Fluctuating water temp in shower

[KeithXP]

Apologies for the long post, and thanks in advance for reading it!

Problem: Shower water temperature drops to lukewarm, then returns to hot on 1-2 min cycle. The problem is pronounced in winter, presumably because incoming water is colder. Almost unnoticeable in summer.

This has been happening in our main bathroom for some time, where we have a thermostatic shower valve ("QL4 1-way" from tapstore.com). We have good mains pressure, all our bathroom facilities are in the basement which helps.

We have a Ferroli F30 combi boiler. Each time we have a service visit we complain about this problem and the engineer will tell us it is not the boiler, partly because when running just a hot tap, it produces consistently hot water. They suspect the thermostatic valve is operating poorly.

We just had a new shower room fitted out and find the problem is the same here, where there is a brand new thermostatic valve (Metro from bathstore.com) in the shower cubicle and also a deck-mounted shower head on a manual valve. Water temperature fluctuates on both of them.

I have been told to try cutting back the flow rate to see if it is more stable - it did not appear to make any difference.

Getting desperate now, to the point of considering a new boiler - but need to know that this is the source of the problem. Can anyone tell me how to get to the bottom of this?

thanks gain.

 

[piscokid]

Sorry, I know this post is a year old, but the answer may help somebody. I recently replaced the flow switch in an F30 with fluctuating DHW, as the thermistor tested ok, but it was an expensive failure at £44. If the boiler fires up with the inner cover off, suspect flue related stuff, in this case it was the plastic venturi in the fan which had crumbled - under a tenner and 20 mins to change.

 

[KeithXP]

We fixed ours by getting a new Worcester boiler... The Ferroli was rubbish and I would not recommend it. The new boiler provides abundant hot water with no temperature fluctuation at all.