high water pressure

Hello,

When taps are described as suitable for high water pressure or low water pressure, or reducers in toilet inlets, etc. what is the definition of high pressure and low pressure, i.e. how many bars does it have to be to be considered high pressure?

Thanks, Stephen

Reply to
Stephen
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As a rule of fumb, I believe they refer to mains as 'high' and gravity as 'low'.

Reply to
The Medway Handyman

I would say that anything over 1 bar counts as high.

Reply to
John Rumm

Relative, or absolute?

Reply to
Sam Plusnet

above atmospheric...

Reply to
John Rumm

But what happens when you have a mixer tap which is fed by gravity hot water but mains cold?

Reply to
Stephen

my kitchen tap works fine.

Reply to
charles

If its a good one, not a lot as they don't mix until the outlet. If its a rubbish one then the high pressure cold forces the hot water back into the tank and it overflows.

Reply to
dennis

If its a proper mixer tap then the cold feed must include a double check valve...

Having said that - they often don't work well in this setup - in some cases just forcing mains cold back up the hot feed!

(typical kitchen mixers are of a concentric spout design where the mixing (such as it is) happens outside the tap.

Reply to
John Rumm

In the kitchen, I have a ball valve restrictor in the cold supply.

Reply to
Michael Chare

Bath mixer taps with shower diverter valve aren't meant to be fed direct from the rising main (ditto for shower mixers). The cold is meant to be fed off the header tank to maintain equal pressure between the hot and cold supplies.

Kitchen 'mixer taps' designed to allow a direct rising main feed for drinking water usage keep the two streams seperate until they emerge from the 'mixer nozzle' specifically to avoid this back flushing issue.

However, it is possible to defeat this simple protection against unintended backflushing by forcing an old fashioned push on washing machine hose over the end of the mixer nozzle outlet as I proved when the flow from the hot tap had dropped to a slow trickle a few months after the kitchen extension had been built.

In this case, the backflushing was an intended result which completely restored the hot flow to better than new where it has remained so for the past 28 years since.

The central heating developed a blockage in the header tank feed a year or so after it had been installed which prevented me from bleeding air out of even the ground floor radiators. Obviously a serious issue which I tackled by pushing a suitable length of pyro cable down the feed pipe from the header tank until it hit and broke the obstruction right at the soldered T joint.

It seemed to me that I must have broken up an encrustation that had formed across the end of the feed pipe just where it met the horizontal run of the CH pipework. Like the kitchen hot water feed, this also proved to be a permanent fix.

In this case, I assumed that dissolved residues from the soldering had precipitated out just where the hot water met the cold header tank feed and had built up a crust across the end until it had completely sealed the cold 'port' at the T joint.

In the case of the ever reducing kitchen tap hot water flow, I'd assumed a concentration of soldering debris building up at a sharp bend/ T joint connection which I'd managed to dislodge by my pulsing the backflush pressure (although ther hadn't been any obvious signs of 'crud' appearing during this process).

At the time, there was no access to information via a search engine as there is today so I never pursued my theories on this 'strange' (to me) phenomena nor ask if this was a fairly common issue with brand new plumbing works.

I do harbour a suspicion that this might have been the result of 'deliberate' sabotage in order to screw extra payment out of post installation work that should have been carried out as part of a 'proper job'. Is there a possibilty of my suspicions having some basis in fact or was I simply being paranoid?

Reply to
Johny B Good

(Opens new can of worms).

Many of the cheap el crappo taps sold by the sheds are designed for mains hot & cold from a combi boiler.

If you fit one to a mains cold, gravity hot system you will only get a trickle out of the hot side.

A 'proper' mixer, from a plumbers merchant, is designed for mains cold, gravity hot & will work a treat.

A proper plumbers merchant will ask the punter the right questions.

Reply to
The Medway Handyman

Still seems to apply to some kitchen mixers, but I did notice, when sourcing taps for the new bathroom that bathroom mixers nearly all seem to be balanced pressure (or unspecified) nowadays.

When I did the bathroom in the old house, circa 2000 the catalogues seem to often list version for both balanced and unbalanced pressures

Reply to
chris French

Assuming that the hot supply is low pressure. When fed from a combi for example it would be normal to supply the tap from the rising main (via a double check).

If fed from an unvented cylinder, the the cold ought to be fed from the pressure balanced output of the PRV.

Forcing mains cold up a vented hot feed is a "standard" trick for clearing an air lock in the hot feed.

Remember its not just a feed tank though, but feed and expansion. So in normal use, hot water will push up into the F&E tank. Its probably the place where you get most heating and cooling cycles of the water, coupled with the location where most new (i.e. oxygenated) water will enter the system, so its a prime location for a scale related blockage. (not to mention any other fungal growth that decides to form a crust over the top of the nice bacterium friendly warm water tank!)

Air locks in gravity fed HW systems are quite common unless the installer has taken great care with all the pipe runs to make sure there are no inversions or up hill sections.

Who knows... could equally be a result of less than perfectly skilled plumbing. Never put down to malice what can be explained by incompetence etc!

Reply to
John Rumm

Perhaps the fact that modern designs are unspecified means that it doesn't matter and there's nothing for me to worry about any more?

Thanks for all the replies. The tap in question was in situ, and I hadn't bought it. I contacted the manufacturer (ideal standard) and they told me they define low pressure as 1 bar and anything above that as high pressure, so I take that to mean gravity HW is low pressure and the cold main is high pressure.

I've only ever had vented HW, so don't know about other systems. I guess HW from a combi boiler classes as high pressure because it comes from the mains. I know there's often concern here about combis not being good enough for showers but I'm guessing that's low flow rather than low pressure? Is the hot water pressure at the tap comparable to the mains cold pressure?

BTW what flow do you need from a combi for a good shower?

Thanks, Stephen.

Reply to
Stephen

Correct (although we get a decent shower off ours unless someone runs a tap at the same time) (*)

Yes

An expert should be along shortly.

*: A builder has suggested that if we replace the century-old lead pipe incomer, we should dramatically improve our flow rates.
Reply to
Martin Bonner

How much of the flow restriction is the hard water scale, which does at least mean you aren't drinking dissolved lead?

Reply to
Alan Braggins

Combis are usually pretty good for showers - they have high presure, but low to medium flow rate. The area that lets all but the most powerful down is bath filling, where you want high flow rate.

Most modern showers are happy with 7 - 9 lpm

Even a 24kW combi will do 9 lpm at final use temperature.

I used to find a 35kW combi would do two acceptable showers at once.

(soaker heads and body jets etc might be more difficult though)

Reply to
John Rumm

I used to find that hard water scale would create a fairly thin yellow/orange coating on the inside of a lead pipe... the main restriction just being the 1/2" diameter of the pipe rather than the scale.

(however different areas of the country may get different types of scale deposit)

Reply to
John Rumm

His implication was that the flow would have been reasonable when the pipe was fitted, but it will have furred up. (Cambridge water is *hard* (320 mg/L of CaCO3), so I'm not worried about drinking lead.)

Reply to
Martin Bonner

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