Why would my shower suddenly start running hot/cold/hot/cold etc?

[Dave7]

It is NOT electric but runs off the boiler + cold water tap ( with adjusters for temperature which I never alter).
Been working fine for 2 years but with certain individual jets that don't get hot.
So.......thinking the head might be iffy I put a new one on this morning. Same hose, I just switched the head to a new one......and the problem started.
It started OK then after maybe a minute it went completely cold.....ran cold for a minute then hot again. It was like that for my whole shower.
Anyone got clue why it would happen ?

 

[derrick]

Heat exchanger possibly.

 

[newts]

Is the hot water supplied from a combi boiler (boiler fires up when you turn the hot tap on)?

 

[Dave7]

Is the hot water supplied from a combi boiler (boiler fires up when you turn the hot tap on)?

Yes it is. All the taps work off if.
Recently had the boiler serviced all CH and hot taps all work fine.

 

[newts]

Sounds like the combi boiler is cycling on/off. This can caused by a boiler issue &/or the thermostatic cartidge in the shower valve cyling through very hot/cold cycles as the boiler struggles. Get a Gas Safe engineer in to check the hot water flow rate on the boiler (not B/Gas they aren't very good!).

 

[MontyVeda]

was the washing machine running at the time?

 

[Dave7]

was the washing machine running at the time?

No. CH was on as normal.

 

[Dave7]

Sounds like the combi boiler is cycling on/off. This can caused by a boiler issue &/or the thermostatic cartidge in the shower valve cyling through very hot/cold cycles as the boiler struggles. Get a Gas Safe engineer in to check the hot water flow rate on the boiler (not B/Gas they aren't very good!).

I wonder if the new shower head is just too small and the pressure is causing the problem.
Think I will give the old head a clean, switch it back and see what happens.

 

[newts]

I wonder if the new shower head is just too small and the pressure is causing the problem.
Think I will give the old head a clean, switch it back and see what happens.

Remove the head & run the shower through the hose to see if the issue still occurs, this help to isolate where the problem is.

 

[CXRAndy]

Turn the central heating to off. Run a hot tap so the boiler kicks on and produces hot water, not full flow.

Then switch on the shower, observe the boiler and hot water tap for major flow variations and check if the boiler is switching on and off.

If the boiler runs constantly, it points to the mixing/thermostatic valve in the shower.

If the boiler is cycling on and off , get an engineer to check it out

Testing made easier with two people (communicate by mobile phone if distance too far shout)

 

[slowmotion]

If you have a thermostatic shower valve ( you may not ) , it could be that the thermostatic wax cartridge in the shower valve is on the blink. I got fed up with our's go wonky every few years and replaced the valve with a manual one.

 

[presta]

My shower's electric, but I have a Worcester Bosch 28 CDi Compact combi boiler, and I've noticed something similar from the hot tap on the washbasin (but not in the kitchen). Normally it'll run cold until the boiler warms up, but occasionally it runs cold hot cold hot for up to 6-8 repetitions before it settles to a constant temperature. It's not very frequent, so I've never bothered chasing it, but the boiler's nearly 7 years old, and it didn't start doing it until at most a year ago.

 

[newts]

If you have a thermostatic shower valve ( you may not ) , it could be that the thermostatic wax cartridge in the shower valve is on the blink. I got fed up with our's go wonky every few years and replaced the valve with a manual one.

There is a very high risk of scalding with a non thermostatic shower valve if a combi boiler cycles & you get a burst of hot water.
When a themrostatic cartridge fails you only get tepid water through (the failsafe inbuilt in the cartridge closes off the hot supply if there is not enough cold water coming through).

 

[Supersuperleeds]

It is NOT electric but runs off the boiler + cold water tap ( with adjusters for temperature which I never alter).
Been working fine for 2 years but with certain individual jets that don't get hot.
So.......thinking the head might be iffy I put a new one on this morning. Same hose, I just switched the head to a new one......and the problem started.
It started OK then after maybe a minute it went completely cold.....ran cold for a minute then hot again. It was like that for my whole shower.
Anyone got clue why it would happen ?

When ours did this, I think it was the heat exchanger. Though I think it impacted the bath as well, the other taps don't get run long enough. I think the the engineer said something about a thermostat control cuts the hot water off as it thinks it is too hot, then it turns it back on when it thinks it is cool enough, and repeat and repeat until the shower/taps are turned off.

It was an easy fix, not sure of the cost has we have British Gas home care and the boiler is covered by that.

 

[slowmotion]

There is a very high risk of scalding with a non thermostatic shower valve if a combi boiler cycles & you get a burst of hot water.
When a themrostatic cartridge fails you only get tepid water through (the failsafe inbuilt in the cartridge closes off the hot supply if there is not enough cold water coming through).

I've got a conventional system with a condensing boiler and DHW cylinder. Combi's have been shown to have part-load summertime efficiencies as low as 45% due to frequent cycling and short DHW draw-offs.