There are various reasons why shower pumps cannot be connected as you ask. The feeds to a pump need to be at identical pressure to work properly and safely. Even if the feed was to vary, it must do so equally on both and this cannot be achieved by using different sources/pressures of water.
The whole point of safety is to ensure the shower has an uninterrupted supply of cold water (which a cold main can't guarantee so if any supply was to fail, it must be the hot that stops first. That is why regulations state the cold connection at the header tank must be lower than the hot.
Another point is to do with cold water supply. Pumps are designed to pump, not suck so are designed to feed from a free flowing supply of low pressure water which won't happen if the supply pressure is controlled by a regulator.
If the shower was being used and the mains supply was reduced or interrupted two things would occur. Firstly, hot water only would be fed to the shower head with obvious results, and secondly, the pump would attempt to draw water out of the main. If the main supplying the house is perforated, and it probably is, ground water could be sucked into the drinking water supply. Remember older mains were often laid next to foul sewers.
Connecting a pump in this manner contravenes WRAS regulations, and will invalidate any pump warranty as instructions will stipulate that these pumps are not suitable for fitting into mains supplies.
Sorry if this post has turned into a bit of a lecture, it wasn't intended to be that way. Hope this info helps.
can you give a little more info on the actuall shower
when you say power shower is there a pump involved,either seperate from the shower unit or is it built in?
the more info the better the answer people can give
many thanks bob
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!