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:
AdamW said:
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...
AdamW said:
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".
AdamW said:
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?
AdamW said:
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.
AdamW said:
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.
AdamW said:
I submit that the post appearing at the top of this page is correct...
No. It isn't.
AdamW said:
...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.
AdamW said:
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?
AdamW said:
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.
AdamW said:
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.
), will reduce the cold pressure/flowrate for the duration of the toilet filling, and thus the hot water flowrate.