations 1999 prohibit it, except where the water undertaker has given specific permission — because pumping the mains would draw down the pressure for every property on it.

Connecting a pump to the cylinder

A detail with a real failure mode behind it, which is why it turns up in questions.

Air collects at the top of a hot water cylinder. Take the pump's hot supply from an ordinary draw-off and that air goes with it, gets trapped around the impeller, and the pump eventually fails.

Two fittings solve it by taking the hot directly from the cylinder rather than from the draw-off:

• Surrey flange
• Essex flange

Both give an air-free connection. The vent connection is made at 30–60° so air can continue up the open vent and dissipate over the cistern, and the shower's hot connection is taken at 90° to that angled connection.

Two more installation points:

• The cold feed to the cylinder connects higher than the cold take-off for the shower, so that if the cistern runs low the hot runs out first. A shower that suddenly goes cold is a nuisance; one that suddenly goes hot is a scald.
• Run 22mm as far as practical before reducing to 15mm, to keep the flow up.

Shower controls

Three named types, and they are really two ideas plus a pressure rating.

• Manual. The user sets the blend by hand. Cheap and simple, and it has no way of reacting if a supply pressure changes — so the temperature moves when somebody runs a tap elsewhere.
• Thermostatic. A thermostatic element inside the valve senses the outlet temperature and adjusts the blend continuously to hold it. If the cold fails entirely it shuts the valve down rather than delivering full hot. This is what makes it the answer to any scalding question.
• Low or high pressure. Not a third mechanism but a rating. A low pressure valve is designed for gravity systems and will barely flow on mains; a high pressure valve is designed for mains or unvented and will barely flow on gravity. Fitting the wrong one is one of the commonest reasons a new shower disappoints.

The exam framing: manual valves react to nothing; thermostatic valves react to temperature; the pressure rating decides whether the valve suits the system at all.

Common exam traps

Trap 1: twin impeller before, single impeller after. Twin impeller boosts hot and cold separately on the way in; single impeller boosts already-mixed water on the way out.

Trap 2: shower pumps are not permitted on mains cold water. Prohibited by the Water Regulations except by permission of the water undertaker.

Trap 3: 1 metre without a pump, 150mm with one. The 150mm is to lift the flow switch, not to drive the shower.

Trap 4: negative head means the cistern is below the outlet. No flow at all, so a standard flow switch cannot start — it needs a negative head pump.

Trap 5: Surrey and Essex flanges are about air, not pressure. They give an air-free hot connection so the pump impeller does not become airlocked.

Trap 6: electric showers take cold only. No hot supply, no cylinder — which is why they still work when the cylinder is cold.

Quick revision summary

Before the mock test, eight things you need to be able to produce from memory:

1. Three types: gravity, pumped, electric — and electric takes cold only
2. Balance means equal pressure and flow on both supplies; whichever supply dominates sets the temperature
3. Never mains cold with cistern-fed hot — the worst possible imbalance
4. Unvented: take the shower's cold after the pressure-reducing valve and before the single check valve
5. Twin impeller fits before the valve (boosts hot and cold); single impeller fits after it (boosts mixed water)
6. Head: 1 metre gravity, 150mm minimum with a pump; negative head needs a negative head pump
7. Surrey or Essex flange gives an air-free hot connection so the impeller does not airlock
8. Controls: manual, thermostatic (shuts down if the cold fails), and a low or high pressure rating to match the system

How PlumbMate puts this into practice

Shower configurations are visual — which is why the app teaches them as labelling boards rather than as text.

• Label the systems. Drag-and-drop boards for the gravity system, single and twin impeller pump arrangements, and the mixed-supply configurations.
• Wrong answers are logged and resurface more frequently in later sessions.
• The 3× rule. Three correct answers before a question clears.
• Explanations on every question, like the ones above.