Problems with hot water since fitting shower mixer valve.

Hi, have a Worcester 35CDi II combi, and everything was fine until plumbed up shower mixer valve. Now, whilst the heating works fine, the hot water only cycles between lukewarm and cold (with the boiler switching the heating off soon after it goes on).
Now there was some vague blurb in the mixer tap fitting instructions about the water pressures. We have very high water pressure here ~5Bar, and I a quick Google has brought up a single mention of someone seeing something similar, that the pressure of the cold water is going through the valve and back down the hot output.
Can someone elaborate on this? What are my options on how to fix this? Fit a pressure reducer to the cold feed? What sort of pressure will my combi hot side have? I always thought it was the same as the mains pressure when you had a combi....
Mixer PDF: http://www.roperrhodes.co.uk/wp-content/uploads/2012/02/DUAL_TRIPLE-INLINE-SHOWER-VALVE-INSTRUCTIONS7.pdf
"Unbalanced Water Supply Valves must always be operated within either the range for BS 1287 OR BS1111 as described in the table below. Valves cannot operate effectively where a hot or cold pressure system crosses the boundaries of the two ranges. In addition the maximum ratio of unbalanced hot and cold water pressures for the valves to operate effectively is 5:1. Hot or cold pressure must be reduced or boosted so as to work within the required range.
Maximum Water Pressure Concealed Dual and Triple Control shower valves are suitable for use with all water supply systems up to a maximum of 5.0 Bar. Operating pressures above 5.0 Bar will require the installation of pressure reducing valves."
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On 01/10/2012 10:00, MarkG wrote:

http://www.roperrhodes.co.uk/wp-content/uploads/2012/02/DUAL_TRIPLE-INLINE-SHOWER-VALVE-INSTRUCTIONS7.pdf
Can you isolate the hot or cold to the shower mixer valve? If so, do that (either of them or both) and if that solves the problem, simply fit two non return valves to the hot and cold feed to the mixer.
If you cant isolate the shower, put the mixer to coldest setting and see if that solves it, if so, fit the non return valves.
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Well, spoke to my bathroom fitter, and whilst the mixer has been plumbed up for a while, he only turned the isolators on last night, which was exactly the time the other problem cropped up, so pretty sure it's related.
Will the non-return valves limit flow in any way? Is there a specific type? Going to pop down plumbase and pick a couple up in a mo....
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On 01/10/2012 15:15, MarkG wrote:

With your high water pressure and it only supplying the shower, I am sure it won't be a problem.
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On 01/10/2012 10:00, MarkG wrote:

Chances are that the combi needs to throttle the water going through it in order to heat it sufficiently. [Otherwise you might have a very high flow of luke warm water].
What flow rate do you get at (say) the kitchen hot tap? How does this compare with the rate at the kitchen cold tap. If the cold rate is much higher than the hot, throttle it so that they are more or less equal. The shower mixer should then work better.
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You seem to have been to posh bathroom supplier, at least by the look of their website. Would it not be their responsibility to have installed a shower suitable for your pre-existing boiler?
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On Mon, 01 Oct 2012 17:49:25 +0100, Phil Addison

