How to determine the right shape and size of a Wirquin Macdee Metro Rapid siphon for a specific cistern

[@TonyC]

I am planning to get a new WC for the downstairs cloakroom in our house. We have two possible WCs in mind. Both have a cistern with a lever rather than a push button on the middle of the lid, which is what my wife and I prefer. I am concerned about the siphons supplied with the cisterns. I have written to the manufacturer of each WC but have not yet received a reply. But I strongly suspect that the supplied siphons will not be of the type that come in two parts, allowing the bell housing assembly to be removed from the siphon leg without having to drain the cistern and remove the cistern from the pan. With this type of siphon, changing a broken diaphragm washer becomes a 15 minute DIY job.

So, I am expecting to have to replace the supplied siphon with a Wirquin Macdee Metro Rapid siphon. The problem then is how to select the shape (of the bell housing) of the siphon (oblong, square, D-shaped or round) and the size of the siphon (6/7 sizes depending the shape, from 7.5” to 10.5”). Does anyone have any experience of how to select the right shape and size for a specific cistern?

Intuitively, I feel that, when you unscrew the nut to remove the bell housing assembly of a Metro Rapid siphon, the exposed top of the siphon leg should be above the waterline so that water does not start to flow down the siphon leg into the pan. So, one question that comes to mind is, knowing the recommended depth of water in the cistern, how do I determine the optimal size of Metro Rapid siphon for that cistern?

I have had some correspondence with Wirquin on this issue, but they have been particularly vague in their replies.

(PS. I know that there are other siphons on the market like the Metro Rapid - the Dudley Turbo 88, for example. But we have a Metro Rapid in another WC in our house and it works very well.)

 

[Ben-gee]

Height adjustable turbo 88 is your best bet here.

 

[@TonyC]

Height adjustable turbo 88 is your best bet here.

Thanks. But does the Turbo 88 support internal overflow, an important requirement for many modern cisterns?

I actually have a very old (dating back to the 1990s) Turbo 88 on another WC in our house. It doesn't work as well as the Metro Rapid. The pin joining the two parts of the siphon is more fiddly than the screwed nut on the Metro Rapid. And, as far as I am aware, it does not support internal overflow because the cistern has an external overflow pipe to the outside of the house. I've had a quick look at the specifications of the Turbo 88 on the Dudley Web site and there is no mention of it supporting internal overflow.

 

[WC1]

Yes it does.
+ 1 for the dudley

 

[Ben-gee]

Any syphon is also an internal overflow - you can’t stop it from being one it’s just physics.

You just have to make sure the spill over level is below any holes in the cistern (screw holes, handle hole etc) - hence get a height adjustable one.

 

[@TonyC]

Any syphon is also an internal overflow - you can’t stop it from being one it’s just physics.

You just have to make sure the spill over level is below any holes in the cistern (screw holes, handle hole etc) - hence get a height adjustable one.

Yes, I can see that.

But it makes me wonder why some manufacturers make a special point of stating that their siphon supports internal overflow and other manufacturers don't. In the latter case, I begin to wonder whether there is something about the design or configuration of the siphon that could make it unsuitable to act as an internal overflow.

And I also wonder why so many cisterns with siphons in the past have been installed with an external overflow pipe. Is it "belt and braces"?

 

[WC1]

The external often just gets left in place when z new flush goes in. Internal (infernal?) overflows are a shocking waste of water. Many people without a meter just let it run, sometimes for years without knowing it isn't meant to do that.

 

[Ben-gee]

They may not state it is internal overflow, but it is!!

External overflow, often better referred to as warning pipe - has three good functions.
1/ serves as a visual warning that something is wrong.
2/ may be necessary if cistern has holes/cutouts below the spillover level of the syphon.
3/ ALWAYS functions as an overflow, internal overflows don’t work - in fact cause a disaster - if the sewer is blocked.