Lukewarm water - not boiler problem

[Manateedugong]

Hi,
Hoping someone can advise before we spend out yet more money on something unnecessary!
Since we moved into our house, the water from the hot taps has only been intermittently hot and mostly lukewarm. Bath taps the worst, kitchen sink tap the best.
Plumber came and diagnosed heat exchanger problem, that was replaced but made no difference. He then changed the diverter valve but again no difference. He searched all around the house and came to the conclusion that it was the valve on the kitchen mixer tap but changing that also made no difference. He is coming around today with a new valve just in case the one he used was faulty but said that if that doesn't work we will need a new tap. I'm concerned about spending that sort of money, especially if it doesn't fix he problem.
This morning, after reading up online (hadn't done so before as thought it was a straightforward boiler problem) I have found that if I turn on the bathroom sink cold tap whilst running a bath the water is properly hot.
Does anyone have any ideas?
Obviously just running a bath with another tap on is fine as a solution, my only concern is that the system may be costing me more in terms of gas if cold water is constantly backing up into it?

I'll be watching this with interest, as I've got the same problem (although I don't have a mixer tap in the kitchen). Repair being done Monday on part on the boiler, as we've been told that's what it was - actually we were told the fact that the water was sometimes lukewarm and occasionally hot was a big red arrow to this particular part. I'll let you know what the part was and whether or not it fixes the problem on Monday, just in case it helps you.

You have a combi boiler, I gather.

A combi boiler can heat X amount of water by Y degrees, in a minute,

If your bath tap delivers 2X amount of water, it will only be warmed by half a Y. In winter the incoming water will be very cold and needs to be warmed by more degrees.

It is usual for bath taps and their pipes to be bigger than sink taps, so will deliver more litres of water per minute than sink taps, so, from a combi, the bathtap will be cooler than the sink tap.

Your method of turning on the cold tap will reduce the amount of flow available, so the bath tap will be hotter.

I expect that if you turn down the flow on the bathtap, the water will come out hotter and slower.

Sadly this is normal for ordinary combis.

There might or might not be some other problem. If you run the bathtap into a bucket, time it to full, and calculate how many litres per minute it delivers, that will give a big clue, but the power of the boiler, and the temperature of the cold and hot water will give more.

p.s.

If you had a hot water cylinder, the cause, diagnosis and fix would all be completely different.

Thank you for the replies (and yes it is a combi boiler). Our plumber has been back with a new non return valve for the mixer tap and that seems to have done the trick! Took him about 10 minutes compared to the several hours he's spent so far replacing bits that didn't need repairing. Just waiting for the bill now...

How does changing the valve on the mixer tap in the kitchen solve the bath hot water temp??

We have the same issue, I've always presumed the prob was as PigletJohn diagnosed.

I think that in this case, water is able to leak, inside the tap, between the hot and the cold pipes (it is probably a ceramic disk tap with a joystick).

When a lot of hot water is being drawn from a large bathtap, the pressure in the hot pipes drops, and water from the cold pipes passes into the hot pipe and mixes. With a combi this will not happen when the taps are off.

Something similar can happen when instead of a combi, you have a hot cylinder at low pressure, fed from a tank in the loft.

Joystick taps are particularly prone to this, but some shower mixers can do it, when worn, or if they have high pressure cold and low pressure hot.

You can detect it by putting your hand on the hot pipe to the suspect mixer, and feeling if they go surprisingly cold (when cold water is flowing up them). British traditional kitchen taps are designed so that the hot and the cold water cannot mix inside the tap or the spout, for hygiene reasons.

The other condition I described in my earlier post will happen with a combi even when there is no fault or leak, so it is more common.