Shower keeps going hot and cold

[Jon hutchinson]

having an absolute nightmare!
The shower in our house keeps going hot and cold. It takes approx 1 min to heat up the 1 min of hot, 1 min of cold 1 min of hot, and keeps repeating.
The shower is run of the combi boiler (Alpha CB28). The shower itself doesn't have a brand name but I think it's a bath store model as the rest of the bathroom suit has that.
We moved into the house in October and it's slowly got worse with lessening times between hot and cold.
I've checked the mains water feed is fully open and turned down the water temp on the boiler but nothing.
The shower also makes a whirring noise when it flips between hot and cold.
Any ideas?
Photo of shower attached

 

[Jon hutchinson]

Forgot to mention the hot water is fine and constant elsewhere in the house (kitchen, bath etc)

 

[Ted777]

Them types of shower units are notoriously useless. The thermostats seem to stick I don't know anyone who's had a perfect experience with them. You could try and spray some wd40 around the thermostat and move it hot to cold a few times it might go good for a while, though they can be bought for about £50 and simple to swap out yourself.

 

[Jon hutchinson]

Thanks Ted. It is still intermittently hot and cold after the WD40. Do you think it maybe worth replacing the whole shower bar?

 

[Roger928]

I have a shower identical to that. Cost £100 quid and run from a gravity cylinder. And no problems. Very even stable temperature.

 

[picasso]

Try changing the shower head for a bigger one , boilers need a minimum flow to work properly.

 

[Agile]

Turn a basin tap on at half flow and see if it still cycles.

If not then it is the boiler needing a greater flow to stay on.

You can test if the boiler is turning on at the right flow rate by SLOWLY opening a tap until the boiler just turns on. Don't open tap any further and measure the flow rate in litres per minute.

Most boilers turn on at about 2 litres per minute.

Tony

 

[MeldrewsMate]

Turn a basin tap on at half flow and see if it still cycles.

You can test if the boiler is turning on at the right flow rate by SLOWLY opening a tap until the boiler just turns on. Don't open tap any further and measure the flow rate in litres per minute.

 

[MeldrewsMate]

I agree completely with Tony, that's the best advice you're going to get.
Not familiar with the Alpha boiler, but if it has a mechanical diverter valve and its diaphragm is perished then you'll need a higher flowrate to initiate hot water production than you did previously. These diaphragms fail gradually, so the symptom will get worse gradually.

When the shower is first switched on it draws mostly from the hot water supply, though the flow through the shower head is quite constant. If we assume 8 litre/minute total flow then the hot side will be supplying the full 8 l/m. Once the water becomes hot the internal thermostat starts to restrict the hot supply in favour of the cold supply, maybe 50:50 hot:cold, thus the boiler flow rate reduces to 4 l/m, maybe low enough to remove the boiler demand (turning the flame off). The hot water takes your notional minute to evacuate the pipework whereupon the shower thermostat detects the drop in temperature and diverts more water from the hot supply....ad infinitum.

You may wish to turn the boiler hot water temperature down to about 40 deg C as a supplementary test to increase the flow from the hot supply whilst waiting for the boiler to be repaired.

As for the whirring noise....I haven't a clue.

MM

 

[picasso]

Or you could just change the showerhead but if you wanna fart about with flow cups and running taps with the shower going while your watching the boiler go for it (best advice your going to get )