Replacing Toilet Cistern Innards

[Roger Mills]

Our bathroom toilet cistern - fitted when the bathroom was refurbished in the early 1990's and not touched since - had become very difficult to flush, and it was pretty obvious that the syphon was shot. It's a close-coupled toilet with a fairly heavy vitreous cistern, which was fitted with a conventional lever-operated flush syphon and bottom entry ball valve, and bottom entry overflow pipe - discharging to the outside world.

I decided to replace the syphon with a button-operated dual-flush valve like this

and the fill valve with one of these:

The job is now done. [I could write a book about why it took me a whole afternoon to get it apart, but I won't bore you with that!]

I do have a few questions, though, for future reference - because I have another toilet in the en-suite which may get the same treatment even though its flush isn't *too* bad as yet.

The dual-flush valve has its own adjustable overflow pipe, which discharges into the pan - potentially doing away with the need for an external overflow. But: (a) I already had an external overflow which would have been more trouble than it's worth to disconnect and blank the hole in the bottom on the cistern, and (b) I think I'm more likely to notice water dripping from an external overflow than dripping into the pan - and I'm on a water meter!

So, what do people normally do when doing a refurb which results in 2 overflows? I've set the adjustable one on the new flush valve a bit

*higher* than the original overflow pipe so that it will only discharge into the pan if the original one can't cope for some reason. Does that sound reasonable?

The new fill valve is adjustable for height. It has a "critical level" indicator and the blurb says that this must be at least 1" above the overflow level. But if I do that, the top of the fill valve is too high to fit the lid! What am I supposed to do? I could potentially cut a bit off the overflow pipe - or adjust the new valve's overflow to a lower level so as to discharge into the pan rather than outside. But the overflow level would then be dangerously near to the normal fill level - so it could only over-fill by a tiny amount before overflowing.

So, what's this "critical level" business all about, and what are the likely consequences of disregarding it?

I imagine that a few of you fit quite a lot of toilets, and must have come across these issues.

[The Natural Philosopher]

I have found these mechanism and indeed much of bathroom plumbing to be a bit of a Pirates Code. More a sort of guideline than and actual rule book.

IN general I take them and modify them and replace crappy seals with good ones and with the use of a bit of brainpower and silicone sealant bugger with them till they work.

[David Lang]

The internal overflow is a backward step IMO, for exactly the reason you state.

It does.

No idea I'm afraid.

[Roger Mills]

Thanks for your comments. Are there any rules of thumb on what the minimum vertical distance should be between the normal fill level and the overflow level?

Mine is currently about 28mm, and this "critical level" mark is also

28mm above the water - making it *level* with the overflow rather than one inch above it, as specified. If I reduce the overflow level to meet this requirement, it will be only about 3mm above the normal fill level

- which sounds far too little to me. [I suppose I could lower the fill level, but this may impact the flush performance.]

[Graham.]

I have fitted both my loos with that same pair of valves, but as I didn't bother to RTFM, and just winged it I was blissfully unaware (until now) of critical levels etc. Only the downstairs cistern has a conventional "warning pipe" overflow.

I'm sure it will be OK.

[spuorgelgoog]

I think there has to be a certain height (25 mm?) between the water inlet and the overflow level so that cistern water cannot back-siphon into the mains supply even in an overflow situation. If not, then use a double-check-valve in the supply.

I think BS 6700 and BS 12121 part 2 or 3 "Type B air gap" may apply, or is that for storage tanks not WC cisterns? Anyway the principle is the same.

Owain

[spuorgelgoog]

I was right.

The physical or piezometric level of the fluid in any part of the receptacle a minimum of two seconds after closing the water inlet, starting from maximum water level.

Interpretation given in Regulators' Backflow Specification - backflow prevention arrangements and devices: terms relating to backflow prevention

Owain

[Roger Mills]

Thanks. In that case, I don't think I'll worry too much. It would take several things to fail at the same time before there was a problem, and even so, the pipework is long enough to ensure that even if there were some backflow, it wouldn't get as far as the public maims.