Showers hot water now cold

[Brian246]

Hi all can anyone help?
I have a Mira Silver mixer shower that is fed by a Stuart turner 1.5 twin pump that is about 6 months old which replaced a leaking pump.
The last couple of weeks the hot water is ok for pressure but the temperature has dropped to tepid at best and sometimes cold.
We don’t have a header tank and our oil fired boiler is serviced regularly and working fine.
Any advice would be appreciated.

 

[John.g]

If you don't have a (cold water?) header tank have you got a unvented HW cylinder and if so why do you require a twin pump?.
Maybe you have a combination cylinder with the cold mounted on top of the hot cylinder.

 

[Brian246]

Hi thanks for the reply.
We don’t have a HW cylinder at all and our boiler is a oil fired external one with no header tank.
We had the shower pump replaced about 6 months ago due to it leaking I assumed the previous occupants had it installed has the pressures not great so I replaced like for like.
The shower is a Mira mixer system which is at least 6 years old and after some home work I’m beginning to suspect that’s the problem.

 

[John.g]

Where are you getting the HW from?, is it a combi boiler??

 

[Brian246]

Yep it’s a combi

Where are you getting the HW from?, is it a combi boiler??

ep

 

[John.g]

Very good, so presume the ST is pumping cold water from both ends from a cold water storage tank (CWST), one end is supplying water to the combi cold feed and the other end cold water to the shower?.
You say that the hot water pressure is OK but only tepid? so is it hot leaving the boiler?.

 

[Brian246]

Yep all the hot water taps are working good.

 

[Brian246]

Very good, so presume the ST is pumping cold water from both ends from a cold water storage tank (CWST), one end is supplying water to the combi cold feed and the other end cold water to the shower?.
You say that the hot water pressure is OK but only tepid? so is it hot leaving the boiler?.

Sorry to sound like a muppet but I think the boiler is fed direct from the mains.

 

[John.g]

Wonder what a ST twin is used for so or any pump for that matter.

 

[ShaunCorbs]

Chances are the digi shower pump eg Mira wasn’t changed when they replaced / upgraded the boiler sounds like you need the new digi hp controller

Does it look something like this ?

https://www.showerspares.com/images/products/large/1/mira-platinum-digital-mixer-unit-and-wireless-controller-high-pressure-1666.003.jpg

 

[Brian246]

Wonder what a ST twin is used for so or any pump for that matter.

Not sure why they installed it I assumed they thought the pressure was low, which is strange cos it’s not that low.
I would have fitted a electric one but I just had like for like replaced.

 

[Brian246]

Chances are the digi shower pump eg Mira wasn’t changed when they replaced / upgraded the boiler sounds like you need the new digi hp controller

Does it look something like this ?

https://www.showerspares.com/images/products/large/1/mira-platinum-digital-mixer-unit-and-wireless-controller-high-pressure-1666.003.jpg

I don’t think we have a digi controller just the pipes from pump connected to the shower.
I think the job is a bit of a lash up.

 

[John.g]

1. where is the cold water to the ST supplied from (cold side of twin)

2. where is the water supplied to the "hot" side of the ST, is it from the hot outlet from the combi.

 

[ShaunCorbs]

What’s your incoming water pressure?

 

[John.g]

Hope the Combi HW outlet is not being boosted by the ST, if so, then possibly a big difference in the HW/CW pressures at the thermostatic mixer which it probably can't deal with?.

 

[Brian246]

All the feeds to cold and hot are plumb in correctly and our water pressure has been checked by the water authority and is ok.
I was asking around and it might be the thermostat mixer cartridge in my Mira silver shower it’s at least six years old to my knowledge and the water is hard here so maybe it full of scale and rubbish.

 

[John.g]

Would still be nice Brian to know where the hot and cold to the ST are piped from.

 

[Brian246]

Very good, so presume the ST is pumping cold water from both ends from a cold water storage tank (CWST), one end is supplying water to the combi cold feed and the other end cold water to the shower?.
You say that the hot water pressure is OK but only tepid? so is it hot leaving the boiler?.

Wonder what a ST twin is used for so or any pump for that matter.

Hi John g
That’s a good point about why they installed a st pump anyway.
installed by previous owners 3 years ago and I replaced it about 5months ago. It seem in can’t cope with the water from the boiler.
The pressure is ok and the heating is fine but a boiler engineer called round and it seem the hot water form the boiler is scaled up so that’s why the hot goes warm then cold.

 

[John.g]

If the hot water tap(s) (post #7) can continuously flow say 15 LPM at 40C (assuming a 30kw combi) then IMO not a whole lot wrong with the Hx. With the shower in service the HW flow rate will be much lower so maybe a flow switch problem but you can check this by reducing the flow rate from the hot water tap to ~ 3 or 4 LPM and the water should still stay (very) hot at ~ 60C. You could also switch off the DHW at the boiler and then measure the shower flow rates when turned to fully hot and then fully cold, might point to something.
Also check pump and shower filters.