New thermo shower cycles cold hot

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Hi gang,

I’ve tried to do my best looking at old threads with similar issues so apologies if this repetition.

I’ve just had quite a bit of work done; a downstairs extension with bathroom. The water supply to the extension was done in 15mm due to spurring off available convenient water supplies in the existing kitchen.

I forked out on a new Grohe Euphoria 260 shower which has a thermostatic bar and 2 separate outlets - the handheld and the rain effect shower head.

All of this was added to a 13 year old Worcester Bosch Greenstar 30i combi boiler.

All seemed well but now the shower cycles between hot and cold. I seem to be able to prompt the warm by turning the shower off and then on in the hope it creates enough demand on the boiler but not sure if it’s just coincidence.

Theres no cycling hot-cold anywhere else in the house and, while the plumber has said the boiler will need replacing eventually, this appears a new problem isolated to just the new shower.

So, is this a flow rate or pressure issue, or is there a cheaper tweak to try before trying to fix it with £2ks worth of new boiler? There’s mention on other threads about restrictors causing the problem, or the temp of the boiler DHW and the shower head.

Your guidance appreciated, as ever.
 
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Could be reversed supplies causing the issue, as well as restrictions, e.g. Blocked filters.
 
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Same boiler in mothers house found a problem in shower would get hot, then go cold, then hot again and stay hot, reason was boiler has a small water store, which would be the first shower going hot, and then that would run out, so shower went cold, then boiler had fired up, so hot again.

If I turned Eco on, it did not use water store, so stopped problem with shower, but then if I turned kitchen taps on to give trickle of hot water the boiler would not fire up, so I just waited for it to go cold and hot again.

When I did bathroom in my house I looked at shower with side sprays, but it used more water than my Main 7 boiler could supply, so simple thermostatic shower, have found over the years must use large shower head, works OK with shower head off an electric shower in winter, but in the summer when in coming water is warmer there is not enough flow, and shower will go cold, but mothers boiler seemed to modulate better than the old Main 7 (1980's model before combi came out) and I could turn shower down a little.

Try switching Eco off, and actually measure how much water a minute the shower lets through and compare with boiler spec.
 
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A few things to try.
1 Set shower temp to cold, do a timed bucket test.
2 Set shower temp to max, do timed bucket test plus see if the thing cycles at max
Compare the 2 flow rates plus do a subjective assesment of hot pressure vs cold pressure. If cold pressure/flow is much higher than hot that could be causing your problem.
3 Reduce dhw temperature on the boiler, see if that improves things
 
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A few things to try.
1 Set shower temp to cold, do a timed bucket test.
2 Set shower temp to max, do timed bucket test plus see if the thing cycles at max
Compare the 2 flow rates plus do a subjective assesment of hot pressure vs cold pressure. If cold pressure/flow is much higher than hot that could be causing your problem.
3 Reduce dhw temperature on the boiler, see if that improves things
What will each of these 3 show me?
 
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The first 2 will give you some numbers to go with any subjective assessment of hot pressure/flow vs cold. Those mixers work best with similar hot and cold pressure. No 2 may uncover a restriction in hot pipework-if
The 3rd may solve the problem- if you reduce dhw temp then the mixer won't be adding as much cold/reducing the hot flow so there may be sufficient flow through the hot to keep the boiler firing
 
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