Venturi Showers

[AdamW]

I believe that the cold supply to the shower is from the mains, whereas the hot water stored. Therefore a transient drop in mains pressure will not affect the hot pressure,

A transient drop in mains pressure WILL produce a transient drop in hot water flowrate. A toilet refilling, which I certainly wouldn't describe as transient (although in plumbing parlance perhaps transients last this sort of time, you tell me ), will reduce the cold pressure/flowrate for the duration of the toilet filling, and thus the hot water flowrate.

althought it will affect the flow of mixed water from the shower head - this is because the flow of mixed water is an aggregate flow dictated by the 3.0 bar of mains cold and 0.3 bar of stored hot.

... the flow rate you experience will be proportional to the temperature of the incoming cold water.

Erm, not by the temperature, but by the pressure of the two supplies.

No, I meant what I said. As a human being, you will set your shower to about 40 degrees celsius. Mine is at 42, I like it hot. Now, your hot water pressure and temperature is constant, and will be about 60 celsius. Your incoming cold will be about 15-20 in summer, and as low as 3 or 4 degrees in winter.

So in summer, your shower will be pumping through 1 litre of hot for very litre of cold to get a mixed temperature of 40. In winter, you will need nearly 2 litres of hot for every litre of cold. Now, with a significantly lower cold flowrate, the venturi assist to the hot water is also significantly decreased. Thus, your overall flowrate will decrease.

Erm, with at least two of the three products that I've read about (one of which I've installed), you can't turn down the cold as you would do on a conventional shower valve.

Well in that case I would like to see just how the hell they've designed the venturi to cope with such a wide range of incoming cold temperatures!

If you're talking about an instantaneous water heater type of electric shower, then I would expect one of this type would be worse than a venturi valve, because all such (electric) showers are quite poor.

I agree electric showers are pants, but I would have to try a venturi shower (good old Bernoulli!) before I'm convinced it is a danger to pump sales.

Although there is a minimum pressure at which they will work, yer actual flow from the shower head is a function of the power rating of the element and the specific rise in temperature needed,

Yeh, thanks, I'll just be sitting here in the corner with dribble coming out of the corner of my mouth occasionally shouting "DUUUUURRRRRRRRRRR!!!" and wearing a big gold badge saying "******". No, really, I hadn't figured that all out. Seriously, thanks Prof!

I think I can now safely pass on my pedant's sceptre to Softus, knowing it is in good hands

 

[Softus]

At the risk of stating the obvious, if I assert something that's the polar opposite of something you wrote, then I'm not stating the obvious. Or maybe I just can't cut through the jungle of sarcasm and "street talk" - your posting was, at best, garbled.

For example, when you say "hot water flowrate", it isn't clear whether you're referring to the rate of hot water coming from the cylinder, or the mixed water from the shower head.

If the former, then, however you choose to define transient, the flow rate isn't affected by a toilet refilling.

If the latter, then you're just arguing semantics, which holds no interest whatsoever for me. Best keep that sceptre.

 

[civory]

Thanks again S - you're really helpful. I live in the north-east so any supplier
between Newcastle and Darlington would do! My initial attempt at ordering was from www.h-i-e-plumbing.co.uk.

Cheers CI

 

[Softus]

Let me know if you want me to get you one.

If you want to pursue this idea then you'll find my Email address in my profile.

 

[civory]

Hi S - your profile doesn't show your email address (or maybe it's hidden).

Problem is I could really do with having the shower by Monday at the latest.

Nobody locally seems to have one in stock - all quoting 7 days or more for delivery.

 

[Softus]

I will check tomorrow and get back to you.

 

[AdamW]

At the risk of stating the obvious, if I assert something that's the polar opposite of something you wrote, then I'm not stating the obvious. Or maybe I just can't cut through the jungle of sarcasm and "street talk" - your posting was, at best, garbled.

For example, when you say "hot water flowrate", it isn't clear whether you're referring to the rate of hot water coming from the cylinder, or the mixed water from the shower head.

If the former, then, however you choose to define transient, the flow rate isn't affected by a toilet refilling.

If the latter, then you're just arguing semantics, which holds no interest whatsoever for me. Best keep that sceptre.

Generally, tangential posts on this forum are written assuming the reader has a certain level of background knowledge on the subject. So, whilst my post may have seemed ambiguous, it is not unreasonable for me to assume that the reader (in your case apparently a plumber) would understand what I mean.

Take the electrics forum. A while back there was a post by someone arguing that the "neutral" in a domestic single-phase supply is not actually a neutral at all. Yet we still call it that, because it's just easy. It's convention (in this country at least) but if the EU changed the name to some other term, we would still call it neutral.

I could have written my answer with mathematical equations. As I said, I have something of an understanding of the theory behind the venturi effect - I do work in the aerospace industry after all, and it is generally held that fluid dynamics are quite important round our way.

