Mixer showers: check the cartridge and isolating valves first
In a standard mixer shower fed by hot and cold water from the household system, a common cause of running cold or fluctuating temperature is a scale-blocked or worn thermostatic cartridge. Cartridges contain small internal parts that mix hot and cold water to a set temperature, and in hard water areas like the Thames Valley, limescale builds up inside them over time and affects how well they regulate flow.
Another straightforward cause is an isolating valve that has been left half closed, often after other plumbing work nearby. This restricts one side of the supply, throwing off the hot-to-cold balance and causing the shower to run cooler than it should, or to become unstable when someone else uses water elsewhere in the house.
Unbalanced supply pressure
Mixer showers rely on relatively similar pressure from the hot and cold supplies to blend properly. If one side has noticeably lower pressure than the other, perhaps due to a partially closed valve, a smaller pipe run, or a fault in the hot water cylinder or system feeding it, the shower may struggle to hold a steady temperature and drift toward cold.
This is worth checking by comparing the flow from a nearby hot tap and cold tap. A big difference in strength between the two points toward a supply-side pressure issue rather than a fault in the shower valve itself.
Electric showers: heating element and flow issues
Electric showers heat cold water directly as it passes through, using an internal heating element, so a shower that runs cold with an electric unit points to either the incoming cold water flow being too low to trigger the flow switch, or an internal fault such as a failed heating element or scaled-up element reducing heat transfer.
Because this involves the mains electrical supply to the unit and, often, the heating element itself, any diagnosis or repair beyond checking incoming water flow needs a qualified electrician. Macrostart do not carry out gas or electrical work, so an electric shower fault of this kind should go to an appropriately qualified engineer.
Combi boilers: flow rate and other outlets competing for hot water
If your hot water comes from a combi boiler, a shower running cold or losing temperature partway through is often linked to the boiler's flow rate or its internal flow switch, particularly if someone else in the house runs a hot tap or the dishwasher at the same time, since combi boilers can only heat a limited flow of water at once.
This kind of intermittent, load-related cold shower is a strong sign the boiler itself needs looking at. Boiler diagnosis and repair is gas work, which is outside what Macrostart carries out, so this should be referred to a Gas Safe registered engineer.
Hard water and scale in the Thames Valley area
Reading and the wider Thames Valley sit in a hard water area, and limescale is a genuine contributing factor across almost all of the causes above, from clogging thermostatic cartridges to reducing flow through shower heads and electric heating elements. Regularly descaling shower heads and hoses, and having cartridges checked periodically, reduces how often these faults occur.
| Shower type | Likely cause of running cold |
|---|---|
| Mixer with thermostatic cartridge | Scaled or worn cartridge, or half-closed isolating valve |
| Mixer without thermostatic control | Unbalanced hot/cold supply pressure |
| Electric shower | Low incoming flow, or element/electrical fault (needs an electrician) |
| Combi boiler fed | Flow rate limits or other outlets running (needs a Gas Safe engineer) |
Your questions answered
Why does my shower go cold when someone flushes a toilet?
This is a classic sign of unbalanced or insufficient supply pressure, often on gravity-fed systems without a thermostatic cartridge to compensate. A thermostatic shower valve is designed to hold temperature steady despite this, so upgrading to one can solve the problem.
Can I replace a shower thermostatic cartridge myself?
It is a plumbing task rather than gas or electrical work, so it is within reach for a confident DIYer with the water supply isolated first, but cartridges are model-specific and getting the right replacement part matters. Many people prefer to have this done by a plumber to avoid leaks from a poorly reseated cartridge.
My electric shower is losing power - is that dangerous?
It can indicate an internal electrical fault, and electric showers should only be opened up or diagnosed by a qualified electrician. Stop using the unit if you notice tripping, burning smells, or inconsistent power, and get it checked before using it again.
Does Macrostart fix boiler or electric shower faults?
No. Macrostart handles bathroom refurbishment, tiling and light plumbing fitting such as tap and shower valve replacement, but boiler work is gas work and electric shower faults are electrical work, both of which need a suitably qualified and registered engineer.
Will descaling my shower head actually help with cold water?
It can help with reduced flow and pressure caused by limescale build-up, especially in hard water areas, but it will not fix a genuinely faulty cartridge, element or boiler flow issue if that is the underlying cause.