Hot tap running luke warm

Hi all, looking for a bit of advice please. I have a valiant combo boiler and the trouble I'm having is the bath hot tap runs only luke warm unless I turn the flow right down. Would anyone have any suggestions please?

Many thanks
Jim.

Do you know what size the pipework is to the bath?

Your bath tap's flow rate is exceeding the rate at which the boiler can heat the water. You might have a scaled heat exchanger, or it could be that your boiler simply isn't powerful enough to keep up with the demand you're placing on it

1. Its Vaillant, not Valiant.
2. All combination boilers are designed to produce a certain flow rate of water at a temperature a specific temperature above the incoming cold water. Generally 35 degrees.
3. So a 24 kW boiler will produce about 12 litres per minute (lpm) of hot water at a temperature of about 45 degrees. Assuming cold water in at 35 degrees.
4. If the outlet uses more than 12 lpm, then the temperature will be reduced, as the water is going through the boiler too fast to be heated to the maximum.
5. An average bath take about 80 litre of water. Assuming an end temperature of 40 degrees, this will require about 70 litres of hot at 45.
6. 70 litres of hot will take 70 / 12 = just under 6 minutes. Probably a bit more allowing for losses en route.
7. Try turning the flow down until the water is hot, then measure the flow rate. If this is significantly different from the manufacturer's figure there is a genuine problem.

Thanks for the replies. The boiler is a "vaillant" eco 831. The pipe work is 15mm. I've just checked the flow rate of the hot water and fully open is about 10lpm, it gets very hot when the flow is reduced to about 3.5 - 4 lpm. This was using the onboard diagnostics

Your pipework may be 15mm, but I bet your bath taps are 3/4". That makes a difference.

Get the boiler serviced, properly, not just a poke and go, a full strip down and clean and see what your engineer finds. My first thoughts are it's under gassed so could be a few different things.

Oilhead, you are correct. Fitted a airater and now all is fine. Hot to the touch water