Didn't mean to offend by offering you the sceptre, but I feel you are throwing some sarcasm back at me with your assertion that you are not a pedant Arguing semantics... right back at you.

As to the flowrate/toilet scenario, I invite you to turn on a mains-fed cold tap, observe or even measure the flow. Now flush the mains-fed toilet and see what happens. Go on, I dares ya.

I submit that the post appearing at the top of this page is correct, and if you can't understand it then I really suggest you just think about it a little. The hot water flowrate IS affected by cold water flowrate. Otherwise they wouldn't have bothered inventing the venturi shower. The greater the cold flowrate, the greater the potential the venturi valve has to "suck in" more hot water. As the venturi valve "sucks in" hot water from the hot water suppply pipe, the hot water cylinder will replenish it.

Flowrate and pressure are very much related here, it's just an application of Bernoulli's equation. So by talking about one we are also talking about the other.

It's really quite simple.

 

[Softus]

Hi S - your profile doesn't show your email address (or maybe it's hidden).

Problem is I could really do with having the shower by Monday at the latest.

Nobody locally seems to have one in stock - all quoting 7 days or more for delivery.

I meant to ask you (but I was tired) which type of valve you need - is it exposed or concealed? And if the former then do you want a fixed outlet or a riser rail?

Please look again at my profile - you should see my Email address.

 

[Softus]

A long post here AdamW, but I'll try to keep things clear...

Firstly, and most importantly, I have no wish to offend you, and would be dismayed if I'd been sarcastic. I'm happily taking the olive branch you're offering with every intention of having a proper forum debate. Secondly, on a previous point:

I agree electric showers are pants, but I would have to try a venturi shower (good old Bernoulli!) before I'm convinced it is a danger to pump sales.

This isn't a discussion on the relative merits of pumped vs. venturi showers - the enquirer isn't trying to make that particular choice. On to your specific points...

Generally, tangential posts on this forum are written assuming the reader has a certain level of background knowledge on the subject. So, whilst my post may have seemed ambiguous, it is not unreasonable for me to assume that the reader (in your case apparently a plumber) would understand what I mean.

You may be forgetting - it's quite important that I'm not the only person who understands what you've written. In the meantime, I understand all of the technical points, I just disagree with some significant parts, probably because you haven't defined what you mean by "hot water flowrate".

I could have written my answer with mathematical equations. As I said, I have something of an understanding of the theory behind the venturi effect - I do work in the aerospace industry after all, and it is generally held that fluid dynamics are quite important round our way.

Hm, perhaps, but that has nothing to do with anything here. One (other) thing I'm particularly struggling with is that you seem to have no experience or knowledge of venturi showers. Am I wrong, or are you basing your argument on your knowledge of the Venturi principle in the context of fluid dymanics in your industry?

Didn't mean to offend by offering you the sceptre, but I feel you are throwing some sarcasm back at me with your assertion that you are not a pedant Arguing semantics... right back at you.

Er, to be precise, I have never said that I'm not a pedant. However, I can't accept such an accolade from someone who has to rely on onomatopoeia to get their point across.

As to the flowrate/toilet scenario, I invite you to turn on a mains-fed cold tap, observe or even measure the flow. Now flush the mains-fed toilet and see what happens. Go on, I dares ya.

Now then, you know perfectly well what will happen, so this is where you're being obtuse. It would be reasonable (and economical of our time here) if you were to assume that I also know, and for you to conclude that we have crossed wires.

I've said, more than once, that the MIXED water flowrate will be affected by the cold flowrate. It will, therefore, be affected by any drop in mains pressure caused by, for example, filling a WC cisterm. However, more than once, you've said that the hot water flowrate will be affected by a change in mains pressure. This is not true.

I submit that the post appearing at the top of this page is correct...

No. It isn't.

...and if you can't understand it then I really suggest you just think about it a little.

You may not be trying to offend me, but most people would find that comment more than a teeny bit belittling. Anway, I've thought about it, for all of the handful of microseconds it needed wink, and you're still wrong.

The hot water flowrate IS affected by cold water flowrate. Otherwise they wouldn't have bothered inventing the venturi shower. The greater the cold flowrate, the greater the potential the venturi valve has to "suck in" more hot water. As the venturi valve "sucks in" hot water from the hot water suppply pipe, the hot water cylinder will replenish it.

Okaaaaay, and so to the nub of it. How is the pressure, or indeed the flow, of hot water at the outlet of the cylinder affected by a drop in mains pressure for the amount of time it takes for a WC cistern to fill?

Flowrate and pressure are very much related here, it's just an application of Bernoulli's equation. So by talking about one we are also talking about the other.

