A mixer shower on a gravity-fed system runs weak and swings between hot and cold because its two supplies are not equal. The hot side arrives from the cylinder at tank pressure, while the cold side is often plumbed to the mains at a far higher pressure, so any change in the house shifts the blend. A balanced supply means both sides come from the same source at the same height, which is what keeps the mix steady. This page explains where the pressure actually comes from, what balanced means in manufacturers' own words, which valves suit low pressure, and which parts of the work are an engineer's job.
Why does a mixer shower on a gravity-fed system go weak and swing hot to cold?
A gravity-fed system makes pressure from height alone, and a mixer shower feels that immediately. The cold water cistern in the loft feeds the hot cylinder, so the only push behind your hot water is the drop from the loft down to the shower. Heatrae Sadia states the conversion plainly: a 10 m difference in height equals 1 bar. A first-floor shower sits only a short drop below the cistern, so the hot side arrives gently.
Temperature swings come from a second problem. A mixer blends two flows, not two temperatures. If the cold side is fed from the mains and the hot side from the cylinder, the two sides arrive at very different pressures. Flush a WC or open the kitchen tap, the pressure on one side moves, the blend shifts and the shower head runs hot or cold. The same effect is described in our guide on a shower going cold when a tap is used.
What does a balanced cold supply mean on a tank-fed mixer shower?
Balanced means both sides of the mixer are fed from the same pressure source, at the same height. On a tank-fed system that source is the cold water cistern, not the mains. Both sides then rise and fall together, so the blend holds when another outlet draws water.
Manufacturers set out where the balanced cold branch is taken from. Heatrae Sadia notes that balanced hot and cold supplies are an advantage in many mixer showers, and on its Coral Aquanox thermal store the balanced cold is taken between the cold water inlet and the cylinder's cold feed connection. On a Megaflo Eco Plus unvented cylinder the balanced cold is teed off after the pressure reducing valve and before the expansion valve. Both are changes to the pipework around the cylinder, so both are work for an installer rather than a householder.
A mixer shower cannot hold a steady temperature while one side comes from the cylinder and the other from the mains. Either the cold side is re-routed to match the hot side, or both sides run at mains pressure from a suitable cylinder. Which route fits your home depends on the pipework already in it, and that is what a free home survey looks at before any written fixed price is given.
Shower pump or balanced cold supply: which one lifts pressure and which one steadies the temperature?
A shower pump and a balanced cold supply answer two different complaints: one adds pressure, the other keeps the two sides equal. A shower that is both weak and unsteady has two symptoms, and they do not share one cure.
| Criterion | Shower pump | Balanced cold supply |
|---|---|---|
| What it changes | Raises flow and pressure on the hot and cold side by pumping from the cylinder and the cold cistern. | Puts both sides on the same source at the same height, so the mix ratio stays where you set it. |
| Effect on temperature swings | Pressure rises, but swings can remain where the hot and cold sides still come from different sources. | Removes the usual cause of swings, because both sides move together when another outlet opens. |
| What the house must have | A hot water cylinder to draw from, plus space for the pump. A combi boiler has no cylinder, so no pump can be fitted. | A cold run from the cistern, or on an unvented cylinder a tee after the pressure reducing valve. |
| Who carries out the work | Pipework and connections are installer work, not a householder job. | Altering supply pipework is installer work, and the backflow protection has to suit the fittings regulations. |
Viessmann confirms that an electric pump can be used on a gravity-fed system to hold shower pressure. Vaillant and Viessmann are equally clear that a shower pump cannot be hung on a combi boiler, because a pump is connected to a hot water cylinder and a combi draws straight from the mains instead.
How much height do you need between the loft cistern and the shower head?
Height is the only thing making pressure on a gravity-fed shower. Heatrae Sadia gives the rule: a 10 m difference in height equals 1 bar, so roughly one metre of drop gives about 0.1 bar. A cistern two or three metres above the shower head therefore delivers a fraction of a bar. Viessmann makes the same point for gravity-fed homes: pressure is poorest at the top of the house, close to the tank, and better further down.
