Mixer shower only works in evening, not in morning

[C M]

In the middle of a total refurb and having some weird bathroom plumbing issues.
Context: it's a new combi boiler, new mixer shower and bath tap. No other taps currently plumbed in.

The shower only kicks the boiler on in the evening, not in the morning.
In the morning I can get hot water for a minute or two by running the hot tap on the bath mixer. But as soon as I switch to the shower, the boiler turns off (despite a great flow rate from the shower).

Plumber says that the shower thermostat is faulty and has removed the valve. (The issues above occur with the valve missing.)
But why would the shower work perfectly in the evening?

There's also an issue with the bath mixer. Hot water kicks boiler on, but flow rate seems low. Adding a bit of cold into the mix seems to turn off the boiler. When the cold is turned off the whole mix goes icy cold. Could these issues be related?

Seems unlikely that I have two new faulty mixers (but possible I suppose), so I'm wondering if it's a boiler issue.

Be glad of any suggestions of things to check...

 

[Rob Foster]

I think we will need a bit more detail here. Make type boiler and shower is a good start ...others will pitch in then
Rob Foster aka centralheatking

 

[Gasmk1]

i think you have lower pressure in the morning due to water demand, this may not be enough to kick in the hot on the shower, less demand in the evening so enough to bring the hot water on on the combi. i suspect it is more than likeley the diverter valve on the combi that needs to be repaired or replaced. you will need your gas safe enhineer to check the boiler

 

[Aire]

I'd agree with gasmk1. Could be an issue with lower flow rates in the morning but then you do say great flow from the shower. The bath tap may have a high pressure filter at the tap outlet, this may be clogged with flux deposits. It may also be flux deposits causing an issue with the boiler if the pipework wasn't flushed properly. I've had it once where a new boiler had intermittent hot water. This was caused by the flow sensor being gunked up with flux deposits.

 

[C M]

i think you have lower pressure in the morning due to water demand, this may not be enough to kick in the hot on the shower, less demand in the evening so enough to bring the hot water on on the combi. i suspect it is more than likeley the diverter valve on the combi that needs to be repaired or replaced. you will need your gas safe enhineer to check the boiler

There's no other water demand in my house - do you mean on the mains?
(I tested it with the CH turned off)

 

[Burger]

Do you have a thermostatic mixing valve fitted under the bath? Could be a twisted/bent flexi hose if fitted. Do you have any check valves fitted under the bath? Could be the diverter on the bath/shower mixer - especially if poor pressure. It Does sound more like a higher demand from the incoming water main in the morning than the evening, especially if your on a common supply (shared supply) not found that often, but still common (pun intended).

 

[Burger]

Also could be boiler related if any dirt/grit/solder/flux has got into any components

Gas safe registered engineer would be required for that though.

 

[Gasmk1]

There's no other water demand in my house - do you mean on the mains?
(I tested it with the CH turned off)

i mean local demand water pressure tends to drop when it is times of high demand i.e. early morning and late evening. also some utilities companies lower the night time pressure to lessen water leaks

 

[C M]

I think we will need a bit more detail here. Make type boiler and shower is a good start ...others will pitch in then
Rob Foster aka centralheatking

It's a Valliant, and a Triton Levano shower

 

[C M]

The consensus from these replies seems to be that I'm right to be suspicious that it's a boiler issue rather than a shower issue..?
(especially with the odd issue with the bath taps)

 

[Burger]

The consensus from these replies seems to be that I'm right to be suspicious that it's a boiler issue rather than a shower issue..?
(especially with the odd issue with the bath taps)

One thing you could try is connecting up a temporary hot water supply and outlet (tap etc) and see if you have the same issue. If not then bath/shower mixer fault or pressure issue. If same issue then boiler and GSR engineer.

 

[C M]

One thing you could try is connecting up a temporary hot water supply and outlet (tap etc) and see if you have the same issue. If not then bath/shower mixer fault or pressure issue. If same issue then boiler and GSR engineer.

The plumber is back soon. He's saying the shower is faulty but I've asked him to plumb the sink first, check that's working first.
It just seems unlikely to me that new bath and shower mixers would both be faulty...

 

[Burger]

From your OP I thought it was a bath shower mixer, but is it a bath mixer tap and separate shower mixer? If this is the case you could have crossed connections on your shower, bath mixer or both.

 

[C M]

From your OP I thought it was a bath shower mixer, but is it a bath mixer tap and separate shower mixer? If this is the case you could have crossed connections on your shower, bath mixer or both.

Yes, separate mixers.

 

[Burger]

Hi CM,

How did you get on? Now I know they are separate I would be pointing more towards the boiler, incoming pressure supply and or crossed connections.

 

[Ben-gee]

Could be a non return valve in the shower is faulty or stuck open with some crud. Easy to check