Lukewarm bath water - thermostatic mixer problem?

[Curioustiger]

I have a bath, a shower and a basin in my bathroom, all run off a combi boiler. The shower and basin water is absolutely fine both in terms of pressure and temperature. However the bath taps run lukewarm at best. If I turn it on to just cold, the pressure is fine. However if I turn it on to just hot, the flow is obviously restricted as it's more of a dribble.

Annoyingly we had this problem in the old bathroom, then had a new bathroom put in, and still have the same problem!

Does anyone know if this is the thermostatic mixer playing up and if so, could we get it fixed without ripping out any of the bathroom? The bath is enclosed but the fitters added a hatch so there is some access to pipe work etc.

NOt sure, but we have a similar issue. Not a combi, but we have just had a thermal store fitted so all the hot water should be at mains pressure. The basin tap is fine, but the bath runs quite slowly. I know the plumbers reduced the flow on another tap that ran at such high pressure it over shot the basin (!) so I do think there is adjustable flow valves in some bath fillers. Our plumbers are coming back on Thursday and this is one of the things on the snagging list.

I had this problem in our last rental house. After ages of going back and forth with the agents, they sent a plumber out and the problem was so easy to fix. Ours used to mainly happen in cold weather, which is a clue to the solution.
This is what the plumber told us was the problem: With the combi boiler, the water runs past the whatever heats it when you demand it by turning on the tap in the bathroom. In cold weather, obviously the water is colder and needs more time to heat it up. By turning the hot tap on full (which would seem the logical thing to do) then the water runs to fast past the heating part (sorry not very technical) in the boiler and doesn't have time to heat up, hence the lukewarm water. So either turn it on halfway (sorry takes a while still for the bath to full though) and the water should be hotter. Or you can adjust the water flow at the boiler as well. Depends on your boiler, but there were some knobs underneath ours near the hot water outflow pipe to upstairs and turning this slightly down also helped the problem.
Hope this works as its a cheaper solution than getting someone in.

Actually just re read your post, not sure if the solution above would help you or not now, as you seem to have more of a pressure than heating problem.

I will give your solution a go havingkitties and report back... I think it is a different problem though as we've had it since the summer and you can really see how little water comes out when it's just turned to hot!

better it would be great to hear if your plber can fix this! Ultimately I'm happy to call them out but would much prefer to know if they're likely to be able to fix it before I commit to the call out charge.

Run the bath tap into a bucket, time it to full, calculate how many litres per minute you get.

If you have a thermometer, see what temperature it is, and what is the temp of the cold water.

Ok will do that and report back