What is gravity head and how do I measure it?

Gravity head is the vertical distance from the bottom of your cold water storage tank to the shower outlet. Measure from the base of the tank in the loft to the shower head position. Every 1 metre of head provides approximately 0.1 bar of water pressure. A typical UK loft tank gives 3-5m head (0.3-0.5 bar) to a first-floor shower, which is usually insufficient for a good shower experience.

What is the difference between positive and negative head pumps?

A positive head pump activates when water flows past it by gravity — it needs at least 600mm of gravity head between the tank and the pump inlet. A negative head pump has a built-in pressure switch and can pull water up to it, making it suitable when the shower is at or above the tank level (e.g. loft conversions). Universal pumps work in both situations and are the safest choice when head is borderline.

Do I need a single or twin impeller pump?

A twin impeller pump boosts both hot and cold water supplies, which is essential for mixer showers (both thermostatic and manual). A single impeller only boosts one supply. Twin impeller pumps are recommended in almost all situations as they maintain balanced pressure across both supplies, giving more consistent temperature control.

What bar rating pump do I need?

For a standard mixer shower, a 1.5 bar pump is usually sufficient. For rain/drencher heads (200mm+), use at least 2.0 bar. For multi-head systems with body jets, use 2.0-3.0 bar. Higher bar pumps are noisier and more expensive, so don't over-specify. Common UK brands include Stuart Turner (Monsoon), Salamander and Grundfos, all available in 1.5, 2.0 and 3.0 bar ratings.

Can I use a pump with a combi boiler?

No. You must NOT connect a shower pump to a combination boiler or an unvented (pressurised) hot water system. Pumps are only designed for use with gravity-fed (vented) hot water systems with a cold water storage tank in the loft and a hot water cylinder. Using a pump with a combi or unvented system can cause dangerous over-pressurisation and will void the pump warranty.

TradeCalculator.co.uk

Shower Pump Calculator — Pressure, Type & Sizing

Calculate the correct shower pump type and bar rating based on gravity head, required outlet pressure, pipe restriction and shower type.

Gravity Head

Vertical distance: tank base to shower outlet

Required Pressure at Outlet

1.0 bar min for decent shower, 1.5 for good flow

Shower TypeAffects minimum pressure requirement

Pipe Size22mm reduces restriction on long runs

Pipe Run Length

Total pipe run from pump to shower

Number of Elbows

Each elbow adds restriction

Number of Tees

Each tee adds restriction

Twin impeller (boosts hot and cold)

Required for mixer showers — boosts both supplies equally

Pump Price (optional)

Price for the pump unit

How We Calculate This

This calculator determines the correct pump type and bar rating based on your gravity head, required outlet pressure, pipe configuration and shower type.

The formula

Gravity pressure (bar) = Head (mm) ÷ 10,000

Pressure deficit = Required bar − Gravity pressure

Pump rating = Pressure deficit + Pipe restriction losses

For 3000mm gravity head (0.3 bar) with 1.5 bar required: deficit = 1.2 bar. With 5m effective pipe run adding ~0.15 bar loss: pump needed = 1.35 bar minimum → select 1.5 bar pump.