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Plumber question - water pressure / shower

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 elliot.baker 03 May 2021

Help please!  

We need to get a new shower. Thinking of changing to one fed off the boiler instead of electric, want one of these ones with a rain head and the separate head, it says "minimum recommended pressure 1.0 bar".

Now our boiler pressure on the front of it is 1.2 bar (you can increase the pressure by putting more water in). I think it's a called a "combi" (it makes hot water on demand), we don't have a tank).

The shower also says maximum flow 9.5ltre/min (rain water head) or 5.7 ltr/min (handheld head)...

oh and FYI the boiler is on the ground floor and the shower will be two floors higher.

... so...

is it that,

a) because the boiler is 1.2 bar we are definitely ok, or 

b) I need to measure the water flow coming out of the bath taps (where the shower will be) using a bucket and stop watch, and if it's more than 9.5 litre/min then I'm definitely ok (and if it's less than that does it mean we definitely shouldn't get that shower?)

c) something else!

Please help! Please don't say "ask your plumber"!

many thanks. I will repay this kindness with answers to things I know about one day!

 deepsoup 03 May 2021
In reply to elliot.baker:

> Now our boiler pressure on the front of it is 1.2 bar (you can increase the pressure by putting more water in).

That's a completely different thing, it's the pressure of the water in your central heating system (and inside the boiler itself, which is a part of that sealed system).  Nothing to do with the cold water flowing in, through the heat exchanger and out again hot. 

The 1.0 bar you need is at the tap in the bathroom.  In a system with a hot water cylinder the pressure of the hot water is determined by the height of the header tank (and it'd almost certainly be inadequate for a big shower head on the top floor).  With a combi-boiler the hot water is pushed out of the boiler by the cold water coming in, and it's unlikely to be a problem unless your cold water supply is a bit dribbly.

> b) I need to measure the water flow coming out of the bath taps (where the shower will be) using a bucket and stop watch, and if it's more than 9.5 litre/min then I'm definitely ok (and if it's less than that does it mean we definitely shouldn't get that shower?)

Strictly speaking at the height the shower head will be above the bath taps, but if it makes a significant difference the pressure's probably not up to it.

Post edited at 15:20
 Pedro50 03 May 2021
In reply to elliot.baker:

I know nothing of the figures but my sister remodeled her bathroom and replaced her lovely electric shower with one fed from the modern combi boiler. To my mind it's barely adequate and disappointing, I'm too polite to mention it. 

 wintertree 03 May 2021
In reply to elliot.baker:

a) As deep soup said the gauge is the pressure of the heating loop.  Mains pressure is almost certainly higher (varies between 3 bar and 4 bar for us).

b) Flow rate results from the choice of shower head, the length and bore of plumbing in there and the mains pressure.  As long as water comes out of a tap faster than that, you’re not going to have a problem.  I can’t imagine it doesn’t.

Some combi boilers thrash temperature up and down for low flow rates which makes them incompatible with some shower heads; they can’t match the continuous low demand.

With a bit of bodgery you can attach a shower head to a mixer tap somewhere in the house via a house pipe and test it outdoors to see if you get a stable temperature.  

 CurlyStevo 03 May 2021
In reply to elliot.baker:

Make sure its thermostatic shower system - most are now I think

 CurlyStevo 03 May 2021
In reply to Pedro50:

Mine is far better than any electric shower I've used but I have high water pressure

 Craigyboy13 03 May 2021
In reply to elliot.baker:

what's your cold water pressure like in the bathroom, that's a good start. 

don't be upping the pressure on the boiler either  

if you do go for a new shower I highly recommend a mira digital shower! so good! 

 elliot.baker 03 May 2021
In reply to elliot.baker:

Thanks for the tips everyone, so I just did a timed bucket fill with the hot and cold taps both on for 30seconds, and it filled up a bucket a little past the 5ltr mark. So it's definitely >10ltre/minute flow at the bath taps (the water wasn't perfect shower warmth but wasn't freezing). Turning the cold on makes the hot tap go a bit weaker. Since the shower's maximum output is 9.6ltr/min I presume we would be ok.

