The shower in my ensuite is the highest flowing water in the house.
Its on the regular hot water system (ie not a powered electric shower)
It has a Mira control for hot and cold water.

Hot water pressure in the shower has always been "weak" and hot water was always slow to start flowing. Saturday morning hot water stopped altogether. Cold works fine in the shower but if I push the lever to full hot - flow drops considerably and just some cold water comes out.

There is plenty of hot water everywhere else - including the sink and bidet beside the shower.

Here is what happened.
I turned on the shower - at hot. Normally, to speed things up I turn on the hot tap in the sink. (I do this because I was told that this gets cooled water out of the hot pipes and lets recently heated water through more quickly)
Anyway this time the result is that the flow in the shower slowed - but the hot water never came through.

Is this an airlock somewhere - or a blockage in the mixing shower control.

Any suggestions?

Blockage in the mixing valve.

Need to shut the hot and cold water off to the mixing valve itself. Then remove the trim plate and the guts inside the valve.

You would then pulse the hot and cold water on/off for a second or two to clear the line of any sediment/debris. Then re-install the guts and see if that helped. If not, then the mixing valve CARTRIDGE itself is clogged and will need to be replaced. Repeat sequence to shut off the hot and cold water to the mixing valve and remove the inner guts again and then take the assembly to your local MIRA supplier and get replacemnet cartridge.

Let me know how you make out... :)

MARK

Thanks Mark

Got it fixed based on a combination of advice.

If its debris in the valve - then all advice is to follow the steps above
(looking at the mira picture of the guts of the valve - I would get an expert to do this)

Before going that route I tried the Airlock solutions.

First attempt
Connect a hose between a hot tap and a cold tap. Turn on shower. Then turn on both of the connected taps. Idea is that cold water pushes back through the hot circuit clearing the airlock.
The only place in the house with separate taps is the sink in the ensuite bathroom. I tried the ssuggestion but it didn't work - probably because the cold water pressure upstairs isn't great.

Second attempt
Try to do the same thing in the kitchen where the cold water supply is at mains pressure.
Here I have separate taps - but just one spout. However hot water comes from the center of this spout - and cold from around this. So they are effectively separate.
So I connected up about 2ft of hose to this spout - with an adjustable garden spray at the other end.
Go upstairs and turn on shower
Back to Kitchen
Turn on hot tap - hot water comes out the spray
Close off the spray
Turn on the cold tap.
Leave it for a minute
Turn Kitchen taps off
Back upstairs
Some improvement in flow
Waited a couple of minues - got luke warm water

Third attempt
Same as second attempt but
When you come to turn on the cold tap in the kitchen I took massplumber2008 suggestion and pulsed the cold water 7 or 8 times. (can feel the hose expanding and contracting while doing this).
Turn everything off in kitchen
Back upstairs
By the time I got up there - shower was running hot with decent flow.

SORTED

Thanks for the help
Joe