Well it's suitable for both hi and low pressure systems, but clearly can get freaked out by large differentials in pressure. They didn't install it, they just supplied the valve, and it's not even theirs, it's a stock concealed thermostatic triple valve that every man and his dog seems to use these days, they just do their own knobs and faceplates to suit...
Been on a trip to Plumbase and picked up a 15mm pressure reducing valve on the cold side.
http://www.ebay.co.uk/itm/Honeywell-3-4-INCH-Pressure-Reducing-Valve-PRV-for-15mm-or-22mm-1-5bar-6bar-/261101591356?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item3ccade1b3c
or a check valve on the hot side to stop the cold going back down.
http://www.ebay.co.uk/itm/BRASS-SINGLE-CHECK-VALVE-15MM-NON-RETURN-COMPRESSION-/250688583906?pt=UK_DIY_Materials_Plumbing_MJ&hash=item3a5e3438e2
Not sure what's going to be best (opinions???), but have both options available.. The other option I have considered is that my boiler might not be pressursing the hot side correctly, and it's only become a problem now, so I guess I need to pressure check both sides to see what I have...
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I am a bit surprised by your problems. One of the main benefits of combi boilers is that you get balanced pressure of both hot and cold water (the hot water comes from the mains too, it just passes thro a heat exchanger).
A quick look at the installation manual for the boiler shows that it is suitable for use on mains supply of up to 10bar without the use of a pressure reducing valve so I wouldn't expect one to be fitted to yours (it would be on the cold feed to the boiler).
That said, the symptoms you describe do suggest back pressure on the hot side, reducing or stopping flow and causing the boiler to shut off.
Have a look to see if you can see a pressure reducing valve before the boiler as that could be an explanation. The manual I looked at did not show an internal pressure reducing valve.
You also say the mixing valve has a maximum pressure of 5bar so there may be an issue there too but the bad news is that if you have over 5bar on both hot and cold then you may need to pressure reduce both.
I don't think a check valve on the hot side will be of benefit as if the fault is a cold to hot pressure differential then the valve will be held closed by the pressure diff, no water will flow end the boiler wont fire.
As an experiment, try opening a hot tap in the bathroom a bit then turn on the shower valve. If the flow from the hot tap increases then you have back flow cold to hot.
HTH
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Well it states a maximum "static" pressure of 10bar and a maximumk "flow" pressure of 5 bar.
What's the difference? - please no wise asses saying 5 bar :-)
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I did the test, and the hot flows correctly out of other taps when the shower valve is on....
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This means that when it is off it wont leak or burst at a pressure up to 10 bar, when it is operating (on) however the pressure limit is lower, 5bar.
No idea what the reason for this is, it may be a pressure balance issue or it may be that post valve sealing is not proof to higher levels.
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On 01/10/2012 19:57, MarkG wrote:

Are you asking why static and flow pressure are different, or why this device has different limits for the two? Someone else has addressed the second possibility, so I will address the first.
Static pressure is what you get when there is no flow, so the full pressure generated by the pump - or whatever is driving it - is seen at the point of application. Flow pressure is what you get when liquid is flowing, and is lower than static pressure because some of the pressure is lost along the way, overcoming flow resistance.
The electrical analogy is to consider the difference of measuring first the Open Circuit voltage at a point in an electrical circuit, and then the voltage when current is flowing. The second measurement is lower due to the internal resistance of the power supply and the resistance of the cables etc.
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On 01/10/2012 18:26, MarkG wrote:

http://www.ebay.co.uk/itm/Honeywell-3-4-INCH-Pressure-Reducing-Valve-PRV-for-15mm-or-22mm-1-5bar-6bar-/261101591356?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item3ccade1b3c
http://www.ebay.co.uk/itm/BRASS-SINGLE-CHECK-VALVE-15MM-NON-RETURN-COMPRESSION-/250688583906?pt=UK_DIY_Materials_Plumbing_MJ&hash=item3a5e3438e2
You want to use the non return valve on both hot sides!
What is happening is this...
Your shower comprises of a mixer then a simple on/off valve, so when the valve is off, this is just stopping the mixed water from exiting through the shower head, it should already have a check valve, the PDF you linked to shows it does, but this doesn't seem to be working, or the shower only has one on the cold, or it has one on the hot, but has been connected the wrong way around (do you get hot water out the shower when you ask it to deliver hot!?)
Fitting a non return valve to the hot feed to the shower will prevent cold water passing through the valve when you open a hot tap, and the boiler restricts the flow to get the temperature you have asked of it, if the boiler didn't restrict the flow, then too much water would pass, and the boiler would not be able to raise the temperature enough, so when the boiler is restricting the flow, water then passes through the shower mixer, as it is an easier path.
If you are just taking cold water at a high rate (bath tap, or multiple taps open on cold possibly), then I expect you may end up with water flowing the other way through the shower valve, this is why I suggested having two non-return valves on the shower, one on each feed.
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OK, picked up a pressure gauge from PlumbCentre, measured 4Bar on cold side and 3.3Bar on the hot side.
So it's below the 5Bar limit, which is good, so the question is, would a 0.7bar difference in pressure cause this sort of problem? Seems like a VERY picky thermostatic mixer, or is this perfectly normal?
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On 02/10/2012 19:08, MarkG wrote:

That's strange, with all the taps off, I would expect the static pressure to be the same on both the hot and the cold if they are both "powered" by the incoming mains supply.
In the PDF you linked to, it shows a non return valve, are you able to see if this is actually present on the unit you have?
I asked earlier if the pipes were connected the correct way around, are they (do you get hot water from the shower if the mixer is set to hot only?)
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