Shower Query

Hi
I recently purchased a refurbished top (2nd) floor flat and have a little issue with the shower, which I would appreciate your opinions on (I would note I have no plumbing knowledge!):
I should set out the following I suspect:
The water main is 22mm in diameter (new stopcock- fitted by our family plumber recently, fully on and back half a turn), this connects to a (sept 2004) Worcester Junior 24i combi, serviced by them last week. This was said to be fine when serviced. Hot water taps and rads all function fine. The problem pre/post-dates the service.
There is what I believe to be a 'thermostatic' shower in the cubicle. It's a chrome bar with a flow knob at one end, temp the other. No brand name apparent. Again late 2004 install.
I have used it everyday for the month we've been in, set at c. 34degrees and flow i guess on just enough to be a sensible power (which is very nice indeed). No problem.
However, when my girlfriend uses it, which of course takes longer, at the same temp and flow rate, the shower goes cold/cool for 10-20seconds, after a while, then warms up again. I would note NO other taps/rads are on at this time of the morning in 'our' flat. This happens every day to her, never me.
I have read various posts/talked to people and various things have been suggested, including
- reducing the mains flow (bolier manual) which will increase water temp? - remove a 'flow restrictor' from the boiler, (worcester engineer) as he thought the hot water tap in the kitchen seemed a little cool/slow (i have not noticed this, but he knows what he's doing!). - replace wax thingy in shower unit??
Thanks for your time.
David
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snipped-for-privacy@hotmail.com wrote:

The shower in my previous house did this. The combi boiler (eurocombi) was not fully modulating, but had 2 settings of 30% and 100%. When using the shower it started off at required temp according to the thermostatic mixer. After a while (when water temperature exceeded the preset hot water output temperature), the boiler modulated to 30%, which was not enough power to heat the water to the thermostatic mixer setting at the flow rate required, so shower went a bit cold. After about 15 secs, the cycle repeated and the hot came back on. The problem is simply due to the boiler not having sufficient range of output settings. The cycling can be worsened by some thermostatic mixers where when the temp goes up, so the mixer reduces the hot flow rate, so the combi on one of its limited range of output levels causes the water to get even hotter, so the mixer reduces rate etc, in a cycle. Eventually the flow of hot is so low that the boiler can switch off entirely, in which case the shower will to totally cold until the boiler relights - that is the worst case. Solutions ? You could have the flow on very high, such that the boiler always remains on its maximum setting, or very low such that the boiler is always on it's lowest setting (except off !). In fact any level where the boiler stays on one of its fixed output levels would work. The problem is that any (random) change in pressure/level can set the cycle going. If you get a mixer design and settting combination, where effectively hot flow rates are not reduced, just the cold is modulated, this can help. (But depending on plumbing, increasing cold flow may reduce hot flow, this the cycle kicks off again ...)
The only real answer is to have a "fully modulating" boiler, i.e. one with many discrete levels of output, such that the thermostatic mixer can always set to the desired temperature. Such boilers will often keep the output temperature pretty constant with varying flow, and you may then ask, do you even need the thermostatic module ? Obviously the module is useful as whenever you switch the shower on you will get the temperature you want (after small adjustment periods). In my new house I am using a shower hose at present, since I am doing up the bathroom. The boiler I have now is a vaillant, and I was surprised so find that whilst using this simple hose, the temperature is rock-solid, even when the loo is refilling. It's just a good boiler I guess. But I have experienced good and bad combis, and the difference in performance can be quite marked.
Hope that helps. Simon.
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The previous poster suggests that the problem is that the boiler is generating more heat than is required leading it to shut itself down. If this is the case then reducing the mains flow will not help - it is equivalent to slowing down the shower flow rate and will exacerbate the situation.
Does your g/f use the shower at a slower flow rate than yourself? If the modulating problem described is taking place then you want to increase the water flow rate so the boiler does not reach the maximum temperature and cut out. Another test would be to leave a hot tap running while having the shower and see if it happens then.
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I find that mine starts very hot (heat rise after the thermostat is satisfied due to mass of metal) goes cool and then the boiler realises and fires up to recover. Just a bit too much lag in the responses I think.
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snipped-for-privacy@hotmail.com wrote:

Is this just once or does it cycle? If once then see what happens if you shower after her. When it is turned of after the first shower, the boiler will continue to pump for a while and cool down the heat exchanger. Next time the tap is turned on there will be some hot water left in the pipe between boiler and shower, but it will take time to heat up, so you will get a slug of coldish water coming through.
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Thanks everyone
To answer your questions:
- the problem does 'cycle' she said it went 'cold' (not cool) twice this morning on a hair wash day!
- flow rate, nope she uses the same as me (on enough to hit sensible power, which is only a slight turn) give or take a slight margin, i will stand over her tmrw morning and double check
- problem occurs no matter who showers first, for example I was first yesterday, she was first today, and the problem occured - for me, blissful shower as ever!
- temp, when I use it, the temp is fine, takes 5-10secs to hit the preset temp and stays constant for the time I am in there - perfect.
I would note that no other taps etc are drawing supply, just the shower.
Thanks
David
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Tell her to use a bit more flow than usual and see what happens.
Christian.
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Christian,
I will do! I will report back tmrw morning, thanks.
Regards
David
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She used the shower this morning, with the flow rate higher, much improved went cool for a few seconds, rather than 10-20,...............what does this mean, do I have to accept it, would a better shower fix the problem?
Thanks
David
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snipped-for-privacy@hotmail.com wrote:

Poor thermostatic valve in the shower, possibly not the type designed for mains pressure hot water, or perhaps just worn out.
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Okay, well thanks eveyone.
Bottom line is she is happy with the shower now & so am I, suffice to say if it every fails I will replace with an entirely suitable/quality version.
Cheers
David
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snipped-for-privacy@hotmail.com wrote:

ks eveyone.

One other suggestion, it helps to prevent the valve sticking and scaling up it is not always left on the same temperature setting: try moving it through the full range of adjustment occasionally. You don't need to be standing in the shower, or even have it running when you do this.
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