Shower: where to begin

Need a surface mounted thermostatic mixer shower. Hot pressure is much lower than cold, ie vented cylinder versus mains. Any recommendations, suggestions etc?
NT
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On 18/07/2017 00:56, snipped-for-privacy@gmail.com wrote:

A few options... running gravity hot and mains cold is always tricky - unless you fit non return valves to the hot you may find you just back fill your hot water cylinder with cold water from the mixer.
Options that would work include using a venturi shower (it uses the cold pressure to in effect boost the hot), or adding a single impeller pump and attempting to mix pumped hot and mains cold, or better, a dual impeller pump and pump both hot and cold (from a dedicated additional feed from the cold cistern).
--
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John.
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On Tuesday, 18 July 2017 01:39:36 UTC+1, John Rumm wrote:

yeah, the old one can do that

pump? no no. Venturi it is. Cheers.
NT
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On 18/07/2017 01:39, John Rumm wrote:

Where does the displaced hot water go? Up into the cold water tank? I do hope it's cast iron, not plastic.
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On 18/07/2017 08:20, GB wrote:

Yup.

You would have to spend a long time standing under a cold shower to displace a whole cylinder full ;-)
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John.
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On Tuesday, 18 July 2017 08:20:43 UTC+1, GB wrote:

Yes, wherefrom it will overflow into the loft.
(Whether it's a hot or cold overflow is another matter, but irrelevant as far as plasterboard is concerned)
Owain
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On 18/07/2017 18:15, snipped-for-privacy@gowanhill.com wrote:

Hopefully the cold cistern would have an overflow pipe out of the loft...
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snipped-for-privacy@gowanhill.com writes

Your header tank will (should) have an overflow out of the house.
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bert

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On 18/07/2017 18:15, snipped-for-privacy@gowanhill.com wrote:

What I had in mind was this frightful case, which was much more extreme than the OP's:-
http://www.telegraph.co.uk/news/uknews/1434588/Woman-killed-by-boiling-water-pouring-on-to-bed.html
A young mother died when boiling water from a faulty heater poured through the ceiling as she lay asleep in bed, an inquest heard yesterday. Sharon Minister, 30, was with her boyfriend, Mark Nicholas, when the plastic water tank in the loft above her melted and dumped 50 gallons of boiling water on to them. Mr Nicholas, 27, survived but Miss Minister died in a specialist burns unit a week later, the hearing in Helston, Cornwall, was told. Miss Minister's children Jessica, nine, and Victoria, six, were not hurt but they ran from the house in Penzance to a neighbour, crying hysterically: "Mark is bleeding and Mummy is burned." Mr Nicholas told the inquest that the couple woke in the dark to find themselves in terrible pain but with no idea what had caused it. He said his girlfriend had spent 17 months complaining about dampness in the loft beams but he claimed Penwith Housing Association, which owned the property, had done nothing about it. Mr Nicholas said he heard strange bubbling noises coming from the immersion heater as he went to sleep an hour and a half before the water came through.
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On 19/07/2017 08:10, GB wrote:

Yup, there have been a few cases like that (hence my comment about my suspicion that there are more serious accidents with gravity hot water systems). Many of these seem to be caused by electrically heated cylinders boiling and overflowing into plastic cold cisterns that are not well enough supported.
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John.
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On 7/18/2017 1:39 AM, John Rumm wrote:

I'd go for the last one. Don't get a cheap shed / SF / TS pump, get a Stuart Turner or at least a metal regenerative one from someone like Grundfoss.
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On 18/07/2017 10:03, newshound wrote:

We had a cheap, noisy and only just sufficient flow pump. It failed a while ago and was replaced with brass Grundfoss Watermill one. Far better.
SteveW
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snipped-for-privacy@gmail.com wrote:

    Use a manual valve. Better reliability and control.
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On Tuesday, 18 July 2017 09:47:45 UTC+1, Capitol wrote:

lower than cold, ie vented cylinder versus mains. Any recommendations, sugg estions etc?

A manual mixer valve is what I'm after. I'm not clear what other option the re would be. Maybe I'm misunderstanding what you mean. It needs a protectiv e thermostat to prevent scalding. And a nonreturn valve. On second thoughts I'm not convinced venturi is needed, there is enough HW delivery there. I definitely don't want a pump.
NT
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On 18/07/2017 12:33, snipped-for-privacy@gmail.com wrote:

A manual mixer is basically just a mixer tap - like the Mirra 88 with no thermostat or pressure balancing "smarts"

Manual mixers may not even have that.

That would be separate from the shower valve usually.

The issue is whether the DHW has adequate pressure to overcome the back pressure from the shower head while also being fed with mains cold, and not permit the cold to flow back up the DHW feed. The double check valve will certainly stop the backflow, but you may find that its very difficult or impossible to balance the temperature you want - a tiny change in cold tap position will have a disproportionate effect on the shower temperature.
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John.
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On Tuesday, 18 July 2017 14:38:56 UTC+1, John Rumm wrote:

that would be ok but for one thing. If cold pressure drops there would be r isk of scald, so really I need a thermostat to minimise any risk.

The main problem with the current aqualisa is that it goes from too hot to too cold over a very narrow range, making it a sod to set the temp usably. The fact that the mechanism has gotten somewhat sticky makes it overly diff icult to set. I can do it ok but others find it a headache. It also backfee ds & goes cold if turned on too far. Really one has to understand the issue s to set it effectively. Add to that the fact that every so often it comple tely changes its setting and it can't just be left alone.
I can't help thinking an easy option might be the old fashioned 2 tap appro ach. But no thermostat :/
I don't really know where to go with it. I don't want to replumb or add a p ump because there is enough hot delivery. I want a shower that won't go fro m ice to steam in one degree of movement, and that has a thermostat for saf ety.
NT
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snipped-for-privacy@gmail.com writes

Not been following but I noticed your comment about Aqualisa. We have one installed 23 years ago:-)
When first used, I called the plumber back because I was convinced he had swapped the hot and cold pipes! In fact the shower control is very sensitive to flow rates and will not control the water temperature reliably if you run at a low setting.
Ours is fed by a Stuart Turner double head pump because the header tank is so low (included attic house). Turning up the wick solved the problem.
I have replaced the thermostatic control once, lime scale, otherwise very reliable.

--
Tim Lamb

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On 18/07/2017 16:28, snipped-for-privacy@gmail.com wrote:

That pretty much describes the behaviour one would expect with significantly unbalanced pressures on the feeds.

Unlikely to be noticeably different.

The delivery rate is not really the issue. Its lack of hot pressure (or too much cold). Taps do an effective job of setting a required delivery rate regardless of the actual pressure.

So you need to fit a thermostatic mixer, and then either boost the hot pressure, or limit the cold pressure.
You could try using a PRV on the cold feed before the shower. However even of you get closer to a balance you then potentially still run into the normal problems with low pressure showers - i.e. you need carefully considered pipe runs and attention to detail in the ways water take-off happens elsewhere in the house to maintain decent steady state conditions.
If however you start with high pressure hot and cold, then they tend to be less influenced by these factors.
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John.
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On Tuesday, 18 July 2017 20:26:36 UTC+1, John Rumm wrote:

It's beginning to sound like what I need is a pump, and keep the existing shower.
I don't suppose an old CH circulator would do the job? I have one. And maybe a valve on the cold to tame it a little, not too much.
NT
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snipped-for-privacy@gmail.com writes

The nice thing about the Stuart Turner pump set up is the *auto start* arrangement.
Somebody in here once explained the mechanism but mine just works! You turn on the shower flow control and the pump starts!
--
Tim Lamb

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