Of course they're related, just like gas pressure, volume, temperature and molar density are all related. But they are distinct attributes and can't be used interchangeably. For example, it's a common layman's fallacy that you can reduce the cold mains pressure by partially closing the stopcock. The fallacy usually arises because people often do it to reduce mains hammer, or to stop unclipped pipes (installed by incompetents) from banging. But when all other valves are closed, the pressure hasn't been reduced at all.

It's really quite simple.

I expect that both of us can mathematically demonstrate the effective pressure resulting from flowing water, which is fairly simple to do but would be strictly unnecessary here.

 

[petewood]

don't understand it but i'm enjoying it

 

[Softus]

don't understand it but i'm enjoying it

Jump in Pete - the water's lovely!

 

[AdamW]

OK, it goes like this:

1) you flush toilet

2) mains cold water pressure and flowrate available to the shower valve decrease whilst the toilet refills.

3) the hot water flowrate, which is of course increased by the venturi valve, will therefore decrease also...

Here is why:

If the cold water heading into the venturi valve decreases in flowrate, you will not get as much of a boost from good ole' venturi effect.

In operation, the venturi valve is essentially sucking the hot water out of the cylinder. This is why they can operate in negative head situations.

Therefore, a decrease in cold water pressure or flowrate will result in a decrease in hot water flowrate and pressure also.

Following this, the mixed flowrate will obviously decrease (as you have said: something we agree on! ), but partly because of decreased cold flowrate and partly because of decreased hot flowrate.

If you get to the situation where the cold water flowrate decreases to such an extent the venturi valve is not able to "suck" hot water out of a tank in the negative head situation, then flushing the toilet will stop hot water flow altogether.

 

[Softus]

OK, it goes like this:

<snipped>

In operation, the venturi valve is essentially sucking the hot water out of the cylinder. This is why they can operate in negative head situations.

Therefore, a decrease in cold water pressure or flowrate will result in a decrease in hot water flowrate and pressure also.

Following this, the mixed flowrate will obviously decrease (as you have said: something we agree on! ), but partly because of decreased cold flowrate and partly because of decreased hot flowrate.

If you get to the situation where the cold water flowrate decreases to such an extent the venturi valve is not able to "suck" hot water out of a tank in the negative head situation, then flushing the toilet will stop hot water flow altogether.

Excellent reply AW, and so much so that I can now see where the wires got crossed.

We've been agreeing by stating our case on precisely parallel paths of reasoning, but each failing to notice the vehicle on the adjacent carriageway.

My point started out as an objection to your assertion that the hot pressure reduced, and I became stuck on that objection even after you changed "pressure" to "flow".

Your point is that the temperature of the mixed water (emerging from the shower head) isn't affected by a mains pressure drop, because the proportion of hot and cold water remains the same, even though the individual (and combined) flow rates reduce.

Have I got it right this time?

On a final, pedantic point, I still have an issue with this bit:

Therefore, a decrease in cold water pressure or flowrate will result in a decrease in hot water flowrate and pressure also.

Since the cold storage cistern will commonly hold fifty gallons, and the WC cistern merely two, the head of water will reduce by no more than 4%. In practice, however, the pressure of hot water will reduce by even less. If we assume that both cisterns will fill at the same rate, then the average reduction in head will be only 2%.

I suggest that this reduction in hot water pressure, whilst real, is much less significant that you're suggesting.

 

[AdamW]

Lovely analogy with the carriageways and so forth

Your point is that the temperature of the mixed water (emerging from the shower head) isn't affected by a mains pressure drop, because the proportion of hot and cold water remains the same, even though the individual (and combined) flow rates reduce.

Spot on!

Since the cold storage cistern will commonly hold fifty gallons, and the WC cistern merely two, the head of water will reduce by no more than 4%. In practice, however, the pressure of hot water will reduce by even less. If we assume that both cisterns will fill at the same rate, then the average reduction in head will be only 2%.

Ah, but is the capacity of the cisterns relevent here? Seeing as the cold input to the venturi shower is mains rather than tanked?

Obviously in practice the inlet to the toilet filler should be fitted with a restrictor when supplied with mains, but knowing several friends with toilets that refill in around 5-seconds it would appear many people forget to fit them.

So, for those 5-seconds or so the pressure to the shower could halve (if the mains pressure was exactly equal to the pressure you were using at the shower prior to the flush)

However, with a 5.8 bar supply I think you could probably pressure wash your patio whilst flushing the toilet and still have enough pressure left over to run such a valve. But in a low mains pressure and/or negative head (for hot tank only, of course, fitting a venturi valve above the potential head of the mains supply will not help you at all) situation you could have the shower "stall", and stop flowing altogether whilst the toilet (presumably lower down than the shower) refills.

Glad we managed to untangle the web of fluid dynamics that was building up around us!

 

[Nige F]

The prize for pedantry should go to PVM,for he fulfills his moniker succinctly. His comments are quickly read and digested by all