A first-floor bathroom can feel weak while a ground-floor tap runs well. Outlets above the cylinder have less head behind them, not more. Shower products carry their own pressure limits, and one Heatrae Sadia shower manual states a minimum of 0.1 MPa and a maximum of 0.7 MPa. As a comparison, a Megaflo Eco Plus unvented cylinder expects at least 0.15 MPa, which is 1.5 bar, and 20 litres per minute from the mains.
Which mixer valve suits a low-pressure gravity-fed shower?
Low pressure thermostatic mixing valves are a separate product family with their own standard. Approved Document G, which applies in England, refers to BS EN 1111 for thermostatic mixing valves at PN 10 and to BS EN 1287 for low pressure thermostatic mixing valves. A valve designed for the higher-pressure family may never see the pressure it needs on a tank feed. Matching the valve to the system is therefore part of the job, not an afterthought.
Approved Document G also expects a blending valve to be shown incapable of exceeding 48 °C in use, and to fail safe. HSE advises fitting a thermostatic mixing valve as close to the outlet as possible where there is a scalding risk. A thermostatic shower is not a cure for an unbalanced supply either. It can only correct the blend within the pressure and flow it is actually given.
Why the cylinder is kept hot while the shower outlet must not be
Hot water is stored hot for hygiene and blended down at the outlet. HSE advises storing hot water above 60 °C and distributing it above 50 °C, with cold water stored and distributed below 20 °C. Legionella bacteria multiply between 20 °C and 45 °C, stay dormant below 20 °C and do not survive above 60 °C. HSE also expects 50 °C to reach the outlet within one minute.
Blending down happens at the tap or shower, which is exactly the job an unbalanced supply upsets. In England, Approved Document G requires hot water to a fixed bath not to exceed 48 °C, and that duty applies when a dwelling is built or there is a material change of use. The same document gives the reason: at 55 °C skin takes around 12 seconds to burn partially, at 48 °C around 10 minutes. Our guide on cylinder temperature covers the storage side in more detail.
What can you check yourself on a tank-fed shower, and what has to be left to an engineer?
Only a short list of checks belongs to the householder, and none of them involves opening pipework. Everything that changes how the shower is supplied is installer work.
- Check the programmer and the cylinder thermostat setting, so the cylinder is genuinely heating before you shower.
- Read the boiler pressure gauge if your boiler has one, and compare it with the manufacturer's instructions.
- Press the reset button once if the boiler has locked out.
- Bleed a radiator that is cold at the top, if the heating is also behaving oddly.
- Taking a balanced cold supply or re-routing the shower's cold pipe: installer work.
- Fitting a shower pump, a thermostatic mixing valve or a backflow prevention device: installer work.
- Any work on an unvented cylinder: Approved Document G in England allows this only for a competent person, with proof such as a competent person scheme or a valid skills card for unvented hot water systems.
- Anything involving gas, the flue, seals, condensate inside the appliance or opening the boiler casing: a Gas Safe registered engineer, under the Gas Safety (Installation and Use) Regulations 1998 as amended, in England.
There is a legal reason the two supplies cannot simply be joined at the mixer. The Water Supply (Water Fittings) Regulations 1999, Schedule 2, which apply in England and Wales, require every water system to include suitable backflow prevention matched to the highest fluid category downstream. If the survey points towards mains pressure on both sides instead, our page on Megaflo and unvented cylinders explains what that change involves, and the guide on converting a vented system covers what has to be checked first.
Sources
- Approved Document G: Sanitation, hot water safety and water efficiency (with 2024 amendments)
- HSE: Hot and cold water systems
- HSE: Thermostatic mixing valves (TMVs)
- Water Supply (Water Fittings) Regulations 1999, Schedule 2
- Heatrae Sadia: Coral Aquanox Thermal Store, direct and indirect
- Heatrae Sadia: Megaflo Eco Plus 250/300 installation manual
- Viessmann: How to increase your water pressure
- Vaillant: Increase water pressure in a shower with a combi