Scheduled visit
The shower runs hot, then cold
Mid-shower, with the controls untouched, the temperature swings — hot one moment, then noticeably cooler, then hot again. It is most obvious when someone else in the villa runs a tap, a toilet refills or the washing machine fills. The heater is still heating; the problem is that the balance of hot and cold reaching the shower is no longer stable.
Plumbing
Active leaks are treated as emergencies. Pressure and drainage complaints are normally same-day or next-day.
Call nowRequest a repair on WhatsApp01LIKELY CAUSES
What usually sits behind this symptom, and what tells one cause from another.
Worn thermostatic mixing valve
The valve no longer compensates when the pressure on one side changes; confirmed by testing the cartridge or the valve's response to a second outlet opening.
Pressure imbalance between hot and cold
When another outlet opens, the cold pressure at the shower drops differently from the hot, and the temperature follows.
Blocked inlet filter in the valve or shower head
Scale has restricted one side of the valve, so the mix drifts as flow through the two inlets changes.
A failing non-return valve on the shower supply
A non-return valve that no longer seals lets water cross between the hot and cold sides, so the mix drifts as pressures change.
02BEFORE ANYONE VISITS
What you can safely check.
- Note whether the swing happens when another tap or appliance runs.
- Check whether it affects every shower or just one — one shower points at that valve, all of them points at the supply.
- Clean the shower head and any accessible inlet filters of scale.
- Avoid turning the control rapidly to fight the swing; it usually makes the next swing sharper.
03WHAT WE MEASURE
We run the shower and open a second outlet to reproduce the swing, then measure the temperature response. We test the valve's cartridge or pressure-balance unit and check both inlet filters, separating a failing valve from a genuine pressure imbalance between the hot and cold supplies.
04IN A VILLA
Villa showers sit at the end of longer pipe runs and share their supply with more outlets than a flat's, so the pressure at the valve changes more when something else opens. A thermostatic valve that coped in a smaller home can be pushed past its ability to compensate in a villa, and the result is a shower that runs hot and cold.
05THE FIX
Which repair, and when.
A shower that swings hot and cold is unpleasant and, at the extreme, a scalding risk. The valve wears further with use, and the swing widens until the shower becomes hard to use at all.
Replace the valve cartridge or pressure-balance unit
The valve itself has worn and can no longer compensate.
Clean the inlet filters and head
Scale on one side of the valve or in the head is unbalancing the mix.
Correct the pressure imbalance
The hot and cold supplies are genuinely unequal and a pressure-limiting or balancing fix is needed.
06WHAT DECIDES THE FIX
The same symptom can lead to different repairs. These are the things that decide which way it goes.
- Whether it is one shower or every shower — one shower points at that valve, while all of them point at the supply.
- What the valve cartridge shows — a worn thermostatic cartridge is a small, common replacement.
- Whether the swing tracks another outlet — a swing that appears only when a tap or toilet runs points at pressure balance.
- How scaled the inlets are — blocked filters on one side unbalance the mix and are cleaned before anything is replaced.
- The age of the valve — an old valve is often cheaper to replace than to keep patching.
- How hot the swing gets — if it ever reaches scalding, treat it as a safety job and stop using the shower until it is fixed.
07HOW A VISIT GOES
What to expect when we come out, and why the order matters.
Reproduce the swing
The shower is run and a second outlet opened to bring the temperature swing back.
Test the cartridge or pressure-balance unit
The valve's response to changing pressure on each side is checked.
Check both inlet filters
Scale on one side of the valve, or in the head, is cleared so the mix is no longer unbalanced.
Separate valve wear from supply imbalance
The measurements show whether the valve or the pressure difference is doing the work.
Replace or balance, then re-test
The cartridge is replaced or the imbalance corrected, and the shower is run through the same test again.
08KEEPING IT FROM RETURNING
Once it is fixed, these steps make the fault less likely to come back.
- Have thermostatic cartridges serviced or replaced when the swing first appears, before it widens.
- Clean shower heads and inlet filters of scale periodically.
- If another outlet triggers the swing, note which one — it makes the diagnosis faster.
- Avoid rapid full-hot to full-cold changes on an ageing valve.
- If the swing becomes sharp enough to scald, treat it as a safety issue, not an inconvenience.
- If a shower is used by children or older people, fix a temperature swing promptly rather than living with it.
09Typical villa plumbing callout
AED 200 – 1,400 depending on access
Tap, trap and fixture work sits low in the band. Pump and vessel replacement sits mid. Anything requiring buried-line access or surface breaking is quoted only after the leak is located, because the location determines most of the cost.
10QUESTIONS
- Is it the water heater?
- Rarely, if the heater still delivers a steady hot supply at the basin. The swing is about the balance of hot and cold at the shower, which is the valve's job.
- Why does it happen when a toilet flushes?
- A toilet or washing machine draws cold water, briefly changing the pressure on the cold side of the valve. A healthy valve compensates; a worn one lets the temperature swing.
- Can I just live with it?
- You can, but it usually gets worse, and the swing can become sharp enough to scald. The fix is usually a valve cartridge, which is a small job.
- Is a thermostatic valve worth repairing or should I replace it?
- Usually repairing is enough — the cartridge is the wearing part and is a small job. Replacement is for a valve that is old, scaled or already failing in other ways.
- Why is the temperature fine at the basin but not the shower?
- Because the shower valve is the balancing point. The basin runs from the same supply, but it is the shower's mixing valve that has to compensate, and that is where the fault is.
- Is this more common in villas than flats?
- It can be. A villa's longer runs and more outlets mean the pressure at the shower changes more when something else opens, which asks more of the valve.