It's a Grohe I'm looking at - seems quite reasonably priced for the brand and the way it looks. Got that nice sturdy hotel shower look about it, doubt it will stay as clean though!

In reply to elliot.baker:

Just to support the above - the statutory minimum water pressure for UK mains is 1 bar and it is usually a fair bit higher.  It is usual to convert everything in the house to mains pressure (no loft tanks required) when installing combi boilers, simply because you then get balanced pressure for installing things like thermostatic showers.  

 Andrew Lodge 03 May 2021
In reply to elliot.baker:

When we were looking at upgrading our shower a couple of years ago we did the boiler at the same time and went for an unvented system, effectively both hot and cold water are at mains input pressure throughout the house.

Miles better flow rate than any combi boiler or electric shower could manage. Probably not the most efficient but it's a great shower every morning which is worth a lot to me. 

 CurlyStevo 03 May 2021
In reply to elliot.baker:

The actual boiler you have can make a difference too. The higher gas flow boilers can put out more hot water. Could be worth goggling that. I deliberately got a bit of a more expensive boiler when I upgraded it for that reason.

In reply to elliot.baker:

There is one problem with boiler fed showers that no one has mentioned yet. I have had boiler fed for the last 30 years through 4 houses, 6 showers and 5 different boilers.

I can't get them to run cool enough in summer, in summer winding the hot back far enough to be comfortable on the thermostatic valve restricts the hot flow that far that the flame shuts off because there isn't enough flow going through to trigger it. 

 CurlyStevo 03 May 2021
In reply to Dax H:

don't think you'll find that with a quality boiler like Worcester Bosch 

In reply to Dax H:

Never had that issue on our combi (worcester greenstar).

 gethin_allen 04 May 2021
In reply to Dax H:

I turn down the hot water temp at the boiler in the summer. 

In reply to CurlyStevo:

> don't think you'll find that with a quality boiler like Worcester Bosch 

That is my boiler

In reply to gethin_allen:

> I turn down the hot water temp at the boiler in the summer. 

Me too, just can't get it low enough. 

 Cobra_Head 04 May 2021
In reply to Dax H:

> There is one problem with boiler fed showers that no one has mentioned yet. I have had boiler fed for the last 30 years through 4 houses, 6 showers and 5 different boilers.

> I can't get them to run cool enough in summer, in summer winding the hot back far enough to be comfortable on the thermostatic valve restricts the hot flow that far that the flame shuts off because there isn't enough flow going through to trigger it. 


You can just have cold showers in the summer though.

In reply to Pedro50:

> I know nothing of the figures but my sister remodeled her bathroom and replaced her lovely electric shower with one fed from the modern combi boiler. To my mind it's barely adequate and disappointing, I'm too polite to mention it.

As electric showers are trickly crap, I guess that means her house has a water pressure problem.  I don't *think* you can fit a combi on a tank so it'll be a mains pressure issue.

In reply to CurlyStevo:

> don't think you'll find that with a quality boiler like Worcester Bosch

Doesn't happen to mine (Worcester Bosch Greenstar 25i).

As others have said the fix is to turn the hot water temperature on the boiler down a bit.  That said mine is on 65 degrees (which I believe is recommended to avoid Legionella) and you can turn the shower down a fair way from how I have it before it "flames out".

What there is an issue with is thermostatic mixers and non-modulating boilers - they "fight" with each other, a bit like you get if you fit a thermostatic radiator valve in the same room as your main thermostat.  If you have a non-modulating (older) boiler you want a non-thermostatic mixer.

Post edited at 16:43
 deepsoup 04 May 2021
In reply to Dax H:

> Me too, just can't get it low enough. 

Any way you can crank up the flow rate so you're mixing your minimum feasible amount of hot water with more cold?

I don't have that problem either (Worcester Greenstar 30si in my case), but I don't have a thermostatic shower so may be doing just that without realising it.

