On Friday, 21 July 2017 12:39:01 UTC+1, John Rumm wrote:
On 21/07/2017 01:48, tabbypurr wrote:
On Friday, 21 July 2017 00:59:21 UTC+1, John Rumm wrote:
On 20/07/2017 23:16, tabbypurr wrote:
On Tuesday, 18 July 2017 00:56:12 UTC+1, tabby wrote:

Need a surface mounted thermostatic mixer shower. Hot pressure
is much lower than cold, ie vented cylinder versus mains. Any
recommendations, suggestions etc?

Am searching for venturi showers. Not seen anything suitable
yet.

But this isn't quite adding up. Specs on the units I've seen say
for 0.3m or 5m head. But the HW head is 3 floors up, so nearly
2 bar,

2 bar would be 60' of head...

no... 3 floors is about 20-25' so 2/3 bar not 2 bar.

With careful piping and a shower designed for low pressure operation,
2/3rds bar would be ok with the same pressure feed and flow on both
feeds. Its going to be very difficult however if you are attempting to
mix with a source at say 3 bar or more.

If you really have that, then unless the mains is at a
significantly higher pressure you should be able to get reasonably
performance out of a mixer with just a bit of pressure reduction on
the cold side.

Have you measured the static mains pressure?

No I don't have a measuring wotsit.

Might be worth getting one, so you have a feel from the magnitude f the
problem you are trying to solve. Probably only 10 to 15 quid from a
plumber's merchant.

But at any given flow knob
setting presumably a service valve could help equalise pressures more
nearly.

A service valve can throttle the maximum flow rate, but it won't reduce
the pressure. So if you attempt to to mix with a low pressure DHW, and
then apply enough flow resistance to the outlet of the mixer, you will
still likely back feed the DHW.

You need a PRV. One like:

https://www.toolstation.com/shop/p11827?table=no

Might be handy since it shows you what output pressure you are getting.

and the pipework 22mm. There's plenty of HW flow to the bathroom
sink, but the shower behaves as if the HW flow & pressure are
poor.

Something tells me this is not the setup venturi showers are
designed for, and that there is maybe a problem with the mixer
rather than the DHW supply. Or perhaps what's on the mixer's
outlet.

Have you checked it with the shower head removed? Is it a hard
water area? Shower heads can restrict flow dramatically, especially
if scaled a bit. The restriction will make the mixing at unbalanced
pressures more "twitchy"

I'm going to check it like that, I suspect that may be part of the
problem. The shower mixer has endured 35 years or so of scaly water,
though the head & hose are recent.

You may find you need to specifically look for a head that is designed
for low pressure operation.

Today it got a new hose & head. The new head has way more holes than the old one, and the hose is 11mm not 5mm bore!. And there were what look like lumps of rust partly blocking the 5mm, so no wonder it was misbehaving. Tomorrow will give it a good test run & see how much better it is. I can see straight away there's way more flow.

Next job is a PRV. The question is: honeywell £24 no gauge, another £6 with, or toolsatan £20 with gauge? Screwfix reviews for the non-honeywell aren't too good, so I'm inclined to get the better one. Is it worth adding a gauge?

FWIW I turned both sink taps on full. The last 5m or so to the sink is plumbed in 22mm hot, 15mm cold, before that it's all 22mm. The hot tap produces more flow at max than the cold.


NT