I presume it's intended as just that (a 'bypass'); Is it closed, or nearly closed? The question obvioulsy is 'why?'.
) - so one would not have thought that it would serve any safety or pump-preserving function (unless the heating coil got blocked, which is very unlikely). AFAICS, the only effect opening that 'bypass valve' would have would be to reduce flow through the DHW coil (when it was 'operational') and thereby cause it to take longer to heat up the water before the cylinder stat was satisfied.
It's closed now. I can't remember how open it was. It cannot be intended to be frequently altered - merely 'set'.
It cannot reduce the overall impedance, can it?As you've said in your response in the plumbing forum, at first sight it doesn't really make any sense. All it is 'bypassing' is the DHW heating coil, which is surely already 'very low impedeance' (to keep the discussion forum-relevant
Yes, quite - but no point.
But it doesn't, does it? It still has to go through the same pipes before and after.
It is and it does not.
Akin to wiring a switch in parallel with a load - pointless?
Yes I think you have.
Sure, but that's the nature of bypass arrangements - 'set' and the time of installation and never altered.
Of course it can - think two resistors (or other loads) in parallel.
Sure it would. The impedance of the pipes before and after obviously wouldn't change, but the most important part of the impedance of the circuit as a whole is the connection between flow and return (through coil and/or bypass) which would change consdierably if you opened that bypass valve. So, if it was doing 'DHW only', with the pump trying to do its 'constant current/flow' thing, opening that valve would result in an appreciable reduction in pressure at the pump side of the coil/bypass. Again, think of electrical anolgies (given a constant-current source).
Well, not just a switch - a switch with a small resistance (aka that valve) in series with it. However, not pointless if one had a constant current source and wanted to reduce the voltage (aka pressure) across the load (hence also across source, aka pump).
I admit it can/would in the by-pass pipe and the coil but not at the pump where the same one pipe is connected nor in the same one pipe back to the boiler.
OK. In contrast with the ('simplified') electrical analogy I posted, you are 'not ignoring the resistance of the wiring'. The analogy (and, indeed, what I was suggesting) relies on the fact that the 'distribution' wires or pipes have fairly 'negligible' impedance in comparison with the 'load' (coil in your case). If it's t'other way around (i.e impedance of coil negligible in comparison with impedance of the rest of the pipework, then, as far as the pump is concerned, reducing the impedance of the (already negligible impedance) coil a bit more with the bypass will clearly make very little difference - which I guess is the way you are thinking.
I would imagine that the original setup may well have been as you describe, but I'm not sure how that would/could explain the bypass thingy. My system here is sort-of like having a 3-port valve, in as much as th DHW and several different CH zones each have their own motorised valves. However, despite that, when the plumber put the system together (about 25 years ago) he actually did almost the opposite of your bypass thing, in that he not only deliberately reduced the feed to the DHW coil to 15mm (like 'yours') but also put a gate valve in that 15mm feed and closed it down a fair bit 'by ear/feel'! His excuse/explanation was that "you'll get far too much flow through that there coil if we don't restrict it a bit"!! It made no sense to me - either then or now. It struck me that "too much flow" merely meant that my hot water would heat up quicker! Maybe he thought that it would be noisy? Anyway, I didn't argue, and have lived with it for 25+ years!
Slowing the flow through the coil down not only reduces the entry/exit delta temperature, but also increases the reheat time of the cylinder, as the end of the coil will be a lot cooler than it needs to be.
Indeed so - I wrote:
I've often thought about that, but my head goes around in circles a bit! With a slower flow, there will be greater extraction of heat per volume of 'coil water' (hence temperature of 'coil water' will drop more during its passage through the coil), but there will be less volume/time of coil water delivered. With higher flow, there will be less extraction per volume of 'coil water' (hence temperature of water will drop less during its passage through coil), but there is a greater volume/time of coil water being delivered. Which of those opposing effects 'wins', I haven't a clue! They might end up much the same!
Indeed. As I said, my immediate reaction, like yours, was that it didn't make sense, for that reason. My 'just another thought' proposal was a desparate attempt to see if I could think of any way in which is could be rationised/justified, but I suspect that's not the reason. As you say, I suspect that it's just a 'mistake'.
Only? What if, like my system, that flow reduction is achieved by a restriction in the feed to the coil?
That's certainly what common sense says, and was the initial reaction of both EFLI and myself. If you haven't already seen it, you may be amused to see the response that EFLI got in the plumbing forum. I wonder if, like electricians, plumbers are meant to understand what they're doing and saying?
Oh sorry, must have skipped that bit. I wondered where I got the idea.
Yes, the only advantage I can think of is less heat loss on the return to the boiler.
Yes, thank you for the input.
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