 Si dH 04 May 2021
In reply to elliot.baker:

I've had a Mira Realm shower like the one you describe installed with two heads at both our current and previous houses. They're not cheap but it's been great in both locations. No issues with flow or temperature control (which is completely continuously variable). From what you said about the taps it sounds like your water pressure is fine, the only other thing is to check your model of boiler and find out the maximum hot water flow rate (probably google-able.) It's likely to have easily sufficient flow except for if you are running something else at the same time, but worth checking if it's a small one.

(Edit, in our old house I the boiler was a standard Worcester combi 28cdi, in our current house it's a Worcester highflow 440 which mostly works like a combi but also has a small integrated water tank that allows it to increase max flow for a period. In both cases I have had no problems.)

Post edited at 18:45
In reply to Si dH:

> (Edit, in our old house I the boiler was a standard Worcester combi 28cdi, in our current house it's a Worcester highflow 440 which mostly works like a combi but also has a small integrated water tank that allows it to increase max flow for a period. In both cases I have had no problems.)

It sounds like there's a range of boiler types under the 'combi' label. I'm on oil rather than gas and have a worcester greenstar 25/32 (25kW hot water /32kw heating). Like yours it has an insulated pressurised water tank, and I wonder if thats why I don't have the issues Dax describes in the summer, if the burner cuts out I'm still getting constant hot water from the tank until the burner fires up again?

 Si dH 04 May 2021
In reply to Ridge:

Not sure but I never had those issues with my old combi either. The highflow design is fairly unusual for gas boilers, they are expensive and only usually recommended for houses with multiple bathrooms that might want to run two showers together.

In reply to Dax H:

> There is one problem with boiler fed showers that no one has mentioned yet. I have had boiler fed for the last 30 years through 4 houses, 6 showers and 5 different boilers.

> I can't get them to run cool enough in summer, in summer winding the hot back far enough to be comfortable on the thermostatic valve restricts the hot flow that far that the flame shuts off because there isn't enough flow going through to trigger it. 

I've only had that problem in somewhere that had poor flow (old lead pipe supply that was furred up). The pressure was good when no water used, but it dropped off to low pressure as soon as you used it. Once the supply from under sink to mains in street was replaced with new blue plastic, all was good with the shower. (I like cool showers too)

 jkarran 12:53 Wed
In reply to elliot.baker:

> a) because the boiler is 1.2 bar we are definitely ok, or

That's your central heating system pressure (at the boiler's elevation). Irrelevant here.

> b) I need to measure the water flow coming out of the bath taps (where the shower will be) using a bucket and stop watch, and if it's more than 9.5 litre/min then I'm definitely ok (and if it's less than that does it mean we definitely shouldn't get that shower?)

Basically. If you get a good (~9,5L/min) flow from your bath taps when they're producing a mix of water that's a nice bath temperature then your new shower should work fine (the shower will have slightly less pressure than bath because it's physically a bit higher, only matters if it's all very marginal). If you want both heads on at once you need to check the system can deliver more like 16L/min of warm water (though a little on the low side would still work, who really needs two full power showers at once anyway). Don't just measure the hot then the cold, have them both flowing while you make your measurements and make sure the mix running into the bath is pleasant, flow restrictions elsewhere in the system can mean you get lots of hot or lots of cold but nowhere near twice as much mixed.

If you're a bit low on flow then before giving up or splurging on a pump it's worth checking you have full bore shut-off valves in place (likely hidden under the bath panel), the small ones are quite restrictive hence not normally found on baths but you never know.

Given they're shit and more expensive there's probably a good reason someone put an electric shower in. It might no longer be an issue of course if for example the boiler has been upgraded since the shower was installed or been swapped to a combi from a tanked system. It might be though that the boiler can't serve two showers/baths at once, if it's a second bathroom and simultaneous use of both shower rooms is needed you'll have to do your flow tests with the other shower running.

> c) something else!

If you're struggling for flow you can pump it but it's a really a job for a plumber and or electrician and it won't solve a too small boiler issue.

jk

Post edited at 12:54

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