I have a Quooker ‘Boiling Water Tap’ that was installed in 2020.  Since the water in Epsom is basically ‘dilute cement’, coming from a borehole in the chalk  (water hardness 24 ° dH), I had a limescale filter (water softener) fitted into its supply line, but this needed changing every 6 months and at £80 a pop, this was an expensive business.

Clearly I wasn’t the only one who thought so, because in 2023 they said they were discontinuing that filter and introducing one with a higher capacity – and a higher price, of course.

One of my New Year jobs was to fit this new filter. Unfortunately, the Quooker instructions were far from clear – mainly consisting of diagrams that had to be read with some rather terse instructions.  I know from my own experience how hard it is to write clear instructions: the lack of labelling on the diagrams gave me the impression that this was to make them ‘language neutral’ – they could use the same diagrams for instructions in various languages.  Moreover, I realised that the instructions that came with the new filter did not include how to take out the old filter.  Naturally, I presumed that I just needed to take out the old one and connect the new one in  its place.

WRONG!  The connectors wouldn’t marry up to the existing pipe ends. So I explored the Quooker website and found a download of how to substitute the new filter in place of the old one.  But this wasn’t straightforward, because there are several different models of boiler and the first task was to identify it from the model number, of which there are quite a lot. Mine was a model PRO3 E and a particular feature of this is a pressure relief valve in the water supply line to the boiler. The original ‘Scale Control Plus’ filter was mounted between the incoming water supply and the inlet of the pressure relief valve, whereas the new Scale Control R had to be fitted between the outlet from the pressure relief valve and the inlet to the boiler. Perhaps we shouldn’t be surprised that this is not mentioned anywhere: you have to deduce it from the diagrams.  But it makes sense because then the various adaptors that they supply do enable you connect the new filter in place.

But now I had a problem – there was a stonking great gap of almost a metre between the water supply inlet and the pressure relief valve.  The instructions casually show a short piece of copper pipe (not supplied) to bridge this gap.  Except that because the old filter had around a metre of plastic tubing between these two points, it would be a major plumbing job to get some piping, cut it to size, solder two 90-degree elbows and fit it all in place with connectors, olives and what-have-you all in an awkward space in the cupboard underneath the sink.

Then I had a brainwave –  I could do this with a flexible pipe.  After all, the other connectors to the filter and boiler all had flexible tubing.

When is ¾ inch actually ½ inch?

Now we enter another maze.  I measured the outside diameter of the threaded (male) connectors of the two ends that I needed to join.

I measured these as having around 18 mm external diameter screw thread, but when I looked on the Screwfix website, the screw thread connectors all seemed to be imperial.  The nearest was fitting was for a ¾-inch external diameter and I ascertained that I would need a pipe about 80 cm long.  Looking through the Screwfix and similar websites, I couldn’t find long flexible pipes: there were some that said ¾-inch to 22 mm, which puzzled me until I realised that the 22 mm was the size of a compression fitting onto 22 mm tubing.

Then I saw that a washing machine hose was plenty long enough and had ¾-inch connectors.  Not ideal for a permanent water connection, but I could find a proper reinforced flexible pipe on-line for later delivery.

Disaster! ¾-inch connectors are actually 1 inch in diameter! They were far too big. How come?  It turns out that  ½-inch connectors are actually ¾-inch diameter! It’s because historically, pipe sizes relate to the internal bore of the pipe, not its outside diameter, and you have to allow for the thickness of the pipe wall.

Internationally Imperial

And even more surprising, despite all the hype about returning to imperial units in the UK, it seems that for pipe connectors we have remained imperial, even to the point that the sizes are internationally standardised as BSP (British Standard Pipe) sizes – except of course in USA where they have their own system.

Confusingly, the external diameter of pipe threads are actually ¼-inch larger than their nominal size!

So a half-inch flexible pipe is required to connect to  the ¾-inch outside diameter of the actual connectors.  Luckily I was able to buy these easily on line. Wey-hey!

Parallel vs Tapered threads

Now we come to another little wrinkle regarding pipe threads.  One end of the connector fitted without problems onto the stop valve for the water supply, but I had difficulty getting the other end to pull up tight onto the inlet to the pressure relief valve.  Eventually, I realised that the inlet at that end had a tapered thread! Yes, BSP threads come in BSPP parallel threaded  and BSPT tapered versions.  The idea of the tapered version is that as you tighten the connector, the tapered thread binds to the connector’s thread and ensures it is water-tight.  However, I had to put some graphite lubricant onto the thread in order to get it to pull up tight to the washer in the connector- even then using two large spanners.

Anyway, thank goodness it all works now. But it was far more difficult than it would have been with clear instructions – including something as simple as details of the pipe thread required – and I literally have the scars to prove it!