Identify your hot water system first – gravity, combi, unvented or electric – because that determines which showers will work. Pressure and flow are different things and a shower needs both.
Identify your system first
Everything follows from this, and it takes two minutes to work out.
| If you have | You have | Pressure at the shower |
|---|---|---|
| A cold tank in the loft and a hot cylinder in a cupboard | Gravity-fed (vented) | Low, determined by the height of the tank above the shower |
| No cylinder, no loft tank, a boiler that heats water on demand | Combi | Mains pressure, flow limited by the boiler |
| A cylinder but no loft tank, usually stainless | Unvented | Mains pressure, flow limited by the incoming main |
| A shower unit on the wall with a cable to it | Electric shower | Mains pressure, flow limited by the heating element |
| A cylinder and a pump in the airing cupboard or loft | Pumped gravity | Whatever the pump delivers |
Pressure is the force behind the water. Flow is the volume arriving per minute. A gravity system usually has good flow and poor pressure – which is why a pump transforms it. A restricted mains supply has good pressure and poor flow – which a pump will not help. Getting this backwards is the most expensive mistake in this area.
The shower types
Mixer shower
Blends stored hot and cold. Simple, reliable and capable of excellent performance if the system supports it. Available in low-pressure versions for gravity systems and mains-pressure versions for combis and unvented cylinders – and the two are not interchangeable.
A thermostatic mixer holds the outlet temperature steady when pressure changes elsewhere in the house, so somebody running a tap does not scald you. It also has a maximum temperature limit. We would treat thermostatic as standard rather than an upgrade.
Electric shower
Takes cold water only and heats it as it passes, using an element. Completely independent of the hot water system, which makes it the right answer where hot water is poor, where a shower is needed somewhere with no practical hot supply, or as a backup for when the boiler fails.
The limitation is physics. The element’s power determines how much water can be raised to shower temperature per minute, and that is a hard limit. Higher-rated units give better flow and need a larger cable and a suitable electrical supply. In winter the incoming water is colder, so the same unit delivers less flow at the same temperature – which is why electric showers feel worse in January and it is not a fault.
Power shower
A mixer with an integral pump, for gravity systems. It solves the low-pressure problem at the shower itself. Needs correct feeds and the right pump type.
Digital shower
Electronic control with a separate processor unit, giving precise temperature and often remote start. Genuinely pleasant to use. More expensive, more to go wrong, and the processor must be somewhere accessible – a decision that has to be made before anything is boarded over.
Matching them
| System | Suits | Avoid |
|---|---|---|
| Gravity (tank and cylinder) | Low-pressure mixer, or pumped mixer / power shower | Mains-pressure mixers, rain heads, body jets |
| Combi | Thermostatic mixer rated for the boiler’s flow rate | High-flow rain heads and body jets the boiler cannot feed |
| Unvented cylinder | Almost anything, if the incoming main measures well | Nothing much – but measure first |
| Poor mains, no useful hot | Electric shower | Anything with a high flow demand |
| Gravity with a pump | Whatever the pump is sized for | Exceeding the pump’s rating |
The one absolute: you cannot pump directly off the mains. Boosting a mains supply with a domestic pump is not permitted under the water fittings regulations, because it can draw pressure down for neighbours. Pumps on gravity systems draw from stored water, which is why they are allowed.
Pumps, properly
For a gravity system, a correctly specified pump transforms the shower. The details that determine whether it works:
- Positive or negative head. A positive-head pump needs the water level in the tank to be above the pump. A negative-head pump can start with the pump at or above tank level and has a sensor to detect demand. Fitting the wrong type means a pump that will not prime or will not start.
- Dedicated feeds. The pump needs its own draw-off from the cylinder – typically a flange or an Essex or Surrey fitting – not a tee into an existing pipe, which draws air.
- Tank capacity. A pump moves a lot of water. A small cold tank will empty during a long shower and the pump will run dry.
- Position and noise. Pumps are not silent and they are frequently sited in an airing cupboard next to a bedroom. Anti-vibration mounts and sensible positioning matter.
- Single or twin impeller. Twin pumps both hot and cold, which is normally what you want for a mixer.
- Correct rating. Bigger is not better – an oversized pump can outrun the tank’s ability to refill.
Why a shower disappoints
- Weak from day one – a system mismatch. The shower was specified for more pressure or flow than the system delivers.
- Declined over years – scale, in the head, the hose or the thermostatic cartridge. Descale the head first; it is free and it frequently fixes it.
- Temperature swings when a tap runs – a non-thermostatic valve, or shared feeds.
- Runs hot then cold on a combi – the boiler struggling to modulate down to the shower’s flow rate, or a scaled plate heat exchanger.
- Fine in summer, poor in winter, on an electric shower – normal physics, colder incoming water.
- Pump cutting in and out – usually a feed problem or the wrong head type rather than a faulty pump.
- Good pressure but poor flow – a restriction somewhere: a partly closed valve, a scaled tail, or an undersized supply.
Weak shower? Descale the head first. If that does not fix it, the answer is in the system rather than the shower.
Questions we get asked about this
Why is my shower so weak?
Most often the shower is specified for a different water system from the one you have. Second most often it is scale in the head, hose or cartridge. Descaling the head costs nothing and is worth doing first.
Will a pump fix my weak shower?
If you have a gravity system with a loft tank, yes, and it will transform it. On a combi or unvented cylinder, no – and pumping directly off the mains is not permitted.
Can I have a rain head?
Only if the system can feed it. A large rain head needs substantial flow. On an unvented cylinder with a good main it is excellent; on a combi or a marginal supply it disappoints. Measure before buying.
Is an electric shower a good option?
It is the right answer where hot water is poor or unavailable, and it is a genuinely useful backup for when the boiler fails. It will never match a good mains-pressure mixer for flow, because the element sets a hard limit.
What is a thermostatic shower?
One that holds the outlet temperature steady when pressure changes elsewhere in the house, with a maximum temperature limit. It is both a comfort and a safety feature and should be considered standard.
My shower runs hot then cold on a combi.
Usually the boiler struggling to modulate down to the shower’s flow rate, or a scaled plate heat exchanger reducing both temperature and flow. In a hard water area, the second is very common.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.