Mira shower only lukewarm

[MColeman]

Hi, hopefully someone can help.
We have two Mira thermostatic showers one has started to only reach lukewarm temperature. the sink in the same room has plenty of hot water. The problematic shower will run hot if the sink cold tap is run at the same time as the shower. I have removed and cleaned all four strainers ( 2 in each ). I have changed the non return valves in the other shower in case they were passing. I have also fitted a new cartridge. The showers were fitted years ago and are hot water fed from a combi boiler. Only one shower is run at any one time.
would anyone have any other suggestions?

 

[Johntheo5]

I think most thermostatic valves (when cold) are spring loaded to push the piston/spool (if installed in shower cartridges) to shut off the cold supply by moving the piston and allowing full HW flow until the thermostatic element forces the piston to throttle the hot flow and allow more cold from the other end, this element can produce very high force so surprising, especially with a new cartridge that its behaving this way, even if the cold water supply shut off completely then the thermostatic element should completely shut off the HW. If you turn the temperature control to its coldest setting does it give cool/cold water with the cold sink tap shut off and hot water when you open the cold tap?. Can you open the old cartridge to see how it works, might give some clues.

 

[MColeman]

Thanks for the quick reply,
when trying to blow through the cartridge it does appear to stop both the hot side and the cold side when turned fully one way or the other I’ll have a go at refitting this one again tomorrow.

 

[Johntheo5]

So, if turned fully to hot and blow in the cold end nothing comes out the mixed outlet (or the hot end) and when to fully cold and you blow through the hot end nothing out the mixed outlet or the cold end?.

 

[MColeman]

No if the cartridge is turned to fully hot and I blow in the hot side air passes to the back of the cartridge and out. if I blow in the cold side it has no passage through. Like wise if I turn it all the way to cold then air passes through but the hot input becomes sealed. I’m guessing that in an ideal situation both would be slightly open to achieve the desired temperature and that is why non returns are fitted.

 

[Johntheo5]

Yes, that seems fine , the only one I opened had a one piece plastic piston with a sealing rubber O ring in the valve body to stop the hot & cold water leaking past the piston and short circuiting, the piston only moved ~ 4 or 5 mm and if both pressures are exactly the same, then both will only supply exactly the same volume of water when the piston is mid position.
For interest I will post a few photos of the one I opened up, shortly.

 

[Johntheo5]

The Altenec One seems to have a adjustable piston stroke via the top cap.

 

[MColeman]

 

[Johntheo5]

Where are the hot & cold ports located??.

 

[MColeman]

The first photo shows the hot inlet. the ‘o’ ring that is round it seals it against the brass body of the shower.

 

[Johntheo5]

Before refitting the old cartridge you might turn it to fully hot then pour very hot water down the mixing port, water should then stop flowing out the hot port and should flow out the cold port if the thermostatic element is working and hasn't lost its liquid or wax charge.