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First things first.
An age old question. By convention, as you are facing the sink, the hot tap should be on the left and the cold on the right. It is not uncommon in older instalations to see the hot tap on the right hand side. Many people who are used to that configuration still prefer it. For kitchen taps, in England and Wales there are no current regulations to stop you doing just that but that is not the case for bathroom sinks.
In the Building Regulations for England 2010 and in the Building Regulations for England 2010, states Part G 5.5 Where hot and cold taps are provided ona sanitary appliance, the hot tap should be on the left. These regulations are not retrospective, there is no obligation to change taps that are already fitted although it only seems a matter of time before kitchen sinks are brought under the same regulation. Rules for Scotland are different, we will try and clarify the position under Scottish regulations. Regulations in the USA, states hot should be on the left.
No one can be entirely sure. At one time most sinks had no taps or just had one tap for cold water. This was often in the centre but sometimes placed to the right. When the hot tap was introduced it was placed to the left of the cold tap. It is generally assumed cold to the right as most people are right handed but any accurate evidence for this seems to have been lost with time.
All drawings by the WRAS (Water Regulatory Advisory Service) For English and Welsh regulations and Scottish bylaws, show cold water to the right and hot to the left.
Monoblock mixer taps are deigned for hot left and cold right. Most however will work either way. If your tap, or any other device, has a thermostaic valve fitted, then it is essential that the hot is connected to the left hand side to ensure the device oparates safely.
Sometimes you may be required to cut holes for your tap, but at what distance apart ? The industry standard distance between the hot and cold water taps is 180mm between the hole centres. Although there is no regulation to say you can't put them any distance you like if the situation suits, keeping to this 180mm distance will enable you to fit a bridge type mixer tap should you require it in the future.
There are many styles of bridge mixer taps should you choose to change your a pair of pillar taps. Keeping to the 180mm standard centre improves your choices.
If you do not have standard spaced holes and you wish to fit a mixer tap all is not lost. Some bridge taps are available with a variable offset mount. You can adjust them to fit your holes. The only drawback is that the spout of the tap may end up further forward or back than you would prefer.
The hole size to accommodate the vast majority of monoblock taps is 35mm in diameter. Check this before you cut your hole if you are fitting a new tap, especially if you have a third connection for a rinse head. Some of these can be considerably larger.
Having a standard for hot and cold obviously makes sense from a fitting point of view. Many plumbers can work simulataneously on a larger system without having to second guess another plumber's work or to cross over pipes to marry hot and cold . There are also safety considerations. Much is made today, especially in the US, of blind people knowing which tap is which without having to resort to trial and error.
A set of pillar taps and a pair of bib taps on an extended mounting. Ideally, seperate hot and cold taps, should be 180mm apart.
It may come as surprise to many people that the UK was almost unique in the world has having traditionally used searate hot and cold water taps. But for what reason ? Well actually for two very good reasons. Firstly unlike many other countries, from early into its introduction, piped water in the UK was classed as potable, meaning suitable for drinking. Cold water was driven through the mains, to the cold tap either by pumps or gravity using high altitude reservoirs. Hot water pressure on the other hand was generally supplied by a storage tank in the loft. Storage tanks in the loft where left open, subject to contamination by dust and insects and were not therefore deemed as drinkable water. This means that it was very common to have a big difference in pressure between the hot and drinkable cold supply.
Mixer taps, especially early ones, do not generally work well with a big difference in supply pressure between hot and cold, with a simple mixer design the high pressure of the cold water restricts the flow of the hot water into the tap. You may have encountered this effect on an old system, the mixer tap feeds cold water for the majority of the turn and switches quickly to hot at the last bit of movement with very little adjustment in between. To combat this, most modern mixer taps have an improved method of operation for an improved control of water temperature.
This difference in pressure also has the potential to cause a hazard to health. In rare but theoretically possible situations, water from the hot supply can be drawn into the cold water supply and could in theory, contaminate the cold water supply. This effect is taken very seriously by the water authorites these days who insist on non return valves in any situation where it can occur.
With the introduction and ever increasing popularity of combi boilers, the design of which ensures that there is no exposed storage tank and the suppy of equal, mains pressure to both hot and cold taps, this is becoming much less of an issue. This may account for the increase in popularity of mixer taps in the UK.