Baths Luke warm, other hot taps red hot

[CaptainJack88]

Brother has an issue where all of his hot taps and showers are red hot, apart from his baths where the water is like warm.

They've got a huge combi boiler system. He's checked under the bath and there's no sign of a mixer valve anywhere.

Any idea what could be causing it?

Update: It's a Jacuzzi bath style, although the air system is separate.
Hot and cold feeds go into separate valves, that then feed into a diverter valve. Turn left for shower right for bath

 

[JohnD]

combi boiler.

How many litres per minute does his hot bathtap deliver, and what is the temperature?

When he turns the flow down to half, what is the temperature?

BTW, has he got bathtaps with two knobs? Or a ceramic mixer with a joystick? A photo of the tap might be informative.

 

[CaptainJack88]

combi boiler.

How many litres per minute does his hot bathtap deliver, and what is the temperature?

When he turns the flow down to half, what is the temperature?

BTW, has he got bathtaps with two knobs? Or a ceramic mixer with a joystick? A photo of the tap might be informative.

Just updated with more info on the tap setup. Not sure on the lpm, however it looks like the feed is via 15mm copper

 

[ianmcd]

All combi boilers raise a given flow rate of water by a certain temperature, if the water flows too fast the water will not heat up properly, as already advised try running the hot slower and if it gets hotter doing that then that is your problem

 

[undertrained]

Is it a new build? If so definitely a TMV somewhere. If not try what @ianmcd suggested.

 

[CaptainJack88]

Is it a new build? If so definitely a TMV somewhere. If not try what @ianmcd suggested.

Not a new build, but had new bathrooms and boilers fitted. He's checked all his cupboards and under the baths for one but can't see one anywhere.

Have told him to check the half flow to see if its any better.

The boiler they have is a Worcester 440CDi with a flow rate of 20lpm. Our boiler is 13lpm without issues. Logically my mind says that if 15mm pipe has a flow rate of 18lpm, then the 20lpm boiler isn't the issue?

Not sure of relevance, but some pics

Pipework from boiler

Pipework to hot/cold valves

Pipework to shower head/bath tap diverter

 

[Gasguru]

Pipes don't have specific flow rates...with sufficient pressure a 6mm pipe could have an excessive flowrate.
Ensure the boilers thermal store is up to temperature (ie the burner has shut down) and run the tap at a typical bath temperature. Time the flow into a 10 litre bucket to give a litres/min flowrate as a starting point.

Remember once the store is depleted the boiler performs much like any other middle range combi.

 

[undertrained]

If it has internal check valves, one could have failed, therefore allowing cold to force hot out of the way.

 

[undertrained]

If it has internal check valves, one could have failed, therefore allowing cold to force hot out of the way.

If you can find a way to isolate the cold, eg an isolation valve, then try that, if it works you know that the check valve or something else has failed.

 

[CaptainJack88]

If you can find a way to isolate the cold, eg an isolation valve, then try that, if it works you know that the check valve or something else has failed.

Actually that's a good point. There are 2 separate hot cold taps that feed into the mixer. Even with cold off, it's still lukewarm

 

[undertrained]

There are 2 separate hot cold taps that feed into the mixer.

? Not sure I follow?

 

[CaptainJack88]

? Not sure I follow?

There are 2 taps, hot and cold. They then feed into the mixer valve. So the mixer valve will only be hot, if the hot tap is on and not the cold. Take a look at the pictures above.

If you have Hot and Cold on but without the mixer valve, no water will come out