Why is my bath water not hot enough?

Question

The bath water starts warm but goes cold quite quickly, even when running it on full hot. The shower seems fine. Is this a cylinder issue or something else?

Liam · Accepted Answer

Is this on a new-build by any chance? If so, it's now regulations that a thermostatic mixing valve is fitted to a bath which causes water temps of roughly 42c.

If it isn't a new build it could well be a heat source issue but if you're getting hot water to the kitchen and bathroom basin then that would indicate that the cylinder or boiler is fine.

Ionut-tiberiu · Answer

If you’ve got enough hot water in the cylinder, it might be worth checking the tap cartridge.

Michael · Answer

sounds like a mixer issue on the bath but I would get it checked to confirm

David · Answer

Check your mixer otherwise call a plumber to look at it

Aaron · Answer

Could you check whether the hot tap at the basin is running at a good temperature, and also make sure the boiler’s hot water setting is turned up high enough? It’s also worth trying the bath tap on a slower flow to see if the temperature improves. If everything else is working fine, the bath mixer is likely blending in too much cold, which is a common issue.

Current UK regulations require bath hot water to be limited to 48°C for safety, which is controlled by a thermostatic mixing valve. If the bath isn’t getting hot enough, it may be the mixer over‑blending cold water.

Matthew · Answer

If it's hot every where but the bath I would expect it has a thermostatic mixer valve fitted under the bath, this is becoming more common, especially in new build properties.  If it's causing a big issue it can be removed with some alteration to the pipework or the valve itself can be adjusted to let more hot water through, if isolation valves are installed before the valve you could even shut the cold water off so it only allows hot water through

Curtis · Answer

🔧 1. Hot water cylinder running out (most common if you have a tank)
If you’ve got a hot water cylinder (not a combi):
The bath uses a lot more hot water, quickly
You might be emptying the tank faster than it can reheat
The shower feels fine because it uses less water (or may be thermostatically controlled)
👉 Signs:
Bath starts hot, then fades within a few minutes
Hot water comes back after waiting/reheating
Fix:
Check cylinder size, thermostat setting, or whether it’s fully heating (could be immersion or boiler timing issue).

Mihai · Answer

The bath fills fast, so it uses hot water quicker. If the mixer is worn, it lets in more cold.

Lake · Answer

It’s a classic plumbing puzzle, but the fact that your shower is working fine is actually a massive clue. If it were a cylinder issue (like a faulty immersion heater or a depleted hot water supply), your shower would usually be running cold too.

Since the problem is isolated to the bath, it’s likely one of three things:

1. The Thermostatic Mixing Valve (TMV)
Most modern baths are fitted with a TMV, a safety device designed to prevent scalding by mixing cold water with the hot before it hits the tap.

The Issue: These valves can fail or become clogged with limescale. If the internal cartridge is stuck, it might be letting in too much cold water or "hunting" for the right temperature and failing.

The Test: If your bath taps are separate (not a mixer), and both start hot but one dies out, it’s less likely to be a TMV. If it’s a single lever or a deck mixer, the TMV is the prime suspect.

2. High Flow Rate vs. Cylinder Capacity
The bath tap usually has a much higher flow rate than a shower head.

The Issue: When you run the bath "on full," you are pulling water out of the cylinder significantly faster than the boiler or immersion heater can replenish it.

The Reality: You might be "drawing off" the layer of hot water at the top of the cylinder so quickly that the cold water entering the bottom creates turbulence and cools the remaining hot water down (known as "thermal layering" failure).

The Fix: Try running the bath at half-speed. If it stays warm longer, your cylinder simply can't keep up with the high demand of a wide-open tap.

3. Airlocks or "Dead Legs"
If the pipework to the bath is long or poorly routed, you could have a partial airlock.

The Issue: Air trapped in the pipes can restrict the flow of hot water, causing it to stutter or lose pressure, leading to an inconsistent temperature.

Scott · Answer

Given:
Shower = fine
Bath = starts warm → goes cold
Happens even on full hot
👉 I’d bet on: ➡️ Faulty bath mixer cartridge or TMV (80% likelihood)

Why is my bath water not hot enough?

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