Shower water temp issue

Philcan

After a few minutes of how water flowing in my shower the water starts turning cold. It stays cold for a minute and then starts to heat up again. Water stays warm for a few minutes and then turns cold again. This cycle repeats.

Shower is fed from a combi boiler which is less than 18 months old.

Anyone any ideas on what is happening?

Thanks.

John.G

Has the combi got separate CH (heating) and DHW (hot water) control settings on the boiler font.
Could be a number of things including comi flow switch problems, unequal cold and hot pressures to the shower, assume its thermostatic?., shower head blockage or thermostatic cartridge failure.
You don't say what the kw rating of the combi is but I would think that the minimum flowrate would be ~ 2 to 3 LPM to activate the flow switch and call for burner cut in.
I would suggest doing the easy things first.
Ensure CH off and while slowly opening a hot water tap only get someone to observe the burner cutting in, stick a 1 litre jug/bottle under the tap and measure the flowrate, again with someone observing, slowly close the hot tap until burner cut out and again measure the flowrate. If this is satisfactory, power off the boiler and put the shower on its maximum temperature setting and flow and measure its flowrate. Don't know if you can then turn the temperature setting down to say 10C 15C but if you can repeat this test and again measure the flowrate at this setting. If the shower is non thermostatic just measure the flowrates turned to fully hot and fully cold.
Some, as a temporary measure have got the showers to operate by first opening a hot water tap then then switching on the shower to its normal settings and then closing the hot water tap.

Is the combi mains fed or from a pump via a header tank.?

Sleeper12

Mains fed combi boiler can cause this issue. It could be uneven hot and cold pressure going through a thermostat shower

John.G

I hope its not the boiler Hx getting blocked with limescale/debris even though only 18 months old but a full flow hot water test from a hot tap only should tell.
I do know of a case where someone was getting a flowrate of 10 LPM (hot) from the shower with shower head removed but with the head back on and even though still flowing 8 LPM it was turning and remaining cold, eventually when they found that a hot tap had to be almost full open to establish enough flow to get the burner to fire up, cured by fitting a new flow switch, while waiting for this to be installed the hot tap was partially opened and left this way while showering.
I must look up a few combi user manuals to see what the normal pressure loss is through the DHW side of a clean Hx.

Philcan

Thanks for the comments.

Some more info.:

- Boiler is 24 KW and has separate DHW and CH settings.
- Shower is thermostatic and looks old.

Also, when shower is turned on and the cold water starts to flow, the water pressure is strong. But as soon as the water starts to get hot the water pressure drops significantly. And when the water turns cold again the water pressure increases.

Hot water runs to the sink tap without any issues - stays hot and good pressure.

Thermostatic cartridge failure?

John.G

Could be since the flow rates are changing but a 24 kw combi should only need a flowrate of 2 LPM to keep firing, only other suggestion is to increase the Combi DHW setting to min (and then max) and see what happens. if you run the shower with the hot water tap partially open does it and the hot water tap run hot.
As stated above it could also be the flow switch so might be worth carrying out that hot water only test to check burner cut in/ cut out.

Philcan

Thanks John.
So as you suggested below, running the hot water tap at the same time as the shower solves the problem (water stays hot).

What does a flow switch do by the way?

John.G

The flow switch tells the boiler that you require DHW, also if the boiler is running on CH mode at the time then the flow switch changes over a 3 way valve and diverts the boiler water to a plate type heat exchanger which heats the incoming cold mains water for your hot water requirements. This flow switch will switch in at ~ 2 to 2.5 LPM & switch off at ~ 1.5 to 2 LPM. Your test above still doesn't prove conclusively IMO that you have a flow switch problem so definitely try and establish what flow rate it is switching in/out at by doing that simple hot water tap test above and you can then move on from there.

Also can you say oil or gas boiler and make/model.